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Abstract

This paper presents new type curves for analyzing slug tests in hydraulically fractured coal seams. The type curves were developed using a finite-conductivity, vertical fracture model and are presented in terms of three parameters — dimensionless wellbore storage coefficient, dimensionless fracture Conductivity, and fracture-face skin. With these new curves, we may estimate the hydraulic fracture half-length, the formation permeability, and the fracture conductivity. We also present a procedure for using the new curves and illustrate the procedure with an example.

Introduction

Slug testing has been proven to he an effective method for Characterizing the production potential of coal seams. A slug test involves the imposition of an instantaneous change in pressure (or fluid head) in a well and the measurement of the resulting change in pressure as a function of time. This change in pressure is created by either injecting into or withdrawing from the well a specific volume of fluid (i.e., a slug). From this measured pressure we may estimate the permeability and pressure we may estimate the permeability and near-wellbore conditions.

Initially, slug testing methods and analysis techniques were developed for estimating the transmissivity of shallow, underpressured aquifers, but also have found applications in the petroleum industry, especially for analyzing the flow period during drill stem tests. Recently, slug testing has been extended to the evaluation of the production potential of coal seams. Since most coal seams are saturated initially with water, slug testing provides a simple but effective method for estimating flow properties early in the productive life before the initiation of gas production. Reference 7 provides an overview of conventional slug testing in coal seams.

Conventional slug test analysis techniques are based on radial flow models. However, many wells completed in coal seams require hydraulic fracturing in order to become economically viable producers. Therefore, these conventional slug test analysis techniques cannot be used to either assess the success of the fracture treatment or to evaluate the post-fracture potential of these stimulated coal seams. Karsaki, et al. studied the pressure response of slug tests in infinite-conductivity vertical fractures, but they did not investigate the behavior of finite-conductivity fractures. The purposes of this paper are to develop a model for slug testing in coal seams with finite-conductivity vertical fractures and to illustrate application of this model to the analysis of slug tests.

MATHEMATICAL MODEL

We developed our slug test model using Cinco-Ley, et al.'s model which considers a well intersected by a fully-penetrating, finite-conductivity, vertical fracture. The reservoir is assumed to be an isotropic, homogeneous, infinite medium having a uniform thickness, h, permeability, k, and porosity, o. In addition, the reservoir contains a slightly compressible fluid of viscosity, u, and compressibility, c. that are independent of pressure.

Cinco-Ley. et al's model assumes the fracture to be a homogeneous, uniform slab with height. h, width, bf, and half length, L. Because the fracture width is much smaller than fracture length and height, the model assumes the flow in the fracture is linear and that fluid influx at the fracture tips is negligible. In addition, the model assumes that fluid production from the reservoir to the wellbore occurs only through the fracture. Further, since the fracture volume is small, the model neglects the fracture compressibility and assumes flow within the fracture is relatively incompressible. Additional details concerning the model formulation and problem solution may be obtained in Ref. 9 and Appendix A.

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