Common Mistakes Associated With Diagnostic Fracture Injection Tests

Abstract

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 169539, ’Diagnostic Fracture Injection Tests: Common Mistakes, Misfires, and Misdiagnoses,’ by R.D. Barree, SPE, J.L. Miskimins, SPE, and J.V. Gilbert, SPE, Barree & Associates, prepared for the 2014 SPE Western North American and Rocky Mountain Joint Regional Meeting, Denver, 16-18 April. The paper has not been peer reviewed. During the last 20 years, diagnostic fracture injection tests (DFITs) have evolved into commonly used techniques that can provide valuable information about reservoirs and hydraulic-fracture-treatment parameters. Thousands of tests are run every year in both conventional and unconventional reservoirs. However, many factors associated with data collection and analysis can result in poor or incorrect results. This paper attempts to describe some of the common problems and to help prevent some common errors often observed in DFIT execution and analysis. Introduction DFITs are small pump-in treatments performed to gather data to help design follow-up hydraulic-fracturing treatments and characterize a reservoir. A generic DFIT is shown in Fig. 1. The wellbore is first filled at a low to moderate rate until a positive surface pressure response is observed. With low- to- moderate wellbore compressibility, the pressure should rise quickly until initial breakdown occurs. Breakdown will be indicated either by a sharp drop in pressure as a new fracture initiates (as at Number 1 in Fig. 1) or by a plateau in pressure as existing fractures are opened and extended. Once a breakdown event is observed, the injection rate should be increased to the maximum rate allowed by available horsepower or to 75% of the planned treatment rate of the main fracture treatment, whichever is achievable. A constant rate is then held for 3–5 minutes (Number 2 in Fig. 1), after which a rapid step-down can be conducted. The step-down test is separate from the actual DFIT but is commonly run at the end of the pump-in to determine perforation and near-wellbore frictional pressure losses. The rate is then immediately reduced to zero to obtain the instantaneous shut-in pressure (ISIP), shown as Number 4, and the falloff pressure is monitored for as long as possible or as long as necessary to acquire the desired data. Even when high friction is not expected, the step-down is recommended to make identification of ISIP easier.