Investigating C02 Storage Potential of Carbonate Rocks during Tertiary Recovery from a Billion Barrel Oil Field, , : Part 2 - Reservoir Geology (IEA Weyburn C02 Monitoring and Storage Project)

G. Burrowes'

Burrowes, G. (2001 ): Investigating C02 storage potential of carbonate rocks during tertiary recovery from a billion barrel oil field, Weybum, Saskatchewan: Part 2 ~ reservoir geology (!EA Weybum C02 Monitoring and Storage Project); in Summary of Investigations 2001, Volume I, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.1.

1. Introduction permeability lower limestone member of the carbonate. After achieving maximum rates of more The Weybum Field (Figures 1 and 2) covers an area of than 7000 m3/day by 1966, pool production declined to about 180 km2 (70 square miles) and produces 29° API less than 2000 m3/day by 1985. An aggressive vertical gravity crude oil from Mississippian shallow-water infill drilling program (to 40° to 60° acre spacing) was carbonates at a mean depth of 1450 m. Pay thickness initiated in 1986. It arrested production decline, ranges from 5 m to over 30 m with an average of about returning production to almost 4000 m3/day. 10 m. Following its discovery in 1954, the Weybum Horizontal drilling, introduced at Weybum in 1991, Field was produced by primary depletion for about became the preferred production strategy over the IO years (Figure 3 ). In an effort to arrest declining ensuing IO years. Unlike the earlier vertical phase, production, an inverted nine-spot waterflood scheme horizontal drilling targets the largely unswept reserves was introduced in 1964 targeting the higher in the low-permeability, high-porosity dolostone within

HUDSON 'I BAY ALBERTA MANITOBA I

• HELENA

WYOMING !<:;:::=::! WILLISTON BASIN

Figure l - Location of the Weyburn Fie/ti, Williston Basin.

I PanCanadian Resources, 150. 9th Avenue SW, P.O. Box 2850, Calgary, AB T2P 2S5: F-mail: [email protected].

64 Summary ofInvestigations 200/, Volume I R14 R13 R12W2M

Discovered: 1954 22km Area: 70 sq. miles T7 Current oil rate: 18,000 BOPD Number of wells: 1016 total 660 vert. oil 158 hz. oil 197 inj. 18km T6 Sour crude: 25-34 API Depth: 1450m (4760 ft) OOIP: 1,400 MMbbls Cum. Prod. (12/98): 351 MMbbls Total recovery to date: 5% T5

Figure 2- Summary of Weyburn reservoir data showing outline offield and initial Phase IA area ofthe C01 flood.

->, C'a '"C M -E

2000

...... - ...... 0 0, v 0, v 0, v CJ) v CJ) v CJ) v CJ) v CJ) It) It)

D Base Waterflood D Infill verticals D Horizontals D Forecast co2 EOR Figure 3 - Production history and forecast, Weyburn Field, Saskatchewan. the upper Midale. By the mid-1990s, planning for a 2. Regional Setting-The Trap C02 miscible flood was well underway and C02 3 injection began in the Fall of2000 in the 19 pattern, The 1.4 billion barrel (0.2 billion m ) Weyburn Field in Phase l A area (Figure 2). A variety of engineering, southeastern Saskatchewan is one of several large oil geochemical, and seismic efforts are currently fields discovered along the Mississippian subcrop belt underway to monitor progress of the miscible front and of the northern Williston Basin (Figure 4). Reservoired optimize sweep efficiency. hydrocarbons have accumulated in a series of combined stratigraphic, diagenetic and hydrodynamic traps. Progressive northeasterly incision of the Mississippian succession below the sub-Jurassic

