Micro- and Macroscale Consequences of Interactions between CO2 and Shale Rocks

Abstract

In carbon storage activities, and in shale oil and gas extraction (SOGE) with carbon dioxide (CO2) as stimulation fluid, CO2 comes into contact with shale rock and its pore fluid. As a reactive fluid, the injected CO2 displays a large potential to modify the shale’s chemical, physical, and mechanical properties, which need to be well studied and documented. The state of the art on shale–CO2 interactions published in several review articles does not exhaust all aspects of these interactions, such as changes in the mechanical, petrophysical, or petrochemical properties of shales. This review paper presents a characterization of shale rocks and reviews their possible interaction mechanisms with different phases of CO2. The effects of these interactions on petrophysical, chemical and mechanical properties are highlighted. In addition, a novel experimental approach is presented, developed and used by our team to investigate mechanical properties by exposing shale to different saturation fluids under controlled temperatures and pressures, without modifying the test exposure conditions prior to mechanical and acoustic measurements. This paper also underlines the major knowledge gaps that need to be filled in order to improve the safety and efficiency of SOGE and CO2 storage.

Acid Treatment as a Way to Reduce Shale Rock Mechanical Strength and to Create a Material Prone to the Formation of Permanent Well Barrier

“…Mixing of the shale pore fluid with the exposure fluid will thus cause the system to undergo changes toward a new equilibrium state. These changes may thus include: (1) ion exchange between clays and the new pore fluid followed by volumetric response of swelling clays (swelling, Energies 2021, 14, 2342 6 of 14 shrinkage), (2) changes in the interparticle interactions, (3) dissolution of some minerals due to contact with undersaturated solution [16,20]. minerals due to contact with undersaturated solution [16,20].…”

“…Indeed, an X-ray tomography cross-section image through sample The observed significant difference between the maximum peak force after exposure to HCl compared to NaCl is most likely due to leaching out of some cementing material at acidic conditions. It has been shown that exposure of some shale rocks to acidic environment leads to significant deterioration of shale mechanical properties [16,17,[22][23][24]. The loss of mechanical strength of shales upon exposure to acidic fluids was ascribed to dissolution of such minerals as: carbonates like calcite, dolomite, magnesite, and siderite; phyllosilicates like e.g., chlorite and K-feldspar [16,25].…”

“…In fact, calcium was almost entirely removed from the shale sample during the acid treatment. Carbonate minerals are very susceptible to acid attack [16]. It has been shown that even weak acids like carbonic acid can efficiently etch carbonates from various types of shales [16,25].…”

“…Carbonate minerals are very susceptible to acid attack [16]. It has been shown that even weak acids like carbonic acid can efficiently etch carbonates from various types of shales [16,25]. The most susceptible to acid attack is calcite followed by dolomite while siderite and magnesite show lower dissolution rates [26].…”

“…Acid treatment of shales has been of interest to scientific and oil industry environments in view of so called "matrix acidizing" [11][12][13], shale gas extraction [14], as well as CO 2 storage [15,16]. Matrix acidizing of gas and oil-bearing shales is a stimulation method used often as a pre-treatment before hydraulic fracturing.…”