Abstract

Analysis of pressure data immediately subsequent to a fracturing stimulation treatment gives indications of the fracture length and representative conductivity. A comparison of these post-treatment values allows corrections to be incorporated into treatment design studies and thus improves design optimization.

As a first step, the calculations of fracture length and conductivity using two case histories of wells fracture stimulated are included in this study. Some analysis considerations for designing or evaluating post-frac test are also included.

Introduction

The contribution of hydraulic fracturing to the petroleum industry can be summarized by its conspicuous role in production increase and addition to recoverable reserve estimation. It has been reported that about 25 to 30% of total U.S. oil reserves have been mode economically producible by hydraulic fracturing. It is also known that 35 to 40% of all currently drilled wells are hydraulically fractured.

In order to maximize the increase in well productivity from hydraulic fracturing and to optimize the treatment design for subsequent wells having similar characteristics, it is necessary for reservoir engineers to evaluate the effectiveness of fracturing and make an estimation of the future performance of the well. The effectiveness of fracturing in terms of changes in reservoir characteristics and production enhancement in long term prospective can only be judged by the well test analysis. The well test interpretation will be able to identify the system before treatment and will tell what fracturing has done to the reservoir after treatment. The increase or change in the productivity of a well by fracturing is known to be dependent upon fracture characteristics such as conductivity, length and a degree of damage surrounding the fracture.

Considering the cost involved in fracturing operations and the limited technology development associated with fracturing evaluation, there has been a considerable interest in the determination of the characteristics of the fracture by means of transient pressure analysis in past decade. Most of these methods can be classified in three categories - infinite conductivity fracture, uniform flux fracture and finite conductivity fracture model. Although the assumptions of infinite and uniform flux fracture conductivity are reported to be adequate for some cases, a model using a finite conductivity fracture concept has been found to be most applicable to analyze the behaviour of a well intersected by a vertical fracture.

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