Investigating C02 Storage Potential of Carbonate Rocks during Tertiary Recovery from a Billion Barrel Oil Field, Weyburn,

Saskatchewan: 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

The Weybum Field (Figures 1 and 2) covers an area of about 180 km2 (70 square miles) and produces 29° API gravity crude oil from Mississippian shallow-water carbonates at a mean depth of 1450 m. Pay thickness ranges from 5 m to over 30 m with an average of about 10 m. Following its discovery in 1954, the Wey bum Field was produced by primary depletion for about IO years (Figure 3 ). In an effort to arrest declining production, an inverted nine-spot waterflood scheme was introduced in 1964 targeting the higher

ALBERTA

• HELENA

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

' I

WYOMING

permeability lower limestone member of the Midale carbonate. After achieving maximum rates of more than 7000 m3/day by 1966, pool production declined to less than 2000 m3/day by 1985. An aggressive vertical infill drilling program (to 40° to 60° acre spacing) was initiated in 1986. It arrested production decline, returning production to almost 4000 m3/day. Horizontal drilling, introduced at Weybum in 1991, became the preferred production strategy over the ensuing IO years. Unlike the earlier vertical phase, horizontal drilling targets the largely unswept reserves in the low-permeability, high-porosity dolostone within

MANITOBA I

HUDSON BAY

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: Gcoffrcy.Burrowes@pancanadian.ca.

64 Summary of Investigations 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 of field and initial Phase IA area of the C01

flood.

->, C'a

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E

2000

0 It)

I

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. . . . . . . . .

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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 C02 miscible flood was well underway and C02 injection began in the Fall of2000 in the 19 pattern, Phase l A area (Figure 2). A variety of engineering, geochemical, and seismic efforts are currently underway to monitor progress of the miscible front and optimize sweep efficiency.

Saskatchewan Geological Survey

2. Regional Setting-The Trap

The 1.4 billion barrel (0.2 billion m3) Weyburn Field in

southeastern Saskatchewan is one of several large oil fields discovered along the Mississippian subcrop belt of the northern Williston Basin (Figure 4). Reservoired hydrocarbons have accumulated in a series of combined stratigraphic, diagenetic and hydrodynamic traps. Progressive northeasterly incision of the Mississippian succession below the sub-Jurassic

65

unconfonnity has created ideal trapping conditions where porous shallow-marine carbonate is regionally juxtaposed against relatively impenneable, fine­grained 'red bed' siliciclastics of the Triassic to

105•

Saskatchewan

,., ,·

.. ~

..

•

' ·~.;, \ ,,, :·A

'

Jurassic Watrous Fonnation (Figure 5). The efficiency of hydrocarbon trapping at the Mississippian subcrop is further enhanced by widespread, primary, cycle­capping anhydrite within the Mississippian section, as

Manitoba

~ \., '·

well as by widespread, sub­unconfonnity, diagenetic "caprock" of anhydritized 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 low­permeability cryptocrystalline dolostone underlying the unconformity. The development of such subtle hydrodynamic traps is documented by Coalson el al. ( 1994) .

Montana .. North Dakota

The presence of bottom seal across the Weybum Field is problematic. With the exception of northerly margins of the field where the Frobisher Anhydrite is present, Midale reservoir carbonate directly overlies dolostone of the Frobisher Marly . Furthermore, historical drillstem test results and log evaluations suggest that the original free­water level within the field extended well into the Frobisher beds (Weyburn Sub-Committee, 1962). In reality, however, significant communication with Frobisher reservoirs is tempered by widespread metasomatic anhydrite replacement at the base of the Midale and by scarcity of reservoir-quality units within the upper Frobisher beds (Frobisher Marly beds under Weybum are generally dominated by non­reservoir argillaceous, silty, and anhydritic dolostone and marl). On the reservoir production time scale, therefore, communication with Frobisher reservoirs is probably local and limited.

