Review ArticleTheoretical overview of hydraulic fracturing break-down pressure
Introduction
Most potential tight gas reservoirs have a low permeability, so that an appropriate production enhancement technique is required for an economical oil and gas production. The hydraulic fracturing technique of well stimulation is such effective extraction technique widely used in petroleum industry to enhance the oil and gas production from low permeable reservoirs (Gregory et al., 2011; Wiseman, 2009; Haimson and Fairhurst, 1969; Cui et al., 2018; Singh, 2018). Hydraulic fracturing is a mechanical process, whereby a pressurized fluid injected through the wellbore causes unstable fracture propagations into rock mass. These mechanically induced fractures enhance the permeability of the rock formation, thereby facilitate an easy oil and gas flow through the reservoir towards the wellbore (Barati and Liang, 2014; Howard et al., 1970). The break-down is an important process in hydraulic fracturing that decides the economic as well as the safety factors of the fracking process. The break-down pressure is simply defined as the peak pressure that is reached during the pressurization of the wellbore (Detournay and Carbonell, 1997). A number of theoretical, experimental and numerical analyses have been done to interpret the break-down pressure in different rock types, under different in-situ conditions with different fracturing fluids (Warpinski et al., 1981; Schmitt and Zoback, 1993). The process is extremely complex, which depends on several factors such as in-situ horizontal stresses, pressure rate, rock mass properties, fracture-fluid properties and wellbore size and orientation (Haimson and Zhao, 1991; Schmitt and Zoback, 1989; Schmitt and Zoback, 1992).
Considering different approaches, several theoretical models were developed to predict the break-down pressure in hydraulic fracturing process. The tensile strength-based approach, energy release rate-based approach, stress intensity factor-based approach and shear failure-based approach are common theoretical approaches used to derive break-down models and to evaluate the break-down pressure under different conditions. The analytical models have been evolved over time and associated with several complex parameters to predict the break-down pressure more precisely. However, still these models fail to explain the abnormal variations of break-down pressure observed in laboratory and in-situ experiments, which have been conducted under various conditions. This paper is focused on reviewing the basis of the existing theoretical break-down models and to discuss their limitations on the applications in real life hydraulic fracturing process. The paper comprehensively discusses the evolution of break-down models, which are based on different approaches, starting from the simplest theories and their derivations. The possible reasons for the differences between theoretically predicted values and the experimental results, and the causative assumptions have been comprehensively discussed, and the suggestions on extending the studies to predict more accurate results are provided, based on current theoretical, numerical and experimental knowledge. Also, the similarities and equivalences of the discussed approaches are highlighted, and the recent advanced numerical models developed by combining those approaches are briefly discussed.
Section snippets
Tensile strength-based approach
Tensile strength-based approach is a common approach, which basically relies on the stresses generated on the rock mass upon fracture fluid injection. Most of the common break-down models have been developed based on this approach and are comprehensively discussed under this section, including the derivations of the fundamental theories.
Energy release rate-based approaches
The energy balance criterion or the ‘theory of rupture’ proposed by Griffith (1921) laid the foundation on this approach, where the proposed theory describes the unstable fracture propagation in an underground rock formation. The derivation of the theory was based on the Inglis (1913)'s solution, in which the effect of the presence of a crack on the energy of an elastic body was calculated considering the two-dimensional equations of elastic equilibrium in the space bounded by two concentric
Stress intensity factor-based approaches
The stress intensity factor-based approaches give the most promising explanations for hydraulic break-down of rock masses. They are generally associated with fracture toughness, fracture surface energy, stress intensity functions, etc. The approach was first initiated by Hardy (1973) and later improved by many researchers (Van Eekelen, 1982; Abou-Sayed et al., 1978; Rummel and Atkinson, 1987). Stress intensity factor-based approach basically assumes a rock mass with a penny-shaped, symmetrical
Shear failure-based approaches
Some researchers argued that the hydraulic fractures are induced due to shear failure, rather than tensile failure (Morgenstern, 1962; Callanan, 1981; Ljunggren et al., 1988; Panah and Yanagisawa, 1989; Lo and Kaniaru, 1990; Mori et al., 1990). In shear failure-based approach, the failure criterion is defined, such that the break-down occurs, when the stress in the wall of a well reaches the shear strength of the rock. The theory is in agreement with the tensile strength-based approach
Summary
It is important to predict the break-down pressure of the hydraulic fracturing process, in order to define effective operational parameters and safety factors. A number of theoretical models have been developed based on different approaches to predict the break-down pressure of a reservoir upon fluid injection through a wellbore. This paper comprehensively reviews four approaches of the existing break-down models, including their fundamental theories, derivations, underlying assumptions and the
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