Fluid-driven tensile fracture and fracture toughness in Nash Point shale at elevated pressure (NERC grant NE/L009110/1)

A number of processes, both natural and anthropogenic, involve the fracture of rocks subjected to tensile stress, including vein growth and mineralization, and the extraction of hydrocarbons through hydraulic fracturing. In each case, the fundamental material property of mode-I fracture toughness must be overcome in order for a tensile fracture to nucleate and propagate. Whilst measuring this parameter is straightforward at ambient pressure, estimating the fracture toughness of rocks at depth, where they experience a confining pressure, is technically challenging. Here, we report a new analysis combining results from standard thick-walled cylinder burst tests with quantitative acoustic emission to estimate the mode-I fracture toughness (KIc) of Nash Point Shale at elevated confining pressure, simulating in-situ conditions to approximately 1km. In the most favorable orientation, the pressure required to fracture the rock shell (injection pressure, Pinj) increases from 6.1 MPa at 2.2 MPa confining pressure (PC), to 34 MPa at 20 MPa confining pressure. When fractures cross the shale bedding, the required injection pressures are 30.3 MPa (at Pc = 4.5 MPa) and 58 MPa (at Pc = 20 MPa), respectively. Applying the model of Abou-Sayed (1978) to estimate initial flaw size, we calculate that this pressure increase equates to an increase in KIc from 0.6 MPa.m1/2 at 3.2 MPa differential pressure (Pinj - PC) to 4.1 MPa.m1/2 at 22 MPa differential pressure. We conclude that the increasing pressure due to depth in the Earth will have a significant influence on fracture toughness, which is also a function of the inherent anisotropy.
 
Citation proposal
. Fluid-driven tensile fracture and fracture toughness in Nash Point shale at elevated pressure (NERC grant NE/L009110/1). https://metadata.bgs.ac.uk/geonetwork/srv/api/records/974cea20-0190-5541-e054-002128a47908

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Date ( Creation )
2019-10-31
Identifier
http://data.bgs.ac.uk/id/dataHolding/13607555

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notApplicable notApplicable
GEMET - INSPIRE themes
  • Geology
BGS Thesaurus of Geosciences
  • Anisotropy , Toughness , Hydrostatic pressure , Shale , Fractured rock , Bedding planes
Keywords
  • NERC_DDC
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otherRestrictions Other restrictions
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The copyright of materials derived from the British Geological Survey's work is vested in the Natural Environment Research Council [NERC]. No part of this work may be reproduced or transmitted in any form or by any means, or stored in a retrieval system of any nature, without the prior permission of the copyright holder, via the BGS Intellectual Property Rights Manager. Use by customers of information provided by the BGS, is at the customer's own risk. In view of the disparate sources of information at BGS's disposal, including such material donated to BGS, that BGS accepts in good faith as being accurate, the Natural Environment Research Council (NERC) gives no warranty, expressed or implied, as to the quality or accuracy of the information supplied, or to the information's suitability for any use. NERC/BGS accepts no liability whatever in respect of loss, damage, injury or other occurence however caused.
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Available under the Open Government Licence subject to the following acknowledgement accompanying the reproduced NERC materials "Contains NERC materials ©NERC [year]"
Metadata language
English English
Topic category
  • Geoscientific information

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Hierarchy level
nonGeographicDataset Non geographic dataset
Other
non geographic dataset

Conformance result

Date ( Publication )
2011
Explanation
See the referenced specification
Pass
false

Conformance result

Date ( Publication )
2010-12-08
Explanation
See http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2010:323:0011:0102:EN:PDF
Pass
false
Statement
Confining pressure is first applied to a pre-determined level and allowed to equilibrate. Water is then injected at a constant flow rate of 1mL/min into the sealed-off section of the borehole. After approximately 150s, the injection pressure in the borehole starts to increase quasi-linearly. An axial stress is applied to the sample and is set, via servo control, to always slightly exceed the confining pressure or the borehole pressure, whichever is the greatest. This ensures effective sealing of the steel guides throughout the duration of the experiment. Injection pressure increases until it reaches the breakdown pressure, where a hydraulic fracture is initiated at the borehole wall and propagates through the sample thickness.

gmd:MD_Metadata

File identifier
974cea20-0190-5541-e054-002128a47908   XML
Metadata language
English English
Hierarchy level
nonGeographicDataset Non geographic dataset
Hierarchy level name
non geographic dataset
Date stamp
2021-07-30
Metadata standard version
2.3

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http://data.bgs.ac.uk/id/dataHolding/13607555
 
 

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