A practical approach for optimization of infill well placement in tight gas reservoirs
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Despite their low production rates, tight gas wells contribute significantly to the Nation's energy supply. Because permeabilities in tight reservoirs can be as low as fractions of a millidarcy, or even in the microdarcy range, drainage areas are small and many more wells are needed to drain tight gas fields than conventional gas fields. Infill drilling has been the most common and effective means to revitalize tight gas fields, by adding new reserves and accelerating recovery. Given the marginal nature of tight gas fields, optimization of infill well placement is extremely important to ensure economic viability of infill drilling programs. However, optimal placement of infill wells in tight gas fields is challenging. First, the reservoirs are usually quite complex and reservoir characteristics are often not well understood, even though most of these fields are mature. Second, data are usually scarce. It is not uncommon for only production data to be available in a marginal, tight gas field. Third, these fields often contain a large number of existing wells, which can require the evaluation of hundreds of potential infill drilling candidates. Finally, interference between wells affects placement of infill drilling wells and must be considered in the evaluation. Given the marginal nature of these gas fields, a conventional evaluation approach, such as detailed reservoir characterization and simulation, is usually prohibitively time-consuming and costly. Thus, a rapid and cost-effective approach to optimal infill drilling design that adequately addresses these issues would be quite valuable to operators. In this paper, we present a systematic methodology for efficient design of an infill drilling scheme for marginal gas reservoirs. The approach consists of two major components. The first is a sequential inversion algorithm for rapid history matching. The algorithm is conditional to the correlation between permeability and porosity, if any. The inversion provides not only the spatial distribution of both permeability and pore volume, but also the spatial distribution of remaining gas in place. The second component of the approach is a successive selection strategy for infill candidate locations. The method fully addresses well interference between existing and infill wells, as well as interference between infill wells. It is rapid and cost effective. Synthetic and field examples are provided to demonstrate the applicability and power of the method. 2009 Elsevier B.V. All rights reserved.
