Enhanced Oil Recovery Best Practices SPE 118055 PA

8/9/2019 Enhanced Oil Recovery Best Practices SPE 118055 PA

1/12February 2010 SPE Reservoir Evaluation & Engineering 143

Enhanced Oil Recovery Pilot TestingBest Practices

G.F. Teletzke, SPE, R.C. Wattenbarger, SPE, and J.R. Wilkinson, SPE, ExxonMobil Upstream Research Company

Summary

Enhanced-oil-recovery (EOR) implementation is complex, andsuccessful applications need to be tailored to each specific reser-voir. Therefore, a systematic staged evaluation and developmentprocess is required to screen, evaluate, pilot test, and apply EORprocesses for particular applications. Pilot testing can play a keyrole in this process. Before field testing, pilot objectives need tobe clearly defined and well spacing, pattern configuration, andinjectant volumes determined.

This paper outlines a staged approach to EOR evaluation andfocuses specifically on pilot testing best practices. These bestpractices were derived from ExxonMobil’s extensive pilotingexperience, which includes more than 50 field pilot tests cover-ing the full range of EOR processes. Topics covered include: (1)determining whether a pilot is needed and defining pilot objec-

tives, (2) considerations for successful pilot design, (3) typesof pilots and their advantages and disadvantages, (4) tools andtechniques for assessment of key reservoir mechanisms, and (5)minimizing uncertainty in pilot interpretation. Key issues that areoften addressed by pilots are discussed, including areal sweepand conformance, gravity override, viscous fingering, and loss ofmobility control. Also included are aspects of instrumentation andmeasurements in pilot injection, production, and monitoring wells.Several ExxonMobil piloting examples are used to illustrate thebest practices, including a single-well injectivity test, an uncon-fined pilot with observation wells, a small-scale confined pilot, anda large-scale multipattern pilot.

Staged Process for EOR Project Evaluation

and Development

The complexity and cost of EOR requires a disciplined workprocess for project evaluation, design, and implementation. To putpilot testing best practices in perspective, Fig. 1 outlines a stagedworkflow that ExxonMobil has used for evaluation and design ofEOR projects. The role of field tests and pilots in this process ishighlighted in the yellow box.

EOR evaluation starts with screening-level data collection,candidate process selection, injectant source identification, andscreening economics. If these are favorable, design and imple-mentation of an EOR project then requires in-depth analysis ofthe most promising processes. In addition to standard laboratorytests, specialized fluid characterization and reservoir-conditionscoreflood tests using in-situ fluids and a range of injectants areperformed to customize a process for each reservoir. Reservoir

characterization studies are conducted concurrently to identifythe key geologic controls on field-scale sweep efficiency. Thelaboratory experiments and reservoir characterization studies arethen used as input to geologic and dynamic reservoir-simulationmodeling of the process at various scales to evaluate options, definea preferred process design, and provide input to screening-leveldevelopment and facilities planning. If anticipated rates, recover-ies, and economics are favorable, pilot testing in the target field isoften undertaken to resolve uncertainties and fine tune operationaland execution details. Additional laboratory, reservoir characteriza-

tion, and simulation work may be undertaken after pilot testing to

resolve uncertainties further, as indicated by the feedback loop inFig. 1. If the technical and commercial outlook is still positive, thisis then followed by commercial-scale implementation. Stakeholderreviews, indicated by stars, are held after each stage of this process.Additional detail on the staged evaluation process, as applied topolymer flooding, is provided by Kaminski et al. (2007).

Pilot Objectives

Defining clear pilot objectives is the first step in designing andexecuting a successful pilot. Pilots are conducted to address keytechnical and business uncertainties and risks associated withapplying an EOR technology in a specific field. The benefits ofpiloting, however, need to be weighed against the time and expenseof piloting and against other available alternatives. Conducting a

pilot is one of several options for reducing risk that might includeadditional data gathering/appraisal or phased development. If thereare better alternatives to address uncertainty and risk, then a pilotmay not be required. Clearly stating the key uncertainties and pilotobjectives early in the evaluation process helps determine if a pilotis the best approach for addressing these risks and helps guide pilotdesign and execution.

Care should be taken when developing pilot objectives to ensurethat the pilot is appropriately used as a component of an overalllong-term field-development strategy. Pilots should not be a “trial-and-error” test of various field recovery processes; rather they areselectively applied to field test recovery processes that have beentechnically and economically evaluated beforehand. Additionally, therecovery process to be field tested should be optimized through bothlaboratory and reservoir-simulation studies in order to maximize oilrecovery at the lowest possible cost. Before field testing, the mostappropriate well spacing, pattern configuration, length and orienta-tion of wells, injectant, and injection strategy [e.g, continuous gasinjection, water-alternating gas (WAG), simultaneous water and gas(SWAG)] should be defined. Pilots are not run simply to gain experi-ence with application of technology, although training of operatorsmay be an important component of the pilot testing program.

With these comments in mind, specific piloting objectives mayinclude the following:

• Evaluate the EOR process recovery efficiency in the fieldof interest.

• Assess effects of reservoir geology on process performance,particularly sweep efficiency.

• Improve field-production forecasts to reduce technical and

economic risk.• Obtain data to calibrate reservoir-simulation models for full-

field predictions.• Identify operational issues and concerns for full-field devel-

opment.• Assess the effect of development options on recovery (e.g.,

well spacing, processing rate, and completion strategy).• Guide improvements in current operating strategy to improve

economics/recovery.

Considerations for Successful Pilot Design

Once pilot objectives have been defined clearly, sufficient timeand effort need to be expended in designing a pilot to ensurethat the pilot objectives can be achieved. Time spent up front in

pilot design and optimization usually leads to earlier full-fieldimplementation. Poorly designed pilots could potentially lead to

Copyright © 2010 Society of Petroleum Engineers

This paper (SPE 118055) was accepted for presentation at the Abu Dhabi InternationalPetroleum Exhibition and Conference, Abu Dhabi, UAE, 3–6 November 2008, and revised

for publication. Original manuscript received for review 9 September 2008. Revisedmanuscript received for review 3 April 2009. Paper peer approved 9 April 2009.


