A three dimensional visualized physical simulation for natural gas charging in the micro-nano pore system

A micro-nano pore three-dimensional visualized real-time physical simulation of natural gas charging, in-situ pore-scale computation, pore network modelling, and apparent permeability evaluation theory were used to investigate laws of gas and water flow and their distribution, and controlling factors during the gas charging process in low-permeability(tight) sandstone reservoir. By describing features of gas-water flow and distribution and their variations in the micro-nano pore system, it is found that the gas charging in the low permeability(tight) sandstone can be divided into two stages, expansion stage and stable stage. In the expansion stage, the gas flows continuously first into large-sized pores then small-sized pores, and first into centers of the pores then edges of pores;pore-throats greater than 20 μm in radius make up the major pathway for gas charging. With the increase of charging pressure, movable water in the edges of large-sized pores and in the centers of small pores is displaced out successively. Pore-throats of 20-50 μm in radius and pore-throats less than 20 μm in radius dominate the expansion of gas charging channels at different stages of charging in turn, leading to reductions in pore-throat radius, throat length and coordination number of the pathway, which is the main increase stage of gas permeability and gas saturation. Among which, pore-throats 30-50 μm in radius control the increase pattern of gas saturation. In the stable stage, gas charging pathways have expanded to the maximum, so the pathways keep stable in pore-throat radius, throat length, and coordination number, and irreducible water remains in the pore system, the gas phase is in concentrated clusters, while the water phase is in the form of dispersed thin film, and the gas saturation and gas permeability tend stable. Connected pore-throats less than 20 μm in radius control the expansion limit of the charging pathways, the formation of stable gas-water distribution, and the maximum gas saturation. The heterogeneity of connected pore-throats affects the dynamic variations of gas phase charging and gas-water distribution. It can be concluded that the pore-throat configuration and heterogeneity of the micro-nanometer pore system control the dynamic variations of the low-permeability(tight) sandstone gas charging process and gas-water distribution features.

Hydrocarbon accumulation conditions and key technologies for exploration and development of Yuanba gas field

This paper discusses the exploration and development history and lessons of the large ultra-deep biogenic reef gas field in Yuanba area,where the previous“Kaijiang-Liangping shelf”model provides an insight for the discovery of the biological reef and bank on the platform margin.Systematic analysis of key geological conditions for hydrocarbon accumulation shows that the reef and bank reservoirs in Yuanba area are characterized by row and zonal distribution along“Kaijiang-Liangping shelf”,the bank in early and the reef in late,the reef in the front and the bank in the back,and dominated by lateral accretion and progradation.The major exploration target of high-quality reef-bank reservoirs are developed in Changxing Formation instead of in Feixianguan Formation,Three stages of fractures and dissolution,and dolomitization control the development of the high-quality reservoirs.Two sets of effective source rocks(Permian Dalong Formation and Wujiaping Formation)provide sufficient gas source for the large gas field.Three dimensional migration pathway system composed of micro-faults,micro-fractures and interlayer fissures facilitates the hydrocarbon migration and accumulation.The accumulation model of Yuanba gas field is characterized by the near-source accumulation,three-micro migration pathway system,lithological-stratigraphic reservoirs and tectonics controlling enrichment.According to the complex geological conditions such as ultra-deep reservoirs and multiple pressure systems in Yuanba gas field,several key technologies are developed during hydrocarbon exploration and development,including fine prediction of ultra-deep reef-bank reservoirs and identification of gas and water,fine-scale reservoir description and characterization of complex thin and small reef gas reservoirs,optimal&fast drilling of ultra-deep horizontal well,geo-steering of ultra-deep horizontal well for complex reef and bank.All these technologies resulted in the efficient exploration and development of Yuanba gas field and accelerated the innovation of theoretical technology and methods for ultra-deep reservoirs.