Abstract

A computational model for development and optimization of the gas reservoirs with or without water inflow was presented in Part I of this series of two papers. In this Part II, a sensitivity analysis will be presented. The most important variables involved in the development of a Gas Reservoir with or without aquifer will be studied. Furthermore, the effect due to neglecting the rock and water compressibility terms for volumetric, steady state, and steady state system on the development of a gas reservoir will be studied. The model used for the sensitivity analysis was written in Fortran 77 and run by using the Multics System. The main purpose of the model is to determine the number of wells to be produced, and reservoir pressure at each period of time that depends upon the production schedule. The conclusions from the sensitivity analysis performed are as follows: permeability and water viscosity do not have a great effect on finite aquifer but create high affect on infinite aquifer, especially low permeability and high water viscosity; rock and water compressibility affect finite aquifer more than infinite aquifer; each value of porosity will limit the minimum value of reservoir pressure for an infinite aquifer; and the rock and water compressibility term in material balance equation must be considered in the case of problems involving economic decisions.

Introduction

This paper will use the model presented on Part 1 of this series to perform a sensitivity analysis. It will be shown how the parameters that balance equations and the dimensionless variable in Van Everdingen and Hurst unsteady state equation affect the performance of pressure in the reservoir. This analysis is one of the most interesting points for an economic decision in developing a reservoir, because changes in reservoir and aquifer parameters will affect the pressure performance of reservoir that relate to the number of wells to be drilled. It will also tell us which is the best way to improve the reservoir performance.

The paper will focus on the unsteady state radial aquifer and the effect of the rock and water compressibility in material balance equation for volumetric, steady state, and unsteady state systems.

The parameters for an unsteady state radial aquifer, that we are interested in, are: production rate, permeability, water viscosity, porosity, encroachment angle, rock compressibility, and water compressibility.

EFFECT DUE TO VARIATION IN PRODUCTION RATE FOR UNSTEADY STATE AQUIFER

To study the effect of production rate to the decline pressure, the calculation will be based on the reservoir and aquifer data which is given in appendix A. The production rate will be varied from 2.19 × 10-5 scf/d to 3.616 × 10-6 scf/d. The graph pressure vs. time at each production rate for finite and infinite aquifer is shown in Figure 1.

When looking at an infinite aquifer system, pressure will decline during the early period of time and after that it tends to stabilize as seen in Figure l. For the finite system, the pressure will decline at all time. Another interesting point is that during the early period of time, the decline of pressure of. finite and infinite aquifer is almost the same.

The decline of reservoir pressure, at various production rate dues to the finite and infinite aquifers for gas reservoir is the same as the decline of pressure due to water drive for an oil reservoir.

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