Effect of Pore Size Distribution on Phase Behavior of Sour Gas and Hydrocarbon Mixtures in Tight Oil Reservoirs

Abstract

Carbonate reservoirs in North Caspian Sea region of Kazakhstan produce high concentrations of sour gas or acidic gas species, including H2S and CO2. Effect of sour gas components on phase behavior of hydrocarbon mixtures in these fields needs to be clarified. In addition, some of these reservoir formations are very tight with majority of pores falling in the nanometer range. Therefore, nanoconfinement or capillary effect could play an important role on phase behavior during sour crude oil production and sour gas injection processes. Gas chromatography was employed to analyze the composition of a live crude oil obtained from a Caspian reservoir and a nanopore system with a pore size distribution (PSD) that is characteristic of tight carbonate reservoirs was set up to regulate the vaporization sequence. Using a flash calculation procedure that incorporates capillary effect, phase behavior of sour crude oil in nanoporous media was studied. Specifically, pressure and temperature (PT) diagrams, constant composition expansion (CCE), constant volume depletion (CVD), and CVD with a critical gas saturation (CGS) were simulated and analyzed. The live crude oil has an average molecular weight of 56.12 g/mol at bubble point pressure and reservoir temperature, containing 13.32 mol% H2S and 2.90 mol% CO2. PT diagrams showed that the bubble point pressure of sour crude oil is much lower than crude oil analogies that use CO2 and CH4 as substitutes by following the order: H2S<CO2<CH4. Nanoconfinement suppressed liquid vaporization in nanopores in CCE and CVD processes, thus keeping liquid phase at low density and viscosity. Setting a CGS helped expel liquid out of the nanopore system during depletion. CCE and CVD processes indicated that more than 42.0 vol% of the nanopores, including part of the 30 nm and all smaller pores, could be always occupied by liquid phase. These findings are useful for designing, implementing, and simulating sour crude oil production and sour gas injection processes in tight carbonate reservoirs in the North Caspian Sea region.