Volume Issue 0 1996 [Doi 10.2118_35433-Ms] Davies, David; Vessell, Richard -- [Society of Petroleum...

7/28/2019 Volume Issue 0 1996 [Doi 10.2118_35433-Ms] Davies, David; Vessell, Richard -- [Society of Petroleum Engineers S

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13DOE 35433low Unit Characterization of a Shallow Shelf Carbonatenit, West Texasavid K. Davies, SPE, and Richard K. Vessell, David K. Davies& Associates, Inc.

Reservoir: North Robertson

pyright 1996 SOCmtyof Petroleum Engineers Inc(spaw was WW.WW for Pesefuabon at the 19% SPE/DOE Tenth Symposium on Improved

covery held m Tulsa OK 21.24 April 19S6m paper was selected for presentakm by an SPE Program Comm,ftee fol lowtng rewew offcfmatw contained !nan abstract submllted by the authqs) Contents ofthe paper have noten rev iewed by the Sooety of Petro leum Engineers and are sub)ect to carecfion by thelhc i[$) The materta l, as pfesented does not necessary ref lect any pcdon M the Socie ty oft ro leum Engmeem I ISoffmsrs w members Papers presented at SPE meett~s are 8ublecf

watwn rewew byEdflcoalCommittees of the Soaely of Petroleum Enguteers PermmsionCCPYISrestrcfed to an abstracf ofnOtmore lhao 3C0 words llluS!raOOnSmay not M Copmdm abslraci should ccmtamccmspwous acknowledgment of whwe and by whom the paper ISresented Wrde L!brar !an SPE P O Box 83383S Richa rdson TX 793 .93-3836 U S A Tel ex245 SPEUT

model is developed of a heterogeneous carbonate reservoirased fundamentally on measurement of pore geometricalarameters. Pore level reservoir modeling results in improvedccuracy in the prediction of rock types, permeability andidentification of flow units. Pore geometrical attributes arentegrated with wireline log data to allow for i) log-basedidentification of intervals of rock with different capillaryharacteristics, and ii) field-wide, log-based prediction ofermeability. Twelve hydraulic flow units are identified throughintegration of data concerning rock type distribution. andepositional environments. Maps of permeability thickness (kH)for each flow unit reveal significant stratigraphic compart-entalization. Future development drilling using uniform wellpacing patterns is not appropriate. The location and design ofinfill drilling patterns should be geologically targeted forrudent, cost-effective field development.Introductionhis study focuses on the use of pore geometrical attributes to

predict permeability and define hydraulic flow units in amature, heterogeneous, shallow shelf carbonate reservoir. Thepurpose of this study is to identify and map individual hydrau-lic flow units to aid determination of the potential for contin-ued development drilling. The study is funded by the USDepartment of Energy as part of its Class II Oil Program forshallow shelf carbonate (SSC) reservoirs. One objective of theprogram is to demonstrate advanced reservoir characterizationtools that will result in a significant increase of reserves.

SSC reservoirs in the USA originally contained >68 BBO(about one-seventh of all the oil discovered in the Lower 48

States). Recovery efficiency is low; some 20 BBO have beenproduced and current technology may only yield an additional4 BBO, The problem of low recovery efficiency in SSCreservoirs is not restricted to the USA -- it is a worldwidephenomenon. SSC reservoirs share a number of commoncharacteristics, including:

1. A high degree of areal and vertical heterogeneity, rela-tively low porosity and relatively low permeability2. Reservok compartmentalization, resulting in poor vertical

and lateral continuity of the reservoir flow units and poor sweepefficiency

3. Poor balancing of rates of injection and production, andearly water breakthrough in certain areas of the reservoir. Thisindicates poor pressure and fluid communication and limitedrepressuring4. Porosity and saturation as determined from analysis of

wireline logs do not accurately reflect reservoir quality andperformance5. Many injection and production wells are not optimally

completed with regard to placement of perforations, and thestimulation treatment can be inadequate for optimal productionand injection practicesThe North Robertson Unit exhibits all of these characteristics.North Robertson UnitThe North Robertson Unit (NRU) was the single largest water-flood installed in the onshore, lower 48 states of the USA duringthe 1980s. The unit covers 5,633 acres, has 259 wells and usesa 40 acre 5-spot waterflood pattern with 20 acre nominal wellspacing. The field was on primary production from 1954 to1987: the secondary waterflood has been in place since 1987.Currently, the field has 144 active producing wells, 109 activeinjection wells and 6 water supply wells. An objective of thiscurrent study is to identify the areas of the unit with the bestpotential for additional infill drilling (planned 10 acre spacing).The original oil in place is estimated at 260 MMSTB with an

estimated ultimate recovery factor of 1J.SO/O (primary recovery= 7.5?40,secondary recovery= 6%) based on the current produc-tion and workover schedule. Current unit production is approxi-mately 3,000 STB/D and I I ,000 BW/D at a water injection rate

