Application of Pressure Transient Testing for Stimulation Decisions
Authors: Chu Wei-Chun 
Published: December 2007

The decision on stimulating a well is usually derived by analyzing transient tests. However, should a damaged well be capable of producing at rates greater than the minimum rate that satisfies the criterion for economic production, it is difficult to persuade the operator to consider any form of treatment. Conversely, needless remedial treatments may result from reacting to a steep decline in production without considering causes other than wellbore damage. The objective of this paper is to present field examples to demonstrate conditions that do not require remedial treatment. The examples discussed here are presented with an eye towards resolving reservoir-management issues.

Introduction

This paper addresses the role of pressure-buildup behavior on managing the performance of a well. The principal contribution of this work is to emphasize the importance of identifying the true causes of reduction in production rate (for example, wellbore damage vs. boundary effects) before committing resources to a stimulation treatment. This end is achieved by considering a number of field-case illustrations. These examples demonstrate the utility (overcome damage, improve perforations) and futility (layering, boundaries) of stimulation. Recognition of the latter is particularly important because causes other than wellbore damage may result in a steep decline in rate.

Examples

Examples 1 and 2 considered below demonstrate our ability to improve productivity by effecting changes over which one may have control. Examples 3-6 demonstrate the need for a careful evaluation of the variables that affect productivity and illustrate the role of reservoir properties, that may not be altered by a remedial treatment, on well performance.

Role of Damage (Example No. 1). Well A produces a sandstone reservoir located at a depth of 12,700 feet. The entire pay zone (11.5 feet) was perforated and the formation was broken down with crude oil before it was put on production. The stabilized production rate was 600 STB/day. Fig. 1 is a log-log plot of the well response of a buildup test conducted after a few weeks of production. A well-defined, semilog straight-line is clearly evident; the large separation between the p and p' curves suggests that the damage may be severe. In our experience, a vertical separation of over one and one-half cycles is an excellent indicator that the subject well is a potential candidate for stimulation. A quantitative analysis justifies these observations: kh = 1,840 md-ft, s = 90. The value of s, though not large in and of itself, is uncharacteristic of wells producing this reservoir. Because of the well's production rate, a significant amount for a well in the Continental United States, the conclusion that the well is damaged and a candidate for a stimulation treatment was viewed with considerable skepticism and there was considerable reluctance to workover this well was understandable. Nevertheless, the success of fracture treatments in the other wells of this field provided the formation-evaluation team with enough data to make a persuasive case that this well was such a candidate.

Accordingly, a fracture treatment was conducted one year after the well was put on production. Following the fracture treatment, intended to overcome wellbore damage, the well produced at a stabilized rate of 1,700 STB/day, justifying expectations that a higher productivity was possible. Fig. 2 is a log-log plot of the pressure and pressure-derivative curves of a buildup test after stimulation.

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