2/12144 February 2010 SPE Reservoir Evaluation & Engineering

the wrong conclusion or even to no conclusions at all. A poorlydesigned and executed pilot may lead to condemning an appropri-ate EOR process incorrectly or promoting an inappropriate EORprocess, incorrectly; both of which will result in suboptimal fielddevelopment.

By their nature, pilots are a scaled down version of the fullcommercial implementation of an EOR process. This scalingdown is brought about to reduce key uncertainties for decisionmaking in a manner that is as timely and cost-effective as possible.When designing a pilot, care should be taken to both understand

and minimize the impact of the scaled down nature of the pilot.Reduced well spacing, judicious placement of observation wells,and elevated injection rates are techniques that have been used toprovide information on process-recovery performance in a reason-able time frame. However, it is important that the pilot be designedto be scalable to the conditions for full-field application. Patternconfiguration, well design, the chosen injectant, and processoperations should allow for confidence in scale up to the field-wideimplementaton of the process. Finally, the pilot location should bechosen to ensure as much as possible that it can be well character-ized and is representative of the broader EOR target.

Reservoir simulation and geologic modeling, which incorporatethe best available reservoir description and are history matched topilot performance, are the most effective tools for designing andinterpreting pilot performance and translating that performance tofield-scale predictions. A properly designed pilot should ensurethat the pilot area is sufficiently characterized and sufficient pilotdata are collected to underpin reservoir modeling. Without properpilot design, however, reliable data for history matching field per-formance will not be gathered, and, therefore, confident assessmentof field-scale performance will be at risk.

EOR pilots should typically be designed to provide insight onboth the local displacement efficiency of the injectant at the porescale and the volumetric sweep efficiency at the reservoir scale. Afrequent challenge is to obtain a volumetric sweep efficiency thatadequately captures the improved local displacement efficiencyobserved in the laboratory.

With these comments in mind, the following are the require-ments for a successful pilot test:

• Pilot objectives should be clearly defined in advance. Thekey questions to be answered before doing a pilot are: (1) What

results are needed to facilitate full-field investment and operatingdecisions and (2) when are results needed?

• The pilot should be designed and operated to meet the objec-tives, aided by a predictive reservoir-simulation model. The pilotshould be able to distinguish between local reservoir/well effectsand general process mechanisms.

• Available reservoir characterization information should bereviewed to define key geologic factors that may affect injectivityand sweep efficiency and to identify a pilot site having represen-tative geology. Additional geologic studies may be required in

advance of the pilot to define the reservoir description to a suf-ficient level of accuracy.

• A surveillance and monitoring plan should be developed thatensures that data are of high quality and that all needed data areobtained on a timely schedule. Data should be gathered on opera-tional factors such as downtime and backpressure.

• The pilot should be designed and operated to ensure that itis interpretable. It is important that surrounding operations do notaffect pilot results. In addition, high-integrity well completionsare essential to understand and control sweep efficiency in thereservoir. Finally, a reliable injectant supply is required.

Types of Pilots and Their Advantages and

Disadvantages

Before discussing the types of pilots, it is important to clarify thedistinction between data gathering, pilot, and phased implementa-tion. The following is offered as a simple distinction:

• Data gathering: The primary purpose of data gathering is tocollect field data to address specific key uncertainties that couldhave a significant impact on a business decision. Example: If injec-tivity is a key uncertainty in assessing feasibility of a waterflood,then conduct a field test(s) to measure injectivity under a definedset of conditions.

• Pilot: The primary purpose is to validate the performance ofa particular EOR process in the field. Example: Laboratory testsand simulation studies indicate that a CO2 WAG project is likely toyield the highest recovery and best overall economic value amongrecovery processes considered. Before making a huge investment

required for a large-scale application, a pilot is conducted at a wellspacing scalable to that expected for full-scale application.

Staged Process for

EOR Project Evaluation and Development

Stakeholder review/approvals

Lab Data

Reservoir Characterization

Reservoir Simulation

Pilot Testing

Flood Management

Surveillance

Commercial Project Plan Field-wide project design and costs

• Full-field or multiple segment models

• Field-wide development/depletion plan and economics

Implementation, Surveillance, and Operations

Screen Candidate Processes• EOR process identification

• Injectant sources

• Screening economics

Evaluate Most Promising Processes In Depth• Fluid and rock property data collection/laboratory studies

• Reservoir characterization studies

• Mechanistic/fine-scale modeling

• Screening-level development/depletion/facilities plan

Field Tests and Pilots to Address Key Uncertainties• Objectives and design

• Data collection and interpretation

• Facilities reliability and wellbore integrity verification

Fig. 1—Staged process for EOR project evaluation and development.



• Phased implementation: The primary purpose is to manageuncertainty by implementing a project in phases, with appropriateadjustments in scope and optimization of design between phases.Example: A new reservoir development with limited injectantsupply planned as phased development, with the scope of thesecond phase (i.e., wells, facilities, recovery process) adjusted toincorporate learnings from the first phase.

With these definitions in mind, the types of pilots can be

grouped into four configurations:1. Nonproducing pilot.2. Small-scale unconfined pilot.3. Small-scale confined pilot.4. Multipattern producing pilot.While each pilot configuration has its place and purpose, it is

generally true that a more complex, and therefore, more costly,configuration will yield more data and be easier to scale up to com-mercial conditions. Therefore, a balance must be struck betweenthe risks of a commercial project and the cost of ensurance pro-vided by data from a pilot.

