Optimization of Water Injection Policy for Horizontal Wells in Tight Oil with Low Pressure

Abstract The tight oil is usually developed by horizontal well with volumetric fracture, using depletion type of drive. The recovery rate of tight oil with low pressure is unsatisfactory, because of low natural reservoir drive energy. The horizontal well with volumetric fracture is stimulated traditionally by water-flood huff and puff. However, as the decrease of the permeability and productivity of wells, high decline of produced liquids and reservoir pressure is observed by laboratory and dynamic data, which is called stress sensibility effect. This paper presents an integrated method to supplement formation energy and prevent the stress sensibility. The method mainly involves laboratory and numerical model. Firstly, to investigate the possibility of using areal water injection method, two cases are simulated in laboratory. One is simulated as square five-spot well pattern, and another one is horizontal well pattern. Both injector and producer are horizontal well. The injector can be instead of vertical well with large scale hydrofracture for cost saving. Finally, numerical simulation with Non-Darcy flow is carried out. The agreement is obtained with numerical simulation and laboratory. This new method is applied to Chang-7 tight oil in China, with the lower formation pressure factor about 0.8. The reservoir's mid-depth is around 2000m. The tight oil is currently under primary depletion development with an annual production declining rate higher than 50% at the first year. The water-flood huff and puff in Chang-7 tight oil increase productivity to some degree, however, the recovery rate is low finally. The stress sensibility of Chang-7 is measured from lab, and the permeability decrease around 20%. To solve this problem, the method of linear water-drive for tight oil is suggested, using horizontal wells injection-production pattern. The oil productivity increase 3~4 times than vertical wells, and recovery rate increases 5~10% in lab. The reasonable technical policy of linear water-drive is optimized by numerical simulation. This systematic technique provides a reliable method for decision making of energy supplement for tight oil with low pressure, which is helpful for effective and economic development of tight oil with volumetric fracture.