Abstract

Many operators do not routinely run prestimulation pressure buildup tests in prestimulation pressure buildup tests in low permeability gas reservoirs. Frequently, all that is available is a single-rate flow test. To obtain permeability estimates for FERC filings and to evaluate the reservoir, a new method has been developed for analyzing typical prefracture production data.

Permeability estimates obtained from this method have been used as a major source of data in numerous successful tight gas determinations, including the Cotton Valley, Wilcox Lobo, and Canyon formations in Texas.

The paper shows that this simple method can be quite reliable. This conclusion is based on a comparison of permeabilities estimated with this technique and with more rigorous determinations from pressure buildup tests.

Introduction

Accurate reservoir description is necessary to develop tight gas reservoirs economically. A critical formation property is in-situ permeability. An accurate estimate of formation permeability is needed for fracture design calculations and for predicting future production as a function predicting future production as a function of time. Permeability estimates are also needed to determine if a reservoir qualifies for FERC's "tight gas formation" classification.

In many cases, operators do not run pressure buildup tests on low permeability pressure buildup tests on low permeability gas wells prior to hydraulic fracture stimulation. Frequently a well is perforated, broken down and produced for only a perforated, broken down and produced for only a few days prior to performing a massive hydraulic fracture treatment. Chokes are changed frequently in this testing program and there are often shut-in periods of varying lengths between flow periods. This pre-fracture flow data cannot be analyzed pre-fracture flow data cannot be analyzed using conventional pressure transient analysis techniques; therefore, pre-stimulation permeability estimates are not pre-stimulation permeability estimates are not available. To compound the problem, post- fracture pressure buildup tests leading to unambiguous interpretations are often prohibitively time consuming. Thus, an prohibitively time consuming. Thus, an alternate method to estimate formation permeability, even if it is less accurate permeability, even if it is less accurate than the buildup tests, is needed to evaluate tight gas reservoirs using available pre-fracture flow data. pre-fracture flow data. This paper presents an approximate method (ONEPT) for determining formation permeability from single-point flow data. permeability from single-point flow data. This new method uses conventional transient flow equations in situations where only the initial formation pressure, the final flowing pressure, the cumulative gas production, and the final flow rate are known. production, and the final flow rate are known. The ONEPT method is appealing because, in contrast to pressure buildup data, which are often not available, single-point flow data are nearly always available on gas wells. This paper describes the ONEPT method and documents its validity in two field studies in which it is compared to the results of pressure buildup tests. In addition, the pressure buildup tests. In addition, the paper shows the effect on permeability paper shows the effect on permeability estimates of wellbore storage, variable rate history (including shut-in periods), and uncertainties in skin factor. These effects were determined using finite-difference reservoir simulation.

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