SPE 137071

Real – Time Geomechanics: Applications to Deepwater Drilling S. Bagala, SPE, I. McWilliam, SPE, Chevron; T. O’Rourke, SPE, C. Liu, SPE, Schlumberger

Copyright 2010, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Deepwater Drilling and Completions Conference held in Galveston, Texas, USA, 5–6 October 2010. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Lithologies under deepwater conditions usually show relatively reduced effective stress, due to the reduced lithostatic column. This translates into relatively narrow mud weight windows, driven mainly by shear failure or pore pressure in overpressured conditions, and by minimum horizontal stress gradients. Drilling operations should consider wellbore collapse, kick and losses as the primary geomechanics-related drilling hazards. These should be investigated and predicted during well planning, and should also be appropriately monitored during drilling, especially when an appraisal campaign will require highly deviated wells. Real-time geomechanics is defined as a workflow that takes into consideration mud weight window planning, identification of geomechanics-related drilling hazards and possible mitigation actions, and, while drilling, operations monitoring by real-time data acquisition and interpretation, drilling occurrences detection, drilling practices revision, and the real-time update of mud weight window for further drilling. The authors present the case study of a drilling campaign in Chevron operated Rosebank Lochnagar Discovery, deepwater West Shetland, in almost 3,700-ft water depth. This campaign had the goal of proving the development concept of drilling horizontally in a field where the previous maximum inclination was only 35 degrees. The planning phase consisted of mud window modeling using a mechanical earth model from offset wells. Potential drilling hazards were then identified and synthesised using a Drilling Roadmap as a drillling planning and management tool. The monitoring phase consisted of real-time detection, from analysis of logging-while-drilling and wireline data, of drilling hazards typical in the area, such as cavings, losses, and packoffs. Data interpretation required a multidisciplinary team of geologists, petrophysicists, geomechanics engineers, and drilling engineers. The application of real-time geomechanics allowed an improvement in operations, safe drilling practices, and refined calibration of the 1D geomechanical model for further drilling campaigns. Introduction Deepwater drilling activities are nowadays frequently carried on in several exploration and production provinces with water depth in excess of 7,000 ft. Drilling under these conditions poses a series of challenges that require both significant and specific planning efforts. From a geomechanical point of view, the deepwater environment is characterized by relatively low state of stress, see Figure 1. In this figure, Leak – Off Tests are indicative of magnitude of horizontal stress gradients. In some cases, isotropic state of stress conditions occur, with horizontal stress equal to the vertical one. The low state of stress, due to the reduced thickness of the lithostatic sequence, translates into relatively low effective stress. This results in several conditions, each of which is associated with specific drilling hazards:

• high shear failure gradient curve. This would be responsible for the occurrence of wellbore collapse phenomena; • low minimum horizontal stress gradient. This would be responsible for the occurrence of losses in a naturally

fractured rock; • low fracture gradient. Induced fractures with total losses may occur.

Because of these, drilling operations in deepwater would require a planning phase where drilling hazards and their mechanism would be carefully investigated, then, during drilling phase, an accurate management of Equivalent Circulation Density (ECD) would be required.

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The optimization of drilling parameters and the validation of the geomechanical model is attained through the analysis of real-time data. The paper will describe an example of deepwater operations in Chevron operated Rosebank Lochnagar Discovery, where the acquisition of real-time data and their interpretation for the development of real-time geomechanics, permitted to safely conduct drilling operations and validate, or revise, drilling practices and the geomechanical model.

Figure 1 – State of stress in deepwater environment (West of Shetland) compared with state of stress in Central North Sea. Leak – Off Tests are indicative of magnitude of horizontal stress gradients.

Rosebank Lochnagar. Aim of the Drilling Campaign Rosebank Lochnagar Discovery is located in blocks 205 and 213, 12 miles east of the International Median Boundary with the Faroe Islands. Exploration well 213/27-1z discovered oil in August 2004, in a multi – layer reservoir. After that, exploration drilling continued throughout 2007 and 2008, when a total of 6 main wellbores and sidetracks have been drilled. The area is characterized by a deep offshore environment where water depth is around 3,700 ft (see Figure 2).

Figure 2 – Rosebank Lochnagar Discovery location.

