Study of interaction mechanisms between multiple parallel weak planes and hydraulic fracture using the bonded-particle model based on moment tensors

https://doi.org/10.1016/j.jngse.2020.103176Get rights and content

Highlights

  • Interaction modes between hydraulic fracture and multiple parallel weak planes are reproduced.

  • Interaction mechanisms for different interaction modes are studied to reveal the driven force.

  • Hydraulic fracture arrested and crossing with offsetting behaviors are usually associated with shear slippage of weak plane.

Abstract

Outcrop and core observations show that discontinuities are well developed in unconventional reservoirs. Presence of these discontinuities can have beneficial or detrimental effect on hydraulic fracturing behaviors. Previous studies summarized several interaction modes between hydraulic fracture and a single-weak plane based on a large number of laboratory tests and theoretical analysis. In some cases, the reservoir contains a considerable number of parallel weak planes that make the fracture propagation more complex. The interaction mechanisms between hydraulic fracture and parallel weak planes remain unclear. In the present study, a bonded-particle model (BPM) coupling fluid-mechanical is used to study the driven force of those interactions. Moment tensor inversion is used to analyze the failure mechanisms of those weak planes. The results indicate that as the number of weak plane increases, the interaction mode becomes complex. Several modes (e.g. arrested, arrested with offsetting, crossing, and crossing with offsetting) are observed in the multiple parallel weak planes contained models. The interaction mode is closely related to a stress concentration around the intersection area between hydraulic fracture and weak planes. The arrested or crossing with offsetting mode is mainly caused by the shear slippage of the weak plane through moment tensor inversion analysis.

Introduction

Hydraulic fracturing has been widely used to improve the permeability of unconventional reservoirs by creating fracture networks (Mahanta et al., 2017). The unconventional reservoirs (e.g. shale, tight sandstone) are one of the most complex geo-materials in the earth with heterogeneity in terms of their composition, burial history, stratified structure and intrinsic anisotropy (Favero et al., 2016; Hsu and Nelson, 2002). Field observations show that unconventional reservoirs have common occurrence of discontinuities, e.g. in Barnett shale, USA (Fig. 1(a)) (Gasparrini et al., 2014), in the Vaca Muerta formation shale, Argentina (Fig. 1(b)) (Gale et al., 2014), in the Nikanassin formation tight-gas sandstone, Canada (Fig. 1(c)) (Ukar et al., 2019), in the Yanchang formation shale, China (Fig. 1(d)) (Li et al., 2018), in the Paleogene organic-rich shale, China (Fig. 1(e)) (Ma et al., 2016).

The interaction between hydraulic fracture and discontinuities is the key to creating complex fracture networks in those reservoirs. Previous works related to the interaction mostly focused on a single pre-existing fracture (Blanton, 1982, 1986; Daneshy, 1974; Lamont and Jessen, 1963; Warpinski and Teufel, 1984; Zhou et al., 2008). Experimental and theoretical studies showed that approaching angle, confining stress, pre-existing fracture properties controlled whether a hydraulic fracture would be arrested, diverting into, or crossing a pre-existing fracture.

The interaction mode is closely related to the mechanics of hydraulic fracture initiation and propagation. For a better understanding of the interaction mechanism, some researchers investigated the interference mechanism related to pre-existing fracture acting on hydraulic fracture. Jeffrey et al. (1987) used a displacement discontinuity technique in two-dimension to discuss the possibility of shear slippage in hydraulic fracture. The result indicated that shear slippage was controlled by far-field stress, pressure in the fracture and orientation and frictional properties of pre-existing fracture. Murphy and Fehler (1986) indicated that shear slippage along a pre-existing fracture was more easily induced than conventional tensile failure, particularly when the horizontal differential stress was large, approaching angles ranged from 30° to 60°, and low viscosity fluid was injected. Germanovich et al. (1996, 1997) indicated that the main reasons induced bifurcation and branching of hydraulic fracture were the heterogeneity of the stress field, the instability of the fracture propagation and the geometry of a pre-existing fracture. Barree and Winterfeld (1998) indicated that shear slippage more likely occurred as the vertical stress decreased. Gu and Weng (2010, 2012) concluded that shear slippage was caused by a low effective vertical stress. Zhao and Young (2011) indicated that shear slippage increased in the possibility of pre-existing fracture arrested. In addition, the hydraulic fracture can be arrested temporarily by opening and dilating the pre-existing fracture, which may result in the increasing leak-off into the pre-existing fracture. Zhang et al. (2019) indicated that under low approaching angle (30°) the pre-existing fracture failure nature changed from shear to tensile as the differential stress increased.

