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Natural fractures in some US shales and their importance for gas production

Julia F. W. Gale and Jon Holder
Natural fractures in some US shales and their importance for gas production (in Petroleum geology; from mature basins to new frontiers; proceedings of the 7th petroleum geology conference, B. A. Vining (editor) and S. C. Pickering (editor))
Petroleum Geology Conference Series (2010) 7: 1131-1140

Abstract

Shale gas reservoirs are commonly produced using hydraulic fracture treatments. Microseismic monitoring of hydraulically induced fracture growth shows that hydraulic fractures sometimes propagate away from the present-day maximum horizontal stress direction. One likely cause is that natural opening-mode fractures, which are present in most mudrocks, act as weak planes that reactivate during hydraulic fracturing. Knowledge of the geometry and intensity of the natural fracture system and the likelihood of reactivation is therefore necessary for effective hydraulic fracture treatment design. Changing effective stress and concomitant diagenetic evolution of the host-rock controls fracture initiation and key fracture attributes such as intensity, spatial distribution, openness and strength. Thus, a linked structural-diagenesis approach is needed to predict the fracture types likely to be present, their key attributes and an assessment of whether they will impact hydraulic fracture treatments significantly. Steep (>75 degrees ), narrow (<0.05 mm), calcite-sealed fractures are described in the Barnett Shale, north-central Texas, the Woodford Formation, west Texas and the New Albany Shale in the Illinois Basin. These fractures are weak because calcite cement grows mostly over non-carbonate grains and there is no crystal bond between cement and wall rock In bending tests, samples containing natural fractures have half the tensile strength of those without and always break along the fracture plane. By contrast, samples with quartz-sealed fractures do not break along the fracture plane. The subcritical crack index of Barnett Shale is >100, indicating that the fractures are clustered. These fractures, especially where present in clusters, are likely to divert hydraulic fracture strands. Early, sealed, compacted fractures, fractures associated with deformation around concretions and sealed, bedding-parallel fractures also occur in many mudrocks but are unlikely to impact hydraulic fracture treatments significantly because they are not widely developed. There is no evidence of natural open microfractures in the samples studied.


ISSN: 2047-9921
Serial Title: Petroleum Geology Conference Series
Serial Volume: 7
Title: Natural fractures in some US shales and their importance for gas production
Title: Petroleum geology; from mature basins to new frontiers; proceedings of the 7th petroleum geology conference
Author(s): Gale, Julia F. W.Holder, Jon
Author(s): Vining, B. A.editor
Author(s): Pickering, S. C.editor
Affiliation: University of Texas, Jackson School of Geosciences, Bureau of Economic Geology, Austin, TX, United States
Affiliation: Baker Hughes, London, United Kingdom
Pages: 1131-1140
Published: 2010
Text Language: English
Publisher: The=Geological Society of London, London, United Kingdom
ISBN: 978-1-86239-298-4
Meeting name: 7th petroleum geology conference; petroleum geology; from mature basins to new frontiers
Meeting location: London, GBR, United Kingdom
Meeting date: 20090330March 30-April 2, 2009
References: 40
Accession Number: 2012-023195
Categories: Economic geology, geology of energy sources
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, geol. sketch maps
N30°52'00" - N31°30'00", W99°07'60" - W98°25'00"
Secondary Affiliation: Schlumberger, GBR, United Kingdom
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute.
Update Code: 201213
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