Drilling was performed with vertical trajectories or with maximum inclination of 35 degrees.

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Another drilling campaign was conducted in 2009, with the main aim of demonstrating the technical feasibility of horizontal drilling, as it would be required by field economics for the field development phase. The main challenge of well stability with horizontal drilling in this deepwater environment was highlighted by geomechanical modelling performed in the early stages of the exploration campaign. This geomechanical study predicted a steep increase of the shear failure gradient for well inclinations higher than 45 degrees, in the overburden and at reservoir level, with a resultant narrowing of the mud weight window, constrained to the upper end, by relatively low values of the losses curve (minimum horizontal stress). These geomechanical features would strongly restrict the mud weight choice. Therefore, throughout the 2009 drilling campaign, it was important to achieve stable wellbore conditions at high angle whilst continually monitoring wellbore conditions. Well Planning The 2009 drilling campaign was preceded by integrated, multi – functional and detailed well planning studies. They consisted in the design of a pilot well, where maximum inclination of 55 degrees would be reached in the Upper Volcanic Sequence, then the appraisal well itself, where maximum inclination of 90 degrees would be reached at lower reservoir level. Figure 3 and Figure 4 summarize the design, section by section, for the pilot and appraisal wells. Moreover a geomechanical study has been conducted, according to the following steps:

• by using open hole logs from offset wells, a mechanical earth model has been built along the planned trajectories of pilot and appraisal wells. The mechanical earth model comprises deformation, strength and stress properties of the lithological sequence of interest. These properties are calculated as continuous logs;

• the drilling history of the whole Rosebank Lochnagar exploration was reviewed, in order to detect the potential drilling risks and their mechanism. Moreover, calibration data such as mud weight and formation tests (Leak – Off data), were extracted for wellbore stability analysis;

• the wellbore stability produced the mud weight window, the drilling hazard prediction, and the suggestions for possible mitigation actions.

In particular, for the drilling operations of pilot and appraisal wells the following main drilling hazards were considered: • wellbore collapse. High shear failure gradient in shaly formations; • mud losses. In naturally fractured, competent volcanic rocks, where minimum horizontal stress is expected to be

relatively low; • hole packoff. This would be a potential hazard especially in the avalanche window of well deviation, i.e. for well

deviations comprised between 45 and 60 degrees, that would induce instability of cutting beds with accumulation of these at the bottom of the hole.

Figure 5 shows an example of drilling hazards from Rosebank Lochnagar drilling history. Drilling history and drilling hazards are represented by means of the mud weight window (see central track in Figure 5). The mud weight window commonly displays the following 5 curves, all in lbm/gal:

• The equivalent circulation density (ECD): this is the mud weight density under dynamic conditions, i.e. during active mud circulation;

• The kick gradient (or pore pressure gradient): given by the pore pressure gradient log, it can be defined as the minimum mud weight pressure to avoid formation fluid flow into the wellbore;

• The collapse gradient (or breakout, or shear failure gradient): it is the log of the rock shear failure gradient around the well section. If the mud weight is lower than this, shear failure induces breakouts and angular caving production during drilling;

• The losses gradient (or minimum horizontal stress gradient): it is the log of the minimum horizontal stress gradient. It can be also defined the mud weight value that induces the re – opening of pre – existing fractures with consequent mud losses;

• The fracture gradient (or breakdown or tensile failure gradient): it is the log of rock tensile failure gradient. It can be defined as the mud weight value that induces fractures on intact rock, with consequent total losses.

Figure 5 shows, from real – time data, the occurrence of mud losses in a competent, fractured formation. In the mud weight window track, it is evident that equivalent circulation density (ECD) was higher than losses (minimum horizontal stress), see red circles. In the right hand side log track, the collapse of standpipe pressure (green log curve) and the attempt of staging pump to regain circulation is evidence of losses, see black circle. The results of the geomechanical study were reported and displayed in the Drilling Roadmap for the pilot well. In this, drilling hazards are listed by hole section and / or formation together with severity degree, and the suggested mitigation actions. Drilling hazards are predicted from the mud weight window.

