Modeling Gas Water Contact Effects in Pressure Transient Behavior using Analytical Aquifer modeling and Numerical Simulation

Abstract

Interpretation of reservoir boundary conditions and well drainage areas have been historically done conventionally; using analytical and numerical simulation approaches with constant pressure, no flow, leaky and conductive boundaries. This paper investigated the effect of Gas Water Contact (GWC) boundary on the pressure transient behavior. The novel approach adopted involves building numerical simulation well test models that investigate the effect of using Carter Tracy analytical aquifer model to simulate the response at the aquifer interface of a gas reservoir using SAPHIRE software. Bourdarot1 and Kuchuk2 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. However, in the case of a water drive system, they may be incorrect. The result of this investigation indicates that pressure transient at Gas Water Contact boundary behaves like a constant pressure boundary for gas reservoir with small sized aquifer or a radial composite system for gas reservoir with large or infinite aquifer respectively. The former is due to the change in fluid diffusivity and very high mobility contrast. (Khμ)1→(Khμ)2 (with 2>>1) coupled with expansion of active gas cap while the later is due to mobility contrast coupled with expansion from the active water influx. The result of this investigation was compared with the conventional analytical method of using a constant pressure boundary assumption. It is recommended to apply the results of this investigation in: (i) estimating the Gas Water Contact in a down-dip reservoir and therefore help in the quantification of reservoir volumes to support existing reliable technology for determining the Lowest Known Hydrocarbon (LKH) in a Gas Down To (GDT) scenario by applying shrinking box technique to a recognized onset of constant pressure or radial composite effect in a DST or multi rate test in a down dip gas well (ii) Analytical Aquifer boundary modeling in well test designs as it affects drainage area and volume.