Injection-production Mechanism Of Underground Gas Storage Based On The Fractured Buried Hill Oil Reservoir

Buried hill fracture oil reservoirs are mainly located in Circum-Bohai-Sea Region, andbecome one of the most important goals for our country in building underground gas storage(UGS). Buried hill oil reservoir is very different from conventional single-medium reservoir,because not only geometric complexity of2discontinuous mediums, fracture and rock, but alsoflow characteristic complexity of oil, gas and water in the system of fracture and porousnetworks. Therefore, during building UGS based on the fractured dual-media carbonatereservoirs, one of key technical problems to be solved in the first place is verifying clearly themacro and micro percolation mechanism of multi-phase fluids under complex multi-phasepercolation conditions in dual-media, for it is the precondition and theoretic foundation ofoptimal design of this UGS. Basing on the case of buried hill oil reservoir in Huabei oilfield,injection-production physical simulation and numerical simulation of buiding UGS are used inthis paper. It is intended to investigate the key issues during multiple injection-production cyclesof UGS based on the fractured dual-media oil reservoirs, including multiphase percolationmechanism during gas drive, main spaces for storage and percolation, macro and micromovement rules of fluids, change rules of capacity and injection-production capability, and theirinfluence factors. There are no precedents of UGS based on the fractured dual-media oilreservoirs at home, with the lack of necessary theory and practical experience, so the valuabletheoretic basis is provided in this paper. The main research achievements are as following:(1) A set of physical and numerical simulation approaches were formed forinjection-production cycles of UGS based on the fractured dual-media oil reservoirs.(2) The main control factors deciding macro and micro gas-drive effects in the UGS basedon the buried hill reservoirs include drive force, gravity, gas/water capillary force and evidentpermeability differences between fracture and rock system.(3) Fracture system is the main gas-drive target, but effective gas-drive didn’t occur to rocksystem. Meanwhile, some dead spaces are left in the low gas-drive efficiency due to theheterogeneity of fracture system. (4) Gas injection rate shall be kept comparatively low during building UGS. Aftersecondary gas cap reached a certain scale, UGS gradually moves into the transitional stage ofinjection-production cycle for enhancing gas-drive effect.(5) Operating pressure range and working gas proportion should be optimized rationally, sothat excessive changes of gas-liquid interface in fracture system can be prevented during theintensive injection-production process of UGS, and the risks of serious gas channeling andconing of oil/water during the injection-production cycle can be under control.(6) During the process of building UGS and injection-production operation, oil and gascontact, dissolution, then diffusion. Lost injection gas dissolving and diffusing in the residual oilis limited as this process occurs at a slower rate.