Estimating in-place volume associated with each well, leading to estimation of total reservoir in-place volume, is the cornerstone to any reservoir-management practice. Yet conventional methods do not always lend themselves for routine applications, particularly when used in singular fashion. However, combining them on the same plot has considerable merit in that they converge to the same solution when material-balance-derived average-reservoir pressure is used.
This study presents a systematic procedure for estimating the initial gas in-place volume (GIIP) when real-time surveillance data of pressure, rate, and temperature are available at the wellhead. Specifically, we show that log-log diagnosis, followed by combined static- and dynamic-material-balance analysis, and transient-productivity-index (transient-PI) analysis lead to consistent solutions. Thermodynamic behavior of fluids is also explored to ensure that the converted pressures at the bottomhole and measured rates have consistency and accuracy for reservoir-engineering calculations.
Layered systems were selected for this study because they represent most situations. Two synthetic cases probed issues pertaining to average-reservoir pressure computation with the pseudosteady-state (PSS) approach, and two field examples validated the approach presented here.