Image Based Characterisation of Structural Heterogeneity within Clastic Reservoir Analogues

Abstract

The presence of subseismic scale faulting within high porosity sandstone reservoirs andaquifers represents a significant source of uncertainty for activities such as hydrocarbonproduction and the geologic sequestration of carbon dioxide. The inability to resolvegeometrical properties of these smaller scale faults, such as size, connectivity and intensity,using conventional subsurface datasets (i.e. seismic reflection tomography, wireline log andcore), leads to ambiguous representations within reservoir models and simulators. Inaddition, more fundamental questions still remain over the role of cataclastic faults in thetrapping and transfer of mobile geofluids within the subsurface, particularly when two ormore immiscible fluid phases are present, as is the case during hydrocarbon accumulation,waterflood operations and CO2 injection.By harnessing recent developments in 3D digital surface and volume imaging, this studyaddresses uncertainties pertaining to the geometrical and petrophysical properties ofsubseismic scale faults within porous sandstone reservoirs. A novel structural featureextraction and modelling framework is developed, which facilitates the restoration of faultand fracture architecture from digital rock surface models. This framework has been used toderive volumetric fault abundance and connectivity from a normal sense array of cataclasticshear bands developed within high porosity sandstones of the Vale of Eden Basin, UK.These spatially resolved measures of discontinuity abundance provide the basis for thegeostatistical extrapolation of fracture/fault intensity into reservoir modelling grids, whichpromises the introduction of a much higher degree of geological realism into discretefracture network models than can currently be achieved through purely stochastic methods.Moreover, by establishing spatial correspondences between volumetric faulting intensityand larger scale features of deformation observed at the study area (cataclastic shear zones),the work demonstrates the potential to relate reservoir equivalent measures of fault orfracture abundance obtained from outcrop to seismically resolvable structures within thesubsurface, aiding the prediction of reservoir structure from oilfield datasets.In addition to the derivation of continuum scale properties of sub-seismic scale faultnetworks, a further investigation into the pore-scale controls which govern the transfer offluids within cataclised sandstones has been conducted. Through X-ray tomographicimaging of experimental core flood (scCO2-brine primary drainage) through a cataclasticshear band bearing sandstone, insights into the influence that variations in fault structureexert over the intra-fault drainage pathway of an invading non-wetting fluid have beengained. Drainage across the fault occurs as a highly non-uniform and non-linear process,which calls into question the practice of using continuum methods to model cross faultflow. This work has also provided an improved understanding of the role that high capillaryentry pressure cataclised regions play in modifying pore-fluid displacement processeswithin the surrounding matrix continuum. In particular, the high sweep efficiency andenhanced non-wetting phase pore-wall contact relating to elevated phase pressure observedduring drainage points towards favourable conditions for wettability alteration withincataclised sandstones. This is likely to negatively impact upon the effectiveness of oilrecovery and CO2 sequestration operations within equivalent reservoir and aquifer settings.

Date of Award	17 Nov 2015
Original language	English
Awarding Institution	The University of Manchester
Supervisor	David Hodgetts (Main Supervisor) & Jonathan Redfern (Co Supervisor)