Saskatchewan Geological Survey 65 unconfonnity has created ideal trapping conditions Jurassic Watrous Fonnation (Figure 5). The efficiency where porous shallow-marine carbonate is regionally of hydrocarbon trapping at the Mississippian subcrop is juxtaposed against relatively impenneable, fine­ further enhanced by widespread, primary, cycle­ grained 'red bed' siliciclastics of the Triassic to capping anhydrite within the Mississippian section, as well as by widespread, sub­ unconfonnity, diagenetic 105• Manitoba "caprock" of anhydritized Saskatchewan dolostone (Kendall, 1975; Kent, 1987, 1999). More locally, such ~ \., as at Weybum, a 'hydrodynamic' '· trapping component is also created by high irreducible water saturations in updip, very low­ permeability cryptocrystalline dolostone underlying the unconformity. The development ,.,,· of such subtle hydrodynamic traps is documented by Coalson ' el al. ( 1994). ..~ ·~.;, The presence of bottom seal across the Weybum Field is \ problematic. With the exception ,,, of northerly margins of the field :·A where the Frobisher Anhydrite is ' present, Midale reservoir .. carbonate directly overlies dolostone of the Frobisher Marly . Furthermore, historical drillstem Montana ..• North Dakota test results and log evaluations suggest that the original free­ .. water level within the field . ,~· 0 30mi extended well into the Frobisher . I beds (Weyburn Sub-Committee, ' o 50km 1962). In reality, however, 10s· ' 10i4• 102' significant communication with Frobisher reservoirs is tempered by widespread metasomatic Figure 4 - Mississippian subcrop trends and oilfields, Williston Basin (from Kent, anhydrite replacement at the base 1983). of the Midale and by scarcity of reservoir-quality units within the Jurassic upper Frobisher beds (Frobisher Irlai;sic______WATROUS FORMATION Marly beds under Weybum are KIBBEY FORMATION -~ generally dominated by non­ reservoir argillaceous, silty, and c U) 0 anhydritic dolostone and marl). 4> .::: On the reservoir production time .::: n:i n:i scale, therefore, communication .t:. E... c: '2. (.) 0 with Frobisher reservoirs is cu ::s LI. probably local and limited. ·a. 0 ... -,. Q. c., c 0 \ Ill c >, c FROBISHER BEDS Ill 0 c O .!!! cu ·- 3. Midale Stratigraphy Ill "C u-:;; KISBEY SANDSTONE n:i c E 0 .. ALIOA .. I At Weyburn Field, oil is trapped -~ ~ ·- 0 ~ : LI. within Osagean-age Midale strata :::1: TILSTON of the Madison Group (Figure 5). "c &.~ As is common throughout the "~ SOURIS VALLEY Upper Mississippian succession 'O"'E ~ 0 0 _JU. of the Williston Basin, alternating porous carbonate and Bakken Formation impermeable anhydrite record Devonian fluctuating sea levels and episodic isolation and evaporation within a Figure 5 - Mis.sissippian stratigraphy, southeast Saskatchewan. shallow water intracraton ic basin.

66 Summary of Investigations 2001, Volume I top seal across the field. Figure 5 Cl> illustrates Midale stratigraphy Cb~.... ~ -nl 0 relative to a typical reservoir log u ,::, c.. response. ro i nl n::: w> "Three The Midale carbonate reservoir is Fin ers" infonnally subdivided into a lower limestone-dominated unit called the "Vuggy", and an upper, mainly dolostone unit referred to as the " Marly" (Figure 6). Thickness ranges from 10 to 22 m in the Vuggy and I to 11 m in the Cl) Marly. Core-based sequence -ca stratigraphic interpretation by "C >, PanCanadian (Figure 7) indicates c, that the Vuggy can be subdivided ·-::1! c, ::, into two sequences separated by a > mappable surface of subaerial exposure and erosion. Heterogeneous calcareous, algal/coated-grain/pisolitic wackestone, packstone and Frobisher grainstone, with visible "vuggy" and fenestral porosity, are _ characteristic of the lower Vuggy. O Gamma Ray (API) 150 45 Density Porosity % 15 ~ 5m ('7ft) Patchy cementation by calcite, anhydrite, and dolomite adds to 45 15 Neutron Porosity % - the overall heterogeneity. Internal higher order cyclicity within the Figure 6- Typical log re.fponse and major reservoir zones within the Mida/e re.5ervoir. lower Vuggy permits further The reservoir lies between low penneability dolostone breakdown of this unit for and anhydrite of the Frobisher and anhydrite of the purposes of reservoir modelling (V2 to V6; Figure 7). Midale Evaporite. Though locally reduced in thickness Mapping of the lower Vuggy reveals a generally to only a few metres by erosion on the sub-Jurassic northeasterly trend of 'shoal' thicks and ' intershoal' unconfonnity, the Midale Evaporite forms a regional thins (Figure 8).