Fig. 2 illustrates factors that should be considered when selectingpilot type and scale. Two extreme cases are shown. In the first case,the recovery process is well understood because it has been provedcommercially in other fields, the reservoir is well understood becausethere is a nearby analog or existing application in the same field, andthere is low economic and injectant supply risk. In this case, commer-cial application without pilot testing may be considered, with someadditional data gathering or phased implementation to manage risk, asdiscussed earlier in this section. In the second case, the recovery pro-cess is untested, the reservoir is complex or not understood, and thereis significant economic and injectant supply risk. In this case, small-scale pilots, followed by a larger commercial demonstration pilot, arefrequently used to manage risk before commercial application. Clearly,a range of alternatives between the two extreme cases is possible.

The following is a discussion of pilot designs that have beenused to gather the necessary performance data to make commer-cial-scale implementation decisions, particularly for gas injectionand WAG processes. Both producing and nonproducing pilot

designs have been used successfully. Fig. 3 summarizes the non-producing configurations.

Nonproducing Pilots. The simplest design is a single-well injectiv-ity test to determine the ease at which gas can be injected into theformation and to evaluate injectivity losses resulting from WAGprocesses. By adding an observation well, the vertical sweep and thelocal displacement efficiency of the gas at the observer location canbe determined. Addition of a second observer permits the assessmentof the vertical sweep over the distance separating the two observers.The locations of the observation wells will need to account for both

reservoir heterogeneities and near-well pressure gradients (drift) thatmay result in the injected fluids moving away from rather than towardthe observation wells. Because gas injectants are frequently less densethan the in situ oil, observation wells will provide valuable informationon gravity override that may lead to poor sweep efficiency.

One key to successful gasflooding processes is achieving highvolumetric sweep efficiency. Placement of multiple observersaround the injector permits an assessment of not only the verticalsweep efficiency at the injectors but also the areal sweep efficiency.The product of the vertical and areal sweep efficiencies gives anestimate of the volumetric sweep efficiency for the pattern.

Fig. 4 summarizes the advantages and disadvantages of nonpro-ducing pilots. This type of pilot may be useful for providing quickand inexpensive estimates of injectivity and vertical sweep effi-ciency, but it does not provide quantitative data on overall volumet-ric sweep efficiency and ultimate recovery efficiency. In addition,it provides no operational experience with handling and recyclingproduced fluids and is extremely sensitive to fluid drift.

Producing Pilots. Pilots that incorporate production wells, other-wise known as “oil-in-the-tank” pilots, provide the most direct dataon oil recovery, fluid transport through the reservoir, and pressuredrop between injectors and producers. Important factors to con-sider when designing and interpreting producing pilots include:

• Drift: Is the pattern acting as a truly confined flow system?• Balance: Are the relative rates of injectors and producers

allocated to maximize areal sweep efficiency in the pilot area?• Isolation: Is the zone taking injection the only zone that is

producing?

The cost of running a pilot that is truly confined, balanced,and isolated may be considerable because offset production may

Pilot Size Should be Consistent with

Process/Reservoir Knowledge, Available Time, and Risk

Process Untested

Reservoir Complex

or not Well Understood

Significant Economic/

Injectant Supply Risk

Small-Scale Pilot

Large Demonstration Pilot

Commercial Application

Process Well Understood*

Reservoir Well Understood**

Low Economic/

Injectant Supply Risk

Commercial Application

* Process has been proven commercially in other fields** Nearby analog or previous application in same field

Fig. 2—Factors to consider when selecting pilot size and type.



need to be curtailed. This is especially important in systems withgas or light oil in which pressure gradients across the pilot sitemay result in significant fluid flux that will compromise pilotinterpretation. A compromise may have to be struck between thebest possible data and a situation that can be simulated later withreasonable confidence.

Another opportunity provided by a producing pilot is theexperience with separation and handling of produced fluids. Small-scale facilities can be constructed, and easily modified, to gainexperience with separation and recycling of fluids. If the pilotis successful, then the experience gained with facilities design

will translate into cost savings associated with construction of thecommercial facilities.

Observation wells provide a means of monitoring fluid movementat various points intermediate to the injector and producer. Valuableinformation on conformance, fluid transport in the reservoir, and

fluid mobilities can be gained from observation wells. Methods fordata acquisition from observation wells typically include logging,sampling, and pressure measurements.

Fig. 5 summarizes some representative producing pilot con-figurations. Producing pilots provide not only an understanding ofthe injectivity of fluids into the formation, but more importantly,some quantitative data on the production potential of the recoveryprocess, and subsequently a rough estimate of oil recovery. Single,inverted five-spot patterns are often used to provide such informa-tion. Observation wells are often included to evaluate the verticalsweep and displacement efficiency at the observers, vertical and

areal sweep at a distance, fluid mobilities within the formation,and to estimate oil recovery.

As indicated in Fig. 6, although unconfined producing pilotscan provide some production experience rapidly and at relativelylow cost, the swept volume can be difficult to evaluate and perfor-

Pilot Types: Non-Producing

Single Well Injectivity Test

Determine: Injectivity

Injector Offset with Static Observer


Vertical sweep at observer

Displacement efficiency at observer

Reservoir description between injection

and observation well

Injector Offset with Multiple

In-Line Observers


Vertical sweep at observers

Displacement efficiency at observers

Vertical sweep vs. distance

Reservoir description between

injection and observation wells

Injector Offset with Multiple

Areal Observers


Vertical sweep at observer


Areal sweep

Reservoir description between

injection and observation wells

Fig. 3—Nonproducing pilot designs.

Non-Producing Pilots

Advantages

• Low cost

• Quick estimate of oil mobilization vs.

distance

• Estimate of vertical conformance

• No production facilities required

• Estimate of injectivity

• Fast results

Disadvantages

• No oil in tank

• No operational experience with

production

• No confirmation of swept volume

• Limited data on mobility control,

overall conformance, chemical

retention

• Extremely sensitive to fluid drift

Fig. 4—Advantages and disadvantages of nonproducing pilots.



mance may not be representative of a repeated pattern and may bedifficult to scale. In addition, they are sensitive to fluid drift andcan take as long to run as a true pattern flood.

