Pressure-Transient Test Design in Tight Gas Formation
Authors: Lee W.J. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

This paper outlines a procedure for pre- and postfracturepressure-transient test design in low-permeability (tight) gas formations. Theprocedures proposed are based on many years' experience in evaluatinglow-permeability formations, and particularly on recent experience with GasResearch Inst, (GRI) programs in eastern Devonian gas shales and in westerntight-gas formations.

Introduction

Postfracture tests are frequently needed to evaluate stimulation treatmentsfor one of two major reasons:to diagnose the cause of disappointingproductivity following a fracture treatment, with possible causes includingshort propped-fracture length, low fracture conductivity, low formationpermeability, or low formation pressure, orto optimize fracture treatmentdesign in a new or important area by examining the detailed results oftreatments pumped.

Several authors have observed that the ambiguity of postfracture testanalysis is diminished considerably if a prefracture test has been run so thatan estimate of effective permeability to gas is available for use in thepostfracture test analysis. Although the possibility of communication with morenet pay following a fracture treatment is real, the reduction in error possiblewith a prefracture test permeability estimate is still essential. The importantpoint is that proper postfracture test analysis requires the results of aprefracture test; thus, a logical design for the prefracture test is animportant part of the testing program.

Knowledge of discovery pressure in the drainage area of a tested well isimportant for a number of reasons, not the least of which is to help ensurevalid prefracture test analysis. In low-permeability formations, a well willrarely build up completely to discovery pressure following a production periodof any significant length in a buildup test of practical length. However, knowledge that the test data are on the proper semilog straight line (on aHomer graph) headed toward a known discovery pressure can help ensure thecorrectness of an otherwise uncertain test analysis. Thus, good test designwill attempt to obtain a reliable value of discovery pressure, pi, before aflow period that is followed by a shut-in test.

To achieve the objectives of estimating permeability to gas, kg, anddiscovery pressure, pi, in a prefracture test, and fracture half-length, Lf, and fracture conductivity, KtD, in a postfracture test, the tests must bedesigned thoughtfully. For example, a common problem in transient tests inlow-permeability formations is that the test is too short- not only too short ashut-in period, but a flow period preceding a buildup test so short thatwellbore unloading is incomplete (mass rate out at the surface exceeds massrate in through the perforations). This results in uninterpretable andessentially useless buildup data even when a bottomhole shut-in is used tominimize the duration of wellbore storage (afterflow).

This paper presents some field-tested thoughts on test design that willusually provide test data that allow the engineer to achieve the testobjective. More specifically, after a brief review of the modest theoryrequired for proper test design, this paper presents procedures (1) to estimatea value of kg sufficiently accurate for test design purposes, (2) to design aprefracture pressure-buildup test, and (3) to design a postfracturepressure-buildup test. The paper concludes with an application of theseprocedures to an example well.

JPT

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