As permeability of tight oil reservoir is generally less than 0.1md, diameters of pore throats are primarily at the micrometer- and nanometer-scale. Different from conventional reservoir, the percolation occurs mainly as non-linear flows so that the threshold pressure has a significant effect on the flow of fluids within the reservoirs. In this paper, a fracture optimization method for SRV Fracturing of tight oil reservoirs that considers threshold pressure introduced. Through numerical simulation, the fracture parameters of the tight oil in the Mahu reservoir in western China is optimized.
Based on the experimental results of constant-rate mercury injection experiment on cores collected from Mahu reservoirs in Xinjiang oilfield, the relationship between the core and the threshold pressure at different permeability was obtained. Later, a mathematical model of reservoir and fractures was created based on the characteristics of tight oil reservoir and of percolation through artificial fractures. By using the CMG simulator, three level of permeability, 10md, 1md, and 0.1md, were applied to the model separately with the corresponding threshold pressure gradients were 0.01MPa/m, 0.1MPa/m, and 1.0 MPa/m, respectively. Then the model's oil productions were estimated in two cases: when the threshold pressure was considered and not considered, to examine the influence of threshold pressure on oil production. Further research was then carried out to explore proper SRV fracturing methods for reservoirs with different physical properties.
The constant-rate mercury injection experimental results show that when the core permeability is less than 0.1md, the threshold pressure gradient is 3.11 times larger of the core with 1.0md permeability, and 9.65 times larger of the core with 10md permeability. The influence of threshold pressure on oil production should be considered during the numerical simulation of the fracturing of the tight oil reservoir. Compared with the numerical simulation results without considering the threshold pressure, the optimization of artificial fracture parameters with same production are quite different by considering the starting pressure. The cumulative production of 1 year reduced 84.77% when the threshold pressure gradient is about 0.14MPa/m, with the length of the horizontal section is 400m and the permeability is 0.1md, and also under the same crack length, spacing and diversion capacity. Considering the threshold pressure gradient, it is necessary to shorten the intervals between fractures and reduce the flow distance of tight oil from matrix to fractures, so as to increase the production and thereby increasing matrix's contribution to production capacity.
The threshold pressure gradient has a great influence on optimization results of multistage fracturing in tight oil reservoir. The results of paper are applied to the development of Ma-18 well block in Mahu tight oil reservoir. The research findings and methods provide guidance for multistage fracturing optimization in similar tight oil reservoirs.