Mechanical behaviour of Longmaxi black shale saturated with different fluids: an experimental study

Abstract

The fluid-shale interaction during carbon dioxide (CO 2 ) enhanced shale gas recovery processes may have significant influences on the mechanical behaviour of shale, and it is also of great importance for the longterm safety of CO 2 geological sequestration in shale formations. In this work, shale samples from the Longmaxi formation in the Sichuan basin were used to investigate the mechanical properties and the acoustic emission (AE) behaviour of shale upon saturation with different fluids (water, brine, and CO 2 -brine mixture with different CO 2 phase states) at 45 C. X-ray fluorescence (XRF) analysis results show that major element alterations occur after shale is treated with supercritical CO 2 -brine and subcritical CO 2 -brine, the element content of Ca, K and Al in supercritical CO 2 -brine saturated shale was decreased by 16.7%, 5.8% and 6.3%, respectively. Field emission scanning electron microscopy (FESEM) analysis results indicate that significant surface structural changes occur in shale after saturated with CO 2 and brine. Furthermore, the uniaxial compressive strength (UCS) and elastic modulus (E) of shale with different fluid saturation were decreased. Supercritical CO 2 cause a greater reduction of UCS and E compared to subcritical CO 2 due to its higher adsorption capability and larger CO 2 adsorption induced swelling. The maximum reduction in UCS and E of shale was observed in the case of CO 2 -brine saturated samples, and the influence of the CO 2 phase on the mechanical behaviour of shale under brine conditions is not negligible. The fracture propagation pattern changes in shale after fluid saturation are also explained by the AE analysis.

A Damage Constitutive Model for the Effects of CO2-Brine-Rock Interactions on the Brittleness of a Low-Clay Shale

“…Therefore, the investigation of shale's mechanical properties after fluidrock interactions is very important for drilling and fracturing industry. Many scholars have done a brilliant research about the variation of shale's strength and Young's modulus after the adsorption of fluids, such as water [16,17], saline solutions [18,19], CO 2 [20][21][22], CO 2 -dissolved water [11], and CO 2 -dissolved brine [23,24]. However, the studies of the brittleness of shale after fluid saturation are limited.…”

Characterizing the Dissolution Rate of CO2-Brine in Porous Media under Gaseous and Supercritical Conditions

“…After CO 2 is injected into target reservoirs, it migrates in permeable rock under a supercritical state and dissolves into brine to form carbonic acid [2]. The dissolution process reduces the risk of CO 2 leakage through the caprock of the reservoir due to the upward movement of the CO 2 plume under the buoyancy effect [3].…”

“…During the CO 2 dissolution, brine saturated with CO 2 will be removed more effectively from the CO 2 -brine interface and more unsaturated brine comes into contact with the CO 2 . This process will be affected by the actual formation environment, including the reservoir temperature and pressure, flow development, heterogeneity, pore structure, and gas distribution [10].…”

“…The dissolution process reduces the risk of CO 2 leakage through the caprock of the reservoir due to the upward movement of the CO 2 plume under the buoyancy effect [3]. The dissolution rate of CO 2 -brine, which represents the mass transfer process, serves as an important parameter to immobilize the free-phase CO 2 .…”

“…Through these mechanisms, the CO 2 near the two-phase interface is dissolved and transported away. Then, further fresh brine imbibes into the in situ pores and sequentially makes contact with free-phase CO 2 , accelerating the dissolution process.…”