Strain Characteristics of Pore-Throat Structures in an Ultradeep Dolomite Gas Reservoir

There is little research on the strain characteristics of ultradeep dolomite rocks with deep burial, high stress, and developed fractures and macropores. This article comparatively studies the influence of stress and fracturing on the pore-throat structure of different types of ultradeep dolomite rocks by setting up stress experiments and fracturing experiments and using computed tomography (CT) scanning. In the stress experiment, the fluid-solid coupling effect on pore space was considered. The study shows that unlike shallow to medium carbonate reservoirs, when the stress is high but lower than the rock yield stress and in a flowing state, the studied ultradeep dolomite rock has a higher permeability after recovery from compression. When the stress is high, brittle deformation occurs inside the rock sample, and dolomite particles in the macropores are crushed and detached, blocking the middle pores with the gas flow; the two adjacent mesopores are merged into one macropore. In this way, the average pore size of the rock sample increases and the proportion of macropore volume increases. The increase in macropores plays a leading role in improving the reservoir's permeability. The edges of dolomite crystals are compressed open under high stress, resulting in more developed intercrystalline fractures, larger fracture widths, and strengthened communication between fractures. Besides the effect of stress, the fractures generated by fracturing greatly expand and connect the existing fractures, and then improve the connectivity of the reservoir, which is the main factor leading to the improvement of reservoir permeability after fracturing. In addition, rock mechanical parameters were obtained through triaxial mechanics experiments. Dolomite rock samples that have undergone multiple cycles of stress cycling have more developed microfractures and macropores and weakened axial compression resistance and are more prone to be fractured. This study can provide a theoretical basis and guidance for the efficient development of ultradeep dolomite.