Abstract
Natural fractures are a ubiquitous feature of unconventional reservoirs as evident from well logs, core studies, and micro-seismic interpretation. Hydraulic fracture (HF) generally intersects natural fractures (NF) leading to relatively complex geometry of the stimulated volume and the complications in proppant transport and deposition. In this paper, we simulate hydraulic fracturing in the presence of natural fractures in 3D and investigate key mechanisms in successfully stimulating and propping naturally fractured reservoirs. To our knowledge this if the first time the problem has been treated in 3D while considering HF/NF mechanical interaction. Several stimulations are considered using the state-of-the-art simulator “GeoFrac-3D” that can consider irregular fracture geometries and non-orthogonal intersection between the HF and NF, thereby realistic flow and proppant transport pathways and deposition sites. The “GeoFrac-3D” is based on the combination the displacement discontinuity method for the rock deformation, and the finite element method for the fracture fluid and proppant transport simulation. The deformation of the natural fractures is implemented using a linear joint model. The proppant transport and deposition within the fractures is modeled by treating the mixture of fluid and proppant particles as slurry. Example simulations are presented to explore the effective stimulation of fractured reservoirs using 100 mesh proppant. When the proppant can enter secondary fractures without extensive settling in the main HF, the propped surface area is maximized. Proppant settling velocities and thus proppant distribution is affected by fluid velocity, micro-proppant size, fluid rheology, fracture aperture, hydraulic and natural fracture interaction and near wellbore tortuosity.
Introduction
In hydraulic fracturing of unconventional reservoirs, the propagating hydraulic fracture (HF) generally intersects natural fractures (NF) complicating the geometry of the stimulated volume and the estimation of proppant flow and transport. The effectiveness of a hydraulic fracturing job depends on the resultant flow area and proppant pack permeability of the fracture system; therefore, a good understanding of the proppant transport and deposition is an essential component of hydraulic fracturing design. Several experimental studies (Sahai et al., 2014; Tong and Mohanty, 2016) and numerical studies (Weng et al., 2011; Tang et al., 2015; Han et al., 2016; Izadi et al., 2017) have been presented for the proppant transport and deposition in hydraulic and natural fractures (HF-NF) networks. These studies assume a stationary fracture network or a pre-defined propagation path. Recently, Kumar et al. (2019) presented a numerical study of the proppant transport and deposition in the HF-NF network and explored potential benefits of using of micro-proppant in the conductive fracture networks and demonstrated that due to induced stress shadowing effect near the intersections of the HF and NF's, the fracture openings are reduced which creates “choke or bottleneck points” as a resultant the bigger size proppants are prevented to enter into the natural fractures. In this paper, we have extended our earlier work to account for the potential propagation of the natural fracture wings and the impacts of NF's propagation on the proppant transport in the HF-NF networks. The objective is to explore and clarify the potential mechanisms involved in the successful stimulation of naturally fractured reservoirs with proppant deposition. We use the Eulerian-Eulerian approach to simulate proppant transport and deposition using a fully coupled 3D hydraulic fracture network model. We use “GeoFrac-3D” which can consider irregular fracture geometries and non-orthogonal intersection between the HF and NF, thereby capturing realistic flow and proppant transport pathways and deposition sites. A brief discussion of the mathematical formulations and numerical implementation are presented first, followed by several examples to illustrate some important phenomena in the proppant transport in the HF-NF networks in unconventional reservoir stimulation.
