Abstract
We investigated the impact of water weakening on the mechanical behavior of Obourg Chalk and Ciply Chalk (Mons Basin, Belgium). Different mechanical tests were conducted to estimate the 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. This extensive dataset allowed us to calculate wet-to-dry ratios (WDR), i.e., the ratio between any property for a dry sample to that for the water-saturated sample. For both chalks, we found that water has a strong weakening effect with WDR ranging from 0.4 to 0.75. Ciply Chalk exhibits more water weakening than Obourg Chalk. The highest water weakening effect was obtained for UCS, critical pressure, and Young’s modulus. Weakening effects are still present in brine-saturated samples but their magnitude depends on the fluid composition. 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.
Highlights
The mechanical properties of fluid-bearing porous rocks depend on pore fluid properties
Other mechanisms have been proposed to explain water weakening in porous rocks, we show that the decrease in surface energy and the existence of repulsive forces are likely to be the dominant mechanism in our experiments
The parameters involved in this compilation are unconfined compressive strength (UCS) and Young’s modulus derived from uniaxial compression tests, tensile strength obtained from Brazilian tests, compressive strength and Young’s modulus from triaxial tests at 1.5 MPa effective confining pressure, critical pressure derived from isotropic compression experiments, cohesion and internal friction coefficient inferred from
Summary
The mechanical properties of fluid-bearing porous rocks depend on pore fluid properties. The concept of effective stresses implies that variations of pore fluid pressure will impact most of the mechanical and physical properties of porous rocks [1]. Many experimental studies have shown that the mechanical properties of porous rocks are lowered when the pore fluid is water. Experimental studies on sandstones have shown that, like carbonate rocks, water weakening can significantly lower the mechanical strength and elastic moduli of siliciclastic rocks [6,7]. Using a protocol intended to mimic water flooding operations in reservoir to increase the production, David et al [8] have shown that when dry or oil-saturated Sherwood sandstone samples are critically loaded, a small amount of water injected under low pressure is enough to trigger mechanical instabilities leading to failure. It seems important to relate mechanical weakening to variations of the solid–fluid interface properties for different pore fluids
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