Title: The effects of nearby fractures on hydraulically induced fracture propagation and permeability changes

Abstract

Fracture propagation caused by hydraulic fracturing operations can be significantly influenced by adjacent fractures. In this paper, we present a detailed coupled hydro-mechanical analysis to study the effects of nearby natural fractures on hydraulically induced fracture propagation and changes in fracture permeability. Two rock domains were considered in comparison: FD1, with one fracture, and FD2, with two adjacent parallel or non-parallel fractures. It was assumed that water injection occurred in a borehole that intersected the single fracture in FD1 and one of the two fractures in FD2. Simulations were made for a time period of 3 h with an injection period of 2 h followed by 1 h of shut-in. An elastic-brittle model based on the degradation of material properties was implemented in a 2D finite-difference scheme and used for elements of the intact rock subjected to tension and shear failure. The intact rock was considered to have a low but non-negligible permeability. A verification study against analytical solutions showed that the fracture propagation and stress concentrations due to differential boundary stresses could be accurately represented by our model. Next, a base case was considered, in which the stress ratio (SR) between the magnitudes of the maximum and minimum boundarymore » stresses, the permeability k_R of the intact rock and the initial permeability k_TF of the tension failure regions were fixed. In FD2, the distance d_F between the two natural fractures defined by the closest distance was also fixed. The results showed that in both rock domains, the fracture started to propagate when the pore pressure was approximately 85% of the magnitude of the minimum boundary stress. The propagation of a single fracture was significantly greater than the propagation of a double fracture system because, in the latter case, the pore pressure decreased when the two fractures connected. As a result, changes in permeability in FD2 were smaller than in FD1. At the end of injection, the maximum ratios between the final and initial permeability of the natural fractures were found to be approximately 3 and 2 for rock domains FD1 and FD2, respectively. For non-parallel fractures, the controlling factor for fracture propagation was the separation between the tips of the pressurised fracture and the neighbouring non-pressurised fracture. A sensitivity study was conducted to study the influence of the key parameters d_F, SR, k_R and k_TF on the simulation results. Lastly, fracture propagation showed more sensitivity to dF and SR than to the other parameters.« less

Authors:

Figueiredo, Bruno ^[1]; Tsang, Chin-Fu ^[2]; Rutqvist, Jonny ^[3]; Niemi, Auli ^[1]

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+ Show Author Affiliations

1. Uppsala University (Sweden)
2. Uppsala University (Sweden); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
3. Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)

Publication Date:

Sat Aug 12 00:00:00 EDT 2017

Research Org.:

Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); Swedish Geological Survey (SGU); European Union (EU)

OSTI Identifier:

1476553

Alternate Identifier(s):

OSTI ID: 1549867

Grant/Contract Number:  

AC02-05CH11231; 1724; 640979

Resource Type:

Accepted Manuscript

Journal Name:

Engineering Geology

Additional Journal Information:

Journal Volume: 228; Journal Issue: C; Related Information: © 2017 Elsevier B.V.; Journal ID: ISSN 0013-7952

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

58 GEOSCIENCES; hydraulic fracturing stimulation; coupled hydro-mechanical effects; fracture propagation and connectivity; permeability changes; elastic-brittle model