In 1996, the Gas Research Institute (GRI) performed a scoping study to investigate the potential for natural gas production enhancement via restimulation in the United States. The results indicated that the potential was substantial, particularly in the tight sands of the Rocky Mountain, Mid-Continent and South Texas regions. However it was also determined that industry's historical experience with restimulation is mixed, and that considerable effort is required in candidate selection, problem diagnosis, and treatment selection/design/implementation for a restimulation program to be successful. As a result GRI initiated a subsequent two-year R & D project with the objectives: 1) to develop efficient, cost-effective, and reliable methodologies to identify wells with restimulation potential, 2) to identify and classify various mechanisms leading to well underperformance, and 3) to develop and test various restimulation techniques tailored to different causes of well underperformance.

The approach adopted for the R&D program is a combination of conceptual methodology development, laboratory studies, and actual field testing of restimulation treatments in tight gas sand reservoirs. In total, four test sites are planned for the project; each site consists of about 300 total wells in a contiguous area and completed in a consistent producing horizon, out of which five are selected for actual restimulation. The first two sites are in the Rocky Mountain region, the first being in the Big-Piney/LaBarge Producing Complex of the northern Moxa Arch in the Green River Basin (Frontier Formation), and the second being the combined Rulison, Parachute and Grand Valley fields of the eastern Piceance Basin (Williams Fork Formation). At present, restimulation candidates have been selected, verification testing completed, and two restimulations performed at the first site; preliminary candidate selections have been made at the second site.

The results to date from this project show why methodologies to accurately select "high - potential" restimulation candidates have eluded previous investigators. Each of the analytic procedures utilized thus far in the project is providing a different list of candidates, each based upon different criteria and with uncertainty as to the validity (or lack thereof) of each. While the situations being addressed in this project are understandably complex, specifically multilayered, heterogeneous reservoirs that are completed and stimulated in a wide variety of ways, the project experience to date supports that no acceptable methodology currently exists to universally select restimulation candidates across different geologic settings (which is the objective of this project).

What is clear is that restimulation potential does appear to exist, as evidenced from the restimulation treatments performed to-date, but that some degree of site-specific knowledge and methodology customization is required. This is supported by the findings from the individual well reviews and laboratory studies. Clearly, more results from this project are needed to better understand the methodologies being investigated, and how they should be integrated to develop the efficient yet robust methodology sought.

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