Better recovery estimates can be obtained by using a single,normal five-spot pattern. In this design, water or gas is injected atthe four corners of the pattern to provide confinement of the oilwithin the pattern and, therefore, improved estimates of recoverycompared to an unconfined pattern. To reduce pilot duration, con-fined pilots are typically run at a closer well spacing than plannedfor commercial application. Advantages and disadvantages of suchsmall-scale confined pilots are summarized in Fig. 7. This type of

pilot can provide good estimates of oil displacement and, whencoupled with the use of observation wells, vertical sweep efficiencyas a function of distance from the injection well at modest cost.In addition, detailed data on pressure gradients, fluid mobilities,and fluid transport can be obtained that enable rigorous calibrationof simulation models. However, the small size of the pattern may

not sample representative heterogeneities, reflect the balance of arepeated pattern flood, scale to wider well spacings, or indicatelong-term problems.

For improved confidence in scaling pilot results to potentialfull-field applications, repeated inverted five-spot patterns havesometimes been used. This arrangement provides the best estimatesof oil recovery and sweep efficiency, the best data for calibratingsimulation models, and the most direct scaleup to commercialoperations. However, this type of pilot will have the longestduration and will require extensive evaluation time. Naturally,piloting costs increase with the number of patterns placed on test.

Advantages and disadvantages of large-scale, multipattern pilotsare summarized in Fig. 8.

Assessment of Key Reservoir Mechanisms

The specific tools used to assess key reservoir mechanisms willdepend on the EOR process being pilot tested. For illustrative

Pilot Types: Producing

Single Inverted 5-Spot

Determine: Injectivity and productivity

Approximate estimate of oil recovery

Single Normal 5-Spot


Improved estimate of oil recovery

Single Inverted 5-Spot

With Observers


Estimate of oil recovery

Vertical sweep at observers


Vertical sweep vs. distance

Areal sweep

Repeated Inverted 5-Spot


Oil recovery from multiple

confined patterns

Fig. 5—Examples of producing pilots.

Unconfined Producing Pilots

Advantages

• Estimate of injectivity

• Low cost

• Rough estimates of mobility control,

oil mobilization, chemical retention

• Some production experience

• Fast results

Disadvantages

• Swept volume difficult to evaluate

• Streamlines, pressure gradients, oil

recovery not representative of

repeated pattern

• Performance difficult to scale

• Sensitive to fluid drift

• Takes as long to run as a pattern

flood

Fig. 6—Advantages and disadvantages of unconfined producing pilots.



purposes, this section will focus on the key reservoir mechanismsassociated with gas injection EOR. Fig. 9 summarizes three sig-nificant problems that can arise in horizontal gas injection andWAG EOR projects (Healy et al. 1994). This figure focuses onproblems associated with horizontal floods because these makeup the majority of gas injection EOR pilots that have been con-ducted to date.

First, in some situations, it may not be possible to inject waterand gas at the desired rates. Reservoir variables that control injec-tivity are effective permeabilities and near-wellbore damage. Waterinjectivity has been a problem in some floods, especially in low-

permeability reservoirs. If injectivity is a potential problem, it canbe evaluated in the design phase through careful laboratory measure-ments, and by conducting pilot injectivity tests.

A second problem is that gas can channel through high-per-meability “thief” zones, leading to poorer-than-expected sweepefficiency. Channeling is controlled by permeability distribution.Gas channeling can be evaluated in the design phase by conductingthorough geological and reservoir description studies along withsmall-scale reservoir simulation studies that properly account forthe governing geologic heterogeneities. Also, the sweep experi-enced in a prior waterflood will provide a strong indication ofthe degree of channeling to be expected in a gas injection project.Thus, an accurate reservoir description combined with historymatching prior waterflood performance can help evaluate the

potential for channeling in the gasflood.The final potential problem is that gas, which is usually lessdense than oil or water, can gravity override or flow to the top of

Small-Scale Confined Pilots

Advantages

• Good estimate of oil displacement,

and vertical conformance vs.

distance

• Detailed data on mobility control,

pressure gradients, and chemical

transport

• Data for simulator calibration

• Easier to scale-up to commercial

• Modest cost

• Moderately fast results

Disadvantages

• May not sample representative

heterogeneities

• May not reflect pattern balance of

repeated pattern flood

• May not scale to wider well spacings

• May not indicate long-term problems

Fig. 7—Advantages and disadvantages of small-scale confined pilots.

Large-Scale, Multipattern Pilot

Advantages

• Best estimate of oil recovery and

sweep efficiency

• Confirmed “oil-in-the-tank”

• Best data for calibrating simulators

• Easiest to scale-up to commercial

performance

• Commercial-scale operating

experience and cost data

Disadvantages

• Very expensive

• Extensive evaluation time

Fig. 8—Advantages and disadvantages of large-scale, multipattern pilots.



a reservoir unit as it moves away from injection wells. When thisoccurs, it will sweep only the very top portion of the zone. Gasoverride is highly sensitive to vertical permeability and to the lateralextent of barriers to vertical flow. Again, geological and reservoirdescription studies and perhaps pilot tests can help to identifyconformance problems and thus avoid a surprise. Because grav-ity override is sensitive to the viscous-to-gravity ratio (VGR), it isimportant to operate a gas injection or WAG pilot at water and gasthroughput rates and well spacing that result in a VGR comparableto that which could be achieved in a commercial-scale project (Stone1982; Jenkins 1984).

In summary, the key mechanisms to be assessed during pilottesting of gas injection processes include injectivity, gravity over-ride, channeling, viscous fingering, and areal sweep. Table 1

outlines the data needed for interpretation of each mechanism andmonitoring tools and techniques that can be used to acquire therequired data.

