Identification

Title

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

Abstract

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.

Resource type

nonGeographicDataset

Resource locator

https://www.bgs.ac.uk/services/ngdc/accessions/index.html#item131125

name: Data

function: download

Unique resource identifier

code

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

codeSpace

Dataset language

eng

Spatial reference system

code identifying the spatial reference system

Classification of spatial data and services

Topic category

geoscientificInformation

Keywords

Keyword set

keyword value

originating controlled vocabulary

title

reference date

date type

publication

effective date

2008-06-01

Keyword set

keyword value

Hydrostatic pressure

Fractured rock

Anisotropy

Toughness

Shale

Bedding planes

originating controlled vocabulary

title

reference date

date type

revision

effective date

2011

Keyword set

keyword value

NERC_DDC

Geographic location

West bounding longitude

East bounding longitude

North bounding latitude

South bounding latitude

Temporal reference

Temporal extent

Begin position

2014-10-01

End position

2018-10-01

Dataset reference date

date type

creation

effective date

2019-10-31

Frequency of update

notApplicable

Quality and validity

Lineage

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.

Conformity

Conformity report

specification

title

reference date

date type

publication

effective date

2011

degree

false

explanation

See the referenced specification

Conformity report

specification

title

reference date

date type

publication

effective date

2010-12-08

degree

false

explanation

See http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2010:323:0011:0102:EN:PDF

Data format

name of format

PDF

version of format

Constraints related to access and use

Constraint set

Limitations on public access

Constraint set

Limitations on public access

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.

Responsible organisations

Responsible party

contact position

School of Earth and Environmental Sciences

organisation name

University of Portsmouth

full postal address

Burnaby Building

Portsmouth

PO1 3QL

email address

not available

responsible party role

pointOfContact

Responsible party

contact position

School of Earth and Environmental Sciences

organisation name

University of Portsmouth

full postal address

Burnaby Building

Portsmouth

PO1 3QL

email address

not available

responsible party role

principalInvestigator

Metadata on metadata

Metadata point of contact

organisation name

British Geological Survey

full postal address

Environmental Science Centre,Keyworth

NOTTINGHAM

NG12 5GG

United Kingdom

telephone number

+44 115 936 3100

email address

enquiries@bgs.ac.uk

responsible party role

pointOfContact

Metadata date

2022-09-20

Metadata language