Predicting opening mode fracture patterns from diagenesis (OpenFrac)
Lead Research Organisation:
University of Leeds
Abstract
All rocks in the subsurface are fractured to some extent. Fractures form in response to stress and chemical change and provide important fluid pathways (or storage sites), and sometimes barriers, trapping fluids such as water and mineral-rich fluids. Fracture-filling cements precipitated from these fluids play a crucial role in propping these fractures open and recording the physical-chemical conditions at which the fracture opened and grew.
As societies and economies gear up to achieve Net Zero, we urgently need to expand subsurface exploration and production. Therefore, fractured, shallow crustal rocks are expected to play a central part in the Energy Transition. To develop and manage geological resources in fractured rocks safely and cost-effectively, we must be able to predict the pattern of fractures at depth since they fundamentally control fluid flow. Despite a rigorous understanding of rock behaviour under stress, and how fractures conduct fluids, our ability to quantitatively predict subsurface fracture patterns remains poor. This is because fracture prediction has been overwhelmingly dominated by mechanical and geometrical models that are based on the central concept of a critical stress or rock strength measured in ‘fast’ laboratory tests for rock failing in tension or shear. However, we know that, in nature and in the laboratory, rocks fail in the presence of chemically active fluids at stresses significantly below these critical levels; thus, our knowledge base necessary to model subsurface fracture networks remains incomplete. To address the challenge of fracture prediction for the Energy Transition, this proposal seeks to couple the mechanical and geometrical evolution of fractures with the chemical evolution of the fluids contained within fractures.
OpenFrac will develop a new fundamental model to quantify the evolution of opening mode fractures – such as economically important joints and veins – through the diagenetic evolution of sedimentary rocks. We will measure the growth of single sub-critical fractures in sandstone and limestone as a function of the chemical composition of pore water, temperature, and pressure. Our new understanding of the physico-chemical processes of sub-critical fracture in rocks will provide parameters for numerical models designed to explore the evolution of more complex patterns of multiple fractures in space and time.
We will test and apply our predictive model of fracture pattern development in two case studies of direct relevance to the Energy Transition. Firstly, we will compare model predictions against fracture patterns mapped at different scales in selected sandstones around West Yorkshire (UK). These rocks comprise the target reservoirs for a pilot geothermal study underway at the University of Leeds campus. Eight boreholes on University of Leeds campus have been drilled, cored, and instrumented to provide data on the subsurface behaviour of these rocks. Our second case study is the limestones of the Peak District in Derbyshire (UK). This formation underlies vast areas of current UK heat demand and has the potential to deliver sustainable and secure geothermal energy. This project will generate an improved fundamental understanding of how fracture patterns are coupled to diagenesis, with predictions of fracture distributions in the subsurface underpinned by a physico-chemical model. We will deliver our results through open source code and open access data, readily extensible by other researchers working on different rocks, in partnership with our Stakeholder Advisory Board to deliver impact for the clean Energy Transition.
As societies and economies gear up to achieve Net Zero, we urgently need to expand subsurface exploration and production. Therefore, fractured, shallow crustal rocks are expected to play a central part in the Energy Transition. To develop and manage geological resources in fractured rocks safely and cost-effectively, we must be able to predict the pattern of fractures at depth since they fundamentally control fluid flow. Despite a rigorous understanding of rock behaviour under stress, and how fractures conduct fluids, our ability to quantitatively predict subsurface fracture patterns remains poor. This is because fracture prediction has been overwhelmingly dominated by mechanical and geometrical models that are based on the central concept of a critical stress or rock strength measured in ‘fast’ laboratory tests for rock failing in tension or shear. However, we know that, in nature and in the laboratory, rocks fail in the presence of chemically active fluids at stresses significantly below these critical levels; thus, our knowledge base necessary to model subsurface fracture networks remains incomplete. To address the challenge of fracture prediction for the Energy Transition, this proposal seeks to couple the mechanical and geometrical evolution of fractures with the chemical evolution of the fluids contained within fractures.
OpenFrac will develop a new fundamental model to quantify the evolution of opening mode fractures – such as economically important joints and veins – through the diagenetic evolution of sedimentary rocks. We will measure the growth of single sub-critical fractures in sandstone and limestone as a function of the chemical composition of pore water, temperature, and pressure. Our new understanding of the physico-chemical processes of sub-critical fracture in rocks will provide parameters for numerical models designed to explore the evolution of more complex patterns of multiple fractures in space and time.
We will test and apply our predictive model of fracture pattern development in two case studies of direct relevance to the Energy Transition. Firstly, we will compare model predictions against fracture patterns mapped at different scales in selected sandstones around West Yorkshire (UK). These rocks comprise the target reservoirs for a pilot geothermal study underway at the University of Leeds campus. Eight boreholes on University of Leeds campus have been drilled, cored, and instrumented to provide data on the subsurface behaviour of these rocks. Our second case study is the limestones of the Peak District in Derbyshire (UK). This formation underlies vast areas of current UK heat demand and has the potential to deliver sustainable and secure geothermal energy. This project will generate an improved fundamental understanding of how fracture patterns are coupled to diagenesis, with predictions of fracture distributions in the subsurface underpinned by a physico-chemical model. We will deliver our results through open source code and open access data, readily extensible by other researchers working on different rocks, in partnership with our Stakeholder Advisory Board to deliver impact for the clean Energy Transition.