Abstract

Tight gas reservoirs present unique challenges to the reservoir engineer. Applying classical reservoir engineering techniques to these reservoirs is problematic due to the length of time required to reach pseudo-steady state flow and/or establish a constant drainage area. This leads to an inability to accurately estimate the recoverable reserves in a timely and consistent manner. This paper focuses on reserve estimation techniques for tight gas reservoirs.

Both decline curves and material balance methods were found to have serious drawbacks when applied to tight gas reservoirs that had not established a constant drainage area. Gas production analysis (GPA) utilizing a combination of the decline curves and material balance in conjunction with classical pressure transient analysis was found to be more accurate in determining reserves.

Introduction

For the purposes of this paper, gas reservoirs with a permeability less than 0.1 md are considered "tight gas" reservoirs. When tight gas reservoirs are produced, a significant amount of time is required before the pressure transient is affected by all the boundaries of the reservoir, which can often take years. This length of time is the principle reason why the determination of reserves for tight gas reservoirs is problematic. Tight gas reservoirs appear to be more heterogeneous than systems with higher permeability. Relatively small changes in permeability can result in uneconomical flow rates from a given well while the same percentage variations in higher permeability systems would result in little or no change in well performance.

The paper provides a critical review and explanation of common techniques used to estimate tight gas reserves including rate-time decline curve analysis, material balance and the combination of type curve and decline curve analysis commonly referred to as gas production analysis. Discussion topics are supplemented with conceptual simulation work and field examples. The conceptual simulation study cases are prepared as a control set to test the accuracy of each analytical method. Well performance will be evaluated during both early time transient and late time boundary dominated flow (BDF) to determine the accuracy of each technique as compared to a known recoverable reserve and/or original gas- in-place. Four field examples are included comparing the results of applicable techniques through time.

Background Theory

The following is a brief summary of the three reserve estimation techniques detailing their assumptions, basic theory and implementation.

Rate-Time Decline Curve Analysis

Decline curve analysis is an extensively used methods of estimating future performance and ultimate recoverable reserves. The data required to perform this analysis is readily available. This method is based on the empirical equations of Arps1. Table 1 provides a brief summary of useful decline curve equations2.

Rate-Time Decline Curve Analysis

Decline curve analysis is an extensively used methods of estimating future performance and ultimate recoverable reserves. The data required to perform this analysis is readily available. This method is based on the empirical equations of Arps1. Table 1 provides a brief summary of useful decline curve equations2.

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