Abstract:

Five typical continental reservoir crude oils in China were selected as research objects. Single-cycle and multi-cycle injection and production experiments were conducted using a simulation platform for oil-gas phase behavior during injection and production in gas storage reservoirs, which was developed based on ultra-high-pressure fluid phase analysis. Oil-gas properties under multi-stage pressures during single-cycle injection and production were investigated for three types of crude oils with different properties. Oil-gas characteristics during multi-cycle injection and production were examined for two types of crude oils with different properties. These revealed the relationship between interphase mass transfer patterns and the number of construction cycles in the oil and gas system of continental reservoir gas storage construction, providing theoretical guidance for determining the number of construction cycles for converting reservoirs of different types into gas storage. The experimental results showed that: (1) During gas injection, mass transfer was primarily driven by dissolution and diffusion, supplemented by evaporation and extraction. This led to a decrease in the density and viscosity of crude oil, a left-upward shift of the phase envelope, and a shift of the critical point towards lower temperatures and higher pressures. (2) During gas production, mass transfer was dominated by evaporation and extraction, supplemented by dissolution and diffusion. This manifested as increases in crude oil density and viscosity and decreases in gas density and viscosity. When the pressure of the oil and gas system reached the lower limit, the density and viscosity of crude oil reached their maximum values, while the density and viscosity of gas reached their minimum. Meanwhile, the phase envelope shifted left and upward, and the critical point moved towards lower temperatures and higher pressures. (3) Within a single cycle, the gas injection process lightened both volatile oil and heavy oil, reducing the content of heavy components. After gas production down to the lower limit pressure, the content of black oil C₇+ components decreased compared to the initial stage, while the content of volatile oil C₇+ components increased compared to the initial state. (4) Over multiple cycles, as the number of injection and production cycles increased, during the gas production process down to the lower limit pressure, the content of heavy components in black oil first decreased and then increased until it stabilized. The extraction capacity of the gas cap for crude oil was basically zero, indicating the completion of the reservoir construction. For volatile oil during the gas production process down to the lower limit pressure, the content of heavy components in volatile oil first increased and then stabilized. The extraction capacity of the gas cap for crude oil was also basically zero, marking the completion of gas storage construction.

Key words: reservoir-type gas storage, multi-cycle, multi-stage pressure, interphase mass transfer, evaporation and extraction, dissolution and diffusion