Understanding injectivity changes requires measurement of notonly the injectivity index, but also the permeability distribution andfluid mobilities near the injection well. Frequent measurements ofinjection rates and bottomhole pressures are used to provide high-resolution injectivity data. Flow profiles, fall-off tests, and step-ratetests have been used to characterize the near-well permeabilitydistribution and fluid mobilities. Permanent downhole monitoringtools are now being used routinely to obtain high-resolution real-

time temperature and pressure data.To assess gravity override properly, the change in oil saturationwith depth and distance behind the passing gas displacement front

Potential Problems with

WAG and Gas Injection Processes

Potential Problem

• Cannot inject gas or

water at desired rates

Evaluation of Problem

• Lab measurements

• Pilot injectivity tests

• Geological, reservoir

description studies

• Simulation studies to test

completion strategy and

injection rate

• Pilot test for vertical sweep

• Geological, reservoirdescription studies

• Pulse (interference) testingprior to gas injection

• Waterflood history matching

• Pilot tests for conformance

Gas

• Gas channels through

high-permeability

zones

Gas• Severe gravity override

of gas occurs

I n j e c t i v i t y

Water GasWater

Fig. 9—Potential problems with WAG and gas injection processes.

TABLE 1—TOOLS FOR KEY-RESERVOIR-MECHANISM ASSESSMENT

Mechanism Data Needed for Interpretation Tools/Techniques

Injection ratesInjectivity index

Bottomhole pressure

Flow profiles

Fall-off tests

Injectivity

Permeability distribution near injection well

Step-rate tests

Time-lapse logging in monitor wellsOil saturation change with depth anddistance from injector Core after passage of flood front

Core data

Vertical pulse tests

Gravity override

Vertical permeability

Cross-layer pulse tests

Oil saturation change with depth anddistance from injector

Time-lapse logging in monitor wells

Gas/oil ratio or water cuts vs. time atproducers

Sample producers for early breakthroughof injectant

Interwell tracers Sample producers

Channeling/viscous fingering/loss ofmobility control

Pressure surveys Flowing and shut-in pressures

Areal sweep/conformance Volume balance of oil, gas, water, andtracers produced to determine swept pore

volume

Sample producers for injected tracers



and the effective pattern vertical permeability are needed. Time-lapse logging, coring behind the flood front, and either verticalor cross-layer pulse tests have been used to provide this informa-tion. Cased-hole logging tools used for time-lapse logging includenuclear logs (steel and nonmetallic casing) for gas saturation andtotal porosity and induction logs (nonmetallic casing) for watersaturation. Fitz and Ganapathy (1993) provide an example of quan-titative monitoring of fluid saturation changes during a gas injectionEOR project. Post-flood core wells have been used to measure verti-cal conformance and remaining oil saturation. In some cases, spotfluid samples for composition have been collected at observationwells, but usually after critical log data have been obtained.

Channeling and loss of mobility control or viscous fingering arethe other key mechanisms affecting sweep efficiency. In additionto assessing the change in oil saturation behind the flood front,the gas/oil ratio and water-cut behavior of producers over time,interwell tracers (radioactive or chemical) and pressure surveys arecommonly used to estimate the degree of channeling and viscousfingering. Careful and regular sampling of produced fluids, flowingand static bottomhole pressure surveys, and time-lapse logging areavailable techniques for acquiring such data.

Finally, flood conformance or areal sweep is needed to comple-ment the channeling and gravity override data and determine thevolumetric sweep efficiency within the pattern. Swept pore volumecan be determined by carefully tracking the movement and break-through of tracers at production wells and keeping accurate records

of oil, water, and gas production

Pilot Interpretation

Successful pilot interpretation requires advance planning. It isessential that a detailed reservoir simulation model of the pilotarea (with appropriate boundary conditions) be built in advance tooptimize the pilot design and monitoring program, anticipate dataneeded for history matching the pilot, enable timely interpretationof pilot, and assess the need for selective use of additional observa-tion wells and post-flood coring. The geology of the pilot area anda good understanding of the target oil distribution are critical inputsto the simulation model. Pilot wells should be cored and logged,if at all possible. Core, log, and pressure transient data should beintegrated into a consistent reservoir description.

The following pilot design and operational best practices helpto minimize uncertainties in test interpretation and facilitate historymatching of pilot results:

• Production facilities, well completions, tubulars, and artificiallift should be representative of the anticipated commercial-scaledevelopment.

• Several good baseline logs and possibly a single-well tracertest should be run in wells before the pilot begins and at regulartime intervals to verify reproducibility of the log measurements andensure accurate determination of saturation changes during time-lapse logging at observation wells. Having logging tools dedicatedto the project also helps to ensure reproducibility.

• An adequate period of steady baseline injection and produc-tion should be achieved before initiating the EOR process. Thiswill reduce uncertainty in interpretation of injectivity, saturationchanges, and incremental oil production.

• Fluid drift should be minimized so that the pilot area acts asa truly confined system. This can be accomplished by regulatingrates in the surrounding patterns or locating the pilot in an areawithout strong pressure gradients.

• The relative rates of injectors and producers should beallocated to maintain pattern balance and maximize areal sweepefficiency in the pilot area.

• Steady and uninterrupted injection and production ratesshould be maintained. This is important to maintain the desiredVGR, maintain pattern balance, and minimize the effects of exter-nal influences.

• Injection and production zones should be isolated so that onlythe targeted production zone is taking injection.

• An adequate volume of EOR fluid should be injected toreduce uncertainty in interpretation of sweep efficiency, satura-

tion changes, and incremental oil production. Experience indicatesthat the volume of EOR fluid injected needs to be at least 20%of the pattern hydrocarbon pore volume before the pilot can beinterpreted adequately.

• The original pilot operating and monitoring plan should becontinued until sufficient data are acquired to validate simulationmodels; do not attempt to optimize on the basis of early results.

Assessing incremental oil recovery over waterflood should be akey objective of a pilot. This can be accomplished in several ways,each of which has advantages and disadvantages:

• In cases where the waterflood is very mature (>90% watercut), an increase in oil cut can provide a direct measure of improved

recovery. A disadvantage is that this may delay the pilot, or thewaterflood may contact only part of the target zone.