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In addition, predictions on drilling mechanics are also reported, such as prediction on torque and drag, stick – slip and shock. The Drilling Roadmap is an important design tool for prediction and monitoring. It constitutes an integrated work and communication tool for drilling engineers, geomechanics engineers, geologists and petrophysicists. The Drilling Roadmap for pilot well, 2009 Rosebank drilling campaign, is shown in Figure 6. Real - Time Data Acquisition and Real - Time Geomechanics Workflow The acquisition of a specific set of logging-while-drilling (LWD) data during 2009 Rosebank Lochnagar drilling was considered crucial to the following:

• monitoring and optimization of drilling parameters and performance; • detection of drilling hazards; • validation of geomechanical model.

The following set of LWD data has been acquired: • well geometry: density and ultrasonic caliper. • formation evaluation: compressional sonic, density, gamma – ray, resistivity. • drilling parameters: weight on bit, rate of penetration, equivalent circulation density (ECD), standpipe pressure, pump

flow, hookload, shock, drag, torque, stick - slip.

LWD data are usefully interpreted with the addition of wireline data that may become available after the completion of each well section. Moreover, the combination of data acquired at different times provides a very useful indication of hole stability over time and after subsequent operations (e.g. bottom hole assembly trips). A wellsite connectivity system permitted a continuous flow of data from downhole to the desktop of a dedicated team composed of professionals from Rosebank Lochnagar Operator and Contractors, such as drilling engineers, geologists, petrophysicists and geomechanics engineers. Real-time data acquisition and transmission are at the root of the real-time geomechanics workflow, as it is shown in Figure 7. The mechanical earth model contained in the Drilling Roadmap constitutes the prediction that, together with the real-time data, permits to monitor drilling operations. Any difference between the design and the as drilled, would prompt a revision of the model. This workflow permits to take decisions during drilling, with respect to drilling hazards occurrence, and to geomechanical model revision. In the specific case of 2009 Rosebank Lochnagar drilling, pilot hole execution was very important for the validation of available geomechanical model. Due to limited offset wells control, there was a very high degree of uncertainty in the initial geomechanical model.Any revision of this would be performed, first of all according to rig schedule for the subsequent appraisal well, but also in the light of the following development drilling campaign. Real – Time Geomechanics during Rosebank Lochnagar Drilling Campaign The results of the drilling monitoring are herewith described for the pilot hole and the appraisal, by drilling section. Pilot Hole: 16” section In this section, the well was designed to build angle up to a maximum of 55 degrees, throughout a shale sequence and at the top of a volcanic sequence. The predicted geomechanical model showed the increase of the shear failure gradient, up to 10.6 lbm / gal, in argillaceous formations, with a relatively narrow mud weight window constrained to the upper end by the losses curve, with a minimum of 10.5 lbm/gal, see Figure 8, that shows the Drilling Roadmap for the 16” section with the list of all drilling occurrences predicted during planning phase. Real-time acquisition in this section witnessed trouble free drilling, with the following outcome:

• The drilling team performed an accurate management of the ECD, kept stable around 10 lbm/gal in the lower section at maximum angle, see Figure 9;

• only a negligeable amount of cavings of uncertain origin was reported at the top of Balder Formation. Cavings were described as stress relief cavings with a blocky morphology and very rare elongate, splintery cavings, see Figure 10. This permitted to establish that shear failure curve, as planned, was too conservative;

LWD formation evaluation logs showed same trend as logs from offset wells used for geomechanical modelling in the planning phase. This was an indication of the accuracy of the predicted geomechanical model. Figure 11 summarizes main events of 16” section.

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Figure 3.Pilot well design.

LWD:

LWD

Wireline TBC. May opt for TLC

Directional MWD

MW: 9.6-10.5 ppg

Vertical(MWD)

Vertical(MWD)

None(Riserless)

Normal 8.6ppg EMW

26" Hole

Normal 8.6ppg EMW

10ft sampling intervals from start 8 1/2"

section to TD of the well.

