Coupled Geomechanical and Reservoir Modeling Investigating Poroelastic Effects of Cyclic Steam Stimulation in the Cold Lake Reservoir
- D.A. Walters (Taurus Reservoir Solutions Ltd.) | A. Settari (Taurus Reservoir Solutions Ltd.) | P.R. Kry (Imperial Oil Resources)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- December 2002
- Document Type
- Journal Paper
- 507 - 516
- 2002. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing, 5.2 Reservoir Fluid Dynamics, 1.6 Drilling Operations, 1.2.2 Geomechanics, 2.2.2 Perforating, 5.1.2 Faults and Fracture Characterisation, 1.2.3 Rock properties, 4.3.4 Scale, 5.1.5 Geologic Modeling, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.5.8 History Matching, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.4.6 Thermal Methods, 5.3.4 Integration of geomechanics in models, 2.4.3 Sand/Solids Control, 5.8.6 Naturally Fractured Reservoir, 5.5 Reservoir Simulation
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The heating of bitumen reservoirs by cyclic steam stimulation (CSS) requires high pressures and temperatures that disturb the soil matrix. This disturbance results in a dilation of the soil matrix, creating regions of enhanced permeability and porosity, which impact both the injection and production cycles of steam stimulation and the porous zones not hydraulically connected to the reservoir. The volumetric strain changes reach far beyond the region of injection and include areas of both expansion and compression. The magnitude of the volumetric strains may result in significant displacements and strains being transferred to porous zones outside the reservoir. This can result in observable pressure changes in an otherwise hydraulically static system.
Pressure data at an observation well drilled to monitor aquifer pressures showed a significant response to the CSS process occurring in the reservoir approximately 300 m below the aquifer. Coupled geomechanical and reservoir modeling and an analytical calculation both show that this pressure behavior is attributed to the poroelastic response of the aquifer caused by the CSS process occurring in the reservoir. This paper describes the field data, the methodology of the coupled reservoir and geomechanical modeling, and the result of the analysis of the field data with the coupled model. The analysis shows clearly that the aquifer response is not caused by a hydraulic communication with the reservoir. It also shows the capabilities of coupled modeling, as this type of a problem could not be analyzed with conventional reservoir simulation tools.
Bitumen deposits found in the Clearwater formation near Cold Lake, Alberta are being produced with CSS. Typically, this process consists of scheduled cycles of injection and production from vertical wells. High injection pressures and temperatures are used to maximize injectivity and heat the reservoir to increase the mobility of the bitumen for production. Initially, high injection rates are sufficient to fracture the reservoir immediately. However, significantly more volumes of injected steam are required with each successive cycle before the pressure reaches a level at which the reservoir fractures. 1 These high pressures and temperatures, while primarily helping injection and production, also cause poro- and thermoelastic effects in the reservoir that reach far beyond the production zone.
It is accepted that initial injection into the Clearwater formation at Cold Lake is accommodated by hydraulic fracturing, while later injection is dominated by the propagation of localized shear failure. 2 The poro- and thermo-elastic effects of this high-temperature injection result in significant dilation in the formation. The effects of the volumetric expansion within the Clearwater are transferred to the material surrounding the reservoir, extending to the surface. Evidence of this has been observed primarily as surface ground heave.3
Pressure fluctuations observed in an aquifer overlying the Clearwater formation were suspected to be the result of such poroelastic effects of the CSS process occurring below. The observation well, TH-1, was completed in the Muriel Lake sands of the Quaternary Aquifers in the Cold Lake field area. The well was drilled originally to assess water availability for the camp supply of Cold Lake bitumen development and subsequently became an important groundwater monitoring well. In June 1997, pressure changes (2 to 3 m water head) were observed at TH-1 that caused concern. Large pressure changes also had been observed earlier in 1994 and 1995. Several possible mechanisms were proposed to explain the pressure responses observed at TH-1. One of those possibilities was that the pressure changes occurring in the aquifer were a result of geomechanical effects caused by the dilation and recompaction of the Clearwater formation under the action of CSS.
The potential for the geomechanics effects of CSS causing such pressure changes in surrounding isolated hydraulic systems needs to be demonstrated. Therefore, a coupled reservoir/geomechanics simulation study was completed to quantify the possible pressure changes in the aquifer caused by the geomechanical effects of CSS in the Clearwater.
In the next section, the pressure observations at Well TH-1 will be presented, along with net injection plots of the underlying CSS wells defining the problem. Next, the methodology of the coupled reservoir and geomechanical modeling will be described. A singlewell model used to conceptualize the problem will be presented first, followed by a full-field simulation. Finally, some conclusions will be drawn from the simulation study.
Well TH-1 is completed over a 4.6-m interval near the top of a Muriel Lake sand that extends from 76.2 to 100.6 m. The Muriel Lake sands containing the aquifer vary considerably in thickness throughout the Cold Lake field, and the local thickness at TH-1 is greater than average. Also, the quality of the sand and, therefore, its permeability is highly variable throughout the field.
The CSS process is conducted within the Clearwater formation, which has a thickness of approximately 35 m and a structure top approximately 430 m below ground level.
Records of water-level observations at Well TH-1 exist from when it was drilled in 1980 to the present. The data up to 1994 show small seasonal fluctuations of approximately 0.2 m, with a general decrease in water level of approximately 3 m. Anomalous responses began in early 1994, as shown in the data plotted in Fig. 1. This shows the field observations at TH-1, plotted together with the net injection (total volume injected or produced) of 140 wells undergoing CSS in the Clearwater below. In this paper, the water level is expressed as an equivalent pressure assuming a normal freshwater density. It may be observed that the pressure response in TH-1 is closely related to the net injection taking place in the Clearwater.
Initially, several mechanisms for such behavior were investigated, including detailed inspections of all steaming wells to ensure that casing integrity existed. The possibility of communication with the reservoir was also investigated. It was thought that the most logical explanation could be attributed to geomechanical effects, and the present study was then initiated to quantify such a scenario.
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