SW NE

Ratcliffe

Midale Evaporite ------~~~i#?':ie~~**~ Midale Marly

(upper) Midale Vuggy w...-,.;.-;..~~

Frobisher © Major depositional sequences ! fractures

1.6 Km mmm anhydrite, P?J.··.. do1ostone o· limestone I Lllli1Ll anhydrite cemented 1 mile

Figure 7 - Midale geological model illustrating depositional seque11ces a111J reservoir flow units. T.F., Three Fingers; Ml and MJ, Marty units as di.5c11s.wd ill text.

Saskatchewan Geolo~ical Survey 67 studies by PanCanadian suggest R.14 R.13 R.12W2M the presence of four distinct depositional settings that reflect responses of an intracratonic T.7 epeiric ramp to significant eustatic fluctuations. The inferred spatial relationships of these environments are summarized in Figure 9.

Initial lower Vuggy deposition occurred in response to the early T.6 phases of a major transgression over the Frobisher surface. Stacked, strongly progradational cycles with evidence of repeated subaerial exposure record deposition within a marginal marine, peritidal island/lagoon T.5 complex. This style of sedimentation ceased abruptly in response to major sea-level fall and subaerial exposure. Up to 8 m of relief on the lower Vuggy Figure 8 - Generalized distribution ofMidale Vuggy shoal and intershoal areas, surface was developed by a Weyburn Field (from Churcher and Edmunds, 1994). combination of depositional topography and erosional The overlying upper Vuggy sequence (V 1) is incision. Considerable porosity enhancement probably distinguished in core by the general absence of coarse also occurred at this stage. vuggy pores and by depositional fabrics dominated by abraded skeletal packstone and wackestone. Upper Significant sea-level rise resulted in major inundation Vuggy thickness generally varies inversely with_ that of of the shelf over the eroded lower Vuggy and in a the lower Vuggy, with thick upper Vuggy associated change in depositional setting to dominantly subtidal, with thin, 'off-shoal' lower Vuggy. open-marine mid-ramp influen_c~d by sto~- ~nd :wave­ reworking. Blanket-like depos1t1on of the d1stmct1ve High porosity, micro_sucrosic Marty d~lostone with non-vuggy, skeletal-rich beds of the upper Vuggy mud-dominated fabncs abruptly overhes the upper largely infilled the lower Vuggy topography. Vuggy. Isopachs of the Marly illustrate_ a generally. southwestward thickening wedge that, m cross sect10n, A widespread Thalassino~c{es-burrowed_ layer mar~s a appears to onlap the upper Vuggy surface to.the third major shift in depos1t1onal style w1thm the Mtdale northeast. Two Marty layers (MI and M3; Figure 7) beds. Upper Vuggy grain-dominated textures a?ruptly are distinguished based on a widespread, low porosity, give way to extensively bioturbate~, mud-dominated calcareous marker bed (M2) within the Marty. Below textures in the Marty. The predommance of skeletal the capping Midale Evaporite, ~ thin (I to 3 m ~h_ick) and non-skeletal algal remains, ostracods, and the . transitional unit of laminated, silty, and anhydnttc introduction of terrestrial silt and clay suggest a shift to dolostone is commonly referred to as the "Three a more proximal, quiet-water subtidal setting. The Fingers" (due to its characteristic gamma response; _see apparent regional extent of these environments Figure 6). This unit is considered to be non-reserv01r seaward of an evaporitic coastal plain, represented by facies in most areas. the overlying upper Marly and Midale Evaporite, argues for deposition within an extensive epeiric lagoon that may have devefoped in response to .. 4. Depositional Environments regional isolation of the ~tlhston Basm._ Depos1t1~nal environments may have mcluded both distal, relatively Kent (1984, 1999) and Lake (1998) have discuss~~ deep, outer ramp equivale~ts and pro~i~al, very regional aspects of Mississippian Madison deposition shallow restricted euryhalme bays. W1thm the Marly, within the Williston Basin and generally agree that the an overall brining-upward and shallowing-upward characteristic style of sedimentation indicates a succession is suggested by the upward change from a periodically restricted, shallow epeiric sea. !~pica! diverse Cruziana ichnofacies to a Planolites/ features include extremely low-angle depos1ttonal Chondrites-dominated ichnofacies with algal slopes coupled with predominance <;>f shallm-".-water lamination (see Keswani and Pemberton, 1993). Marty facies exhibiting abrupt vertical fac1es transitions on a deposition culminated in silty, anhydritic tidal flat and regional scale. The carbonate reservoir at _Weybum sabkha dolo-laminites of the upper Marly "Three represents an erosional remnant of a considerably more Fingers" zone. Subsequent deposition of the Midale_ extensive marginal ramp on the northern flank of the Evaporite records the final shift to fully coastal manne Williston Basin. Phase IA area sedimentological core to continental evaporitic salinas and playas.

