Abstract
Ultralow-permeability reservoirs are difficult to effectively develop using conventional technologies, so it generally produces using horizontal wells’ volume fracturing. Besides, oil wells also face the problems of short stable or no production period, rapid decline rate, and extremely low development degree. Water huff-n-puff of oil recovery technology is a quality and efficiency technology to improve the development effect of an ultralow-permeability reservoir. A systematic study on the related key influential factors of water huff-n-puff is done for improving the development effect of ultralow-permeability reservoirs. This manuscript studied the key influential factors of water huff-n-puff and related improving recovery methods using physical simulation experiments to put forward a beneficial condition for water huff-n-puff, combining with the field practice. The results show that imbibition and energy supplement are main mechanisms of water huff-n-puff; imbibition oil recovery is influenced by rock wettability, permeability, boundary condition, fracture length, water injection speed, and imbibition solution type; the stronger the water-wet reservoir rock is, the bigger the core permeability is, the longer the fracture length is, the larger the contacting area between imbibition solution and rock is, the stronger the ability of imbibition solution to change wettability, and to reduce the interfacial tension force between oil and water is, the better effect of water huff-n-puff is. So far, the field practice of water huff-n-puff has been conducted in 38 wells; the success rate can reach 92.1%, cycle oil increment of single well can reach 972 t, and accumulated oil in evaluation period can reach 36936 t, which further proves that water huff-n-puff can achieve a good effect. The effective methods to improve the development effect of an ultralow-permeability reservoir are changing reservoir physical property and supplying reservoir energy. Higher reservoir energy is good for oil flow, and better physical property can improve the displacement effect and imbibition function of injection solution. Artificial fracturing, higher injection-production ratio, and pretreatment of temporary stoppage fracturing are good methods to improve the development effect of water huff-puff.
Highlights
With the decrease of conventional oil and gas resources [1,2,3], unconventional reservoirs [4,5,6], such as ultralowpermeability and tight oil reservoirs, have gradually become an important field for oil and gas development in the world
Depletion development is usually adopted after fracturing without energy supplement for reservoir, leading to a short or no stable production period, rapid decline rate, and extremely low-stage development degree; oil recovery generally is less than 5%
Due to the special characteristics, such as low permeability, small pore throat size, and complex heterogeneity, of the ultralow-permeability reservoir [13], water injection or gas injection, which is suitable for conventional lowpermeability reservoir, has a poor applicability for ultralow-permeability reservoir
Summary
With the decrease of conventional oil and gas resources [1,2,3], unconventional reservoirs [4,5,6], such as ultralowpermeability and tight oil reservoirs, have gradually become an important field for oil and gas development in the world. The technology of water huff-n-puff in the ultralow-permeability reservoir has developed rapidly, and field practices have been carried out successfully in Changqing, Daqing, Tuha, and other oil fields in China, and obvious effects have been achieved. During the process of water huff-n-puff, the injection water can displace the oil existing in the fracture and matrix and supply reservoir energy; the imbibition function, controlled by the capillary force and gravity, benefits from the oil-water replacement effect [17,18,19]. The both functions can effectively enhance oil recovery. The beneficial condition for improving the recovery of water huff-n-puff was put forward; the whole study could provide a theoretical and practical guidance for the popularization of water huff-n-puff
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