Abstract
Abstract Determination of gas in place (GIP) is among the hotspot issues in the field of oil/gas reservoir engineering. The conventional material balance method and other relevant approaches have found widespread application in estimating GIP of a gas reservoir or well-controlled gas reserves, but they are normally not cost-effective. To calculate GIP of abnormally pressured gas reservoirs economically and accurately, this paper deduces an iteration method for GIP estimation from production data, taking into consideration the pore shrinkage of reservoir rock and the volume expansion of irreducible water, and presents a strategy for selecting an initial iteration value of GIP. The approach, termed DMBM-APGR (dynamic material balance method for abnormally pressured gas reservoirs) here, is based on two equations: dynamic material balance equation and static material balance equation for overpressured gas reservoirs. The former delineates the relationship between the quasipressure at bottomhole pressure and the one at average reservoir pressure, and the latter reflects the relationship between average reservoir pressure and cumulative gas production, both of which are rigidly demonstrated in the paper using the basic theory of gas flow through porous media and material balance principle. The method proves effective with several numerical cases under various production schedules and a field case under a variable rate/variable pressure schedule, and the calculation error of GIP does not go beyond 5% provided that the production data are credible. DMBM-APGR goes for gas reservoirs with abnormally high pressure as well as those with normal pressure in virtue of its strict theoretical foundation, which not only considers the compressibilities of rock and bound water, but also reckons with the changes in production rate and variations of gas properties as functions of pressure. The method may serve as a valuable and reliable tool in determining gas reserves.
Highlights
The determination of gas in place, as a basic problem in the field of oil and gas reservoir engineering, is related to the development planning and production design of gas wells
The conventional material balance method needs average reservoir pressure data by shutting the gas well or conducting a well testing destined for static pressure
This study is aimed at offering a reliable approach for determining gas reserves applicable to abnormally high-pressure gas reservoirs with desirable accuracy and low cost
Summary
The determination of gas in place, as a basic problem in the field of oil and gas reservoir engineering, is related to the development planning and production design of gas wells. A variety of existing GIP determination methods based on the material balance equation, are inconvenient and uneconomical to implement owing to the demand for accurate average formation pressure data often obtained by shutting-in the wells or well testing. For gas reservoirs without water drive, Mattar and McNeil (1995, 1998) [8, 9] proposed the flowing material balance procedure when the gas well is flowing at a constant rate. It is, less desirable to obtain convincing results because the variations of gas property parameters (such as gas viscosity and compressibility factor or Z-factor) with pressure or time are not considered
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