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 (CSS) 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 modelling. 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 modelling 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 area(1). More than 3,000 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 the 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.
Hydrogen sulfide (H2S) generated by aquathermolysis—a high-temperature reaction of condensed steam (water) with sulfur-bearing bitumen in the reservoir rock—may increase the risk of sulfide stress cracking (SSC) in cyclically steam stimulated (CSS) wells. In a given field, H2S levels and wellbore conditions vary significantly among wells and so do their SSC-susceptibility. Identifying the SSC-susceptible wells is important in terms of reducing SSC risk by allocating resources and implementing pro-active intervention measures to the SSC-susceptible wells. A comprehensive research program, with a dedicated instrumented CSS well as the centerpiece, has been undertaken by Imperial Oil Resources with the objectives of characterizing H2S evolution in the wellbore and developing a tool for identifying the SSC-susceptible wells. The research includes laboratory and field tests, and statistical, phase behaviour and kinetic modelling. The SSC-susceptible zone for Cold Lake CSS has been established from Cyclic Slow Strain Rate (CSSR) laboratory tests incorporating CSS fluid chemistry, stress-strain environments, casing metallurgy, and variable temperature and H2S partial pressure. A statistical logistic model matches the experimental CSSR data well. The instrumented well data validate the phase behavior model, which in turn explains the measured H2S profile in the wellbore. An aquathermolysis kinetic model has been developed for the instrumented well and validated with data from nine other CSS wells. The research has led to the development of an engineering tool for identifying the wells at the risk of falling into the SSC-susceptible zone.
Imperial Oil's heavy oil operation in northern Alberta utilizes Cyclic Steam Stimulation (CSS) to extract bitumen from the oilsand reservoir (~400 mKB). The operation currently produces over 120,000 bpd from approximately 3000 deviated wells. During CSS, the well casings are subjected to thermal stresses due to the cyclic, high-pressure, high-temperature nature of the process. In addition, shear stresses develop in the overburden due to volumetric dilation of the reservoir during steam injection. The combination of thermal fatigue and shear deformations can occasionally result in casing failures at some location along the wellbore, but typically at the top of the oilsand reservoir. Imperial Oil and CANMET developed the application of passive seismic systems to monitor a volume of the overburden shale for casing failures, as well as for other events. As a result of this monitoring, a model for the seismic signature of well casing failures was developed. The seismic energy and radiation pattern of casing failure events has enabled the passive seismic systems to detect the occurrence of casing failures at the reservoir top, beyond the intended monitoring zone in the overburden shale. Subsequent casing checks have identified that the model detection is correct 88% of the time. In addition, event source location accuracy can be established based on the location of the actual casing failures determined from well workovers. This tool has allowed Imperial Oil to decrease well downtime and develop steaming strategies to reduce the occurrence of casing failures at the top of the reservoir. Introduction Imperial Oil uses Cyclic Steam Stimulation (CSS) at its heavy oil operation in Cold Lake, Alberta. In the CSS process, the same wells are used for high-pressure steam injection, (typically 10–12 MPa), and for production. A typical stratigraphic section for Cold Lake is shown in Figure 1. Steam injected into the Clearwater Sand (CW) is accommodated by hydraulic fractures and non-linear dilation, which result in deformation of the CW1,2. Numerical modeling has demonstrated that shear stresses, which develop at the boundary between the CW sand and the overlying shale, occasionally exceed the expected shear strength of the boundary3. In addition, thermal stresses on the casing impact its ability to withstand shear loads4. These factors are manifested in the field as casing deformations and failures located at or near the top of the CW.
This paper describes Imperial Oil's success in identifying and remediating poor steam conformance in a horizontal well used in Cyclic Steam Stimulation operations. Imperial Oil is conducting Horizontal Well Cyclic Steam Stimulation (HWCSS) at nine horizontal wells located at two pads in Cold Lake. Steam conformance along horizontal wells is a significant issue in these types of thermal applications. The horizontal wells are completed with Limited Entry Perforations (LEP) to improve distribution of steam along the liner. One of the HWCSS pads, D36, has been the subject of both 4D seismic and injectivity analyses to characterize steam conformance along the horizontal section over the fi rst three cycles. These analysis techniques showed that four out of the five wells on the pad had excellent steam distribution along the horizontal liners. However, one of the wells, D36-H1, showed little or no steam conformance along the last half of the liner. This lack of horizontal steam conformance put the recovery expectations of this well at risk. A workover conducted on D36-H1 successfully removed sand that had been obstructing the liner. Steam injection subsequent to the clean-out showed steam distributed across the majority of the liner (most of the LEPs accepting steam). Introduction Imperial Oil has been conducting commercial operations at Cold Lake using Cyclic Steam Stimulation (CSS) since the mid- 1980s. CSS involves injecting steam at fracture pressure (>10 MPa) and producing a mixture of water, bitumen, and gas from the same wells. Approximately 3,000 wells are in operation at Cold Lake, the vast majority of which are deviated vertical wells. These wells are drilled from common surface pads, typically in groups of 20. As an enhancement to the CSS process, Imperial has been piloting Horizontal Well CSS (HWCSS) at nine 500 m long wells located at two pads. One of the main challenges of the HWCSS process is the control of steam conformance in the reservoir. To aximize recovery, it is necessary to contact as much of the reservoir as possible with steam. This becomes more difficult with advancing cycles because steam preferentially accesses channels that have already been heated and depleted of oil. Horizontal wells with conventional perforations or slotted liners could be further compromised by preferential steam injection into the heel of the well. To overcome this problem, Imperial Oil uses its patented Limited Entry Perforation (LEP) design to control the distribution of steam along the horizontal well liners(1). A schematic of the well completion and LEP design is shown in Figure 1. A previous publication has described the use of 4D seismicnd injectivity analyses to evaluate the effectiveness of LEPs at a 1,000 m infill horizontal well injector(2). This paper follows a similar analysis technique for D36 pad, one of the HWCSS pads at Cold Lake. Excellent steam conformance over the first four cycles is inferred from this analysis at four of the five wells on the pad. However, the analysis identified a steam conformance problem atne of the wells designated as D36-H1.
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