Developing Upscaling Approach for Swarming Hydraulic Fractures Observed at Hydraulic Fracturing Test Site through Multiscale Simulations

Abstract

Summary This work aims to address a challenge posed by recent observations of tightly spaced hydraulic fractures in core samples from the hydraulic fracturing test site (HFTS) in the Middle Wolfcamp Formation. Many fractures in retrieved cores have subfoot spacing, which is at odds with conventional models in which usually one hydraulic fracture is initiated per cluster. Models assuming a single fracture at each cluster, although a common practice, often predict excessive fracture propagation that is inconsistent with microseismic observation. Here, we aim to develop a numerical approach to effectively account for densely spaced hydraulic fractures in field-scale simulations. Because it is impractical to explicitly model all aforementioned fractures, we develop a new upscaling law that enables existing simulation tools to predict reservoir response to fracture swarms. The upscaling law is derived based on an energy equivalence argument and validated through multiscale simulations using a high-fidelity code, GEOS. The swarming fractures are first modeled with a spacing that is much smaller than the cluster spacing; these fractures are then approximated by an upscaled, single fracture based on the proposed upscaling law. The upscaled fracture is shown to successfully match the energy input rate and produce the total fracture aperture and average propagation length of the explicitly simulated swarm. Afterward, the upscaling approach is further implemented in 3D field-scale simulations and validated against the HFTS microseismic data of a horizontal well. Our results show that hydraulic fracture swarming can significantly affect fracture propagation behaviors compared with the propagation of single fractures as assumed by conventional modeling approaches. Under the considered situations, the conventional treatment yields fast propagation speed that far exceeds that indicated by the microseismic data. We also illustrate that this discrepancy can be reduced readily through the implementation of the upscaling law. Our results demonstrate the importance of accounting for the fracture swarming effect in field-scale simulations and the efficacy of this approach to enable realistic predictions of reservoir responses to fracture swarms, without the need to model tightly spaced fractures individually.

An Integrated Multiscale Modeling Framework for Unconventional Stimulation and Production (Final Report)

“…Recognizing that fracture swarms may be the key mechanism required to explain the geophysical observations at HFTS, we created a new and highly efficient method to model fracture swarms in GEOS. A detailed description of the approach was published in the journal article Fu et al (2021).…”

“…Instead, we developed an approach to treat fractures as upscaled swarms by modifying the viscosity of the fracturing fluid and toughness of the surrounding rock. Following the arguments we published in Fu et al (2021), for a swarm consisting of N tightly spaced, parallel hydraulic fractures that take equal amounts of fluid, the effective viscosity, µ*, will be:…”

“…It has been observed that multiple parallel, closely spaced hydraulic fractures are generated from single perforation clusters (e.g. Gale et al, 2018;Fu et al, 2021). The upscaling approach by Fu et al (2020) is applied to form exactly one upscaled fracture for each cluster.…”

“…We only considered the "optimistic" extreme scenario. It entails a plane-strain static finite element model preconditioned to an in situ Shmin profile calculated for the HFTS site based on ISIP (instantaneous shut-in pressure), DFIT (diagnostic fracture injection test), and sonic well logs (Fu et al, 2021). A main fracture, extending 90 m in the vertical direction, is placed in the model, and subjected to a hydrostatic pressure that dilates the fracture to a stress intensity factor SIF = 2.0 MPa• m0.5 at the top tip in the absence of any child fractures.…”

Understanding Unconventional Reservoir Fracturing: A Transdisciplinary Methodological Approach

“…"In field-scale simulations and the efficacy of this approach to enable realistic predictions of reservoir responses to fracture swarms, without the need to model tightly spaced fractures individually." 23 A swarm frac model can be simulated similarly to the main fracture associated with a fracture system, which consumes more energy than the main fracture alone.…”