Study On Formation Characteristic And Flow Mechanism Of Complex Volcanic Gas Reservoir In Yingtai Gasfield

The complex volcanic gas reservoir of Yingcheng Formation in Yingtai Gasfield is characterized by varying lithologies, diversified pore structures, minor pore-throats and high content of irreducible water. In this paper, the experiments like SEM, cast section, conventional mercury injection, constant-rate mercury injection and core displacement are conducted to identify the physical properties and micro-pore structures of the gas reservoir and the flow mechanism under reservoir pressure. The following results are obtained:(1) The Yingcheng Formation volcanic rocks are divided into five types: volcanic lava, pyroclastic lava, pyroclastic rock, sedimentary pyroclasticrock and pyroclastic sedimentary rock. The reservoir lithology presents as rhyolite, sandstone, conglomerate, tuff, and volcanic breccias, etc.; rhyolite is prospective for reservoir development for its good porosity and permeability.(2) Pores in volcanic reservoir are mainly primary pores and secondary pores. The former, slightly developed, is mainly composed of secondary dissolved pores and structural fractures. Pore-throats are very narrow in the reservoir-those with size less than 0.25μm contribute more than 90% of the pore volume, while those with size larger than 1μm are seldom developed.(3) Through stress sensitivity analysis of samples with different lithologies and space types (pore or fracture), it is found that the stress sensitivity of fracture-type samples is higher than that of matrix samples, and the stress sensitivity of pyroclastic sedimentary rock is higher than that of volcanic lava. Furthermore, the stress sensitivity increases as the permeability decreases and the irreducible water saturation increases. Accordingly, the productivity equations for fractured vertical wells and horizontal wells that consider the effects of stress sensitivity are established.(4) For water-containing gas reservoir formation, the relative permeability of gas phase is relatively low, the gas-water flow area is narrow, and the irreducible water saturation is high-usually about 50%. In the comparative experiments of gas-water relative permeability for long cores and small-diameter cores, the long cores reflect much lower water phase permeability and higher gas phase permeability, and the equal permeability point moves leftwards. For volcanic rocks that feature complex pore structures and strong anisotropy, the relative permeability curves of their long cores can more effectively and truly reflect the gas-water flow characteristics of the reservoir due to larger core size.(5) Based on the results of experiments on flow mechanism and depletion development experiments of long cores, the formation pressure spread formula is derived, which take the influences of both stress sensitivity and water saturation into account. Then, the formula is substituted for numerical calculation. The results show that, for gas wells with constant flow rates, the lower the reservoir permeability is, the higher the water saturation is, the shorter the formation pressure spread distance is, the clearer the pressure drop funnel is, and the smaller the rational well spacing and production should be. Moreover, for reservoirs with permeability less than O.O1mD or water saturation more than 60%, the gas phase flowability is poor, and only less reserves can be recovered by conventional fractured vertical wells.In this case, the multi-stage fractured horizontal wells are recommended.(6) Based on the results of experiments on flow mechanism, the mathematical model for development by fractured horizontal well with constant flow rate is established, numerical calculation is conducted to analyze the production performance and influential factors for productivity of fractured horizontal wells under different physical properties, and the formulas are proposed to calculate rational fracture spacing and length of horizontal section of fractured horizontal wells.