Physics-based well performance modeling technology has been developed that establishes Well Operability Limits (WOLs) to mitigate well failures due to the complex geomechanical loads resulting from compacting reservoirs. This new technology assists in active reservoir management by both limiting the producing drawdown pressure on wells most susceptible to failure and providing confidence in increasing drawdown pressure on wells determined not to be susceptible. In 2000–2001, ExxonMobil experienced three unexpected reservoir compaction well failures during the start-up of the deepwater Diana/Hoover project in the Gulf of Mexico. This experience led to the development of advanced well performance modeling technology which was used to assess ExxonMobil's worldwide portfolio for potential compaction related well failures. WOLs have now successfully been used to manage nearly 100 deepwater wells without failure since 2001 and are actively used to establish completion and production strategies for wells in many other types of producing assets worldwide. Implementation of this technology has not only prevented well failures and associated production disruption due to reservoir compaction, but also enabled safe increases in well drawdown and accompanying increases in production rates beyond previous industry experience. This paper focuses on the application of WOL technology to ExxonMobil deepwater wells. The general methodology and development of the reservoir compaction WOL technology will be introduced. Selected case studies will highlight the successful application of the technology as well as illustrate the operating strategies and the subsequent production benefits. WOL technology has significantly improved ExxonMobil's ability to operate wells without incurring costly well failures by more clearly defining the complex limits of compacting reservoirs. Background Ensuring long-term well integrity and optimum completion performance is important for the economic development of any field. As fields are developed with fewer wells and in more technically challenging environments, analysis is required to deliver reliable wells that can be operated at the high rates required to meet today's aggressive production targets. Long-term well integrity is essential for commercial deepwater field development. Operators cannot rely on historical industry practices based on aggregate historical data, anecdotal evidence, and simplified analytical models to prevent well failures while simultaneously maximizing well production and reservoir recovery. This is especially true in deepwater environments, where reservoir characteristics are significantly different than in the fields that serve as the basis for historical practices and the cost of well intervention to recover from a failure is substantial. Well failures due to compaction and subsidence have been experienced for decades in various environments and are summarized briefly below:Wilmington Field - One of the first ExxonMobil experiences with subsidence occurred at the Wilmington field located onshore California. Production began in 1936, and 7 feet of subsidence occurred by 1947, the time of the earliest casing failures. Horizontal movements of 9 inches were documented in three weak shale layers above the producing reservoir (Allen, 1959).
The Cold Lake heavy oil development is located in northeast Alberta, Canada. It began commercial operation in 1985 and uses a thermal recovery process called cyclic steam stimulation (CSS). During steaming cycles, the dilation and re-compaction that occur within the reservoir cause the overburden to deform much like the motion of flexing a thick telephone book. At weak overburden layers, shear slip plane(s) can form due to excessive shear stress overcoming the interlayer cohesion. Over multiple steaming/production cycles, the cyclic flexing and associated shear slip may lead to overburden casing fatigue failures. In this paper, a multi-scale geomechanics modeling methodology is presented to predict the onset of failure due to CSS-related ultra low cyclic fatigue (ULCF). The modeling methodology consists of: (i) converting geological data into a representative finite element model of a single or multiple CSS pads, (ii) constructing a near-well submodel that includes thermal cement and casing, and (iii) constructing a detailed casing and connection submodel to predict the ULCF life of a pipe body or connection. To predict the ULCF life of the casing and connection, an algorithm based on the concept of cyclic void growth is incorporated into the submodel. It provides the capability to predict the number of steam cycles to failure using the concepts of demand and capacity. This enables studying the effects of alternative steaming practices on overburden shear slip and casing/connection life. Based on the learning from the multi-scale modeling, it is found that shear displacements on a shear slip plane can be superimposed using a single-well solution. By applying steaming and production scaling functions, the shear slip can be determined at any location and time. Integration of the single-well solution with ULCF algorithm has facilitated development of a new software tool that can be used to manage CSS operations in Cold Lake.
Stand-alone screens are often a simple and cost-effective sand control solution for long, multi-zone openhole completions. A recent technology advancement is a self-mitigating MazeFlo™ screen for enhanced sand screen reliability. Redundant screens and compartment mazes are incorporated to capture local sand ingress caused by screen damage without interrupting the overall well production. A common screen damage mechanism is screen erosion. The new screen extends the production capacity by increasing erosion resistance and providing self-mitigating functionality. A self-mitigating screen includes an outer primary and an enclosed offset inner secondary screen. A maze compartment stands between the two screens to enable self-mitigation of screen damage. Without the primary screen damage, the secondary screen must withstand continuous fines production to maintain sand control capability in the well. If the primary screen is damaged, the erosive sand will enter the maze compartment and deposit on the secondary screen. The integrity of the maze compartment must be maintained until the compartment is fully packed and the well production is diverted to adjacent undamaged screens. This paper discusses a methodology to evaluate the erosion resistance with regard to operating conditions in a self-mitigating screen. The methodology includes a suite of laboratory tests to establish an erosion baseline for a wire-wrapped screen. The erosion baseline, along with fluid flow modeling will provide design guidelines for a given operating condition. A proper MazeFlo screen design sustains the self-mitigation capability throughout the life of the well. The design envelope can be further expanded when combined with other technologies such as zonal isolation and inflow control devices.
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