Understanding Hydraulic Fracture Growth: Tricky but Not Hopeless (SPE 56724)

Hydraulic fracturing has proven to be a fruitful well stimulation technique in an ever-increasing range of environments. Application has spread from the original target of enhancing production rates from low permeability reservoirs to fracturing of poorly consolidated high perm reservoirs, fracturing of horizontal and deviated wellbores, fracturing of unconventional (often naturally fractured) reservoirs, fracturing for waste disposal, etc. While significant progress has been made in engineering hydraulic fracturing treatments, we are still often humbled and alarmed by the apparent complexity of the process.

This paper does not present “the answer” to this complex problem. Instead it attempts to shed some light on areas where we must be careful with our assumptions. This paper presents examples of measuring (inferring) hydraulic fracture growth in a number of different environments. For each example we discuss the implications of the observed fracture growth. For example, how accurate were the fracture model predictions? How effective is the current completion / fracturing strategy in achieving the design goals? What are we learning about the factors or mechanisms that govern hydraulic fracture growth? How can these new insights aid production enhancement?

While most of the direct observations of hydraulic fracture growth are from downhole-tilt fracture mapping, data from many additional diagnostics are also included (post-frac logging, production response, intersections with offset wells, microseismic mapping, etc). Perhaps the most surprising observation is the observed range of fracture height growth: spanning from well-contained fractures in the absence of significant formation stress barriers, to extremely uncontained fracture height growth. Observations of fracture width, length, and asymmetry will also be presented and discussed.

Introduction

This paper attempts to step back a bit from the flurry of new diagnostic observations of hydraulic fracture growth that have appeared over the last few years. We present examples of observed fracture growth and discuss the implications of what was observed. These are not unique examples, or even the best examples. These are simply examples that allow us to make some general observations about fracture growth that we believe apply to far more than the examples presented.

The examples presented are not detailed case studies. They do not contain answers for how to best fracture stimulate wells in the respective areas. They are used only to illustrate how fracture growth is more complicated than we - particularly us modelers - like to think it is. However, this does not mean that modeling or attempting to understand and predict fracture growth is hopeless. It simply means that we need to re-think our assumptions, and maybe our confidence levels, about how we expect fractures to grow. Many of the realizations of the last few years, greater fracture height confinement for example, are not random events. Instead, they are possibly just physical phenomenon, like tip blunting across material interfaces, playing a much more significant role than we had previously assumed. If we can begin to understand where and why these “other” factors play a major role in fracturing, we can begin to incorporate them into our thinking, design processes, and even fracture models.

The goal of this paper is to raise awareness of the fact that we cannot just take an “uncalibrated”, theoretical fracture model result as a definitive answer. Of course, this does not mean that fractures always behave very differently from what fracture growth models tell us and that models are always wrong. To the contrary - we have observed that models often do provide results that are quite consistent with direct observations. But perhaps more often we observe significant shortcomings in fracture model predictions. This paper aims to provide a “flavor” of the differences between direct observations and fracture growth model predictions.

Category 1: Fracpro
Sub Category 1: Application Examples

Paper Number: SPE 56724

Source: SPE Annual Technical Conference and Exhibition, 3-6 October 1999, Houston, Texas

URL: http://www.onepetro.org/mslib/app/Preview.do?paperNumber=00056724&societyCode=SP...