2 sets of unwashed bulk cutting samples

3 sets of washed &

dried cuttings samples

Normal 8.6ppg EMW

DIR / ARC (=RES+GR)

SONICVISION /

KCL / NACL WBM Barite free system

10ft sampling intervals from

start 16"

2 sets of unwashed bulk cutting samples

3 sets of washed &

dried cuttings samples

None(Riserless)

MW: 9.8-10.8 ppg

MW = 8.6 - 11.5 ppgSeawater and Guar Gum /

Bentonite Sweeps

11.5 ppg KCl Polymer displacment fluid

MWD/GR

Hard drilling in Volcanic interval

Tool vibration/shock

Directional Problems (Holding

angle)

Fractured Basalt / Lost Circulation

Bit ballingWellbore instability

8 1/2" HoleTD at 55 deg

incl.

Normal 8.6ppg EMW

ALAP while avoiding Wellbore stability Issues

Mud Mud Logging SurveyFormation

Pressure

MW = 8.6 - 10.5 ppgSeawater and Guar Gum /

Bentonite Sweeps10.5 ppg Bentonite displacement fluid

Formation Evaluation

Boulders

Strong currents

Well Schematic Hole Size / Depth & BOP

16" Hole55 deg @ TD

Potential Drilling Problems

42" x 36" x 26" Hole

Directional Problems (Kick-off)Hole problems in highly reactive

shalesHole Cleaning / Bit

BallingLost Circulation

(Balder)Hole enlargement in

unconsolidated sands

Differential sticking in sandsFaulting

KCl / Polymer / Glycol WBM

Directional MWD & GWD & Gyro on W/L

Boulders

Mobile sands

Hole Washout and/or Caving

Strong currents

Low Shallow gas risk

MWD/GR

ft MDRKBSea-bed

20"

36" x 30" '

Possiblefault

zone &losses

13.3/8" shoe

8.1/2" TD

Caution: Potentially

reactive shale/sandsequence

6 SPE 137071

LWD:

LWD

DIR / GVR(=GR+Res) / ADN

(backup Stethoscope on board if unsuccessful MDT)

LWD Ecoscope / Periscope / GVR

Stethoscope Run later on seperate run for pressures

Vertical(MWD)

Vertical(MWD)

Directional MWD

Directional MWD & GWD & Gyro on W/L

10ft sampling intervals from

start 16"

2 sets of unwashed bulk cutting samples

3 sets of washed & dried

cuttings samples

Hard drilling in Volcanic interval

Tool vibration/shock

Directional Problems (Holding angle)

Fractured Basalt / Lost Circulation

Bit ballingWellbore instability

Normal 8.6ppg EMW

10ft sampling intervals from start 12.1/4"

section to TD of the well.

2 sets of unwashed bulk cutting samples

3 sets of washed & dried

cuttings samples

Less than 9ppg EMW

DIR / ARC (=RES+GR) SONICVISION / PWD + Q-BAT

MW = 8.6 - 10.5 ppgSeawater and Guar Gum /

Bentonite Sweeps10.5 ppg Bentonite displacement fluid

MWD/GR

Directional Problems (Kick-off)

Hole problems in highly reactive shalesHole Cleaning / Bit

BallingLost Circulation

(Balder)Hole enlargement in

unconsolidated sandsDifferential sticking in

sandsFaulting

KCl / Polymer / Glycol WBM

MW: 9.5-9.7 ppg

Boulders

Strong currents

26" Hole Normal 8.6ppg EMW

None(Riserless)

Normal 8.6ppg EMW

None(Riserless)

ALAP while avoiding Wellbore stability Issues

ALAP while avoiding Wellbore stability Issues

KCL / NACL WBM Barite free system

MW = 8.6 - 11.5 ppgSeawater and Guar Gum /

Bentonite Sweeps

11.5 ppg KCl Polymer displacement fluid

MWD/GR

MW: 9.8-10.8 ppg

Mud Mud Logging SurveyFormation Pressure

16" Hole55 deg @ TD

Bit Balling

Directional problems

Geosteering

Getting weight to bit

T&D Issues

12 1/4" Hole87.5 deg @ TD

KCL / NACL WBM Barite free system

Formation Evaluation

8 1/2" Hole90 deg Inc @ TD

Well Schematic Hole Size

Boulders

Mobile sands

Hole Washout and/or Caving

Strong currents

Figure 4. Appraisal well design.

