An Integrated Geomechanical Evaluation of Cap and Fault-Seal for Risk Petroleum Trap Integrity using Distinct Element and Boundary Element Numerical Methods
Doctor of Philosophy, Engineering Degree
University of Adelaide
This thesis comprises nine published papers on an integrated geomechanical evaluation of cap and fault-seal for risking petroleum trap integrity using distinct element and boundary element numerical methods. Paper 1 provides back-ground information and an introduction to the body of research presented in this thesis. In some parts of the Penola Trough, South Australia, the seal lithotype is fractured providing structural permeability and thereby compromising seal competency. This work inferred that existing geomechanical techniques, which only considered stresses on the fault plane, had limited application in the prediction of fracture generation within the country rock away from the well-bore. It also suggested that computational stress modelling techniques may provide a useful tool in this area and similar tectonic provinces.
An important stage of the modelling workflow is analysing the sensitivity of the numerical models to various input parameters. Papers 2 and 3 show that the models are particularly sensitive to fault parameters such as friction angle ( ) and cohesion (C). However, fault rock properties are not well understood in petroleum exploration due to depths of investigation and the expense of acquiring core samples.
This thesis develops a new technique, using widely available dipmeter data for the entire borehole. In this, rotations in borehole breakouts caused by discontinuities, in the vertical sense, are used to give qualitative indications of fault rock behaviour (Paper 3). These observations were used to make decisions about fault rock input parameters into the numerical stress models. Paper 8 showed the results of varying fault rock stiffness moduli and fault zone width on the predicted stress within the surrounding rock mass. Where the prevailing stress conditions border between stress regimes, a new and unconventional technique whereby is used to increase confidence in understanding the stress regimes active at a particular depth and/or location (Paper 7). A comparative study of a single fault using three different methods of stress modelling, the distinct / discrete element (DEM), boundary element (BEM) and finite difference (FDM) methods (Paper 7) showed that the DEM underestimated the stress perturbation relative to the other models. Therefore where a clear variation is shown by DEM, there is increased confidence that it does exist and will be enhanced using other codes. Where there is the requirement to model multiple faults, DEM is preferred as the other methods trialled either have restrictions to the number of faults incorporated into the models (FDM) or does not account for full fault interaction and possible moment rotations between fault blocks, such as in the case of BEM.
The application of computational stress modelling offers a new workflow to fully integrate stress studies, cap-seal analysis, fault-seal analysis and structural interpretation to improve the understanding of hydrocarbon leakage risk at the prospect and play scales and was illustrated by way of multiple case study examples (Papers 4, 5, 6, 7 & 8).
The research presented in this thesis has development new concepts and additional workflows which add to the ‘tool-box’ that may be used by those researchers and consultants working in the field of petroleum geomechanics (Paper 9).