The injection of fluid into granular media can create fluid-driven fractures. Suspended solid particles in the fluid reduce fluid injectivity because of particle plugging. The objective of this work is to analyze the grain-scale mechanisms of suspended solid particles on the initiation and development of fluid-driven fractures in granular media. We use the discrete element method (DEM) coupled with computational fluid dynamics (CFD) to perform numerical simulations and determine the effect of suspended solid particles on fracturing. We model a half-domain that represents the near-cavity region. Simulation results show that higher solid particle concentration promotes fractures at lower injection rates, in agreement with expectations. The injection of suspended solid particles can generate external and internal regions of plugged particles, which dramatically reduce the permeability near the injection cavity, increases the injection pressure and promotes fractures at lower injection rates than without suspended solid particles. Results from these simulations present a physics-based basis to determine the mechanisms that explain and permit predicting the injection rate to prevent fractures during injection of fluids with suspended solid particles.

1. Introduction

The injection of fluid into a granular medium can create fluid-driven fractures (Detournay, 2004; Shin and Santamarina, 2010; Sarris and Papanastasiou, 2013; Sarris and Papanastasiou, 2015). The initiation of fractures is determined by the properties of the granular medium (e.g. permeability, stress condition, particle mechanical properties, etc.) and the fluid injection conditions (e.g. injection rate, viscosity of injected fluid, etc.). Suspended solid particles in the fluid can also promote fracture initiation during the injection process (Suarez-Rivera et al., 2002).

Suspended solid particles can clog the granular medium, which leads to an internal and an external filter cakes in wellbores. The external filter cake forms preferentially at the wellbore wall. The internal filter cake forms within the injected porous medium. The time when the external cake begins to form is referred to as the transition time (Zechner et al., 2014). The formation of filter cake depends on factors that include grain size, solid particle size and deposition velocity of solid particles (Rajgopalan and Tien 1976). The permeability reduction at filter cakes is caused by reduced pore throat size, increased surface area and increase tortuosity (Pang and Sharma, 1997). In enhanced oil recovery (EOR), re-injected fluids in reservoirs (such as formation brine) often carry suspended solid particles (Khan et al., 2017; Suri et al., 2019). The injected water usually comes from fresh water, seawater and produced water, which may contain suspended solid particles, insoluble carbonates, sulfates, iron compounds and oil droplets (van Oort et al., 1993). The suspended solid particles may be added artificially to improve reservoir development, such as adding proppant to the fracturing fluid to improve the conductivity of fractures or adding nanoparticles for conformance control (Barree and Conway, 1994). Large polymer molecules used for chemical EOR can also clog pore throats similarly to suspended solid particles. However, the presence of suspended solid particles may also be harmful to an injection project (Khatib, 1994; Roque et al., 1995; Moghadasi et al., 2002). Suspended solid particles deposited on the grains can change the polymer flow resistance when polymer is injected into the wellbore (Zechner et al., 2014). During produced water reinjection, total suspended solids and oil particles in the injected water gradually lower the permeability of the formation near the wellbore, which increases the bottom-hole injection pressure for a fixed rate (Fischer et al. 2017). The presence of filter cakes increases the injection pressure, which increases the pressure requirements of the injection equipment and reduces the injectivity accordingly. During hydraulic fracturing, suspended solid particles can trigger fracturing at a lower rate than fluid without solid particles (Suarez-Rivera et al., 2002).

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