A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering in Offshore Technology and Management, School of Engineering and Technology

Thesis (M. Eng.) - Asian Institute of Technology, 2012

Material balance equation or the P/Z plot method is a well-known application for original gas in place (OGIP) estimation. Therefore, it can be used to apply classic material balance equation to determine the original gas in place of multi-layered gas reservoirs, such as two layers, five layers, and ten layers. To apply the material balance model to estimate the OGIP of multi-layered gas reservoirs, Multi-layered gas reservoirs that are heterogeneous make OGIP estimation erroneous. This study uses a reservoir simulator to simulate cases with multi-layered gas reservoir in order to demonstrate the application of the method. The simulated production and pressure profiles will be used as input data for material balance to determine total OGIP for all layers and different scenarios which appear from various reservoir properties, such as area, thickness, porosity, and permeability. The P/Z will be plotted versus cumulative gas production generated from numerical simulation to estimate OGIP of multi-layered gas reservoir. Moreover, gas rate of each layer is relative with pressure in each layer. It affects the change of average field pressure in well. The percentage error of OGIP estimation is small when pressure of each layer is close to average field pressure. Average field pressure that comes from weight average of hydrocarbon pore volume makes P/Z plot linear on material balance in every case of the change contrast. OGIP estimation is close to actual OGIP. However, average field pressure cannot be used in practice because parameter of hydrocarbon pore volume has difference in each place of reservoir. Therefore, this study has assumption about using proportional gas rate for weight average in pressure of each layer instead of average field pressure. Pressure that get from weight average by proportional gas rate is used for P/Z plot on material balance to find OGIP estimation. Pressure that comes from weight of proportional gas rate is similar to pressure that comes from average field pressure. This OGIP estimation will still be close to actual OGIP. Results from this study will help us to determine conditions that material balance provides acceptable estimation of total OGIP from multi-layered reservoirs.