PUBLICATION RIGHTS RESERVED PUBLICATION RIGHTS RESERVED THIS PAPER IS TO BE PRESENTED AT THE INTERNATIONAL TECHNICAL MEETING JOINTLY HOSTED BY THE PETROLEUM SOCIETY OF CIM AND THE SOCIETY OF PETROLEUM ENGINEERS IN CALGARY, JUNE 10 TO 13, 1990. DISCUSSION OF THIS PAPER IS INVITED. SUCH DISCUSSION MAY BE PRESENTED AT THE MEETING AND WILL BE CONSIDERED FOR PUBLICATION IN CIM AND SPE JOURNALS IF FILED IN WRITING WITH THE TECHNICAL PROGRAM CHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING.

Abstract

In conventional oil and gas reservoirs, the use of prefrac pump-in tests, in conjunction with analysis of the prefrac pump-in tests, in conjunction with analysis of the shut-in pressure response, has been a tremendous asset in improving fracture stimulation designs. Since its introduction in 1979, many modifications and improved analysis techniques have evolved to increase the value of "minifrac" tests. In recent years, the popularity of completing wells in coal formations has led to the use of minifrac applications in these wells as operators attempt to better understand how coals respond to hydraulic fracturing. Although some operators have enjoyed success with minifracs, the usefulness of these types of prefrac tests has been limited as compared to prefrac tests has been limited as compared to applications in conventional gas reservoirs.

In fracture stimulation of coalbed methane wells, the use of minifrac tests and other pump-in tests has sometimes resulted in abnormally high injection pressures during the subsequent fracturing treatment. Many times the result has been an early screenout during the proppant placement. Some operators have abandoned proppant placement. Some operators have abandoned the use of minifrac type pump-in tests for fear of creating pressure problems that would jeopardize the stimulation pressure problems that would jeopardize the stimulation treatment.

In this paper, problems attributed to minifrac tests are described, along with explanations for many of these occurrences. Also included are successful applications of pump-in tests and a modification to improve fluid loss pump-in tests and a modification to improve fluid loss calculations. Recommended procedures to minimize the possibility of creating high injection pressures are given. possibility of creating high injection pressures are given. Field examples from the San Juan Basin, Piceance Basin, Black Warrior Basin and the Central Appalachian Basin are used to illustrate problems and support the recommended procedures presented.

Introduction

A historical review of minifrac pump-in testing in coalbed methane wells will reveal three general classifications as to the use of this process:

  1. successful applications where treatment designs have been significantly improved by the results of the analyses;

  2. applications where no testing abnormalities were noted, but the resultant data could not be meaningfully interpreted by classical analysis methods, and

  3. instances where pressure abnormalities occurred either during or following the minifrac test.

When fracture stimulating coal seams, treating pressure gradients observed may vary from unusually low pressure gradients observed may vary from unusually low (0.5 psi/ft) to very high >1.8 psi/ft).

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