Abstract

Because many oilfield workers see matrix treatments of wells as a low-tech operation, they often fail to pay attention to candidate selection and treatment design. This lack of attention may have led to the relatively low success rate of these treatments. A 1997 survey in a major oil company indicated that one out of every three to four jobs fails to produce more oil or gas after the treatment. This failure represents a loss to the company of over $10 million (U. S.), plus a missed extra production capacity of nearly 40,000 BOPD. The probable main cause for this poor performance is the lack of a structured approach to the following:

  • selecting the right candidate wells and the appropriate treatment

  • defining and implementing a structured treatment design procedure

To improve the situation, a task force investigated the problem and mapped out a total process, which consists of the following steps: A candidate well is selected by comparing its actual performance against its theoretical potential. Then, the source of poor performance is identified, if applicable. Based on this information, the treatment type can be identified and designed, ultimately resulting in an operational stimulation program.

The task force concluded that individual pieces of design software and some design rules existed for many elements, but they lacked an integrated overall approach. The team decided to create a software package by integrating "fuzzy" rules with appropriate mathematical models to guide field engineers through the individual design steps in a consistent, structured manner. This paper describes various elements of the integrated software package that was designed to meet these needs.

Background

Soon after the first wells were drilled, people started to look for methods to improve the production of new and existing wells. In 1895, a well in Ohio, U.S.A. was successfully treated with hydrochloric acid (HCl) for the first time. However, until the early 1930's, when arsenic inhibitors emerged, the lack of good corrosion inhibitors prevented acid from being widely used to stimulate wells. The use of "mud acid" was then introduced for sandstone wells.

In subsequent years, additional techniques, materials, and equipment were further developed, leading to widespread stimulation activity. Today, several thousand treatments are completed worldwide each year, with a total expenditure of many millions of U.S. dollars. Most of this money is spent on matrix-acidizing treatments.

Stimulation is popular primarily because it is probably the most economical way to generate extra production capacity. For instance, a Dutch oil and gas company carried out a stimulation campaign in the prolific Groningen gas field, although the well rates were already approximately 1. 5 million m3/d. A total of 16 wells were treated with acid, resulting in an approximate net increase of 3.6 million m3 /d of gas (at 100 bar FTHP) at a total cost of $600,000 (U.S.). Had the company chosen to obtain such potential by drilling new (infill) wells, they would have spent approximately $7.5 million.

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