. ,~· .. 0 30mi

' . '

I o 50km

10s· 10i4• 102'

Figure 4 - Mississippian subcrop trends and oilfields, Williston Basin (from Kent, 1983).

Jurassic Irlai;sic ______ _ WATROUS FORMATION

KIBBEY FORMATION -~

c U) 0 4> .::: .::: n:i n:i E

c: .t:. ... '2. (.) 0 cu ::s LI. ·a. 0 ...

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>, c Ill 0 c O

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u-:;; -~ n:i c E

~ 0 ..

~ ·- 0 : LI.

:::1:

FROBISHER BEDS

KISBEY SANDSTONE

ALIOA

TILSTON

"c &.~ "~ "'E 'O ~ 0 0 _JU.

SOURIS VALLEY

Devonian Bakken Formation

Figure 5 - Mis.sissippian stratigraphy, southeast Saskatchewan.

66

-,. \

.. I 3. Midale Stratigraphy

At Weyburn Field, oil is trapped within Osagean-age Midale strata of the Madison Group (Figure 5). As is common throughout the Upper Mississippian succession of the Williston Basin, alternating porous carbonate and impermeable anhydrite record fluctuating sea levels and episodic isolation and evaporation within a shallow water intracraton ic basin.

Summary of Investigations 2001, Volume I

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,::, c.. i nl

> w "Three

Fin ers"

>, c, c, ::, >

Frobisher

~ 5m ('7ft) O Gamma Ray (API) 150 45 Density Porosity %

45 Neutron Porosity %

top seal across the field. Figure 5 illustrates Midale stratigraphy relative to a typical reservoir log response.

The Midale carbonate reservoir is 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 Marly. Core-based sequence stratigraphic interpretation by PanCanadian (Figure 7) indicates that the Vuggy can be subdivided into two sequences separated by a mappable surface of subaerial exposure and erosion. Heterogeneous calcareous, algal/coated-grain/pisolitic wackestone, packstone and grainstone, with visible "vuggy" and fenestral porosity, are

_15 characteristic of the lower Vuggy. Patchy cementation by calcite, anhydrite, and dolomite adds to

-15 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 and anhydrite of the Frobisher and anhydrite of the Midale Evaporite. Though locally reduced in thickness to only a few metres by erosion on the sub-Jurassic unconfonnity, the Midale Evaporite forms a regional

SW

breakdown of this unit for purposes of reservoir modelling (V2 to V6; Figure 7). Mapping of the lower Vuggy reveals a generally northeasterly trend of ' shoal' thicks and ' intershoal' thins (Figure 8).

NE

Ratcliffe

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

Midale Marly

(upper)

Midale Vuggy w...-,.;.-;..~~

Frobisher © Major depositional sequences ! fractures

mmm anhydrite, P?J.·· .. do1ostone o· limestone Lllli1Ll anhydrite cemented

1.6 Km I

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

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

T.7

T.6

T.5

Initial lower Vuggy deposition occurred in response to the early 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 complex. This style of sedimentation ceased abruptly in response to major sea-level fall and subaerial exposure. Up to

Figure 8 - Generalized distribution of Midale Vuggy shoal and intershoal areas, Weyburn Field (from Churcher and Edmunds, 1994).

8 m of relief on the lower Vuggy surface was developed by a combination of depositional

The overlying upper Vuggy sequence (V 1) is distinguished in core by the general absence of coarse vuggy pores and by depositional fabrics dominated by abraded skeletal packstone and wackestone. Upper Vuggy thickness generally varies inversely with_ that of the lower Vuggy, with thick upper Vuggy associated with thin, 'off-shoal' lower Vuggy.