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2 D.K. DAVIES, R.K. VESSELL SPE 35433of20,000 BWI/D.The NRU is located in Gaines County, West Texas on the

northeastern margin of the central basin platform (Figure I).Production is from Lower Permian Glorieta and Clear ForkCarbonates, The reservoir interval is thick (gross interval =1400 ft). More than 90% of the interval has uniform Iithology(dolostone), but ischaracterized by a complex pore structure thatresults in extensive vertical layering. The reservoir is character-ized by discontinuous pay intervals and high residual oilsaturations (35?40o 60/0,based on steady state measurements ofrelative permeability). The most important, immediate problemin the tield is that porosity and saturation determined from logsdo not accurately reflect reservoir quality and performance.MethodologyThe techniques used for reservoir description must meet threebasic requirements to be of value in a mature, heterogeneousfield:

1, Flow units must be readily identified using wireline logsbecause few wells have cores (Figure 1). Thus, the fundamentalreservoir description must be log based. However, becausevalues of porosity and saturation derived from routine loganalysis do not accurately identify productive rock in the NRU,it is necessary to develop a log model that will allow for theprediction of another producibility parameter, in this casepermeability.2. Use only the existing database -- no new wells will be

drilled to aid reservoir description, The existing databaseconsists of conventional cores from eight wells and 120 wellswith a relatively complete log suite (gamma ray, PEF, RHOB,PHIN, dual Iaterolog).

3. The reservoir model must be numeric (simulator-ready).Accurate determination of pore body/pore throat characteris-tics is fundamentally important because hydrocarbon displace-ment is controlled at the pore level.z 3,4 Thus, reservoir model-ing should begin at the pore level. In this study, quantitativeanalysis of pore geometry is used to develop the verticalreservoir layering profile of the reservoir (vertical compart-mentalization). Integration with depositional and diagenetic datafrom geological analysis allows for determination of the arealcompartmentalization and permeability distribution in thereservoir.

Ideally, reservoir models should display the distribution ofhydraulic flow unitss and simulation layer boundaries shouldcoincide with hydraulic unit boundaries.c A hydraulic flow unit(HFU) is a mappable interval of rock characterized by:

1. Sufficient thickness and areal extent to be recognized onlogs and mapped across the field2, Similar averages of rock properties that influence fluid

flow3. All fluids in hydrodynamic communicationThis geological/petrophysical reservoir characterization of the

NRU has involved the following techniques:

a. Development of a depositional/d iagenetic modelb. Definition of rock types based on pore geometry and

utilization of rocldlog modelc, Rock type extension with rock/log model to non-cored

wellsd. Definition of flow units and cross flow barrierse. Mapping of reservoir parameters (thickness, PhiH, kH andHPVH) and rock type for each flow unit

DepositionaUDiagenetic ModelPermian carbonates in the NRU were deposited in severalenvironments related to a low relief shoreline and shallowmarine shelf (Figure 2), Small (few feet) vertical fluctuations insea level caused significant lateral migration of facies due to lackof vertical relief (


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SPE 35433 FLOW UNIT CHARACTERIZATION OF A SHALLOW SHELF CARBONATE RESERVOIR: 3NORTH ROBERTSON UNIT. WEST TEXAS

Pore Geometry ModelingAnalysis of 3D pore geometry allows reservoir characterizationto be pore system oriented. The resulting reservoir models arebased on characteristics of the pore system. Pore geometryanalysis involves identification of pore types and rock types.Pore Types. The determination of pore types in a reservoirrequires the use of rock samples (conventional core, rotarysidewall cores, and cuttings samples in favorable circumstances).[n this study, analysis is based on I -inch plugs removed fromconventional cores. Individual pore types are classified in termsof the following parameters:

Pore Boa Size and S%ape. Determined using scanningelectron microscope (SEM) image analysis of the pore system.

