| China is one of the largest natural gas consumers in the world, however the country has a shortage in natural gas resources. Shale gas occurs to be a clean energy source, which has gradually became a hotspot after its successful exploitation and utilizatioin in the north Amer ica, and is considered as an important part to readjust the energy structure and alleviating the contradiction between supply and demand of natural gas in China. Supercritical CO2(SC-CO2) can be used for fracturing of shale formation, by which clay swelling and water lock effect could be avoided. Meanwhile, shale rock is an ideal medium for underground carbon dioxide(CO2) gas storage, therefore technologies of using SC-CO2 for fracturing and enhanced shale gas recovery are of great importance.Unfortunately, the technology is still immature, mechanism of fracture formation and propagation during SC-CO2 fracturing has not been well understood, and evaluation system for the fracturing effect also encounters difficulties. To solve the above problems, physical properties, porosities, and gas competitive adsorption behavior of typical shales were investigated based on a series of analytical approaches. Furthermore, SC-CO2 fracturing was carried out by using an integrated acoustic wave-acoustic emission system in a lab-scale instrument, based on which the formation and propagation of fractures by SC-CO2 was investigated. The main contents and conclusions are summarized as follows:(1) Elemental compositions, mineral compositions, physical properties and porosities of shale samples were investigated by means of inductively coupled plasma-atomic emission spectrometer(ICP), X-ray photoelectron spectroscopy(XPS), X-ray diffraction(XRD), gas adsorption(N2, CO2) analysis and field emission-scanning electron microscope(FE-SEM). It was found that each sample is composed of quartz, clay(chlorite, illite and kaolinite), feldspar, calcite, dolomite, pyrite, berlinite and other minerals. The SEM results indicated that pores can be classified into four types, namely intragranular pore, intergranular pore, organic matter pore and microfracture.In addition, the porosity of sample is ca. 5% with diameters mainly in the range of 0~30 nm, and the BET specific surface areas and pore volumes are positively related to the total organic carbon content(TOC) and clay content, while brittle minerals had a negative effect to these parameters.(2) Based on results of mineral compositions and porosity, adsorption isotherms for CH4 and CO2 under different temperatures were measured by high pressure adsorption analyzer on Longmaxi Formation shale samples collected from Chongqing region in Sichuan Bas in, factors inducing adsorption selectivity of CO2 over CH4 were clarif ied. It was observed that the adsorption capacity of CO2 is always higher than CH4, therefore CO2 can replace CH4 pre-adsorbed on shale sample surface. Furthermore, Type I isotherms were obtained for the adsorption of both CO2 and CH4, the experimental data can be fitted very well by the Langmuir model. TOC and clay mineral composition affects the gas adsorption through controlling of the porous structure(micron and nanometer scale). Micropore and mesopore were mostly originated from the organic matter and increased with the increasing of TOC. The pore volumes and BET specific surface areas are positively related to the saturated adsorption capacity. In addition, CO2 and CH4 adsorption capacity decreases with temperature rising on account of gas desorption rate increasing, and theirs sensitivity to temperature is mainly related to the heats of adsorption.(3) Uniaxial compression tests were carried out on shales samples from Longmaxi formation of Sichuan basin as well as simulated samples prepared from concrete, rock mechanical parameters and deformation/failure characteristics of the specimens were obtained. It was found that Longmaxi formation shale is characterized by relatively high brittleness, high Elastic modulus and low Poisson’s ratio, this is advantageous for fracturing, the uniaxial compressive strength ranges from 77.8 MPa to 117.9 MPa, and the measured elastic modulus is between 16.73~25.22 GPa with an average Poisson’s ratio of 0.30. In uniaxial compression test, shear rupture and tension rupture were generated, and several shear fracture planes and tension shear structural planes weregenerated.(4) SC-CO2 fracturing simulation experiments of shale outcrops were carried out by a triaxial fracturing equipment test system. These results suggest that SC-CO2 fracturing has a lower fracturing pressure than water fracturing, which means lower energy consumption. Fracture morphology and the number of cracks in the sample show a strong correlation with the measured and localized by acoustic emission. Based on acoustic location and monitoring location technology, time-pump pressure curve analys is and tracer method, fracture generation and propagation mechanism for SC-CO2 fracturing in shale rock were preliminary discussed. The results demonstrate that shale SC-CO2 fracturing can produce a crack perpendicular to the bedding plane, it is easier to form vergens and furcated cracks when the fracture extended to the weak bedding plane or natural fractures. Finally, fracture extending to further and generating crossed and linked fracture system. |
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