Impact of chemical alteration on the poromechanical properties of carbonate rocks

Abstract

A B S T R A C TThe technical and economic success of a CO 2 geological storage project requires the preservation of the site injectivity and integrity properties over its lifetime. Unlike conventional hydrocarbon gas injection, CO 2 injection may imply geochemical reactions between acidified pore fluids and target reservoir formations, leading to modifications of their poromechanical properties. To date, the chemical effects on the host rock mechanical behaviour are not satisfactorily taken into account in sitescale numerical models of CO 2 injection, mainly due to a lack of quantitative data. The present experimental work aims at characterizing the evolution of carbonate poromechanical properties induced by acid alteration. Unlike standard experimental approaches, the implemented alteration method induces a homogeneous dissolution pattern, which ensures reliable poromechanical measurements on altered samples. These well-controlled alteration conditions allow a proper interpretation of the test results through the macroscopic continuous approach of poromechanics. Petrophysical, geomechanical, and petroacoustic properties of outcrop carbonate samples have been measured for different levels of alteration to mimic long-term exposure to reactive brine. The obtained experimental data show clear trends of chemically induced mechanical weakening. Nuclear magnetic resonance measurements and microscanner imaging performed before and after alteration have provided complementary insights into the alteration effects at the microscopic scale.

Consistent experimental investigation of the applicability of Biot-Gassmann’s equation in carbonates

“…1.5 MPa); the density and viscosity values correspond to standard fluid properties at 20°C. We have shown that this set of fluids covers a sufficiently large range of bulk modulus to allow a consistent checking of Biot-Gassmann's equation for an outcrop limestone (Bemer et al, 2016 Carbonate heterogeneity is known to induce scattering effects usually referred to as "path dispersion" (Cadoret et al, 1995). Contrary to the first break picking which may measure the travel time corresponding to fastest paths taken by part of the acoustic energy and thus lead to overestimated acoustic velocities, the computation of the phase velocity minimizes the effect of heterogeneity.…”

“…Contrary to the first break picking which may measure the travel time corresponding to fastest paths taken by part of the acoustic energy and thus lead to overestimated acoustic velocities, the computation of the phase velocity minimizes the effect of heterogeneity. The measured phase velocities are then more representative of the sample macroscopic behavior (Cadoret et al, 1995;Rasolofosaon and Zinszner, 2012;Bemer et al, 2016). (Adelinet et al, 2018).…”

“…The inverse analysis of ‫ܭ‬ ௗ and ‫ܭ‬ ௦ has been performed using the least squares method and Excel solver. The errors on the computed drained bulk modulus and solid matrix bulk modulus are estimated at around respectively 9% and 20% (Bemer et al, 2016). Table 6 gives the drained bulk modulus, shear modulus and solid matrix bulk modulus determined for each sample from the phase and first break velocity measurements.…”

“…and some key groundwater resources (Green et al, 2014). They are moreover considered as potential hosts for CO 2 storage (Brosse et Bemer et al, 2016). Carbonate rocks are characterized by multiscale heterogeneities which control their flow and acoustic responses.…”

Quantifying Rock Weakening Due to Decreasing Calcite Mineral Content by Numerical Simulations

“…Mineral dissolution and precipitation are micro-scale processes, which may significantly change the mineralogical composition and microstructure of rocks, and consequently affect the effective mechanical rock behaviour at the macro scale. Predicting these changes has a wide practical importance in applied sciences and materials engineering, especially for hydraulically conductive materials where reactive transport takes place ( Figure 1 ): hydrothermal alterations may weaken geological fractures [ 1 ] and fault zones up to their reactivation [ 2 ], chemical degradation of cements may influence the integrity of wells [ 3 , 4 ] and mineral dissolution can lead to significant weakening of geological reservoirs [ 5 , 6 ]. Hence, the quantification of this direct chemical–mechanical interaction is of substantial relevance within the context of risk assessment for most applications related to geological subsurface utilisation such as geothermal energy systems [ 7 , 8 , 9 ], enhanced oil recovery [ 10 , 11 ], radioactive waste disposal [ 12 , 13 ], underground coal gasification [ 14 , 15 , 16 ] and CO 2 or geological energy storage [ 17 , 18 , 19 ].…”