The published studies mainly focused on the interaction mode and failure mechanism in a single pre-existing fracture contained specimen. As the number of pre-existing fracture increases, the mechanisms (including mechanical interaction and fluid-driven fracturing) become more complex (Chang et al., 2015; Figueiredo et al., 2017; Liu et al., 2018; Wang, 2019; Yi and Feng, 2018; Zhou et al., 2015). For example, based on micro-seismic monitoring data in field, hydraulic fracture growth in unconventional reservoirs is more likely a complex propagation of multi-fracture network than the extension of a single planar fracture (Weng et al., 2011). Consequently, some researchers have studied hydraulic fracture propagation in multiple pre-existing fractures contained models (Beugelsdijk et al., 2000). Recently, Liu et al. (2017) proposed a new method to model random pre-existing fractures through presetting small cement blocks into the mold during pouring processes. In this way, the interfaces between the fresh blocks and the pre-existing small blocks can be used to simulate random pre-existing fractures. They studied the effect of pre-existing fractures around the wellbore, the dimension and volumetric density of random pre-existing fractures and the horizontal differential stress on hydraulic fracture behaviors. In some cases, the reservoirs may contain parallel pre-existing fractures (Fig. 2). Liu et al. (2014, 2016) investigated the influence of parallel and symmetrical multiple pre-existing fractures and non-planar pre-existing fractures on hydraulic fracture propagation and interaction patterns. Those two studies discussed the possibility of the interaction modes (crossing, arrested or dilated) under different confining stresses. But the interaction mechanism between the hydraulic fracture and multiple pre-existing fractures remains unclear.

To contribute to this research, a numerical research of hydraulic fracturing simulation on tight rock model is carried out, which contains parallel cemented pre-existing fractures. The term “weak plane” will be used to refer the “cemented pre-existing fracture” for the rest of this paper, as opposed to the pre-existing fracture with frictional resistance but without cohesion strength. A bonded-particle model with fluid-mechanical coupling is used to investigate mechanisms of interaction between hydraulic fracture and parallel weak planes. This method has been successfully used to study the interaction between hydraulic fracture and a single weak plane in our previous study (Zhang et al., 2019). In the paper we have discussed the effects of approaching angle and confining stress on the interaction mode between hydraulic fracture and a single weak plane, and studied failure mechanism of a single weak plane by the moment tensor inversion. The study mainly focused on micro-parameter analysis, fluid-solid algorithm and model validation. The driven-force of different interaction modes and cracking processes between hydraulic fracture and weak planes are not fully investigated. In the present study, the same method has been used to investigate effects of confining stress, inclination angle, number of weak planes on the behaviors of hydraulic fracture (e.g. crossing, arrested, offsetting, dilation, etc.). The objective of this study is to reveal the driven force of the interaction mode between hydraulic fracture and multiple weak planes, discuss the cracking processes, and to study failure mechanism of the multiple weak planes. These results could provide some qualitative and quantitative insights into the mechanisms behind the interaction.

Section snippets

Background of BPM

Since the bonded-particle model (BPM) was initially introduced by Cundall (1971) and Cundall and Strack (1979), it has been extensively used to study rock and soil behaviors, e.g. deformation and fracturing (Yang et al., 2014; Zhang and Zhang, 2018; Zhang et al., 2018b), acoustic emission (Hazzard, 2004; Hazzard and Young, 2002; Zhang et al., 2017), strength (Zhang and Wong, 2014) or progressive failure (Zhang and Zhang, 2017a).

Two major types of the BPM are available in the PFC (Particle Flow

Modeling parallel weak planes

Fig. 6 shows a model for hydraulic fracturing with a dimension of 300 mm × 300 mm, which contains about 34,000 particles. The particle radius follows a uniform distribution varying from Rmin = 0.625 mm (minimum value) to Rmax = 1.25 mm (maximum value). The micro-parameters of the parallel bond model (in Table 1) are in accordance with (Zhang et al., 2019; Zhao, 2012; Zhao and Young, 2011). Both the normal bond strength and shear bond strength have a standard deviation, which accounts for 25%

Results

When a hydraulic fracture is approaching to a weak plane, it could cause fracturing fluid loss into the weak plane, dilation of the weak plane, or even branching or alteration of the hydraulic fracture path (Gu et al., 2012; Weng, 2015). Fig. 7 shows the interaction mode between hydraulic fracture and a single-weak plane, which are commonly observed in laboratory tests and numerical studies. There are two possible outcomes of the interaction. One is slippage or arrested (Fig. 7(a)) and the

Discussions

At β = 30°, when two parallel weak planes are located at one side of the borehole, the hydraulic fracture is arrested by the outside weak plane and diverts from the end of the weak plane. However, when the 1st or 2nd weak plane connects to the 3rd or 4th weak plane, several events may occur during the time period of the hydraulic fracture propagating towards the outside weak plane (Fig. 17(a)). When hydraulic fracture propagation proceeds along the direction of the weak plane, the coverage

Conclusions

The present study investigates the interaction mechanisms between hydraulic fracture and parallel weak planes by using the BPM. Interaction modes (e.g. arrested with offsetting, crossing with offsetting, crossing with branching) observed in the simulation studies are comparable with experimental results and theoretical analysis. The corresponding mechanics for new fracture re-initiation and its interaction with parallel weak planes are revealed.

At β = 30°, in the single- and two-weak plane

CRediT authorship contribution statement

Qi Zhang: Methodology, Software, Writing - original draft, Visualization. Xiao-Ping Zhang: Conceptualization, Writing - review & editing. Pei-Qi Ji: Formal analysis. Han Zhang: Investigation. Xuhai Tang: Resources. Zhijun Wu: Resources.

Declaration of competing interest

The authors declare that they have no conflict of interest.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51978541, 51839009), China Postdoctoral Science Foundation Program, China (Grant No. 2019M662711), and State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining and Technology, China (Grant No. SKLGDUEK1901) for funding provided to this work. The authors thank Itasca consulting group and Prof. Jim Hazard for the help of the hydraulic fracturing modeling.

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