Water weakening and fluid rock interactions in Chalks from the Mons Basin
Christian DAVID & Davide GEREMIA,
CY Cergy Paris Université, Cergy-Pontoise, France
Water-weakening plays an important role in several fields like oil production through secondary and tertiary recovery operations, Enhanced Geothermal Systems (EGS), as well as in the mechanical stability of underground quarries. The mechanical weakening and deformation induced by water was investigated on two microporous carbonate rock from the Mons basin, the Obourg Chalk and the Ciply chalk, through conventional triaxial tests and low-pressure injection tests. The injection tests were conducted by waterflooding critically loaded rock samples, initially in dry condition, in a way to minimize the variations in the effective pressures. During the tests, the samples were instrumented with P-wave piezoelectric transducers to provide active ultrasonic monitoring while injecting. The results show a significant reduction in the mechanical strength of the chalks. The injection tests, moreover, revealed that the amount of water injected before triggering mechanical instability decreases exponentially with respect to the applied differential stress on the rock sample.
We also investigated the impact of water weakening on different mechanical properties: unconfined compressive strength (UCS), tensile strength, Young’s modulus, mechanical strength under triaxial loading, critical pressure, fracture toughness, cohesion, and internal friction coefficient on samples either dry or saturated with water or brine. We found that water has a strong effect on Ciply Chalk being more prone to water weakening than Obourg Chalk. The mechanical data were correlated to variations in surface energy derived from three different methods: fracture mechanics, contact angle goniometry, and atomic force microscopy. Water weakening in the tested chalks can be explained by a clear reduction in surface energy and by the existence of repulsive forces which lower the cohesion.
Following stability problems in the underground “La Malogne” quarry in southern Belgium, we conducted cyclic imbibition tests on Ciply chalk (the material mined in the quarry), to test the effect of the water table oscillation on the mechanical strength. The results showed that the saturation-desaturation cycles do not induce additional damage. Rather, the mechanical behaviour is similar to that of the rock that is always water-saturated. An explanation for the observed increase in damage may be a combination of partial water saturation and environmental conditions leading to weathering.