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.

Date (Creation): 2019-10-31

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

Point of contact

Organisation name	Individual name	Electronic mail address	Role
University of Portsmouth	Philip Benson	not available	Point of contact
University of Portsmouth	Philip Benson	not available	Principal investigator

Maintenance and update frequency: notApplicable

GEMET - INSPIRE themes, version 1.0

Geology

BGS Thesaurus of Geosciences

Hydrostatic pressure
NGDC Deposited Data
Fractured rock
Anisotropy
Shale
Toughness
Bedding planes

dataCentre

NGDC Deposited Data

Keywords

NERC_DDC

Access constraints: Other restrictions

Other constraints: no limitations

Other constraints: The dataset is made freely available for access, e.g. via the Internet. Either no third party data / information is contained in the dataset or BGS has secured written permission from the owner(s) of any third party data / information contained in the dataset to make the dataset freely accessible.

Use constraints: Other restrictions

Other constraints: 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.

Other constraints: Available under the Open Government Licence subject to the following acknowledgement accompanying the reproduced NERC materials "Contains NERC materials ©NERC [year]"

Language: English

Topic category

Geoscientific information

Begin date: 2014-10-01

End date: 2018-10-01

Reference System Information

No information provided.

Distribution format

Name	Version
PDF
Compressed zip folder
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SRM file

OnLine resource

Protocol	Linkage	Name
	https://www.bgs.ac.uk/services/ngdc/accessions/index.html#item131125	Data

Hierarchy level: Non geographic dataset

Other: non geographic dataset

Conformance result

Title: INSPIRE Implementing rules laying down technical arrangements for the interoperability and harmonisation of Geology

Date (Publication): 2011

Explanation: See the referenced specification

Pass: No

Conformance result

Title: Commission Regulation (EU) No 1089/2010 of 23 November 2010 implementing Directive 2007/2/EC of the European Parliament and of the Council as regards interoperability of spatial data sets and services

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: No

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.