• In cases in which the waterflood is less mature, the baselinewaterflood recovery can be estimated by using a reservoir simu-lation model to history match the pilot area and extrapolate theprepilot waterflood production trend. This requires an adequateprepilot waterflood period to reduce uncertainty in the historymatch and extrapolation.

Pilot Examples

The best practices described in the preceding text were derivedfrom ExxonMobil’s extensive piloting experience, which includesmore than 50 field pilot tests covering the full range of EOR pro-cesses. Table 2 is a list of representative ExxonMobil pilot tests

that have been described previously in the open literature. FourExxonMobil pilot tests are used below to illustrate (1) definitionof pilot objectives, (2) design of pilots to meet the objectives, (3)tools and techniques for assessment of key reservoir mechanisms,and (4) integrated interpretation of pilot data aided by reservoirsimulation.

Single-Well Injectivity Test. This example is a low permeabilitysandstone reservoir located in Wyoming, USA. Average reservoirpermeability is 6.6 md, average formation thickness is 50 ft, andthe reservoir is being waterflooded on a vertical well spacing of80 acres. The concern was that injectivity would be low duringmiscible CO2 WAG injection. Therefore, an injectivity test wasperformed to determine injectivity before, during, and after CO2 injection and to estimate field-scale injectivity to assist prediction

of miscible process performance.The test consisted of 3 months of baseline water injection fol-

lowed by 2 months of CO2 injection before returning the well towater injection. The radius of investigation of the test was approxi-mately 100 ft. Bottomhole injection pressures and surface injectionrates were monitored continuously during the test to determineinjectivity index changes during injection of water and CO 2. Pres-sure fall-off tests were conducted and injection flow profiles weremeasured during both the baseline water injection and CO2 injec-tion to characterize the permeability distribution and changes influid mobilities in the near-well region. Step-rate tests were alsoconducted to confirm that the formation was not fractured.

The results of the test were used to calibrate a radial simulationmodel of the near-well region. Results of the radial model wereused to guide the construction of a full-field simulation model,which was then used to evaluate WAG injection process options.

Unconfined Pilot With Observation Wells. Evidence of grav-ity segregation between water and an enriched hydrocarbon gaswas observed in a tertiary horizontal miscible WAG flood at theJudy Creek Beaverhill Lake ‘A’ Pool. The gas override resulted inbypassing of potential miscible reserves and decreased ultimateoil recovery. An unconfined producing pilot was undertaken byImperial Oil Resources, a majority indirectly owned affiliate ofExxonMobil, to verify the existence and extent of gravity override,quantify the factors affecting vertical sweep efficiency, identifyoptimum well spacing and pattern size, and determine residual oilsaturations to water and enriched hydrocarbon gas (Pritchard etal. 1990). Results of the pilot were used to calibrate a mechanistic

reservoir simulation model, which was subsequently used to guideoptimization of pattern configuration and WAG operating strate-



gies (Pritchard and Neiman 1992). The field is a limestone reefreservoir located approximately 200 km northwest of Edmonton,Alberta, Canada. Its average horizontal permeability is 43 md andaverage thickness is 68 ft. Gravity override was a concern becausethe reservoir has good vertical permeability. The pilot was situatedin a location that (1) was representative of the reef margin faciesthat was the primary target of the hydrocarbon miscible flood, (2)would ensure an interpretable pilot, and (3) would be an economicventure on its own by accessing unswept reservoir.

The pilot pattern configuration is shown in Fig. 10. The testconsisted of 6 months of baseline water injection followed by 1 yearof WAG injection with enriched hydrocarbon gas at a volumetricWAG ratio of 1.0. This WAG ratio was accomplished by 1 week ofenriched hydrocarbon gas injection at an average rate of 2000 resm3 /d followed by 3 weeks of water injection at an average rate of660 res m3 /d. These rates were chosen to achieve the same VGR asthe planned commercial operation. The gas was injected at a higherrate than the water to maximize vertical sweep at the injector andbe representative of the vertical injection profile of a commercialoperation. A lower water injection rate was used to reduce the totalaverage fluid rate and, thus, achieve the target VGR.

The monitoring program included:

• Induction resistivity and neutron logging to determine oil,water, and gas saturation changes at a fiberglass-cased observation

well (Georgi et al. 1991). The observation well was placed withinthe expected WAG commingled zone on the basis of prepilot reser-voir simulation modeling. The location was chosen to confirm theexpected size and shape of the WAG commingled zone (Fig. 11).

• Production and injection profile logs for monitoring changesin fluid production rates and fluid entry horizons. These consistedof a suite of spinner, density, capacitance, and temperature tools.

• Water and solvent tracer for defining the areal distribution ofinjected water and gas. A gas-phase tracer (sulfur hexafluoride)and liquid phase tracer (tritiated toluene) were used to monitorfluid movement.

Conclusions of the pilot, on the basis of an integrated interpreta-tion of the monitoring data, were that (1) a definite oil bank wasformed by the miscible process, (2) gravity override was consistentwith the simulation model predictions, and (3) a reduction in pat-tern size would improve sweep efficiency and ultimate oil recovery.The calibrated simulation model was used to define an optimizedinjection strategy comprising (1) injection of an initial high-ratebank of the enriched gas before WAG injection, (2) tapering theWAG ratio, (3) proper timing of lean chase gas injection, and (4)tailoring of WAG cycle length and bank size to pattern geology.