Pore if%roaf Size. Determined through capillary pressureanalysis and SEM analysis of pore casts. g

Aspecf Ratio. The ratio of pore body to pore throat size: Thisis a fundamental control on hydrocarbon displacement, go

Coordination Number. The number of pore throats thatintersect each pore. Pore Arrangement. The detailed distribution of pores within

a sample. These parameters are combined to yield a classification of the

various pore types in these rocks (Table I), Pore types areidentified in each core sample (350 samples in this study).Commonly, each sample (l diameter core plug) containsseveral different pore types, It is therefore necessary to grouppore types into rock types.Rock Types. A rock type is an interval of rock characterized bya unique pore structure (not necessarily a unique pore type).In this study, eight rock types were identified, based on therelative volumetric abundance of each pore type (Figure 5).Each rock type is characterized by a particular assemblage(suite) of pore types. 11For example, Rock Type I is dominatedby Pore Type A, while Rock Type 2 contains few pores of TypeA and is dominated by Pore Types B and C (Figure S). Identifi-cation of rock types is fundamentally important because porosityand permeability are related within a specific pore structure. dPorosity/Permeability RelationshipIrr the NRU, the basic relationship between porosity andpermeability exhibits a considerable degree of scatter (up to 4orders of magnitude variation in permeability for a given valueof porosity). However, porosity and permeabi Iity are closelyrelated for each rock type (Figure 6). he rock type relationshipwith permeability has an error range of less than one-half decadefor most samples. Regression equations are developed for eachrock type to quantitatively define each relationship. These areused in the field-wide prediction of permeability (permeabilitybeing a function of porosity and rock type).The slope of the individual regression lines varies among rock

types. This demonstrates the well known independence of pore

body and pore throat size. IJ Some methods of flow unit CkiSifi-cation assume a constant relationship between pore body andpore throat size. This is unfortunate because in such classifica-tion schemes the slopes of the regression lines are identical foreach rock type -- an unusual characteristic in rocks with complexpore systems.Average values of porosity and permeability are given foreach rock type in Table 2. The highest porosity rocks in theNRU do not have the highest permeability, The principal payrocks in the field are Rock Types I and 2. They have signifi-cantly lower values of porosity than Rock Type 4. This hasimportant implications in terms of selecting zones to perforate.Obviously, zones with the highest porosity should not be theprincipal targets in this field.The validity of the geologically determ ined rock types (Figure

5) has been evaluated through mercury capillary pressureanalysis, Results reveal differences between the rock types interms of measured capillary characteristics (Figure 7, Table 3).Such cross checks allow for independent validation of the poregeometrical classification of rock types.Rock/Log ModelPore geometry analysis reveals that eight rock types occur in theNRU: six of the rock types are dolostone, one is limestone, oneis shale (Table 2). Individual rock types can be recognized usingspecific cut-off values based on analysis of environmentallycorrected and normalized log responses and comparison withcore based determination of rock type, The log responses usedin the NRU are:

1. Rhomaa vs Umaa with Gamma Ray (allows discriminationof dolostone, limestone, anhydritic dolostone, siltstone, shale).2. LLs Lld and Porosity (allows discrimination among Rock

Types I through 4),The rock-log model was first developed for 5 cored wells

only, Subsequently the model was extended to the 3 remainingcored wells. Evaluation of cored intervals reveals successfuldiscrimination (>80%) of each of the principal rock types (RockTypes 1 through 4) despite the fact that wells were logged bydifferent companies at different times. Misidentification of RockType I results in identification of Rock Type 2, while mis-identification of Rock Type 2 results in identification of RockType 1. Thus, there is no significant misidentification of thedominant rock types by logs over the cored intervals. The modelis extended to all wells with sufficient log suites in the field (120wells in the NRU). Specific algorithms allow for rock typeidentification on a foot-by-foot basis in each well.As has been shown previously (Figure 6), permeability is a

function of rock type and porosity, Rock type and porosity canbe determined from well log responses alone. Therefore,permeability can be predicted using well log information. Thisallows for the development ofa vertical layering profile basedon rock type and permeability in cored and non-cored wells.The resulting reservoir model is thus numeric and log-based.

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4 D.K. DAVIES. R.K, VESSELL SPE 35433Hydraulic Flow UnitsIndividual HFUS are identified based on integration of dataregarding the distribution of rock types, petrophysical properties(particularly permeability and fluid content) and depositionalfacies. Evaluation of this data for 120 wells reveals that rocktypes are not randomly distributed. The principal reservoir rocks(Rock Types 1 and 2) generally occur in close association, andthey alternate with lower quality rocks (Rock Types 3, 4, 6, 7and 8). Correlation of rock types between wells reveals anobvious layering profile in which 12distinct layers -- hydraulicflow units -- are distinguished in NRU (Figure 8). Correlationis aided by a knowledge of the distribution of depositionalenvironments because there is a general relationship betweendepositional environment and rock type. Rock Types I and 2are more common in high energy deposits (shoals, sand flat,forebank). Rock Types 3 and 4 are common in low energydeposits (supratidal, tidal flat, lagoon).Maps are prepared for each of the HFUs to illustrate the

distribution of important petrophysical parameters (Figure 9).The distribution of the principal rock types for each HFU is alsomapped (Figure 10). This allows for rapid identification of areasof the field dominated either by high quality or low quality rock.There is a general tendency in the NRU for the higher quality

rocks (Rock Types I and 2) to occur in discrete belts on the NEedge of the unit while lower quality rocks (Rock Types 3 and 4)occur in SW portions of the unit. Within this general trend,perturbations exist in the distribution of permeability. Theseperturbations are important because they result in compart-mentalization of the reservoir. There are no faults in the NRU.Compartmentalization is entirely stratigraphic. It is the result ofareal variations in the distribution of individual rock types.