Low Shallow gas risk

Potential Drilling Problems

42" x 36" x 26" Hole

MW: 9.8-10.8 ppg

Sea-bed

20"

36" x 30"

'

13.3/8" shoe

Possiblefault

zone &losses

Caution: Potentially

reactive shale/sandsequence

SPE 137071 7

Figure 5. Rosebank Lochnagar. Exploration well. Example of mud losses in competent and fractured rock, detected from real – time

data.

Gamma Ray0 gAPI 150

Penetr. rate0 ft/h 500

Standpipe Pressure

0 psi 5000

Cal.0 12

Fracture

Equiv Circ Density8 lbm/gal 13

9504

9612

9720

9828

9936

10044

10152

Mar.01-200718:00:00

Mar.01-200719:00:00

Mar.01-200720:00:00

Mar.01-200721:00:00

Mar.01-200722:00:00

Mar.01-200723:00:00

Depth (ft)Time

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Figure 6. Pilot well. Drilling Roadmap.

Drilling Optim

Gas hydrates

Geosteer. Lost Circ.Poor Hole Cleaning

Shoe failures

Sloughing / Packoffs

Unstable Zones

IDHole Depth

Description Actions

R01

Hole washing out and / or caving in. Details: possilbe reactive shales and mobile sands. Shallow gas risk negligble to low . Boulders below seabed. Potential cement blocks from the 30 " shoe

Details: seawater + Hi Vis pills and KCL polymers  displacement mud weight 8.6 ‐ 11.5 lbm / gal. Ensure > 2000 bbls of kill mud weight 

available in the event of shallw gas incident 

R02Reactive shales. Details: too light mud weight leads to collapse in formation A, too high mud weight leads to losses in formation B

Details: monitor and report cavings and cuttings in the amount, morphology, formation and depth. Maintain efficient hole cleaning. Attempt to increase mud weight first sign of cavings and consult with 

office on land

R03Potential cement blocks from 20" shoe / rathole.

Details: limit flow to 650 ‐ 700 gpm while drilling inside casing and rathole.

R04 Hydrate Formation

R05

Directional Control (e.g. tool ‐ face control, achievable DLS, difficulty kicking off). Details: soft shale formation and possible borehole washout

R06Potential for high stick / slip and bit / BHA whirl and bit damage in Volcanoclastics.

Details: in drilling 16" hole , max rate of penetration = 250 ft/hour to prevent cuttings build up in annulus. Min. 120 round / min. for hole 

cleaning and optimum  hole cleaning above 30 deg deviation = 1400 gpm flowrate

R07 Cuttings avalanche zoneDetails: monitor cuttings and ECD trend. In drilling 8.5" hole to TD, min. flowrate for hole cleaning  500 gpm. Optim. flowrate 580 + gpm. Max 

flowrate 650 gpm.

R08Possible mud losses in the formation B. Details: planned mud weight of 9.7 lbm/gal.

Details: start section with 9.6 ‐ 9.7 lbm/gal mud and increase steadily to 9.8 lbm / gal before reaching 45 deg incl. Ensure LCM pills are mixed and 

ready to pump if required.

R09

Tigh / narrow mud weight window . Details: drilling at 55 deg inclination has not been attempted through Volcanoclastic and C series before. Hole collapses in weak zones, C series and Volcanic 

Details: close attention to shakerr cuttings for signs of cavings. Reciording and trending drilling parameters : pressure, torque, pickups / slackoffs fior signs of tight hole. Monitor and communicate drilling trends and LWD data for signs of instability. Record any tight sections and losses 

during trip. Take photo of cavings.

R10Risk of mud losses to Basalt flow if fractured. Details: planned Mud weight of 9.8 lbm / gal. Possible dynamic losses if ECD > 10.5 lbm/gal

Details: monitor pit volumes and ECD / ESD variation. No LCM pill prior to initial LOT / FIT.  LCM pill ready to be used for fractured media

LossesFracture

LightSerious

Catastrophic

Severity

Drilling Risks Risk SummaryMud Weight Window.                    Range: 8 ‐ 13 lbm / gal

Pore pressureBreakout

30” casing

13/3/8” casing

20” casing

SPE 137071 9

Figure 7. Workflow for real-time geomechanics.

