Estimation of original gas in place from short-term shut-in pressure data for commingled tight gas reservoirs with no crossflow

Abstract

The conventional material balance method (M.B.) , or p/z method, is not applicable for commingled fight gas reservoirs, especially when only short-term shut-in pressure data is available. This is because p/z is not a linear function of cumulative gas production (GP) under these circumstances. This research studies different empirical methods to estimate the original gas in place (OGIP) for one-layer or commingled two-layer tight gas reservoirs without crossflow, from short-term (72-hour) shut-in pressure data; namely one-point (secant), two-point (tangent), curve fitting, and type curve methods. A two dimensional dry gas simulator (GASSIM simulator) was used in this study. The wellbore storage, non-darcy flow, and backflow phenomenon during shut-in periods were considered in this work. The non-darcy flow was found to have insignificant effect on the 72-hour shut-in pressure data. The backflow phenomenon is the flow of gas from one layer into another via the wellbore during shut-in periods. The crossflow , which is not subject of this study, on the other hand, is the inter-layer flow in the reservoir. The backflow, wellbore storage, short shut-in time, and low permeability are contributing factors that cause the graph of p/z as a function of cumulative gas production to be a curve instead of a straight line. The one and two-point methods (secant and tangent methods) are easy, fast, but inaccurate, especially at early times. Their accuracy is improved if later times data is used, but then the OGIP estimates would be less useful since the field would be near abandonment. The one-point method always gives conservative estimates of the OGIP, the two-point method is usually more accurate than the one-point method. The curve fitting method can be more accurate whenever it is applicable but its major drawback is that it is not always feasible and can either over-estimate or under-estimate the OGIP. The type curve method is relatively the better one among those considered in this study, because it can use early time p/z72 data with relative reliability; its weakness is that it can have non-unique solutions.