The Effect of Fluid Content on the Mechanical Behaviour of Fractures in Chalk

Abstract

¶The paper presents an experimental study on the effects of fluid content on the mechanical behaviour of natural fractures in chalk. The aims of the study are to provide better understanding of the mechanisms of chalk-fluid interaction, in general, and to explain the behaviour of petroleum chalk reservoirs during water injection, in particular. The experiments were carried out on Lagerdorf chalk using the direct shear apparatus. Two types of fluids were used in the tests: 1) water, and 2) synthetic oil. Lagerdorf chalk is a water-wet material which will develop capillary pressures upon contact with water. Initially saturating the chalk with oil will enhance the water wettability by inducing additional capillary forces between water and the non-wetting oil. In addition to the tests on fractured chalk samples, unconfined compression and direct shear tests on intact chalk samples were performed. The results showed significant differences in the strength and deformation characteristics of intact chalk initially saturated with different fluids. Intact water-saturated chalk showed lower deformation modulus (about 50%) and lower peak (also about 50%) and residual shear strength than the oil-saturated chalk. Water injection in initially oil-saturated fractures resulted in significant normal deformation under constant effective normal stress and shear stress relaxation under fixed shear displacement. The water-induced deformation occurred almost instantaneously after only a few cm3 of water had been injected into the fracture, and further injection of water did not increase the water-induced deformation. After water injection, fractures in initially oil-saturated chalk showed significantly lower normal and shear stiffnesses and lower shear strength. The weakening in shear is attributed partly to the reduction in the basic friction angle, φb, and this reduction was verified in a series of tilt tests to measure the frictional resistance between smooth edges of core samples of chalk. The reduction in the basic friction angle implies that the interaction of chalk with water is governed not only by capillary forces, as postulated in several previous studies, but also by chemical and/or physio-chemical effects.