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.
Highlights
Based on geological statistics, carbonate reservoirs account for about 35.7% of global oil and gas resources, and deep and ultra-deep carbonate gas reservoirs have become one of the popular resources for exploration and development [1,2]
The variable sedimentary environment and accumulation process lead to a complex pore structure of deep carbonate gas reservoirs, including pores, cavities and fractures, and the physical properties are quite different between layers [9,10,11,12]
Wood et al [19] found that the reservoir quality of the Triassic tight gas siltstone area in the Western Canadian Sedimentary Basin was strongly affected by solid bitumen (SB) through organic petrological observation and scanning electron microscopy (SEM)
Summary
Carbonate reservoirs account for about 35.7% of global oil and gas resources, and deep and ultra-deep carbonate gas reservoirs (depth > 4500 m) have become one of the popular resources for exploration and development [1,2]. The variable sedimentary environment and accumulation process lead to a complex pore structure of deep carbonate gas reservoirs, including pores, cavities and fractures, and the physical properties are quite different between layers [9,10,11,12]. Some pores and fractures in the Sinian and Cambrian reservoirs are filled with solid bitumen (SB) [13,14]. To study the influence of SB on a gas reservoir, the first step is to research the distribution characteristics in the pore structure. Wood et al [19] found that the reservoir quality of the Triassic tight gas siltstone area in the Western Canadian Sedimentary Basin was strongly affected by SB through organic petrological observation and scanning electron microscopy (SEM). Gao et al [22] and Mastalerz et al [23]
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