Shale gas fracking: A coupled analysis of fluid transport, storage and flow through fracture networks

Lead Research Organisation: Imperial College London
Department Name: Earth Science and Engineering

Abstract

Research into subsurface characterisation of sandstone and carbonate reservoirs is well developed, along with the methods of modelling fluid storage and transport within them. However some of these methods do not accurately represent the processes within Shale reservoirs.This doctoral research project aims to focus on creating an analytical model for gas flow through a shale reservoir. In Part A of the project, we aim to complete a technically rigorous model for flow through the porous matrix and planar fractures. Part B of the project aims to develop realistic functions for stress dependant permeability and transmissivity, then implement these in the model. In Part C, we aim to incorporate the flow model developed in parts A and B into a geomechanical fracture model. Specifically the multi-institutional numerical framework CSMP++.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509486/1 01/10/2016 30/09/2021
1815494 Studentship EP/N509486/1 01/10/2016 30/09/2020 Jordan Schofield
 
Description A unified semi-analytical model is presented for gas flow from shale matrix into fractures. The model is used to analyse the non-linearities associated with transport and storage of gas in shale, including effects such as surface adsorption, slip flow, and pressure-dependent permeability.
The results from the semi-analytical model are compared with previous numerical, analytical, and field data from the literature. Comparisons are also made between various published apparent permeability functions based on the different flow regimes present in shale. Various sorption models and stress-dependent porosity and permeability models are also investigated, using parameters based on published laboratory results.
The comparison between the semi-analytical model and the analytical and numerical models of Yu et al. (URTC, 2014) for flow to a single fracture show a close match. The findings of the present work show that certain affects that are accounted for in the apparent permeability functions are not highly relevant, whereas others are. Specific attention is focused on the influence of using different adsorption models (Langmuir, BET, etc.).
The use of the standard Boltzmann transformation in the semi-analytical model renders it applicable only to the "early time" regime in which interference between nearby fractures has not yet made itself felt. In this regime, production necessarily scales with the square root of time. Modifications to the model to treat the late time regime are currently being investigated.
Current industry practice when modelling shale reservoirs is to use very fine scaled numerical models, assuming dual porosity, these are computationally expensive to run, especially given the accuracy of the results. The semi-analytical model developed in this work enables users to simulate shale reservoirs more efficiently than when using fine scale discretised matrix blocks. It does this by modelling 1D flow through shale matrix and then applying that across a 2D fracture area for a given time.
Exploitation Route The semi-analytical model has two key limitations: First, the model is invalid during very early time production, when shale gas wells are flowing the fracturing fluid back to surface; Second, it does not accurately represent the flow rate decline during the "late time" flow regime, which occurs later in a shale gas well's life. Both modelling when the likely onset of late time flow may occur and the flow rates once it is reached is of great interest to industry. More accurate production modelling enables industry to better model the economics, with reduced uncertainty. Therefore, a key area of future research should include modelling late time flow analytically.
Sectors Energy