Hydraulic Fracturing: Holistic Fracture Diagnostics
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 107877, "Holistic Fracture Diagnostics," by R.D. Barree, SPE, and V.L. Barree, Barree & Associates, and D.P. Craig, SPE, Halliburton, prepared for the 2007 SPE Rocky Mountain Oil & Gas Technology Symposium, Denver, 16-18 April. The paper has not been peer reviewed.

Since the introduction of G-function derivative analysis, prefracture-treatment diagnostic-injection tests have become a valuable and commonly used technique. Unfortunately, the technique frequently is misapplied or misinterpreted, leading to confusion and misdiagnosis of fracturing parameters. A consistent method of analysis of the G-function, its derivatives, and its relationship to other diagnostic techniques is presented.

Introduction

The prefracture-treatment diagnostic-injection-test analysis provides critical input data for fracture-design models and reservoir-characterization data used to predict post-fracture production. An accurate post-treatment production forecast is necessary for economic optimization of the fracture-treatment design. Reliable results require accurate and consistent interpretation of the test data. In many cases, closure is identified mistakenly through misapplication of one or more analysis techniques. In general, a single unique closure event will satisfy all diagnostic methods. All available analysis methods should be used in concert to arrive at a consistent interpretation of fracture closure.

The relationship of the preclosure analysis to after-closure-analysis results also must be consistent. To perform the after-closure analysis correctly, the transient-flow regime must be identified correctly. Flow-regime identification has been a consistent problem in many analyses. There is no consensus regarding methods to identify reservoir-transient-flow regimes after fracture closure. The method presented in the full-length paper is not universally accepted, but it appears to fit the generally assumed model for leakoff used in most fracture simulators.

Transient-Flow Regimes During and After Fracture Closure

Immediately after shut-in, the pressure gradient along the length of the fracture dissipates in a short-duration linear-flow period. In a long fracture in low-permeability rock, the initial fracture linear flow can be followed by a bilinear-flow period, with the linear-flow transient persisting in the fracture while reservoir linear flow occurs simultaneously. After the fracture transient dissipates, the reservoir-linear-flow period can continue for some time, depending on the permeability of the reservoir and the volume of fluid stored in the fracture and subsequently leaked off during closure. After closure, the pressure transient established around the fracture propagates into the reservoir and transitions into elliptical, then pseudoradial flow. Each of these flow regimes has a characteristic appearance on various diagnostic graphs.

Fluid leakoff from a propagating fracture normally is modeled assuming 1D linear flow perpendicular to the fracture face. It has been pointed out that in some cases of moderate reservoir permeability, the linear-flow regime may not occur, even during fracture extension and early leakoff. During fracture extension and shut-in, the transient may already be in transition to elliptical or pseudoradial flow. In this case, analyses assuming a pseudolinear-flow regime will give incorrect results. In all cases, an understanding of the flow regime and its relation to the fracture geometry is critical to arriving at a consistent interpretation of the fracture falloff test.