Figure 8. Pilot Hole. 16” section Drilling Roadmap.

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Figure 9. Pilot hole. Mud weight window and Equivalent Circulation Density in the 16” hole section.

Figure 10. Pilot hole. 16” section. Cavings from top Balder Formation.

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Figure 11. Pilot hole, 16” section. Summary of real- time data. Legend. DTCO_actual: LWD compressional sonic. DTCO_exp_1: compressional sonic from offset well 1. DTCO_exp_2: compressional sonic from offset well 2.

12

Reported minor cavings when drilling top Balder, unknown origin

No significant stability events with mud weight 9.7 – 9.8 lbm/gal. Drilling to max. incl. 55 degrees. Collapse curve (breakouts) appears to be conservative

LWD Sonic (actual) compared with sonic from offset wells used for geomechanical modeling

Well deviation0 deg 50

DTCO_actualDTCO_exp_1DTCO_exp_2

140 us/ft 40

Pore Pressure

LossesFractureECD8 lbm/gal 13

Breakout

Pilot Hole: 8 ½” section This section, where a deviation of 55 degrees has been mantained, was expected with relatively narrow mud weight window at reservoir level, due to relatively high predicted collapse curve, 10.7 lbm/gal maximum and with losses curve down to 10 lbm/gal, in some volcanic layers. Drilling Roadmap for 8 ½” section is shown in Figure 12. The main occurrences while drilling and logging this section were: • on 14 July 2009, drill down to 10189 ft and backream before pulling out of hole for Bottom Hole Assembly change;

• on 15 July 2009, while tripping down with new Bottom Hole Assembly (BHA), well is in poor conditions from 10117

to 10182 ft. Pack off event while washing / reaming down;

• after completing drilling to section TD at 10246 ft, logging followed. Logging run 2C was stuck at 10170 ft on 20/07/2009. After several attempts, logging tool was run down to 10236 ft;

• pack off, at 10,291 ft, stuck pipe at 10,200 ft, then several pack off events at 10,135 ft, 10,045 ft and 10,029 ft, all on 22 July 2009;

• after several attempts involving a wiper trip and several packoff events, logging program was abandoned.

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LWD logs acquired during the trip in hole with new BHA on 15/07/2009, showed hole enlargement and ledges (washout) in correspondence of a layer of shale between 10060 and 10250, see caliper log in Figure 13. The packoff events were clearly shown by the ECD data, see Figure 14. It was concluded that backreaming operations, prior to pulling out of hole and changing BHA, damaged the shale, creating hole enlargement and ledges, where drilling debris accumulated and triggered the packoff events and, ultimately, the stuck tool event that ended the logging program. This evidence prompted a new drilling practice, to be applied during the subsequent appraisal well: • avoid to perform BHA change and preliminary backreaming in sensitive claystone; • carefully perform run-in-hole operations, monitoring real-time data to avoid surge, especially in sensitive claystone.

Figure 12. Pilot Hole. 8 1/2” section Drilling Roadmap.

Figura 12.

13

FractureLBM/GAL

LossesLBM/GAL

kahead / Drillmap Risks

Focus

CollapseLBM/GAL

Pore PressureLBM/GAL

Hole Cleaning / Avalanche Zone

Hole angle above 45 deg liable to see cuttings bed formation, and risk of avalanche / pack off if hole cleaning is not managed

ECD, T&D, Swab-Surge,

Drilling parameters

Shock, Vibration and Whirl

Potential for high shock levels and BHA whirl in volcanics. Impreg bit damage, drillstring and tool fatigue can occur with high dynamics.

Drilling parameters

Geomechanics Possible Instability and hole collapse in Colsay1 & 4 and Rosebank Middle Volcanics

Shaker returns,smooth

drilling practice, T & D

Losses to fractured formation

Fractured basalt and tight mud weight window across sands can lead to losses and damage to formations.

ECD management

and LCM treatment

Lookahead / Drilling Roadmap Risks

SPE 137071 13

Figure 13. Pilot Hole. 8 ½” section. LWD caliper (left track). The yellow shadow area shows the difference between the bit size (in gauge hole conditions) and the caliper log.

