Pressure Falloff Tests of New Coal Wells

8/3/2019 Pressure Falloff Tests of New Coal Wells

1/13

THE SPE IMAGE LIBRARY SPE 21809

Pressure Falloff Tests of New Coal Wells

21809

Seidle, J.P.,*

Amoco Production Co.;Kutas, G.M.,

Amoco Chemical Co.; and

Krase, L.D.,*

Amoco Production Co.

*SPE Members

Copyright 1991, Society of Petroleum Engineers, Inc.

This paper was prepared for presentation at the Rocky Mountain

Regional Meeting and Low-Permeability Reservoirs Symposium held in

Denver,

Colorado, April 15-17, 1991.

This paper was selected for presentation by an SPE Program Committee

following review of information contained in an abstract submitted by

the

author(s). Contents of the paper, as presented, have not been reviewed

by

the Society of Petroleum Engineers and are subject to correction by

the

author(s). The material, as presented, does not necessarily reflect

any position of the Society of Petroleum Engineers, its officers, or

members. Papers presented at SPE meetings are subject to publication

review

by Editorial Committees of the Society of Petroleum Engineers.

Permission

to copy is restricted to an abstract of not more than 300 words.

Illustrations may not be copied. The abstract should contain

conspicuous

acknowledgment of where and by whom the paper is presented. Write

Publications Manager, SPE, P.O. Box 833836, Richardson, TX 75083-3836

U.S.A. Telex, 730989 SPEDAL.

Abstract

This paper demonstrates field application of

pressure falloff testing and analysis methods to new coal

degas wells in the San Juan Basin. These tests have

provided absolute coalbed permeability, wellbore skin,

and initial reservoir pressure. Effects due to free gas

and gas desorption on pressure falloff tests are considered.

The effects of sorption compressibility are shown to have

a significant impact on skin but not on permeability.

Influence of the duration of the injection and shutin

periods on the design and analysis of coal well PFOTs

is also discussed. Field examples of both pre- and

post-stimulation falloff tests are presented.


2/13


Introduction

Over the past decade, technology for pressure

transient testing of coals wells has developed rapidly.

Much of this work has concentrated on developing techniques

for testing coal wells after gas has been desorbed. A

single interference test in coal wells has been reported.

However, many operators need to evaluate the commercialviability of new coal wells and few investigators have

developed pressure transient tests suitable for such wells.

Use of slug tests in coalbed methane wells was

discussed by Koenig and Schraufnagel and examples of

these tests in Warrior Basin coal wells were presented.

Slug tests were found to be simple, reliable, and

economic tests for characterizing new coal wells. However,

slug tests have two disadvantages. The practical radius

of investigation of these tests is roughly 100 wellbore

radii, which is typically about 30 feet into the coal

seam, and in slug tests the accuracy of the permeabilityis no better than the accuracy of the wellbore storage

coefficient. Slug tests offer no independent method

of wellbore storage determination, and errors in this

coefficient lead to errors in the permeability.

The first effort to consider two-phase flow in

coalbeds was done by Kamal and Six. A new pseudopressure

was defined which incorporates both gas-water relative

permeabilities and sorption isotherm properties. By

assuming gas instantaneously desorbs from the coal matrix,

Kamal and Six developed a method which yields absolute

coalbed permeability and wellbore skin. They applied

the method to two San Juan Basin coal wells and reasonable

results were obtained. This method requires relative

permeability data and the sorption isotherm, both of

which are often not available for coal wells, especially

wildcats.

Design and interpretation of injection tests in the

Black Creek coals of the Warrior Basin of Alabama were

reported by Zuber, et al. Their reported tests appear to

be dominated by wellbore storage, indicating further

consideration needs to be given to test design. Zuber,et al., used a simulator for test design. Credibility of

their results would be increased if the simulator had also

been used to match actual test response.

The purpose of the work reported here was to

develop practical methods for the design and analysis of

pressure falloff tests in new coal wells. Existing

technology for injection well testing was readily adapted

to new coal wells. Many times the analysis of these tests

is quite straightforward.

Discussion


3/13


New coal wells are frequently completed in water

saturated coalbeds with no free gas in the coal fractures

(cleats). In such cases, all gas is held in the coal matrix

by sorption and is released only when pressure in the

cleats has been reduced due to water production. With

no free gas present, water injection into a water saturated

coal exactly satisfies the unit-mobility ratio assumption

of classical pressure falloff tests (PFOTs) and thosemethods can be readily adapted to testing new coal wells.

In conventional falloff tests, injection time is often

much longer than shutin time. In PFOTs of new coal wells,

however, injection time needs to be as short as possible

to minimize testing costs.

P. 53


4/13


5/13


6/13


7/13


8/13


9/13


10/13


11/13


12/13


13/13

Pressure Falloff Tests of New Coal Wells

Documents

Transcript of Pressure Falloff Tests of New Coal Wells