It is obvious that uniform infill drilling is neither prudent norwarranted owing to the stratigraphic compartmentalization andirregular permeability distribution of this reservoir. Infilldrilling should be restricted to i) areas of the field that have RockTypes 1 and 2 dominant, good permeability and hydrocarbonpore volume (HPVH) characteristics, high primary and second-ary recovery, and ii) areas of poor reservoir continuity withacceptable porosity and permeability values, significant abun-dance of Rock Types 1, 2 or 3, and good primary but poorsecondary recovery.Conclusions1. Measurement of pore geometrical parameters allows forimproved prediction of permeability and permeability distribu-tion from wireline logs in partially cored intervals, and inadjacent uncored intervals and adjacent uncored wells. Itimproves prediction of reservoir quality in non-cored intervalsfor improved completions and EOR decisions.2, Pore geometrical attributes allow for definition of hydrau-

lic flow units. These attributes can be related to log response,thus allowing for the development of a field-wide, log-basedreservoir model,

3. Uniform well spacing patterns in heterogeneous reservoirsare not prudent owing to the existence of significant arealvariations in permeability. Infill drill patterns should be based onthe distribution ofkH and HPVH.

4. The reservoir characterization methodology used in thisstudy allows for identification of areas of the reservoir character-ized by i) high values of porosity, permeability, HPVH, ii) thicksequences of potentially productive rock, and iii) compart-mentalization.References1.

2.3.4.

5.

6.

7.

8.

9.

10,

Il.

12,13.

14.15.

Pande. P.K.: The NRU-DOIS prospectus, FinaOil and Chemi-cal Co., Midland, Texas (1995) 8.Muskat, M.: Flow of Homogeneous Fluids throueh PorousIv&@ McGraw Hill, New York (1937) 763.Leverett. M. C.: FIow of Oil Water Mixtures through Un-consolidated Sands-, Trans. AIME(194 I) I-5.Myers. M. T.: Pore combination modeling: a technique formodeling the permeabi Iity and resistiv ity properties of complexpore systems, SPE Paper 22662, Ann. Tech. Conference, Dallas.Texas (1991) 77-87.Krause, F. F., Collins, H. N., Nelson, D. A ., Machemer, S,D, andFrench, P.R ,: Multi scale anatomy of a reservoir: geologicalcharacterization of Pembina-Cardium Pool, West-central Alberta,Canada, AAPG Bull. (1987) v, 71, 1233-1260.Haldorserr. H. H.: Simulator parameter assignment and theproblem of scale in reservoir engineering, in ReservoirCharacterization, Lake L,W. and Carroll , H.B.. Jr., eds, AcademicPress. New York ( 1986) 293-340,Clelland. W .D. and Fens, T. W.: Automated rock character-ization with SEM/image analysis techniques, SPE FormationEvaluation (1991) v. 6, 437-443,Wardlaw, NC.: Pore geometry of carbonate rocks as revealedby pore casts and capillary pressure, AAPG Bull, ( 1976) v, 60,245-257.Li, Y. and Wardlaw, NC.: - The influence of nettability andcritical pore-throat size ratio on snap-off, J. Colloid and inter-face Sci. ( 1986) v, 109,461-472,Wardlaw. N. C.: The effects of pore structure on displacementefficiency in reservoir rocks and in glass micromodels. SPEPaper 8843, First Joint SPE/DOE Symposium on Enhanced OilRecovery. Tulsa, Oklahoma ( 1980) 345-352.Wardlaw, NC, and Cassan. J.P.: Estimation of recoveryefficiency by visual observation of pore systems in reservoirrocks, Bull, Can. Pet. Geol. ( 1978) v. 26, 572-585.Archer, J.S. and Wall. C.G,: Petroleum Erreineering Princicdesand Practice, Graham and Trotman, Ltd. ( 1986) 362.Ehrlich, R. and Davies, D. K.: Image analysis of pore geometry:relat ionship to reservoir engineering and modeling, Proc. SPEGas Technology Symposium,Dallas,Paper 19054(1989) 15-30.Calhoun,J.C.: Fundamemalsof reservoir erwineering. Univ. Ok.Press, Norman. ( 1960) 426.Amaefule, J.O., Altunbay, M., I_iab, D and Kersey, D. G,:Enhanced reservoir description: using core and log data toidentify hydraulic (flow) units and predict permeability inuncored wells, SPE Paper 26436, Ann. Tech, Conference,Houston. Texas ( 1993) 1-16.

