Abstract

A laboratory investigation has been carried out to investigate delayed initiation of hydraulic fractures where wellbore pressures are insufficient to induce instantaneous breakdown. The delayed initiation process we are investigating is related to so-called static fatigue, which refers to a property of many brittle/quasi-brittle materials, including rocks, to fail in a delayed manner when subjected to an applied stress that does not exceed the material strength. In this paper, we experimentally verify a limiting case of a theoretically predicted delay time of initiation for given wellbore pressures. The investigation has two parts: characterization of the static fatigue behavior of a given rock and hydraulic fracture initiation experiments. For rock characterization, three-point and four-point bending tests are used to obtain time to failure versus applied tensile stress relationships for Coldspring Charcoal Granite specimens. Then, using an apparently geometry-independent static fatigue parameter and classical hydraulic fracture breakdown theories, we predict and subsequently test the time to failure for different wellbore pressures in the laboratory. We present our laboratory observations and measurements on delayed failure of axially oriented hydraulic fracture initiation. Our results verify the existence of delayed initiation of hydraulic fractures, and they demonstrate the ability of straightforward theoretical considerations to predict the initiation time. Comparison between experimental data and prediction also suggests that the effect of fluid penetration plays an important role in delayed initiation, evidenced by the tendency for the longer-time initiations at lower pressures to fall below the theoretical prediction.

1. INTRODUCTION

Most approaches to stimulation of horizontal wellbores rely on initiation and growth of more than one hydraulic fracture. Past studies on the topic of initiation of hydraulic fractures predict the wellbore pressure at which initiation will occur [1-6]. In the classical hydraulic fracture breakdown models (Hubbert and Willis [1] and Haimson and Fairhurst [2]), initiation will not occur if the near-wellbore tensile stress induced by fluid pressure is less than the tensile strength of the rock formation. However, it has been observed in many laboratory experiments that brittle/quasi-brittle materials, including rocks, can be caused to fail after a period of time when subjected to a sustained static load that is lower than its strength [7, 8]. This behavior is referred to as "static fatigue." In one recent example, Kear and Bunger [8] show that for a crystalline gabbroic rock, applying a load as low as 75% of its nominal tensile strength onto the sample can cause failure within a few hours. Therefore, the concept of tensile static fatigue in brittle/quasi-brittle rocks leads to the central question of this paper: Is it possible for a hydraulic fracture to be initiated under wellbore pressure less than critical breakdown pressure? We seek an answer by theoretical study and laboratory experimentations.

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