Infill drilling plays an important role in revitalizing marginal oil and gas fields by adding new reserves and accelerating recovery. However, design of an infill drilling program is often a challenging task for these kinds of reservoirs. First, most such reservoirs are mature fields where data are scarce and reservoir characterization is very limited. It is not uncommon for only production data to be available in a marginal, mature field. Second, the design often has to deal with a large number of existing wells and evaluate hundreds or thousands of potential infill drilling candidates. Given the marginal nature of these fields, a conventional evaluation approach, such as detailed reservoir characterization and simulation, is usually prohibitively time-consuming and costly. Furthermore, when designing an infill drilling program, well interference or acceleration effects have to be taken into account to reliably identify the potential for new reserve additions. Thus, an optimal infill drilling design procedure that adequately addresses these issues is extremely important.

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 effects between existing and infill wells, as well as interference between infill wells.

The approach is rapid and cost effective, and provides answers to critical questions such as: given certain economic criteria, how many wells should be drilled; where should the wells be drilled; what production performance can be expected; what will the incremental gas recovery be; and what will the acceleration effect be? Synthetic and field examples are provided to demonstrate the applicability and power of the method.

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