Abstract

The feasibility of detecting and accurately estimating gas desorption parameters from a history match of Devonian shale well-test pressure data is examined. Both drawdown and buildup tests are analyzed, and based on the results of these analyses a desorption-specific well-test design is proposed. The results from a simulated desorption-specific test suggest that it is possible to characterize gas desorption from a well test with reasonable accuracy, even when the effects of desorption are partially masked by well-bore storage and skin effects.

Introduction

The Devonian shales of the eastern United States are considered a major potential source of domestic natural gas, containing an estimated 1860 to 2580 Tscf of gas. It is believed that natural gas is stored in the Devonian shales as both conventional "free" gas and as adsorbed gas, or gas that is physically attached to the surface of the shales by Van der Waals-type forces. The existence of adsorbed gas in the Devonian shales is usually associated with the presence of kerogen, a coal-like organic material, as well presence of kerogen, a coal-like organic material, as well as with certain clay minerals such as illite which may be major constituents of the shale,

Previous investigators have suggested that 50% or more of the gas stored in the Devonian shales may exist as adsorbed gas. These estimates have been derived from laboratory measurements of gas-shale adsorption isotherms as well as from engineering studies based on shale gas-content data. Despite this evidence that adsorbed gas may account for a significant portion of the total gas content of the Devonian shales, however, adsorption is still considered to be an unconventional gas storage mechanism and is typically ignored in reservoir engineering analyses.

It has been observed that when the desorption of this adsorbed gas phase is ignored, significant errors may arise in production forecasts made for Devonian shale wells, particularly if the wells have been stimulated. In fact, it particularly if the wells have been stimulated. In fact, it has been reported that the total gas production for a well producing from a formation containing adsorbed gas can be producing from a formation containing adsorbed gas can be an order of magnitude higher than that for a well producing from a conventional formation of similar properties. It is thus very desirable to properly account for adsorbed gas when modeling Devonian shale reservoirs.

Unfortunately, while thousands of wells have been completed in the Devonian shales, little work has been done to characterize the desorption of natural gas from shale surfaces. Although it is possible to determine gas desorption isotherms from laboratory measurements on shale core samples, such data are very scarce and are not currently being measured on a widespread basis. Alternate sources of information on gas desorption in the Devonian shales are thus being investigated. Two such sources of information are measured production and well-test pressure data.

In a previous study, the authors investigated the feasibility of characterizing gas desorption in the Devonian shales from an analysis of production data. While it was determined that quantitative estimates for gas desorption, parameters could be obtained from a production data history match (and that the presence of gas desorption could be detected statistically under certain conditions), the uncertainty associated with these parameter estimates was judged to be unsatisfactory. In this paper, the feasibility of detecting and estimating gas desorption parameters from an analysis of Devonian shale well-test data is examined. Objective, statistically-based techniques are used for estimating gas desorption parameters, evaluating the accuracy of the resulting parameter estimates, and determining whether the presence of gas desorption can be detected from measured well-test pressure data. Both drawdown and buildup tests are investigated, and a desorption specific well-test design is proposed. The results indicate that it is possible to characterize gas desorption from a well test with reasonable accuracy.

Background

It has been noted that significant quantities of Devonian shale gas may exist as adsorbed gas. P. 113

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