Abstract

Producing natural gas from Tight Gas Sand (TGS) reservoirs is currently very important to the industry in North America and will be of increasing importance to the global energy industry in the coming decades. Productivity index (PI) is an important indicator of a well's production capacity. For conventional reservoirs, well productivity is usually calculated using the pressure response of the reservoir in its pseudo-steady state period. However, for TGS reservoir, the period of transient flow covers a larger part of the well lifetime and the pseudo-steady state productivity calculations become less applicable in prediction of the reservoir's production behavior. The Distributed Volumetric Sources (DVS) method can fill this gap. The DVS method can be used to predict the productivity index of a general well completion scheme for both transient flow period and pseudo-steady state flow period. In the paper, we will generally describe the DVS method and the model we have build to predict the PI of TGS reservoir using DVS method.

In this study, we focus on a typical well completion scheme — vertical well intersected by a vertical fracture of finite conductivity. Parametric study is performed by varying the proppant pack permeability with a linear distribution, varying fracture width with an elliptical distribution and varying fracture height with an elliptical distribution. The details of hydraulic fracture are integrated into the calculation of well productivity. By combining the well productivity with gas material balance, production forecasting of the hydraulically fractured wells could be easily obtained. The result of production forecasting could be used to aid in decision making of choosing the best stimulation treatment. Field examples are presented to illustrate the application of this technology for production modeling the complicated reservoir cases involving fracture stimulation.

Introduction

Productivity index (PI) is an important indicator of a well's production capacity. For conventional reservoirs, well productivity is usually calculated using the pressure response of the reservoir in its pseudosteady-state period. There are numerous studies for different well completion schemes which developed correlations for pseudosteady-state productivity index for specific cases, such as horizontal wells and fractured wells. Most of the developed models for complex well completion schemes use some approximations for productivity index calculation and they have some limitations in use. Furthermore, as the petroleum industry goes toward producing lower quality reservoirs like low- and ultra low-permeability reservoirs, the period of transient flow covers a larger part of the well lifetime and these pseudosteady-state productivity calculations become less applicable in prediction of the reservoir's production behavior. The Distributed Volumetric Sources (DVS) method seems able to fill this gap.

Our method is able to predict the productivity index of a general well completion scheme for transient as well as pseudosteady-state flow periods.

Production Forecasting Model

The pseudo-steady state solution for slightly compressible fluid can be solved as following, if the original diffusivity equation is derived in terms of real gas pseudopressure of Al-Hussainy and Ramey 3

EQUATION...................................................................................... (1)

Where real gas pseudopressure function, m(p), is defined in TABLE 1.

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