Abstract

In this paper, we investigate the shortcomings of available methods for the analysis of gas-well production performance; and we propose new method to more accurately estimate original gas-in-place (OGIP).

Several methods are available in the industry to analyze the production performance of gas wells. The most accurate and dependable of them is the superposition time. This method has the advantage of being able to analyze variable-rate and variable-pressure data, which is usually the nature of field data. However, this method has its shortcomings.

In this paper, we use simulation and field examples to review and compare the application of superposition method and illustrate its shortcomings. We present a new pseudotime plotting function for use in the superposition method to smooth field data and more accurately calculate OGIP. The use of this pseudotime is particularly important in the analysis of highly depleted reservoirs with high compressibility where the superposition errors are largest.

We also present a new tangent method to calculate the OGIP with current reservoir properties.

Introduction

If the average reservoir pressure is known, the p/z versus cumulative gas production (Gp) plot is a commonly accepted method used to obtain OGIP in gas reservoirs1,2 but the average reservoir pressure is not known for tight gas wells. So we are trying to estimate OGIP from production data. The material balance defined as:

Equation (1) (Available in full paper)

The superposition time (Super-t) function has been used as a tool to analyze the variable flow rate3–13 without knowing the average reservoir pressure. The superposition time defined as

Equation (2) (Available in full paper)

The slope (∼m PSS) from plotting [m(pi) ±m(pwf)] ÷g vs.Super-t used to determine the OGIP as in the following

Equation (3) (Available in full paper)

Note that initial properties are used in Eq. 3.

In the present work, we examine the shortcoming in using the Super-t and Eq. 3 with initial properties to calculate the OGIP. It appears that, regardless of the flow regimes exhibited by the well reservoir system the reservoir properties change with pressure. If the reservoir pressure drops more than 10%, the Super-t plot become non-linear, so the OGIP from Eq. 3 can have error more than 300%. The superposition time should be corrected for changing properties.

Agrawal14 and Lee and Holditch15 used a pseudotime to linearize the transient analysis of gas wells with massive hydraulic fracture. They consider variations of gas viscosity and compressibility as a function of bottomhole flowing pressure. Their pseudotime was defined as

Equation (4) (Available in full paper)

Fraim and Wattenbarger16 proposed an iterative procedure using the OGIP to predict average reservoir pressure calculations. They used a pseudotime function to take into account the variation of gas properties and provide an exponential decline behavior for gas wells produced at a constant pwf production. Their pseudotime was defined as

Equation (5) (Available in full paper)

In the present work, we develop a new equation for calculating OGIP with current properties

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