Summary of Investigations 2()() 1, J 'o/ume 1 68

-·····- ...- ..,.. . •,_ .. --·········---·--· ·-- . ...•.. . -·. SW NE

Open Marine Protected Peritidal/lsland Restricted Tidal Flats Salina Mid-Ramp Subtidal Ramp Complex & Sabkha

<}:::::::: Silt I Clay Laminated Silty/ Anhydritic Mdst

Outer Ramp

Skeletal Algal/Pisolitic Burrowed Wkst-Pkst Fenestral Grst-Wkst (Dalo) Mdst/Wkst

Figure 9 - Diagrammatic representation of Midale depo.\'itional environmenu·. Mdst, mudstone; Wkst, wackestone, Grst, grains/one; Pkst, packstone; and Dolo, dolomitizetl.

5. Reservoir Characteristics uniform, finer grained depositional textures and absence of strong diagenetic overprints. Table 1 summarizes th e key reservoir parameters of the Midale beds in the Weybum Field. Markedly different Reservoir characteristics of the Marty (homogeneity, flow characteristics between the lower Vuggy, upper hi gh porosity, low permeability) are, foremost, a Vuggy, and Marly reflect the combination of: a) function of early dolomitization of 'bio-homogenized', different primary depositional settings and textures, b) mud-dominated fabrics. Variations in reservoir quality markedly different secondary diagenetic overprints, within the Marly occur mainly in response to variations and c) varying rheological responses to imposed stress. in degree of dolomitization, and a general upward Lower Vuggy reservoir characteristics (extreme trend to finer crystalline, lower permeability fabrics. heterogeneity, low to moderate porosity, low to high Severely reduced reservoir quality is commonly permeabilities) are predictable products of texturally associated with partial dolomitization within either thin diverse peritidal deposits overprinted by complex near­ skeletal-rich packstones or organic-rich, microstylolitic surface and burial dissolution and cementation. On the beds; both of these are widespread at the top of the M3 other hand, the relatively homogeneous reservoir zone and combine to form a correlatable low­ characteristics of the upper Vuggy (low to moderate pcnneabil ity zone separating Ml and M3. porosity, low permeability) directly reflect more

