The role of rock joint frictional strength in the containment of fracture propagation

The fracturing phenomenon within the reservoir environment is a complex process that is controlled by several factors and may occur either naturally or by artificial drivers. Even when deliberately induced, the fracturing behaviour is greatly influenced by the subsurface architecture and existing features. The presence of discontinuities such as joints, artificial and naturally occurring faults and interfaces between rock layers and microfractures plays an important role in the fracturing process and has been known to significantly alter the course of fracture growth. In this paper, an important property (joint friction) that governs the shear behaviour of discontinuities is considered. The applied numerical procedure entails the implementation of the discrete element method to enable a more dynamic monitoring of the fracturing process, where the joint frictional property is considered in isolation. Whereas fracture propagation is constrained by joints of low frictional resistance, in non-frictional joints, the unrestricted sliding of the joint plane increases the tendency for reinitiation and proliferation of fractures at other locations. The ability of a frictional joint to suppress fracture growth decreases as the frictional resistance increases; however, this phenomenon exacerbates the influence of other factors including in situ stresses and overburden conditions. The effect of the joint frictional property is not limited to the strength of rock formations; it also impacts on fracturing processes, which could be particularly evident in jointed rock masses or formations with prominent faults and/or discontinuities

Investigation of geomechanical responses of reservoirs induced by carbon dioxide storage

Assessment of the suitability of potential sub-surface storage sites for CO2 storage cuts across several issues, a dominant part being the sustainability in terms of the retention capacity of prospective reservoirs. Questions often raised but not properly investigated border on the stability of underground reservoirs during the injection process and the protracted effect after injection is fully completed. A review of studies on CO2 sequestration reveals several uncovered areas with one significant aspect being the geo-mechanical effect of CO2 injection and storage within the underground formation. A computational framework has been built as part of a series of ongoing investigations to ascertain the susceptibility of underground formations during and after CO2 is introduced. This is made possible by adopting a discrete element modelling methodology as a first step in the sequence of a designed procedure. By applying this technique, the formation materials are idealised as an assembly of discrete particles interacting in a manner which allows for specific descriptions of the morphology and fracturing events. Computational tests conducted on several types of models representative of reservoir formations reveal reservoir geo-mechanical responses highly dependent on factors, such as material property of rocks, pressure build-up and injection pressure. An example of this is observed in the mode of fracturing events which is significantly influenced by the rate of fluid injection. The outcome of this study forms a strong basis towards a better understanding of the behaviour of reservoir formations subjected to CO2 injection and storage. In addition, information from these studies could serve as a reference for enhanced oil recovery processes and enhanced coal bed methane productions