Effect of water and formation compressibility and water vapor on original gas in place estimation using material balance calculation

Abstract

In gas field development, one of the most important decision factors is reserve estimation. Inaccurate reserve estimation can lead to incorrect economic decision and production plan. The main reasons that may lead to an error in such estimation are water and formation expansion and water vaporization. Exclusion of such factors can result in error of original gas in place when using material balance equation. The objective of this thesis is to study the effect of water and formation expansion and water vapor in OGIP estimation in dry gas reservoirs having different reservoir conditions. This study also investigates how the length of data affects the estimation.The study consists of two parts: (1) estimation of OGIP for reservoirs with significant water and rock compressibility, the conventional, Ramagost and modified Ramagost methods are used to determine OGIP and (2) estimation of OGIP for reservoirs with significant rock compressibility and water vapor, the conventional, Humphreys and modified Humphreys methods are used to compute OGIP. In the first part, it was found that modified Ramagost method provides the most accurate OGIP estimates when a lot of production data are available and the Ramagost method provides the best results when limited production data are available. Furthermore, the error rises when the initial pressure is higher. In the second part, modified Humphreys method provides the most accurate OGIP estimates when a lot of data are available and the Humphreys method provides the best results when limited production data are available. Similar to the first part of study, higher initial pressure results in higher error of OGIP estimate. The increase in water vapor content does not change the magnitude of the error for reservoirs with low initial pressures but increases the error for reservoirs with high pressure.