Table J - Summary ofMidale reservoir characteristics, Weyhurn Field. 6. Fractures One of the more prominent Reservoir ; Lithology : Porosity i Matrix ! Heterogeneity features of the Weybum Midale 1 Fracture Zone & Permeability Density reservoir is sub-vertical natural Textures (avg.%, Type) (avg.,md) fractures, both open and cemented. Not only are these i Dolostone 20-37 <0.1 · 100 Low Low­ Marly obvious in numerous well cores, (26 ) (10) Moderate (Ml, M3) but are also clearly imaged on mudsto ne, intercrystalline biot11rbatio 11 (2-4 m spacing) borehole image logs. Analysis of wackestone : mo/die j dolomitization >25% 0-unfra ct these image logs indicates that the ~-- ---+-- ~ ··---· dominant fracture orientation is Upper Limestone 2-15 ~--Me~i-um____ rgh - <0.01 - 20 parallel to the current southwest­ Vuggy (10) ( 1) (VI) northeast orientation of maximum 1 : packs tone. interparlicle thick bedded, ( < 1 m spacing) I horizontal stress (Bell and wacke.stone J intrap article btoturbatton --- ····----· •-- . ,. ·- · Babcock, 1986; Adams and Bell, Lower Limestone 5-20 <1 - 500 High Moderate - Vuggy 1991 ). Secondary southeast­ (15) (20) . High northwest fracture trends have (V2-V6) 1 mudstoneto fenesfrfll we ll bed ded . (< 1-2 m spacing) also been defined (Bunge, 2000). grainsto ne vuggy high order cyclicily, ] microfraclu,-ed complex dif:' genesis Churcher and Edmunds ( 1994) have estimated fracture spacing at between 0.3 and 10 m, with

Saskatchewan Geolo[?ical Survey 69 highest densities in the upper Vuggy and lowest in the commercially available C02 surfactants and diverting Marty. Although the low fracture density in the Marty agents for C02 mobility control. appears to be inconsistent with the normally more brittle response of crystalline dolostones, the very high Together with the Colorado School of Mines, porosity and sucrosic textures appear to have induced a PanCanadian will be acquiring four multicomponent 3- more ductile response to stress. D seismic surveys within the initial Phase I A area over a period of about three years; the key baseline survey The degree to which fractures affect directional was acquired in 1999. The seismic surveys will use the reservoir permeability, flow characteristics, and latest techniques to acquire P and S wave data in an producibility is difficult to determine. Perhaps the attempt to monitor the progress of the flood front and strongest evidence for their influence is the pronounced to improve spatial resolution of reservoir properties preferred "on-trend" water breakthrough response (porosity-thickness and fracture-permeability). during waterflood. However, much of this effect is Interpretation of the seismic data is to be enhanced by possibly a by-product of northeasterly propagation of the co-ordinated acquisition of two multicomponent 3- fractures under high injection pressures, combined with D vertical seismic profiles (VSPs) as well as single­ the preferred northeasterly orientation of the high­ well and cross-well (horizontal to horizontal) seismic permeability lower Vuggy shoals. Furthermore, history tomography. matching of the breakthrough response does not require permeabilities significantly higher than average, core-based matrix permeabilities. Also, 8. References Pan Canadian' s analysis of scattered transient pressure data does not lend strong support for dual porosity Adams, J. and Bell, J.S. (1991): Crustal stresses in reservoir behaviour. The classic dual porosity response ; in Slemmons, D.B., Engdahl, E.R., may, however, be partly masked by either the Zoback, M.D., and Blackwell, D.D. (eds.), interaction of the markedly different Marty and Vuggy Neotectonics of North America, Decade Map flow systems, or the homogenizing effect of Volume 1, Geo!. Soc. Amer., Boulder, p376-386. microfracturing, which is widely observed in thin sections of the lower Vuggy. Bell, J.S. and Babcock, E.A. (1986): The stress regime of the Western Canadian Basin and implications These ambiguities have made it difficult to develop a for hydrocarbon production; Bull. Can. Petrol. satisfactory, comprehensive fracture model for the Geol.,v34,p364-378. Weybum reservoir. The extent to which fractures will

