Cenomanian (late Cretaceous) deposits of Western Siberia are now the basic source of natural gas in Russia. They are developing at water-drive regime. When total gas production from these reservoirs reaches 60%, water-flooded zone increases to 15–20% of initial total pore volume filled by free gas.
When a cenomanian reservoir is close to completion of development, water-flooded part should occupy about 50% of initial pore volume. A lot of trapped and occluded gas is still remaining in water-flooded zone. Due to vast initial gas reserves, residual reserves of trapped and occluded gas can be comparable to a giant gas field reserves. Correct assessment of recoverable occluded gas reserves is necessary for definition of prospects of a deposit further development.
But there was paid no attention to these reserves assessment in Russia until now because existing system of natural gas production from cenomanian deposits was oriented to reserves assessment in free gas part of the reservoir. The study of these residual occluded gas reserves assessment possibility by mass balance method is represented.
Total initial proven reserves of cenomanian deposits of Western Siberia are more than 20 TCM (trillion cubic meter). Main part of these reserves (more than 17 TCM) is situated in 4 cenomanian reservoirs of Medvejie, Urengoy, Yamburg and Zapolyarnoye fields (fig. 1). By now about 70% of initial reserves of Medvejie and Urengoy cenomanian reservoirs are already recovered. Largest cenomanian gas reservoirs are unique according to their reserves and dimensions. They are bulk gas reservoirs underlying by underground water. Maximum thickness of the deposits reaches 200 m, largest dimensions are up to 200 km length and 50 km width. Reservoir lithology is represented by interbedding of sandstone and silt with thin layers of clay. Gas is produced at water-drive regime.
First cenomanian reservoirs have been developed since beginning of 70-ties. Reservoir study using exploration wells has shown good hydrodynamic connection of different parts of reservoir in horizontal direction as well as in vertical one. As the result, cenomanian deposits are developed as uniform bulk gas reservoir at water drive regime. Gas-water contact begins to move upward at 2–4th year after production start. Regime observations of gas-water contact movement upward the section during gas production have shown, that 60% of proven reserves extraction resulted in water flooding of 20–24% of initial gas reservoir volume. It is expected that at the late stage of gas reservoir production, when 80–85% of initial gas reserves will be recovered, about 55–60% of reservoir volume will be water flooded.
Thus, at the late stage of reservoir production, a considerable p