Multiscale pore structure characterization of an ultra-deep carbonate gas reservoir

The pore structures of shallow and mid-deep carbonate gas reservoirs have been well explored. Totally different from these reservoirs, however, the pore structure of a target ultra-deep carbonate gas reservoir, the Qixia Reservoir in the Shuangyushi play is still unknown. Affected by tectonic movement and deep hydrothermal transformation, the lithology of this target reservoir has significantly changed and it has developed different scales of pores and fractures. Thus, a multiscale micro-CT (computed tomography) method is needed to combine different scales of its pore space to evaluate carbonate samples more realistically. In this study, the multiscale micro-CT method and other traditional methods including X-ray diffraction (XRD), casting thin section, scanning electron microscope (SEM), routine petrophysical measurements (RPM), and high-pressure mercury intrusion (HPMI) are first combined to quantitatively describe a distribution of multi-scale pores, throats, and fractures in the target ultra-deep carbonate gas reservoir. The results showed that micro pores (70%-90%), micro throats (>90%), and micro fractures (>90%) dominated in this reservoir. Dolomite accounted for more than 90% of the rock mineral composition in the Qixia reservoir with few clay minerals. Intercrystalline pores and micro fractures were the main pore types and the mean reservoir porosity was less than 2%. Micro fractures play a great role in improving reservoir permeability. The HPMI method presented with errors in the classification of ultra-deep carbonate reservoir types owing to the development of vugs, large pores, and fractures. In this study, a new classification method was proposed to classify this type of reservoirs based on porosity, permeability, and the studied parameters from the multiscale micro-CT method such as throat number and connected porosity proportion. The target reservoir was divided into the following four categories according to the new classification method: fracture macropore type (FMA), macropore type (MA), fracture micropore type (FMI), and micropore type (MI). According to the results of multiscale CT scanning, a strong linear relationship was observed between the number of throats, the connected porosity, and the permeability of the ultra-deep carbonate rock samples studied. Fractures improved the permeability of rock samples by aiding the connection of isolated pores and the expansion of the number of throats. Therefore, the MA and MI types of reservoirs should be properly fractured to increase their productivity.