High porosity, micro_sucrosic Marty d~lostone with mud-dominated fabncs abruptly overhes the upper Vuggy. Isopachs of the Marly illustrate_ a generally. southwestward thickening wedge that, m cross sect10n, appears to onlap the upper Vuggy surface to.the northeast. Two Marty layers (MI and M3; Figure 7) are distinguished based on a widespread, low porosity, calcareous marker bed (M2) within the Marty. Below the capping Midale Evaporite, ~ thin (I to 3 m ~h_ick) transitional unit of laminated, silty, and anhydnttc dolostone is commonly referred to as the "Three Fingers" (due to its characteristic gamma response; _see Figure 6). This unit is considered to be non-reserv01r facies in most areas.

4. Depositional Environments Kent (1984, 1999) and Lake (1998) have discuss~~ regional aspects of Mississippian Madison deposition within the Williston Basin and generally agree that the characteristic style of sedimentation indicates a periodically restricted, shallow epeiric sea. !~pica! features include extremely low-angle depos1ttonal slopes coupled with predominance <;>f shallm-".-water facies exhibiting abrupt vertical fac1es transitions on a regional scale. The carbonate reservoir at _Weybum represents an erosional remnant of a considerably more extensive marginal ramp on the northern flank of the Williston Basin. Phase IA area sedimentological core

68

-··· ··- ... -.. ,.. .• ,_ .. --·········---·--· ·-- . ...•.. . -·.

topography and erosional incision. Considerable porosity enhancement probably also occurred at this stage.

Significant sea-level rise resulted in major inundation of the shelf over the eroded lower Vuggy and in a change in depositional setting to dominantly subtidal, open-marine mid-ramp influen_c~d by sto~- ~nd :wave­reworking. Blanket-like depos1t1on of the d1stmct1ve non-vuggy, skeletal-rich beds of the upper Vuggy largely infilled the lower Vuggy topography.

A widespread Thalassino~c{es-burrowed_ layer mar~s a third major shift in depos1t1onal style w1thm the Mtdale beds. Upper Vuggy grain-dominated textures a?ruptly give way to extensively bioturbate~, mud-dominated textures in the Marty. The predommance of skeletal and non-skeletal algal remains, ostracods, and the . introduction of terrestrial silt and clay suggest a shift to a more proximal, quiet-water subtidal setting. The apparent regional extent of these environments seaward of an evaporitic coastal plain, represented by the overlying upper Marly and Midale Evaporite, argues for deposition within an extensive epeiric lagoon that may have devefoped in response to .. regional isolation of the ~tlhston Basm._ Depos1t1~nal environments may have mcluded both distal, relatively deep, outer ramp equivale~ts and pro~i~al, very shallow restricted euryhalme bays. W1thm the Marly, an overall brining-upward and shallowing-upward succession is suggested by the upward change from a diverse Cruziana ichnofacies to a Planolites/ Chondrites-dominated ichnofacies with algal lamination (see Keswani and Pemberton, 1993). Marty deposition culminated in silty, anhydritic tidal flat and sabkha dolo-laminites of the upper Marly "Three Fingers" zone. Subsequent deposition of the Midale_ Evaporite records the final shift to fully coastal manne to continental evaporitic salinas and playas.

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

SW NE

Open Marine Mid-Ramp

Protected Peritidal/lsland Restricted Tidal Flats & Sabkha

Salina

Outer Ramp

Subtidal Ramp Complex

Skeletal Wkst-Pkst

Algal/Pisolitic Fenestral Grst-Wkst

<}:::::::: Silt I Clay

Laminated Silty/ Anhydritic Mdst

Burrowed (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

Table 1 summarizes the key reservoir parameters of the Midale beds in the Weybum Field. Markedly different flow characteristics between the lower Vuggy, upper Vuggy, and Marly reflect the combination of: a) different primary depositional settings and textures, b) markedly different secondary diagenetic overprints, and c) varying rheological responses to imposed stress. Lower Vuggy reservoir characteristics (extreme heterogeneity, low to moderate porosity, low to high permeabilities) are predictable products of texturally diverse peritidal deposits overprinted by complex near­surface and burial dissolution and cementation. On the other hand, the relatively homogeneous reservoir characteristics of the upper Vuggy (low to moderate porosity, low permeability) directly reflect more

uniform, finer grained depositional textures and absence of strong diagenetic overprints.

