A Simple Approach to Determining Original Gas-in-Place Using Production Data

Abstract A simple approach is presented that utilizes production data to determine original gas-in-place (OGIP). In particular, we propose a "modified" Agarwal-Gardner rate/cumulative decline type curves that yield an anchor point of unity, instead of 1/2π. The advantage of this unit anchor point is that, the normalized pseudopressure drop ratio, which is a plotting function on the abscissa, need not be modified; hence, providing a simpler approach than the traditional methods. The computation of pseudopressure drop ratio (or pseudocumulative function) requires a prior knowledge of average reservoir pressure history, or indirectly, OGIP. Consequently, various methods have been proposed in the literature to solve OGIP. This paper utilizes an approximate gas flowing material balance equation, which was previously presented by this author, to make an initial estimate or provide a "true window" for OGIP. This approximate plot yields two negative slopes during boundary-dominated flow regime – early-time pseudosteady state and late-time pseudosteady state lines. The early-time pseudosteady state line is shown to yield a reasonable initial estimate of OGIP while the late-time pseudosteady state line yields an optimistic result. The proposed approach is applicable to general variable pressure and/or variable rate case for single- and multi-well systems during boundary-dominated flow regime. The method is validated with two simulated data; and then, applied to two field data – one from a single-well system and the other from a multi-well system.