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SPE35433 FLOW UNIT CHARACTERIZATION OF A SHALLOW SHELF CARBONATE RESERVOIR: 5NORTH ROBERTSON UNIT, WEST TEXAS-

lAELE1- PORETYPE,CLASSIFICATION,NRUPoreyBcDEFG

Sue Cwldmalrnn &peo Pole w~_uoL* MLKI&eL --&& A-72- ~ Lksmolloil.nXL1OO Ifdefmnncded Pmwyrntaparlidc6&120 llm0u18r


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6 D.K. DAVIES, R.K. VESSELL SPE 35433

llm

0.01

100

100 .. .-1q . ~.4 I,.*.. - , i*_* ......=..*..q

&

1 -.. !-. . .. Io 5 10 15 20Core Porosity (??)

Fig. 3. Porosity vs permeability, all core samples.

1 ---. l-- *Iq

* I .----- -- . . . . . .. Is 10 1s 20

% i

*: :~:-_.. *. ..0 q .- ..q*.. s-..-. . *. .- .-- .. . . . I 1

s fo 15 20cOmrJOmdQfw

J=E=I=F1-------i-i --0.01I I I Io 5 10 15 a

Con PorusUy(WJo

Fig. 4. Porosity and permeability for principal depositional environments.

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SPE35433 FLOW UNIT CHARACTERIZATION OF A SHALLOW SHELF CARBONATE RESERVOIR: 7NORTH ROBERTSON. UNIT, WEST TEXAS

Rock Type 1Rock Type 2Rock Type 3Rock Type 4Rock Type 5Rock Type 6Rock Type 7

Fig. 5. Volumetric proportions of pore typesin each rock type.

0i%25 50 75 100voP%Jf---. =

Is to 15Cm Pc+wny w

Ma WE J Ix ....:.--..........:...........:. . fHI--7....,:. Fig. 6. Porosity and permeabili ty by rock type.ALL ROCK 7YPES = Samples used in poregeometry analysisMl I I 48 cmhwsYKlio 1$ 0,011 10 6 comPonNUVITQ Is*W nOcKnFS6g 10 -- ---[

.-~-. -

f I

~ 0., ------- . . .0.01

III J . . . 1 . . . I - .. .ao i

i +__~ Is-~f..--. .\-..I I 0.010 5 10 15--m6 - -JlJaw4 11301


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8 D.K. DAVIES, R.K. VESSELL SPE 35433 -..

, tbnwLOlwlll -

im mmIibmJ=-89JLMP8

,Fig. 7. Capilla~ pressure curves by rock type.

.lfw 4103 604 2SOJ 2s04 Ws 2T 70

Fig. 8. Cross section of NRU showing distr ibution of HFUS. Dar> re Rock Types 1 and 2. Light zones are Rock Typea 3, 4, 6, 7 and 8.

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SPE 35433 FLOW UNIT CHARACTERIZATION OF A SHALLOW SHELF CARBONATE RESERVOIR: 9NORTH ROBERTSON UNIT, WEST TEXAS

!1 1 ,;...I !F? ,

PIw I1 . . . . .k.. . 41....:.

1. 1. . . .4 I

(

. .,,.. . .:.l-. 4

I\ I I

L .+.. . . . . . I

D ,I. . . . .. ... . . . .. ..~ -L--i.*, Ib\ . . . III.Fig. 9. Contour map of HFU #12 showing distribution of A) thickness, B) poroai~ x thickness, C) permeability x thickness, and D) HPV xthlcknees (HPVH). Note: darker areas have higher values than lighter areas.

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10 D.K. DAVIES, R.K. VESSELL SPE 35433

k).B (~

. . . . . J. . ... . . . .II . . . .1 1 ! 1

,

D [ I

l. l. .1

IIIIIT

Fig. 10. Contour map of HFU #12 shovdng distribution ok A) Rock Type 1, B) Rock Type 2, C) Rock Type 3, and D) Rock Type 4. Nota: dadcerareas have higher values than lighter areas.

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Volume Issue 0 1996 [Doi 10.2118_35433-Ms] Davies, David; Vessell, Richard -- [Society of Petroleum...

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