Test Study On The Effects Of Supercritical CO₂ On Shale Mechanical Properties

Effective development of shale gas is an important strategic need for China to relieve energy imbalance between supply and demand and ensure the energy safety. Due to problems of high water consumption and clay expansion when encountering water horizontal by the current well + multistage fracturing technique in exploring shale gas reservoir in water-deficient areas with a high content of clay in China, research team has put forward the idea of using the supercritical carbon dioxide fracturing method to produce shale gas. However, the carbon dioxide will certainly have a significant influence on physical and mechanic properties of the shale mass under its physical-chemical effect when used in place of conventional water-based fracturing fluid for shale fracturing, thus affecting crack initiation and extension mechanism of fractures, so it is the prerequisite and key to undertake a study on the change law and mechanism of mechanical properties of shale that supercritical carbon dioxide has been pumped into for the follow-up study on supercritical carbon dioxide fracturing mechanism.This paper aims to study the effect mechanism of supercritical CO₂ on the mechanical properties of the shale by conducting supercritical CO₂ immersion tests for the Silurian shale from Sichuan Basin Longmaxi for different durations of time, indirect tensile tests?Brazilian tests? and triaxial compression tests on shales before and after immersion, and XRD tests, porosity testing and SEM results for the analysis of the impact of supercritical CO₂ on the mechanical properties of shale. The main results are as follows:?1? It reveals the evolution of the mechanical properties of shale with supercritical CO₂ immersion time. Under supercritical carbon dioxide, with the immersion time lengthen shale tensile strength loss rate were 18.7%, 19.55%, 22.7%, 27.7%, 22.7%; Triaxial compressive strength loss rate were 6.6%, 9.1%, 10.0%, 13.8%, 10.0%; Uniaxial compressive strength loss rate were 9.8%, 15.9%, 20.2%, 21.3%, 20.2%, the mechanics parameters of yield loss with the increase of immersion time is change rule, but increases the size of the showed a trend of decrease. Showed that supercritical CO₂ immersion has obvious degradation effect on shale mechanical properties.?2? The influence of supercritical CO₂ on the acoustic emission characteristics of shale was analyzed by the acoustic emission test under the uniaxial compression of the shale before and after the supercritical CO₂ immersion. After soaking shale accumulated acoustic emission count average value was significantly greater than that of not soak the shale of the cumulative AE counts and acoustic emission energy signal proportion increases, shows that the supercritical CO₂ under the action of shale strength weakened, increase the internal crack and is more prone to large crack.And the surface is rough topography looks shale, shale and unsoaked clay mineral can also be observed scaly orientation, but the pore diameter of the surface pores is more developmental stage ?m, simultaneously more and more obvious honeycomb pores.This shows that between supercritical carbon dioxide and shale chemical reaction changed the main mineral composition and structure of shale, so that the degree of cementation shale decreases directly lead to a decline in cohesion.?3? It reveals the micro mechanism of the supercritical CO₂ degradation of shale mechanical properties. XRD and SEM test results of shale before and after immersion show that calcite, dolomite and pyrite gradually decreased with immersion time increases, so that the degree of cementation shale decreases directly lead to a decline in cohesion; After immersion shale surface morphology becomes more rough, and appears more obvious honeycomb pores, leading to the degradation of its mechanical properties in different degree.