Modelling of Hydraulic Fracturing in Cement Bentonite Geomaterials

Man Yin Albert NG, Cambridge University
Geotechnical Engineering Group

Abstract

Hydraulic fracturing is one of the most important processes in geotechnical engineering. Despite the fact that the hydraulic fracturing process has been widely applied to a range of operations such as the measurement of in situ stress and enhanced oil recovery, its improper application can result in problems such as the weakening of foundation strata by fissuring the soil. This dissertation aims to advance our knowledge of the principles of fracture initiation and propagation in geomaterials through laboratory experiments and numerical simulations.

To simulate the occurrence of hydraulic fracturing in cement bentonite slurry trench cut-off walls caused by the cavity expansion process of a self-boring pressuremeter test and the leak-off process of a double packer test, corresponding types of laboratory tests have been performed. The results indicate that while the peak cavity pressure increases with the confining pressure in both types of tests, it increases with injection rate only in the leak-off test, highlighting the importance of the interaction of injection fluid with pore water in hydraulic fracturing.

Numerical simulations, which are of continuum analysis and discrete analysis, have been performed to model the fracturing process in cement bentonite material. In the continuum analysis, the fracture is modelled as a continuum effect after the tensile failure occurs. With the use of an in situ cavity expansion test, the undrained shear strength and the shear modulus used in the continuum analysis are shown to be more consistent with the laboratory values than those used in the conventional analysis which does not take the tensile strength of the material into account.

In the discrete analysis, a numerical model assuming discrete fractures in the material is implemented in a finite difference program FLAC. The model is based on coupling of three formulations together mechanical deformation, fluid flow and fracture mechanics. The Dugdale-Barenblatt concept invoking a microcrack zone ahead of the crack has been applied. The model is validated by comparing its rate of fracture propagation with a theoretical calculation based on energy evolution. Through a parametric study of the cavity expansion test simulation in an elastic material, normalised graphs grouping the influence of each parameter on the peak cavity pressure have been developed. The model is then extended to simulate hydraulic fracturing in the leak-off test and in a Mohr-Coulomb material. When compared with the model of continuum analysis, the model of discrete analysis shows a more rigid simulation to the hydraulic fracturing process in both the in situ test and laboratory test. The numerical model has shed light on the principles of hydraulic fracturing process in geomaterials and can be used to simulate other operations which utilise this process.

Keywords: Hydraulic fracturing, leak-off, cavity expansion, fracture mechanics, cement bentonite, discrete analysis, continuum analysis