influence C02 flood performance remains a key issue Bunge, R.J. (2000): Midale reservoir fracture to be addressed by the IEA Weyburn C02 Monitoring characterization using integrated well and seismic and Storage Project. One of the avenues PanCanadian data, Weyburn Field, Saskatchewan; unpubl. and the IEA are pursuing is the acquisition of M.Sc. thesis, Colorado School of Mines, 2 I 8p. multicomponent 3-D seismic data to assist in spatial characterization of the fracture system. Churcher, P.L. and Edmunds, A.C. (1994): Reservoir characterization and geologic study of the Weyburn Unit, southeastern Saskatchewan; 7. Flood Monitoring PanCanadian Petroleum Ltd., internal rep., 80p. A critical element of successful, efficient tertiary Coalson, E.B., Goolsby, S.M., and Franklin, M.H. recovery schemes involves diligent monitoring of flood (1994 ): Subtle seals and fluid-flow barriers in conformance as well as the overall progress of the carbonate rocks; in Dolson, J.C., Hendricks, M.L., flood front. PanCanadian is addressing this process and Wescott, W.A. (eds.), Unconformity-related through monitoring of production fluid composition Hydrocarbons in Sedimentary Sequences, and volumes, chemical tracer studies and Guidebook for Petroleum Exploration and muticomponent 4-D seismic. Exploitation in Clastic and Carbonate Sediments, Rocky Mtn. Assoc. Geol., Denver, p45-58. Production testing on individual wells will be done regularly in five newly constructed satellite stations. Kendall, A.C. ( 1975): Anhydrite replacements of Each satellite is equipped with a three-phase test gypsum (satin-spar) veins in the Mississippian separator for measuring oil, water, and gas flow rates. caprocks of southeastern Saskatchewan; Can. J. Produced gas is monitored for C02 using infra-red Earth Sci., vl2, pl 190-1195. analyzers and dragar tube sampling. Samples of produced oil, water, and gas will be periodically Kent, D.M. ( 1983 ): Mississippian reservoirs in collected for more detailed compositional analysis. A Williston Basin; Comptes Rendues, Ninth tracer study will be implemented in March 200 I using International Congress on Carboniferous perflourocarbons to trace C02 injection gas at seven Stratigraphy, v4, p99- I I 0. injection wells. The study will determine C02 conformance in three flood patterns and assist in _____ ( 1984 ): Carbonates and associated rocks designing flood operations. A screening study has also of the Williston Basin - their origin, diagenesis been initiated at University of Regina to evaluate and economic potential; Rocky Mtn. Sec., Soc.

70 Summary of lnvesl(f?alions 200 I. J 'o/ume 1

·-······.. ·· · .. _,,.. _ ·--·------··------·-·-···· . Econ. Paleont. Mineral., Short Course Notes, 137p.

---,--c------( 1987): Mississippian facies, depositional history, and oil occurrences in Williston Basin, Saskatchewan and Manitoba; in Peterson, J.A., Kent, D.M., Anderson, S.B., Pilatzke, R.H., and Longman, M.W. (eds.), Williston Basin: Anatomy of a Cratonic Oil Province, Rocky Mtn. Assoc. Geo!. , Denver, p 157-170.

- -~- -(1999): A re-examination ofthe process of formation and diagenesis of coated-grain accumulations and contiguous facies in the Mississippian of southeastern Saskatchewan; in Summary of Investigations 1999, Volume I, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 99-4.1, p 17-26.

Keswani, A.O. and Pemberton, S.G. ( 1993): Sedimentology, ichnology, and paleoecology of the Mississippian Midale carbonates in the Williston Basin, area, Saskatchewan: Preliminary interpretation; in Haan, J.D., Jang, K., and Webb, T. (eds.), Carboniferous to Jurassic - Pangea, Core Workshop Guidebook. Can. Soc. Petrol. Geol., Calgary, p206-228.

Lake, J.H. (1998): A Mississippian epeiric shelf model for the Williston Basin; in Christopher, J.E., Gilboy, C.F. , Paterson, D.F., and Bend, S.L. (eds.) Eighth International Williston Basin Symposium, Sask. Geol. Soc., Spec. Pub!. No. 13, p69-71.

Weyburn Sub-Committee (1962): Effective oil pore volume, Midale Beds reservoir, Weybum Field, Saskatchewan; Weybum Working Engineering­ Geological Sub-Committee, Pore Volume Group, 1962 Report, I I Op.

Saskatchewan Cieofogica{ Survey 71