Reservoir characteristics of the Marty (homogeneity, high porosity, low permeability) are, foremost, a function of early dolomitization of 'bio-homogenized', mud-dominated fabrics. Variations in reservoir quality within the Marly occur mainly in response to variations in degree of dolomitization, and a general upward trend to finer crystalline, lower permeability fabrics. Severely reduced reservoir quality is commonly associated with partial dolomitization within either thin skeletal-rich packstones or organic-rich, microstylolitic beds; both of these are widespread at the top of the M3 zone and combine to form a correlatable low­pcnneabil ity zone separating Ml and M3.

Table J - Summary of Midale reservoir characteristics, Weyhurn Field. 6. Fractures

Reservoir ; Lithology : Porosity i Matrix ! Heterogeneity Zone &

1

Permeability Fracture Density

Textures (avg.%, Type) (avg. ,md)

Marly

(Ml, M3)

i Dolostone 20-37

(26 )

mudsto ne, intercrystalline

<0.1 · 100

(10)

wackestone : mo/die j ~-- ---+--~ ··---· Upper Limestone 2-15 Vuggy (10)

(VI) : packs tone. interparlicle I

wacke.stone J intraparticle --- ····----· •-- .

Lower Vuggy

(V2-V6)

Limestone 5-20

mudstoneto grainsto ne

(15)

fenesfr fll vuggy

Saskatchewan Geolo[?ical Survey

<0.01 - 20

( 1)

<1 - 500

(20)

Low

biot11rbatio 11

dolomitization

Low­Moderate

(2-4 m spacing)

>25% 0-unfract

~--Me~i-um ____ rgh -

thick bedded, 1 ( < 1 m spacing)

btoturbatton

1, . ·-··

High Moderate -. High

we ll bed ded .

high order cyclicily, ] complex dif:'genesis

(< 1-2 m spacing)

micro fraclu,-ed

One of the more prominent features of the Weybum Midale reservoir is sub-vertical natural fractures, both open and cemented. Not only are these obvious in numerous well cores, but are also clearly imaged on borehole image logs. Analysis of these image logs indicates that the dominant fracture orientation is parallel to the current southwest­northeast orientation of maximum horizontal stress (Bell and Babcock, 1986; Adams and Bell, 1991 ). Secondary southeast­northwest fracture trends have also been defined (Bunge, 2000). Churcher and Edmunds ( 1994) have estimated fracture spacing at between 0.3 and 10 m, with

69

highest densities in the upper Vuggy and lowest in the Marty. Although the low fracture density in the Marty appears to be inconsistent with the normally more brittle response of crystalline dolostones, the very high porosity and sucrosic textures appear to have induced a more ductile response to stress.

The degree to which fractures affect directional reservoir permeability, flow characteristics, and producibility is difficult to determine. Perhaps the strongest evidence for their influence is the pronounced preferred "on-trend" water breakthrough response during waterflood. However, much of this effect is possibly a by-product of northeasterly propagation of fractures under high injection pressures, combined with the preferred northeasterly orientation of the high­permeability lower Vuggy shoals. Furthermore, history matching of the breakthrough response does not require permeabilities significantly higher than average, core-based matrix permeabilities. Also, Pan Canadian' s analysis of scattered transient pressure data does not lend strong support for dual porosity reservoir behaviour. The classic dual porosity response may, however, be partly masked by either the interaction of the markedly different Marty and Vuggy flow systems, or the homogenizing effect of microfracturing, which is widely observed in thin sections of the lower Vuggy.

These ambiguities have made it difficult to develop a satisfactory, comprehensive fracture model for the Weybum reservoir. The extent to which fractures will influence C02 flood performance remains a key issue to be addressed by the IEA Weyburn C02 Monitoring and Storage Project. One of the avenues PanCanadian and the IEA are pursuing is the acquisition of multicomponent 3-D seismic data to assist in spatial characterization of the fracture system.

