Influence of Pore Structure and Solid Bitumen on the Development of Deep Carbonate Gas Reservoirs: A Case Study of the Longwangmiao Reservoir in Gaoshiti–Longnusi Area, Sichuan Basin, SW China

Abstract

A variable sedimentary environment and accumulation process leads to a complex pore structure in deep carbonate gas reservoirs, and the physical properties are quite different between layers. Moreover, some pores and throats are filled with solid bitumen (SB), which not only interferes with reservoir analysis, but also affects efficient development. However, previous studies on SB mainly focused on the accumulation process and reservoir analysis, and there are few reports about the influence on development. In this paper, through scanning electron microscope analysis, SB extraction, gas flow experiments and depletion experiments, and a similar transformation between experimental results and reservoir production, the production characteristics of carbonate gas reservoirs with different pore structures were studied, and the influence of SB on pore structure, reservoir analysis and development were systematically analyzed. The results show that permeability is one of the key factors affecting gas production rate and recovery, and the production is mainly contributed by high-permeability layers. Although the reserves are abundant, the gas production rate and recovery of layers with a low permeability are relatively low. The SB reduces the pore and throat radius, resulting in porosity and permeability being decreased by 4.73–6.28% and 36.02–3.70%, respectively. With the increase in original permeability, the permeability loss rate decreases. During development, the loss rate of gas production rate is much higher than that of permeability. Increasing the production pressure difference is conducive to reducing the influence. SB also reduces the recovery, which leads to the loss rate of gas production being much higher than that of porosity. For reservoirs with a high permeability, the loss rates of gas production rate and the amount produced are close to those of permeability and porosity. Therefore, in the reservoir analysis and development of carbonate gas reservoirs bearing SB, it is necessary and significant to analyze the influence of reservoir types.

Influence of Pore Structure on Gas Flow and Recovery in Ultradeep Carbonate Gas Reservoirs at Multiple Scales

“…The results of similar transformations provide an effective basis for selecting suitable production pressure differences in reservoirs with varying permeabilities. 41 Combined with the development status of the Carboniferous carbonate gas reservoir, the gas flow characteristics under SWI conditions were closer to the actual characteristics. This is consistent with the relative permeability of the G-W flow, and the gas production rate loss caused by irreducible water was as high as 27% (Figure 12b, Table 6).…”

“…Therefore, an improved depletion simulation method was implemented by reducing the outlet pressure to the waste pressure step-by-step. 41 A gas flow experiment was performed to study flow characteristics under different conditions, focusing on gas supply capacities and production rates, which is a dynamic process. The improved depletion simulation experiment was used to study changes in the pore pressure and gas production rate during the process of depletion, focusing on recovery and other characteristic parameters in different developmental stages, which is a quasi-static process.…”

“…According to the transformation between core experiments and gas reservoir production, the experimental pressure differences and gas velocities were transformed into production pressure differences and gas production rates (eqs and ) . Based on the reservoir geological data, L , d , h , r e , r w , and p e were set to 0.05, 0.025, 15.5, 850, 0.06 m, and 50 MPa, respectively. …”

“…In addition, it is difficult for flowmeters to meet the pressure requirement of 50 MPa. Therefore, an improved depletion simulation method was implemented by reducing the outlet pressure to the waste pressure step-by-step . A gas flow experiment was performed to study flow characteristics under different conditions, focusing on gas supply capacities and production rates, which is a dynamic process.…”

“…For the gas flow and productivity characteristics of carbonate gas reservoirs, scholars have performed a large amount of research through experiments and numerical simulations. − The vertical distribution of multiple layers is a significant feature; some scholars have studied the influence of heterogeneity through depletion experiments . Liu et al studied the development characteristics of low-pressure sandstone gas reservoirs under water-free conditions through depletion experiments under a constant gas velocity with three cores in parallel and analyzed the influence of factors such as pore structure .…”