Estimation of Contacted and Original Gas-in-Place for Low Permeability Reservoirs Exhibiting Linear Flow

Abstract

Abstract Recent advances in drilling and completion technology have allowed commercial exploitation of ultra-low permeability gas reservoirs. Horizontal wells that have been stimulated through multiple hydraulic-fracturing stages are the most popular method for exploitation. Due to the extremely low permeability of these reservoirs, boundary-dominated flow may not be reached for a significant period of time making estimation of original gas-in-place (OGIP) difficult. In these reservoirs, it is important to estimate contacted gas-in-place (CGIP), which is some fraction of OGIP, at any point in time. One of the most popular methods for estimating OGIP in conventional reservoirs is flowing material balance (FMB), which is based on the assumption of boundary-dominated flow. However, this method has been used in the literature to estimate CGIP when dealing with transient data. In this paper, we test the applicability of the traditional FMB plot for calculating CGIP in low-permeability systems. It is found that even when dealing with a gas reservoir that is depleted significantly, FMB underestimates the total OGIP. It is also found that using FMB alone can result in misleading interpretations. For example, the FMB plotting method makes the data look like boundary-dominated flow, whereas other diagnostic tools do not indicate boundary-dominated flow. This means that one can misinterpret the gas-in-place from FMB as total OGIP, which consequently affects the reservoir characterization and reserves evaluation. Finally, we investigated whether using a material balance pseudo-time calculated using the average pressure in the region of investigation, as oppose to average reservoir pressure, will correct underestimating OGIP for the case that the reservoir is significantly depleted. This study is of practical interest for determining how much of the reservoir is contacted at each stage of flow of a multi-fractured horizontal well.