Study of hydraulic fracturing processes in shale formations with complex geological settings

Journal Article · Mon Mar 06 23:00:00 EST 2017 · Journal of Petroleum Science and Engineering

DOI:https://doi.org/10.1016/j.petrol.2017.03.011· OSTI ID:1474993

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

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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)

Hydraulic fracturing has been applied to extract gas from shale-gas reservoirs. Complicated geological settings, such as spatial variability of the rock mass properties, local heterogeneities, complex in situ stress field, and pre-existing bedding planes and faults, could make hydraulic fracturing a challenging task. In order to effectively and economically recover gas from such reservoirs, it is crucial to explore how hydraulic fracturing performs in such complex geological settings. For this purpose, numerical modelling plays an important role because such conditions cannot be reproduced by laboratory experiments. This study focuses on the analysis of the influence of confining formations and pre-existing bedding planes and faults on the hydraulically-induced propagation of a vertical fracture, which will be called injection fracture, in a shale-gas reservoir. An elastic-brittle model based on material property degradation was implemented in a 2D finite-difference scheme and used for rock elements subjected to tension and shear failure. A base case is considered, in which the ratio SR between the magnitudes of the horizontal and vertical stresses, the permeability k_c of the confining formations, the elastic modulus E_p and initial permeability k_p of the bedding plane and the initial fault permeability k_F are fixed at reasonable values. In addition, the influence of multiple bedding planes, is investigated. Changes in pore pressure and permeability due to high pressure injection lasting 2 h were analysed. Results show that in our case during the injection period the fracture reaches the confining formations and if the permeability of those layers is significantly larger than that of the shale, the pore pressure at the extended fracture tip decreases and fracture propagation becomes slower. After shut-in, the pore pressure decreases more and the fracture does not propagate any more. For bedding planes oriented perpendicular to the maximum principal stress direction and with the same elastic properties as the shale formation, results were found not to be influenced by their presence. In such a scenario, the impact of multiple bedding planes on fracture propagation is negligible. On the other hand, a bedding plane softer than the surrounding shale formation leads to a fracture propagation asymmetrical vertically with respect to the centre of the injection fracture with a more limited upward fracture propagation. A pre-existing fault leads to a decrease in fracture propagation because of fault reactivation with shear failure. This results in a smaller increase in injection fracture permeability and a slight higher injection pressure than that observed without the fault. Results of a sensitivity analysis show that fracture propagation is influenced by the stress ratio SR, the permeability k_c of the confining formations and the initial permeability k_p of the bedding plane more than the other major parameters.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE; Swedish Geological Survey (SGU); European Commission (EC)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1474993
• Alternate ID(s): OSTI ID: 1397072
• Journal Information: Journal of Petroleum Science and Engineering, Journal Name: Journal of Petroleum Science and Engineering Vol. 152; ISSN 0920-4105
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
bedding plane
elastic-brittle model
fault reactivation
fracture propagation
hydraulic fracturing stimulation
shale-gas