CHEMICAL ROCK/FLUID INTERACTION: STATIC EXPOSURE OF CARBONATES TO H₂S AND CO₂ UNDER RESERVOIR CONDITIONS

If CO₂ or acid gas (H₂S/CO₂) is injected into a geological formation, the injected fluid is likely to undergo chemical reactions with the reservoir rock/fluid system. Chemical interaction between the rock matrix and the injected fluid will change the basic physical properties of the rock, such as porosity, permeability, but also its mechanical strength, leading to a strong coupling between fluid dynamics, rock mechanics and chemical interactions. To aid the interpretation of complex core flood experiments and hence a proper de-risking of injection projects, the separate investigation of chemical rock fluid interaction under no-flow conditions is required. – This can be done by static exposure of rock samples to the injected fluids under reservoir conditions. In the present study we investigate the chemical interaction between partially brine-saturated limestone and dolostone with H₂S, CO₂ and a mixture of both in the super-critical phase under static exposure conditions. The samples were exposed in gold-tube reactors at 144°C and 250 bar. The chemical and isotopic composition of the acid gas phase was measured before and after the experiment. Mineralogy and rock structure were investigated by environmental SEM measurements and micro-CT scanning respectively. The results provide strong evidence that besides aqueous chemistry, the direct interaction between super-critical H₂S and the dry rock matrix plays a major role, and may even be the dominating mechanism in certain cases.