Abstract

A hydraulically induced fracture containing bitumen was encountered in the Colorado Shale at Imperial Oil's Cold Lake Operation, during development drilling in 1997. The fracture was apparently caused by an inadvertent release of fluids from Cyclic Steam Stimulation operations in the Clearwater formation into the shale about 150 m above the producing formation. Subsequent drilling delineated the fracture to be over 1 km in diameter, extending over five 20 well pads.

Steam injection into the Clearwater formation induces overburden heave and also induces additional shear stresses in the shale. These could cause the shale to slip along the fracture. Depending on the magnitude of the slip, casing strings could be deformed or even failed. Numerical models were developed to assess the risk of future CSS operations and to optimize steaming strategies at the affected pads. A coupled geomechanical-thermal-reservoir simulation code, GEOSIM, was linked to the thermo-elasto-plastic capabilities in the fine element code, ABAQUS, to model the reservoir and overburden, including contact or slip elements in the fracture layer. Field measurements of the fracture pressure and laboratory measurements of the shear strength of the shale were important inputs to the model. Subsequent CSS cycles were conducted with pressure and temperature monitoring of the shale at the fracture depth and microseismic monitoring of the entire shale. Poroelastic fluid pressure and passive seismic responses in the fracture were observed during steaming and were consistent with the numerical modeling. Successful completion of three high-pressure CSS cycles at pads with moderate shale fracture pressure allowed for steaming of a pad with higher shale fracture pressure. This case study is an excellent example of integrating technical geomechanics modeling with operations optimization.

Introduction

Imperial Oil Resources has been conducting commercial operations at the Cold Lake site since 1985. The site is located approximately 300 km NE of Edmonton, Alberta. The Cyclic Steam Stimulation (CSS) process is used to produce bitumen from the Clearwater (CW) formation which typically is found at depths between 420 m and 470 m in this area1. More than 3000 deviated wells, typically arranged in pads of 20 wells, are used to inject high-pressure steam (>10 MPa) to reduce the viscosity of the bitumen. The same wells are then used to produce a mixture of bitumen, water and gas.

A hydraulically induced fracture containing bitumen was encountered in the Colorado Shale (CS), during development drilling of E07 pad in 1997. Fifteen Shale Evaluation Wells (SEW) were drilled through the CS to determine the extent of the fracture. Evidence of the fracture was found in wells drilled from five neighboring pads of CSS wells. The evidence consisted of abnormally high fluid pressures, bitumen in the drilling returns or, in the case of the initial observation, flow of bitumen to surface. An interpolated sketch of the fracture based on the results of the drilling is shown in Figure 1. It was concluded that this was a single contiguous fracture based on the consistency of observed depths and the magnitude of the observed pressures.

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