7. Flood Monitoring A critical element of successful, efficient tertiary recovery schemes involves diligent monitoring of flood conformance as well as the overall progress of the flood front. PanCanadian is addressing this process through monitoring of production fluid composition and volumes, chemical tracer studies and muticomponent 4-D seismic.

Production testing on individual wells will be done regularly in five newly constructed satellite stations. Each satellite is equipped with a three-phase test separator for measuring oil, water, and gas flow rates. Produced gas is monitored for C02 using infra-red analyzers and dragar tube sampling. Samples of produced oil, water, and gas will be periodically collected for more detailed compositional analysis. A tracer study will be implemented in March 200 I using perflourocarbons to trace C02 injection gas at seven injection wells. The study will determine C02

conformance in three flood patterns and assist in designing flood operations. A screening study has also been initiated at University of Regina to evaluate

70

·-·· ···· .. ·· · .. _,, .. _ ·--·-----------··-------·-·-···· .

commercially available C02 surfactants and diverting agents for C02 mobility control.

Together with the Colorado School of Mines, PanCanadian will be acquiring four multicomponent 3-D seismic surveys within the initial Phase I A area over a period of about three years; the key baseline survey was acquired in 1999. The seismic surveys will use the latest techniques to acquire P and S wave data in an attempt to monitor the progress of the flood front and to improve spatial resolution of reservoir properties (porosity-thickness and fracture-permeability). Interpretation of the seismic data is to be enhanced by the co-ordinated acquisition of two multicomponent 3-D vertical seismic profiles (VSPs) as well as single­well and cross-well (horizontal to horizontal) seismic tomography.

8. References Adams, J. and Bell, J.S. (1991): Crustal stresses in

Canada; in Slemmons, D.B., Engdahl, E.R., Zoback, M.D., and Blackwell, D.D. (eds.), Neotectonics of North America, Decade Map Volume 1, Geo!. Soc. Amer., Boulder, p376-386.

Bell, J.S. and Babcock, E.A. (1986): The stress regime of the Western Canadian Basin and implications for hydrocarbon production; Bull. Can. Petrol. Geol.,v34,p364-378.

Bunge, R.J. (2000): Midale reservoir fracture characterization using integrated well and seismic data, Weyburn Field, Saskatchewan; unpubl. M.Sc. thesis, Colorado School of Mines, 2 I 8p.

Churcher, P.L. and Edmunds, A.C. (1994): Reservoir characterization and geologic study of the Weyburn Unit, southeastern Saskatchewan; PanCanadian Petroleum Ltd., internal rep., 80p.

Coalson, E.B., Goolsby, S.M., and Franklin, M.H. (1994 ): Subtle seals and fluid-flow barriers in carbonate rocks; in Dolson, J.C., Hendricks, M.L., and Wescott, W.A. (eds.), Unconformity-related Hydrocarbons in Sedimentary Sequences, Guidebook for Petroleum Exploration and Exploitation in Clastic and Carbonate Sediments, Rocky Mtn. Assoc. Geol., Denver, p45-58.

Kendall, A.C. ( 1975): Anhydrite replacements of gypsum (satin-spar) veins in the Mississippian caprocks of southeastern Saskatchewan; Can. J. Earth Sci., vl2, pl 190-1195.

Kent, D.M. ( 1983 ): Mississippian reservoirs in Williston Basin; Comptes Rendues, Ninth International Congress on Carboniferous Stratigraphy, v4, p99- I I 0.

_____ ( 1984 ): Carbonates and associated rocks of the Williston Basin - their origin, diagenesis and economic potential; Rocky Mtn. Sec., Soc.

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, Radville 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 Engineering­Geological Sub-Committee, Pore Volume Group, 1962 Report, I I Op.

Saskatchewan Cieofogica{ Survey 71