document preview

SPE Members

Abstract

Step rate testing (SRT) has been used for several years to determine the formation or fracture parting pressure (FPP). Unfortunately, little is available in the literature regarding SRT design and analysis. Further, the available guidelines are inadequate and may Lead to questionable values of the FPP.

A systematic investigation of several significant factors affecting SRT design and analysis is presented. The analysis of SRT data influenced by wellbore storage and changing wellbore storage is investigated. The proper application of multirate analysis methods to SRT data is outlined. A new method is proposed for determining parting pressure from SRT data on fractured wells. Field examples are included. This work should significantly enhance our ability to successfully design and analyze the step rate tests.

Introduction

Step rate testing (SRT) is the primary method used to define the maximum safe injection press without fracturing the reservoir rock. This pressure is referred to as the formation or fracture parting pressure (FPP). It is the pressure that will initiate a fracture in an unfractured well will extend an existing fracture in a fracture well. The "FPP" determined from step rate tests is equivalent to the term "fracture extension/ propagation pressure" used in the hydraulic fracturing literature. The FPP is generally greater than the "closure pressure" (defined as the pressure required to initiate the opening of an existing fracture) determined from pump-in/flowback tests.

Determination of FPP is of critical importance for efficient operation of waterfloods and tertiary recovery projects. Injection above parting pressure can result in premature breakthroughs, poor sweep, reduced oil recovery, and loss of costly injection fluids due to uncontrolled fracture extension. On the other hand, injection at pressures far below the FPP implies injecting at a lower rate than an allowable maximum and thereby a reduced rate of oil recovery.

Unfortunately, technology development in the area of step rate tests has not kept pace with field operations even though these tests have been common place in our industry for over 40 years. This is reflected by a surprising Lack of references in the literature on the subject and continued problems with the interpretation of such tests. To our knowledge, there have been only two papers published in the literature, which directly address step rate test (SRT) design and analysis. A brief discussion is also presented in SPE Monograph 5. Our investigation shows that the previously available guidelines are inadequate and may lead to questionable values of the FPP.

The entire subject of multiple rate flow into the reservoir during a SRT, coupled with reservoir fracturing, is a complex one since it requires a knowledge of both rock mechanics and fluid flow aspects of the problem. A recently published paper provides some understanding regarding waterflood induced fracture propagation. The analysis of SRT data is also subject to the type of problems which are common to other pressure transient tests.

P. 491^