10100

10200

10300

Gamma RayGAPI

LWD CaliperIN

Bit SizeIN

14 SPE 137071

Figure 14. Pilot hole 8 ½” section. Real-time data.

ECD (lbm/gal)

11

10

10.5

Time15/07/2009 16/07/2009

15:00 18:00 21:00 00:00 03:00

Depth (ft)

9000

10000

9500

10500

LWD Cali8 in 10

Pore Pressure

Appraisal Hole: 12 ¼” and 8 ½” sections Data from pilot hole drilling permitted to revise the geomechanical model with the timing required by rig schedule, and a new mud weight window was produced for the appraisal. The new model considered a new collapse curve, revised toward less conservative values, so it was possible to predict a wider mud weight window with around 1 lbm/gal gap between collapse curve and losses curve. This even in the 8 ½ in. section, where well geometry, after reaching 90 degrees inclination in the 12 ¼ in. section, would be kept horizontal for 2290 ft, along the lower reservoir, see Figure 15. The 8 ½ in. section was drilled without any drilling hazard.

CollapseLossesFractureECD8 LBM/GAL 18

TimeJul.15-2009 18:15:00

Jul.15-2009 18:30:00

Jul.15-2009 18:45:00

Jul.15-2009 19:00:00

Jul.15-2009 19:15:00

Jul.15-2009 19:30:00

ECD8 LBM/GAL 13

9972

10008

10044

10080

10116

10152

10188

Depth (ft)

ECD spike to 10.9 lbm/gal

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Figure 16 shows real-time data acquired during drilling. LWD calipers and resistivity image show in gauge hole conditions throughout the whole section, with no breakouts. The same data assisted the geologist to keep the appraisal sidetrack within the reservoir section.

Figure 15. Appraisal hole, 8 ½” section. Drilling Roadmap.

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Figure 16. Appraisal hole. 8 ½” section. LWD data. Legend. UCAV: Ultrasonic Caliper. DCAV: Density Caliper. UCAV and DCAV are overlapping.

Resistivity Image

Density and Ultrasonic caliper show in gauge hole and they overlapUCAV

DCAV8 IN 18

Discussion and Conclusion Real-time geomechanics applied to the drilling of pilot and appraisal wells, during the 2009 Rosebank Lochnagar drilling campaign, led to the successful monitoring of drilling occurrences and equivalent circulation density management and contributed to timely decision making and mitigation planning during appraisal drilling. It was also effective in developing and improving drilling procedures, in particular after pack off and stuck pipe events suffered while drilling the 8 ½ in. section of pilot well. Specifically, after these pack off and stuck pipe events, it was decided to minimize backreaming practice in shaly formations, during appraisal well drilling, where no noteworthy stuck pipe events were recorded. Real-time geomechanics during pilot well drilling permitted the revision of the geomechanical model and to make available a revised Drilling Roadmap for the further appraisal activities, in line with rig timing requirements, and with significant modifications. Specifically:

• the collapse gradient curve was re – calibrated to less conservative values; • predicted mud weight window was updated within two days to permit the successful subsequent horizontal drilling of

the 8 ½ in. section in the appraisal. Overall the real-time geomechanics process contributed to prove the feasibility of horizontal drilling in the area in subject, it reduced uncertainty in the geomechanical model and improved confidence in future predictions.

SPE 137071 17

Acknowledgement The authors would like to thank the management of Chevron North Sea Limited, Statoil (U.K.) Limited, Dong E&P (U.K.) Limited, OMV (U.K.) Limited, partners in Rosebank Lochnagar License, for their permission to publish this paper. References

1. Bagala, S.: “Wells 213/27-Y and 213/27-Yz. Pre – drill Wellbore Stability” Chevron Upstream Europe Internal

Report, July 2009. 2. McIntyre, B., Dixon, R., Mohamed, F., O’Rourke, T., Liu, C., Syed, A. and Donald, A.:”Wellbore Instability in

Forties: Diagnosis of Root Causes for Improved Drilling Performance” SPE 124670 (2009). 3. Yarim, G., Ritchie, G.M. and May, R.B.:”A Guide to Successful Backreaming: Real – Time Case Histories” March

2010 SPE Drilling & Completion, pp. 27 – 38.