Characteristics of micron-scale pore-fracture modification in marine shale reservoirs with different combustion explosion energy

Abstract

The combustion explosion fracturing of shale with in-situ methane is a revolutionary fracturing technique, but the modification characteristics of reservoir pores and fractures by combustion explosion energy level are still unclear. In order to study the transformation characteristics of micron-scale pore-fracture systems of marine shale reservoirs with different combustion explosion energies, this study selected a section of shale samples from the Longmaxi Formation in the Luzhou block in the southern Sichuan Basin to carry out different energy gradient combustion explosion test, named W1∼W5 (25–91 MPa). Based on micron CT scanning technology, the three-dimensional reconstruction of shale samples before and after combustion explosion was carried out to analyze the effect of combustion explosion energy on the transformation of the pore-fracture system characteristics. The results show that at the energy between 25 MPa and 45 MPa, the variation characteristics of cracks before and after explosion are not obvious. When the energy of the explosion is in the range of 61 MPa–91 MPa, the cracks gradually extend and become longer, and the distribution density of cracks becomes higher and more interconnected. After the explosion at 91 MPa, it can be seen that a number of annular cracks are generated at the edge of the sample, which are interconnected with radial cracks. Total porosity, connected porosity, pore number, and total pore volume of the shale increased after combustion explosion, where the total porosity at the combustion explosion energy of 91 MPa have the largest increment, while the connected porosity, pore number, and total pore volume in the combustion explosion energy of 71 MPa have the largest relative increments, indicating that there is a fracturing pressure value that strongly breaks through the bottleneck when the explosion pressure is between 61 MPa and 71 MPa. A comparison of different pore size sections before and after the combustion explosion pore volume share found that the explosion pressures of 25 MPa, 45 MPa and 91 MPa tend to expand from 100 to 200 μm pore size to form a larger volume share of >200 μm cracks, while at the explosion pressure of 71 MPa, more inclined to transform the 0–100 μm pores, so that the pores gradually transition to the 100–200 μm. Analysis shows that the gas explosion generates a large pressure, release a large amount of heat. Due to the high temperature and high pressure coupling effect, the pore-fracture in the shale changes significantly. Strong stress waves lead to stress concentration at the tip of shale and crack expansion and extension. At the same time, the local expansion force causes “pore expansion effect”. Various mineral particles in the shale undergo differential expansion, along the crystal rupture or through the crystal rupture. New pore fractures are also generated when the original pores keep expanding, which improves the shale pore connectivity. This understanding can provide a theoretical basis for the selection of the energy level of the in-situ methane combustion explosion in shale.