Review Article
A review of laboratory studies and theoretical analysis for the interaction mode between induced hydraulic fractures and pre-existing fractures

https://doi.org/10.1016/j.jngse.2020.103719Get rights and content

Highlights

  • Interaction mechanism between induced hydraulic fracture and single / multiple pre-existing fractures are discussed.

  • Different theoretical criteria have been discussed for predicating the behaviors of induced hydraulic fractures.

  • Important findings obtained in lab are used to optimize fracturing process and evaluate fracture propagation in the field.

Abstract

Hydraulic fracturing techniques have been widely used to create complex fracture network in unconventional reservoirs. The complexity of hydraulic fracture is significantly influenced by pre-existing fractures that widely exist in reservoirs. A better understanding of the interaction mode between induced hydraulic fractures and pre-existing fractures is helpful to estimate the fracture complexity, stimulated reservoir volume, and completion efficiency. In the present study, a review is conducted on the induced hydraulic fracture behaviours and interaction processes in laboratory test and theoretical analysis. It includes the effects of a series of parameters on the fracture behaviours, interaction modes, and interaction mechanism. Advantages and drawbacks of several theoretical criteria are compared, which are widely used to study the interaction mechanism. The sensitivity analysis of geological condition and hydraulic fracturing parameters on the interaction mode is performed. Then, some important findings in laboratory used to optimize operations of fracture-stimulation in the field are discussed. Based on the literature review, promising studies are recommended, which may provide more profound insights into study of the interaction mode.

Introduction

Hydraulic fracturing technique has been extensively used to increase the permeability of oil and gas reservoirs, which can sharply enhance oil and gas production, especially in unconventional reservoirs (e.g., shale, tight sandstone, and coalbed methane). More than one million wells in the United States have used hydraulic fracturing to enhance productions since the 1940s (King, 2012). To create complex fractures in reservoirs, the fracturing fluid (either water or a water-based fluid with various additives) pumps into reservoirs at a predetermined rate. The entire hydraulic fracturing processes are as follows. As the fracturing fluid pumps down the well, the wellbore pressure sharply increases (in Fig. 1). When the pressure surpasses the rock strength, the reservoir begins to crack, corresponding to a fracture initiation pressure (Pin). After the initiation, the pressure increases to its maximum value, which is named breakdown pressure (Pb). Then, the pressure drops to the propagation pressure (Pprop), which is the steady portion of the pressure curve. As the pumping stops, the pressure instantly drops to a low value. The transition point is called shut-in pressure (Ps) or instantaneous shut-in pressure (ISIP). The fluid leaks off from the fracture surface, the fracture opening width decreases, and the pressure decreases slowly. The fluid pressure inside the fracture eventually reaches equilibrium with the minimum in situ stress. At this point, the hydraulic fracture closes, which corresponds to the fracture closure pressure (Pc). Finally, the pressure decreases to the pore pressure due to fluid leaking off from the fracture and the borehole.

In general, the propagation of the induced hydraulic fracture is along the direction of the maximum confining stress (Zhang et al., 2019c; Wu et al., 2020; Zhao et al., 2020). However, if a pre-existing fracture (e.g. natural fracture, bedding plane, flaw, joint or fault) is near or along the path of the induced hydraulic fracture growth, the propagation direction of the induced hydraulic fracture may be diverted (Zhang et al., 2018a, b). The existence of pre-existing fracture may benefit to improve the stimulated reservoir volume (SRV), or impede the induced hydraulic fracture propagation. It is affected by in situ stress state, geometry and mechanical properties of pre-existing fracture, injection rate and fluid viscosity, etc. (Xu et al., 2020; Yew and Weng, 2014). Many studies have been carried out to study the interaction mechanisms through numerical modelling, laboratory test and theoretical analysis. Comprehensive reviews of numerical modelling used for simulation of hydraulic fracturing and investigation the interaction process can be found in recent published papers (Dahi Taleghani et al., 2016; Kolawole and Ispas, 2019b; Lecampion et al., 2018; Xu et al., 2018; Zhang et al., 2019e). Therefore, the present study is focused on the interaction studies of laboratory test and theoretical analysis.

The objective of the present study is to review the current state-of-the-art hydraulic fracturing studies, discuss advantages, practicality and drawbacks of different methods, and summarize the key findings regarding the interaction mode between the induced hydraulic fractures and pre-existing fractures. Laboratory studies on the interaction mode between the induced hydraulic fracture and a single pre-existing fracture, multiple pre-existing fractures (parallel pre-existing fractures, random pre-existing fractures, and bedding planes) are discussed based on a comprehensive literature review in Section 2. In Section 3, the theoretical criterion for the interaction mode based on different mechanical theories (e.g., linear elasticity theory and fracture mechanics theory) are discussed in detail. Recommendations for further researches are discussed and proposed in section 4. Finally, conclusions are drawn in Section 5.

Section snippets

Hydraulic fracturing in laboratory

Researchers have observed a considerable number of pre-existing fractures in rocks through several techniques, including borehole televiewer (Hickman et al., 1999), geologic sketches (Warpinski and Teufel, 1984), and coring (Warpinski et al., 1993; Gale et al., 2014). These pre-existing fractures significantly influence the stress state. The stress state induced by the pre-existing fracture controls the behaviours and the geometry of induced hydraulic fractures (Warpinski and Teufel, 1984).

Criteria of the interaction mode

To evaluate the fracturing processes and distinguish the failure nature of the pre-existing fractures during the hydraulic fracturing, some researchers have proposed a series of theoretical criteria to predict the interaction mode (in Table 3). Blanton (1982; 1986), and Warpinski and Teufel (1984) proposed a criterion to discuss the interaction mode based on the approaching angle and differential stress. Renshaw and Pollard (1995) proposed a criterion considering crossing an unbounded

Field applications

In the field, there is not so much data that can be obtained like in laboratory tests. The fracturing curve is the main data that can be used to evaluate the fracturing morphology and fracture behaviour (Jiang et al., 2019a; Tan et al., 2020; Zhang et al., 2019a). When a hydraulic fracture propagates in a homogenous rock, the fracturing curve does not have a large pressure fluctuation. As an induced hydraulic fracture encounters a pre-existing fracture (e.g., induced hydraulic fracture arrested

Conclusions and future researches

In the present study, the development of hydraulic fracturing in laboratory was comprehensively reviewed and investigated. The main factors affecting the interaction mode between the induced hydraulic fractures and pre-existing fractures were discussed. The interaction mode is mainly controlled by the confining stress, approaching angle, mechanical properties of pre-existing fractures, injection rate and fluid viscosity. The interaction mode in specimens containing a single pre-existing

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51978541, 41941018, and 51839009), China Postdoctoral Science Foundation (Grant No. 2019M662711), and State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining and Technology (Grant No. SKLGDUEK1901).

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