Small-Scale Confined Pilot. The initial pilot of the solids-sta-bilized emulsion (SSE) heavy-oil-recovery process developed by

TABLE 2—REPRESENTATIVE EXXONMOBIL EOR PILOT TESTS

Field Date Type Pilot Process References

Borregos 1965–66 5-spot Surfactant Pursley and Graham(1975)

Loudon 1969–70 5-spot Surfactant Pursley et al. (1973)

Loudon 1980–81 5-spot Surfactant Bragg et al. (1982,1983)

Loudon 1982–83 5-spot Surfactant Reppert et al. (1990)

Loudon 1982–86 40-acre multipattern Surfactant Huh et al. (1990)

Loudon 1982–86 80-acre multipattern Surfactant Huh et al. (1990)

Means San Andres 1982–83 Nonproducing Co2 miscible Stiles et al. (1983)

Judy Creek “A” 1987 Unconfined pattern Hydrocarbon miscible Pritchard et al. (1990,1992)

Redwater 1988–89 Multipattern Hydrocarbon miscible Wood et al. (1993)

Slaughter 1991–92 Multipattern Co2 foam Hoefner and Evans(1995)

Greater Aneth 1992–94 Multipattern Co2 foam Hoefner and Evans(1995)

East Texas Basin 2001–2005 Single patten Gravity-stable immiscible gasinjection with horizontal wells

Hyatt and Hutchison(2005)

Norman Wells 1986–90 Multipattern Polymer gel Twiedt et al. (1997)

Cold Lake (Ethyl) 1964– Multipattern Cyclic steam stimulation Buckles (1979)

Cold Lake (May) 1972– Multipattern Cyclic steam stimulation Buckles (1979)

Cold Lake (Leming) 1975– Multipattern Cyclic steam and steam drivewith horizontal wells

Buckles (1979)

Cold Lake (H22 Pad) 2002– Multipattern Laser Leaute (2002), Leauteand Carey (2005)

South Belridge 1986–87 Multipattern Steam foam Djabbarah et al. (1990,1997)

South Belridge (Diatomite) 1992–96 Multipattern Steam drive Murer et al. (2000)

Esperson Dome 1984–87 Single pattern In-situ combustion Choquette et al. (1991)

Celtic 1996–99 Single well (horizontal) SAGD Saltuklaroglu et al.

(2000)Celtic 1997–2001 Dual well (horizontal) SAGD Saltuklaroglu et al.

(2000)

Celtic 2002–2005 5-spot SSE Kaminsky andWattenbarger (2008)



ExxonMobil was conducted at the Celtic field in Saskatchewan,

Canada. The SSE process involves the generation and injection ofsolids-stabilized water-in-oil emulsion to more favorably displaceviscous oils (Kaminsky and Wattenbarger 2008). After severalyears of laboratory and theoretical development, the SSE recoveryprocess was deemed ready for piloting in the field. The objectivesof the pilot were (1) to gain operational experience with the SSEprocess, (2) to confirm the ability to generate and inject a solids-stabilized emulsion in the field, (3) to confirm the in situ stability ofthe injectant, and (4) to confirm improved reservoir displacement.After review of several potential pilot locations, the Celtic field waschosen because its reservoir characteristics matched the desirabletarget characteristics for the SSE process, it had existing infrastruc-ture, and it was well characterized with historic performance data.

The Celtic SSE pilot was designed as an isolated five-spot pat-tern with four corner injection wells, a central producing well, andthree observation wells (see Fig. 12). Use of a full, isolated patternminimized interference with existing operations and ensured that oilrecovery during the pilot came from within the pilot pattern. Initialcharacterization of the pilot included logging, coring, extensivecoreflood analysis, a new method to measure steady-state relativepermeabilities for heavy oil systems, fluid characterization, geologic

modeling, and reservoir simulation. Initial reservoir modeling stud-

ies were conducted before the pilot to confirm that the chosen wellspacing and 3-year piloting period would be sufficient to gather nec-essary injection, production, and observation-well data to meet pilotobjectives. Falloff tests were conducted periodically to characterizethe pilot area further and to evaluate changes in well injectivity.

The reservoir surveillance program included: close monitor-ing of injection and production rates, continuous measurementof bottomhole pressures and temperatures, producer samplingand analysis, tracers, and observation well logging. Fiber-opticsensors were placed in each of the observation wells to measurepressure response. Temperature logs were run in the observationwells on a routine basis to help detect the arrival of the slightlyheated injected fluid. Carbon/oxygen and induction logs were runless frequently to detect changes in fluid saturation. Water-phaseand injector-specific oil-phase tracers were added to the injectedfluid to help track the movement of the injected fluids and to aidin the determination of in situ stability. Regular sampling and anin-line viscometer were used to control the quality of the injectant.These quality controls were helpful in identifying and correctinginitial startup problems with injectant preparation. At the end ofthe 3-year pilot, a post-flood well was drilled to take core fromthe swept region of the flood. The ability to generate and injectsolids-stabilized emulsion in the field was demonstrated early onin the pilot. Integrated analysis of the post-flood core-well resultsand extensive surveillance data allowed estimation of the in situ

400 m2-8 4-9110 m Injector

Observation well

16-5

R 10

R 11T 64

T 65

Reef edge

Sweeppilot

Fig. 10—Judy Creek vertical sweep pilot configuration.

Injector Producer

Distance from Injector (m)

F l o w

T h i c k n e s s ( m

)

Observationwell

Gas flowing zone(including dispersion)

Predictedcommingled

zonedimension

Water flowingzone

Commingledzone

Loginterpreted

base of commingled

zone

R 3

R 2

R 1

0 100 200 300 400

24

20

16

12

8

4

0

Fig. 11—Simplified cross section of Judy Creek vertical-sweeppilot showing observation well location.

injection well

production wellobservation well

25ft

45ft

35ft

150 ft

N

40ft

OBS3

INJ4

INJ1

INJ2

INJ3

PROD

OBS2 OBS1

Fig. 12—Celtic SSE pilot configuration.



stability of the injectant and displacement performance, whichwere found to be consistent with prior laboratory corefloods andperformance estimates.

Large-Scale Multipattern Pilot. The first pilot of the liquid-assisted steam enhanced recovery (LASER) process was conductedin the H22 pad of the Cold Lake field in Alberta, Canada (Leaute2002; Leaute and Carey 2005). The LASER process, developed byImperial Oil, involves the addition of an intermediate hydrocarbonsolvent to steam injected in later cycles of a cyclic steam stimu-lation (CSS) operation. Laboratory physical models, theoreticalanalysis, and reservoir simulations provided the confidence to testthis novel recovery concept in the field.

