Review Article
Theoretical overview of hydraulic fracturing break-down pressure

https://doi.org/10.1016/j.jngse.2018.08.012Get rights and content

Highlights

  • Theoretical background of hydraulic fracture break-down pressure models were comprehensively reviewed.

  • Tensile strength-based approach often over/under estimates the actual break-down pressure.

  • Stress intensity factor-based and energy release rate-based approaches provide quite reliable results.

  • Shear failure-based approach is an over simplified one and needs more improvements.

  • Overall, actual break-down process is extremely complex and difficult to precisely predict.

Abstract

The precise prediction of the break-down pressure is imperative to define the pumping schedule and the relevant stimulation parameters of a hydraulic fracturing process. A number of theoretical models have been derived based on different approaches to predict the break-down pressure, under various field/in-situ conditions. Although, the analytical models have been evolved over time, disagreements exist between the theoretical predictions and the laboratory/field results. This paper comprehensively reviews the derivations, evolutions and limitations of most of the existing break-down models and provides suggestions for further improvements. Among a number of theoretical approaches, stress intensity factor-based approach and the energy release rate-based approach give more reliable predictions, which are in line with most of the laboratory and field results. The tensile strength-based approach is commonly used to derive break-down models, but often provides slightly over/under estimations. Shear-based approach is an oversimplified approach and rarely used for the theoretical predictions. The approaches share many similarities, thus advanced models have been developed by combining the theories to precisely predict the break-down pressure. The actual hydraulic fracturing operation is rather a complex process, which involves a number of governing factors including reservoir and fracking fluid properties. Derivation of a global theoretical model is beyond the bound of possibility, as the modelling of break-down pressure for a given reservoir requires specific details of the particular operation and the in-situ conditions. The fracking with non-aqueous or mixture of fracturing fluids can be much complex due to multifaceted fluid properties, interactions, flow behaviour and phase change, thus requires more analytical, numerical simulations and laboratory/field experiments prior to implementation of large scale field projects.

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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