The effect of fluid content on the mechanical behaviour of fractures in chalk

Gutierrez, M Oino, LE Hoeg, K

M. Gutierrez et al., The effect of fluid content on the mechanical behaviour of fractures in chalk, ROCK MECH R, 33(2), 2000, pp. 93-117

33

INGLESE

Article

Geological Petroleum & Minig Engineering

ROCK MECHANICS AND ROCK ENGINEERING

0723-2632 → ACNP

33

2

2000

93 - 117

ISI

0723-2632(200004/06)33:2<93:TEOFCO>2.0.ZU;2-D

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 st udy are to provide better understanding of the mechanisms of chalk-fluid in teraction, in general, and to explain the behaviour of petroleum chalk rese rvoirs during water injection, in particular. The experiments were carried out on Lagerdorf chalk using the direct shear apparatus. Two types of fluid s were used in the tests: 1) water, and 2) synthetic oil. Lagerdorf chalk i s 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 n on-wetting oil. In addition to the tests on fractured chalk samples, unconf ined compression and direct shear tests on intact chalk samples were perfor med. The results showed significant differences in the strength and deforma tion characteristics of intact chalk initially saturated with different flu ids. Intact water-saturated chalk showed lower deformation modulus (about 5 0%) and lower peak (also about 50%) and residual shear strength than the oi l-saturated chalk. Water injection in initially oil-saturated fractures res ulted in significant normal deformation under constant effective normal str ess and shear stress relaxation under fixed shear displacement. The water-i nduced deformation occurred almost instantaneously after only a few cm(3) o f water had been injected into the fracture, and further injection of water did not increase the water-induced deformation. After water injection, fra ctures in initially oil-saturated chalk showed significantly lower normal a nd shear stiffnesses and lower shear strength. The weakening in shear is at tributed partly to the reduction in the basic friction angle, phi(b), and t his reduction was verified in a series of tilt tests to measure the frictio nal resistance between smooth edges of core samples of chalk. The reduction in the basic friction angle implies that the interaction of chalk with wat er is governed not only by capillary forces, as postulated in several previ ous studies, but also by chemical and/or physio-chemical effects.