Revealing of False Response of Constant Pressure Boundary Model for Gas Reservoir Pressure Transient Analysis PTA Enabled in Increased and Realistic in-Place Estimation: A Case Study Using Two Field Data
Authors: Cobanoglu Mustafa 
Published: November 2020

Abstract
Interpretation of reservoir boundary conditions historically have been done and analytical models have been developed for various reservoir boundary conditions including constant pressure boundary.
Bourdarot (1988) and Kuchuk (1996) stated that the effect of an aquifer can be modeled with a constant pressure boundary model. This model assumes that the pressure at the boundary of the reservoir consistently remains at the initial reservoir pressure during the drawdown and build up phases of the well test. Hence, it suggests that the pressure support from gas cap is very strong due to expansion and that the multi-phase flow effects can be neglected. These assumptions work well for gas cap depletion systems since mobility change from oil reservoir to gas reservoir makes sense (i.e. gas reservoir mobility is generally much higher than oil reservoir mobility). This assumption is not valid for gas-water drive system since gas reservoir mobility is much higher than water reservoir mobility. Kehinde (2013) demonstrated this phenomenon by utilizing a numerical simulation for gas reservoir. This paper states the false response of constant pressure boundary model for gas reservoir pressure transient analysis (PTA), explores an alternative solution for this phenomenon and demonstrates the value of new method.
In this study, Analytical model of constant pressure boundary model was reviewed and discussed. Then, two gas reservoir PTA's were interpreted analytically where constant pressure boundary model was required to able to get a match. However, those reservoirs do not have a connected aquifer. The possible explanation of constant pressure boundary response is likely pressure support from neighboring block, which is geologically supported. Study were extended to make a comprehensive numerical well test models where various combination of cross flow models (i.e. partial compartmentalization) were tested and It was confirmed the pressure support from neighboring block is most likely explanation for that PTA's behavior.
This document provides a summary of the constant pressure boundary interpretation models for gas reservoir and discuss the application of analytical models into practical cases. The result of this investigation indicates pitfalls of direct application of analytical models. It was demonstrated that revealing of false response of constant pressure boundary model for gas reservoir pressure transient analysis enabled in increased and realistic in place estimation. Results shown that conventional analytical constant pressure boundary models are underestimating in-place volumes, resulted in ∼2-15 times less than what It should be since it does not model the pressure support properly.
Study results concluded that cross flow from neighboring blocks will give the similar response of constant pressure boundary effect in PTA. However, consequence of 2 scenario (cross flow versus constant pressure) will be totally different in terms of in place volume that is crucially affects the field development opportunities.