Abstract

The study of fracture initiation from a cavity in rock has been largely motivated by the extraction of petroleum and natural gas from a borehole and therefore, is often considered as one of the most challenging problems in rock mechanics. An unresolved issue is relating the fracture parameters from small scale laboratory test to a large scale engineering process. To investigate fracture initiation from a borehole, cavity expansion tests were conducted in the laboratory on specimens of Berea sandstone. To capture the failure process near the circular cavity, digital image correlation (DIC) was used. In addition, a two dimensional bonded particle model was developed and validated. For investigating the effect of scaling on fracture parameters (e.g. critical pressure), several larger specimens with different tension softening behavior were tested numerically. It was shown that size plays an important role. Furthermore, using the DIC measurements, it was found that fracture initiates at 80% of peak internal pressure for the rock tested.

1. INTRODUCTION

The expansion of a circular cavity, such as a production well in rock, has been of considerable interest for decades. However, some aspects of fracturing have not been fully addressed. For example, in the interpretation of the critical (breakdown) pressure for hydraulic fracturing, the peak of the recorded pressure data is often assumed to be associated with tensile fracture initiation. The significant non linearity in the variation of the borehole diameter with breakdown pressure suggests fracturing prior to peak [1].

In this study, using a particle tracking technique named digital image correlation (DIC), a displacement discontinuity at the cavity boundary was identified. It was noticed that a fracture (process zone) develops prior to peak pressure. Another important issue in the problem of borehole expansion is the discrepancy between theoretical predictions and lab/field measurements regarding a size (and rate) effect. Many previous studies demonstrate that a rate effect strongly exists and that the faster the loading, the higher the breakdown pressure [2- 6]. Size effect on the critical load required to initiate a tensile (mode I) fracture in a quasi-brittle material is well known [7], and therefore, understanding the mechanism associated with mode I fracture initiation from a wellbore is important. Previous studies have confirmed that with increase in size, the critical pressure decreases [2, 8]. A recent study [9] demonstrated that the size effect is governed by the ratio between a material length scale and a structural length scale. The material length is defined as the square of the ratio of the fracture toughness to its tensile strength, while the structural length scale is the wellbore radius.

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