Simulation of Gas Well Performance
Authors: Henderson Gerald E. 
Published: December 2007

This paper demonstrates the relevance and practicality of using a single-well numeric practicality of using a single-well numeric simulation model to predict individual gas well performance and deliverability. Techniques for performance and deliverability. Techniques for using the model to match past well test and/or production history prior to making performance production history prior to making performance and deliverability predictions are presented. The use of the simulator is shown to be particularly relevant to the analysis of particularly relevant to the analysis of reservoirs of low permeability, for which conventional performance analysis becomes difficult, if not impossible.

The simulation model used is that originally proposed by Douglas, Dupont and Henderson, proposed by Douglas, Dupont and Henderson, which applies the calculus of variations to the partial differential equations which describe partial differential equations which describe single-phase radial flow.

Introduction

The prediction of gas well performance and deliverability in low permeability reservoirs is a problem which has long plagued reservoir engineers in the natural gas and petroleum producing industry. It has been well established producing industry. It has been well established that the conventional performance analysis tools the four-point open-flow potential test and the P/Z vs cumulative gas curve are not easily applied in low permeability gas reservoirs. Some of the reasons for this difficulty are as follows, although a certain redundancy is risked by their inclusion in this presentation. The principal reasons are (1) the backpressure test principal reasons are (1) the backpressure test must be measured under a flow condition which may be categorized as "stabilized" and (2) effective use of the P/Z vs cumulative gas curve requires that the pressures used in the analysis be the "average reservoir pressure" as might be determined from pressure buildup or drawdown analysis.

Predictions of gas well performance in high permeability reservoirs with these "conventional permeability reservoirs with these "conventional methods" are generally considered quite reliable and are certainly adequate for most reservoir engineering needs. When applied in reservoirs of low permeability, however, it is most commonly recognized that these methods yield quite optimistic deliverability forecasts. The observed results of predictions of this type is that oversized lines, plant capacity, and other equipment are installed, and that the optimum well spacing is not found, all primarily due to overestimation of individual well capacity and reserve.

There is a great deal of information available to the engineer seeking to avoid these difficulties of low permeability gas reservoirs. Most of the techniques offered describe additional testing and analysis methods. There are even a number of applications of "simulator mathematics" to individual gas well analysis, most of which have attempted to preclude using the simulator on each well as analyzed by developing generalized performance through presentation of the dimensionless curves which presentation of the dimensionless curves which relate reservoir geologic parameters and expected flow rates and pressures.

In an earlier paper the author and others presented the results of mathematics and presented the results of mathematics and computing research leading to the development of a single-well reservoir simulator for gas wells. This simulator is mathematically based on application of the calculus of variations to the partial differential equations which describe partial differential equations which describe radial, one-dimensional, one-flowing phase, unsteady-state, heterogeneous gas flow. Basic formation properties such as porosity, net pay and thickness are allowed to vary radially from the axis of the wellbore. The simulator also considers the effect of water and rock compressibility and of varying the gas viscosity and Z-factor as the reservoir pressure changes.