Stress Reversals near Hydraulically Fractured Wells Explained with Linear Superposition Method (LSM)

Ruud Weijermars,Jihoon Wang

doi:10.3390/en14113256

Ruud Weijermars, Jihoon Wang

Open Access

https://doi.org/10.3390/en14113256

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Abstract

Prior studies have noted that the principal stress orientations near the hydraulic fractures of well systems used for energy extraction may wander over time. Typically, the minimum and maximum principal stresses—in the horizontal map view—swap their respective initial directions, due to (1) fracture treatment interventions, and (2) pressure depletion resulting from production. The present analysis shows with stress trajectory visualizations, using a recently developed linear superposition method (LSM), that at least two generations of stress reversals around hydraulic fractures occur. The first generation occurs during the fracture treatment; the second occurs immediately after the onset of so-called flow-back. During each of these stress swaps in the vicinity of the hydraulic fractures, reservoir directions that were previously in compression subsequently exhibit extension, and directions previously stretching subsequently exhibit shortening. The pressure change in the hydraulic fractures—from over-pressured to under-pressured (only held open by proppant packs)—caused the neutral points that separate domains with different stress states to migrate from locations transverse to the fracture to locations beyond the fracture tips. Understanding such detailed geo-mechanical dynamics, related to the pressure evolution in energy reservoirs, is extremely important for improving both the fracture treatment and the well operation, as future hydrocarbon and geothermal energy extraction projects emerge.

Highlights

Stress shadowing refers to the occurrence of stress interference by the superposition of strain due to elastic displacements induced by the propagation of multiple hydraulic fractures in the payzone of subsurface energy reservoirs [1]
If elastic displacements pre-exist in the reservoir, due to the presence of a native tectonic stress anisotropy (Equations (3a)–(3d)), the pre-existing stress state will be overprinted by the elastic displacement field of the fracture treatment operation (Equations (4a) and (4b))
We see that the direction orthogonal to the hydraulic fractures is in compression (Figure 7b), which is a stress reversal as compared to the local stress state prior to the fracture treatment (Figure 7a)

Summary

Introduction

Stress shadowing refers to the occurrence of stress interference by the superposition of strain due to elastic displacements induced by the propagation of multiple hydraulic fractures in the payzone of subsurface energy reservoirs [1]. One of the more intriguing aspects of the stress redistribution process related to stress shadowing is that the minimum principal stress direction prior to the fracture treatment may become a maximum principal stress direction during the hydraulic fracture treatment (and vice versa). Such stress reversals have been reported by numerous authors [4,5,6,7,8,9]. Stress redistribution patterns and reversals vary greatly, depending on the native state of stress prior to fracturing, reservoir pressure waxing and waning during and after the fracture

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