The primary objectives of the LASER pilot were to validatethe improvement in cycle bitumen recovery over the base CSSprocess and to determine the amount of solvent recovery. Becauseof the variability in CSS well performance, both between wellsand in individual wells over time, a large-scale multipattern pilot

design was chosen. In this design, LASER was applied to severalwells in the H22 pad and its performance was compared to that ofa neighboring control pad (H21), where CSS was applied withoutthe addition of solvent. The H22 and H21 pads were chosen forthe pilot and control, respectively, because they had nearly iden-tical pad-level performance through the first six cycles of CSSand because their performance and reservoir characteristics wererepresentative of future LASER targets (see Fig. 13).

Starting in 2000, solvent was introduced in the seventh andeighth cycles into eight wells of the H22 pad, with extensivewell-level and pad-level analysis of injection and production data.Frequent sampling, in-line measurement, and analysis of producedwell streams allowed for accurate determination of the solventproduction. A key element of the sampling protocol was to measurethe solvent in both the produced liquid and the produced vapor

streams. Statistical analysis along with reservoir simulation andhistory-matching, was used to estimate improvements in cycle bitu-men recovery, confirm understanding of the process, and estimateperformance in future cycles and in commercial application.

Summary

A staged approach to EOR development focusing specifically onpilot testing best practices has been outlined. Topics covered include(1) factors to consider when determining whether a pilot is neededand defining pilot objectives, (2) requirements for a successful pilot,(3) types of pilots and their advantages and disadvantages, (4) toolsand techniques for assessment of key reservoir mechanisms, and (5)minimizing uncertainty in pilot interpretation.

Application of these best practices enables the acquisition of

accurate and definitive test data to (1) assess effects of reservoirgeology on process performance, particularly sweep efficiency;

(2) calibrate reservoir simulation models for full-field predictions;(3) improve field production forecasts; (4) reduce technical andeconomic risk; and (5) guide improvements in current operatingstrategy to improve economics/recovery.

Several ExxonMobil pilot tests were used to illustrate the bestpractices and the role of pilots in the staged EOR developmentplanning process. The case histories included a single-well injec-tivity test, an unconfined pilot with observation wells, a small-scaleconfined pilot, and a large-scale multipattern pilot.

Acknowledgments

The authors would like to thank ExxonMobil management fortheir support and permission to publish this paper. In addition,the authors would like to thank the many current and formeremployees of ExxonMobil and its affiliates who have contributedto the development of the pilot testing best practices described inthis paper.

Exxon Mobil Corporation has numerous subsidiaries, many

with names that include ExxonMobil, Exxon, Esso, and Mobil.For convenience and simplicity in this paper, the parent companyand its subsidiaries may be referenced separately or collectivelyas “ExxonMobil.” Abbreviated references describing global orregional operational organizations and global or regional businesslines are also sometimes used for convenience and simplicity.Nothing in this paper is intended to override the corporate sep-arateness of these separate legal entities. Working relationshipsdiscussed in this paper do not necessarily represent a reportingconnection, but may reflect a functional guidance, stewardship, orservice relationship.

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0

75

150

225

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375

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525

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750

M a r - 0

0

A u g - 0 0

F e b -

0 1

A u g - 0 1

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0 2

A u g - 0 2

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0 3

A u g - 0 3

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0 4

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B i t u m e n R a t e s ( m 3 / d )

0

0.05

0.1

0.15

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0.35

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C y c l e O S R

Cycle 6 Cycle 7

5

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1

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11

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12/12

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SI Metric Conversion Factors

°API 141.5 / (131.5 + °API) = g/cm3

cal × 4.184* E+00 = J

cp × 1.0* E−03 = Pa·s °F (°F − 32) / 1.8 = °C

psi × 6.894 757 E+00 = kPa

scf × 2.831 685 E−02 = m3

STB × 1.589 873 E−01 = m3

*Conversion factor is exact.

Gary F. Teletzke currently is team lead for CO2 storage researchat ExxonMobil Upstream Research Company. He holds a BSdegree from Northwestern U. and a PhD degree from the U.of Minnesota, both in chemical engineering. Teletzke’s pastwork has included development of surfactant-based EOR pro-

cesses, development of foam processes for gasflood sweepimprovement, laboratory and simulation studies to evaluateEOR process applications, and leadership of compositionalsimulation and gas injection processes research. He has par-ticipated in design and interpretation of six field pilot tests andhas led projects to evaluate EOR opportunities in the UnitedStates, Europe, Malaysia, and Middle East. Teletzke currentlyis an Associate Editor of SPE Res Eval & Eng and has receivedcommendation as an outstanding Technical Editor andAssociate Editor. R. Chick Wattenbarger currently leads theheavy-oil recovery research team for ExxonMobil UpstreamResearch, which works to develop and apply heavy-oil recov-ery technologies. He holds a BS degree from the U. of Texasat Austin, a MS degree from Texas A&M U., and a PhD degreefrom Stanford U., all in petroleum engineering. Before join-ing ExxonMobil, Wattenbarger worked for Shell Development

Company. His past work has included developing methods formodeling and characterizing deepwater reservoirs, develop-ing methods for evaluating reservoir performance uncertainty,evaluating offshore technologies, and performing integratedfield studies. John R. Wilkinson is currently a senior engineeringconsultant for ExxonMobil Upstream Research. He holds aBachelor’s degree in geological engineering from the U. ofBritish Columbia, Canada. After working summers doing miner-als exploration, Wilkinson’s oil and gas career started with tech-nical positions in petrophysics, drilling, subsurface engineering,and reservoir engineering. He has recently lead an upstreamteam on next generation improved hydrocarbon recovery(IHR) for ExxonMobil and also represents the company in sev-eral industry and governmental carbon capture and seques-tration (CCS) forums.

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