Percolation Mechanism And Development Method Of Fractured Ultra-Low Permeability Reservoir With Pressure Sensitive

The ultra-low permeability fractured reservoir has low matrix permeability,so the main seepage channels often are fractures which have strong pressure sensitivity and anisotropy characteristics.When reservoir pressure changes,fractures in different directions will show different deformation characteristics,which makes the oil-water seepage law more complicated and seriously affects the development effect of water flooding in ultra-low permeability fractured reservoir.This paper studies how fracture direction affects the fracture pressure sensitive deformation characteristics and the seepage law of ultra-low permeability fractured reservoir,so as to provide new ideas for the development and adjustment of such reservoirs.Firstly,based on the similarity theory,a physical simulation method for seepage law in fractured media with constant volume boundary is established,having advantages of easy in fracture generation,visible in model and controllable in fracture.The pressure sensitive experiments of a single group of fractures in different directions were conducted by this method,having results that: Under the same pressure conditions,the flow rate of fractured media decreases with the increase of fracture angle(the angle between fracture direction and macroscopic pressure gradient direction),and the change rate of fracture aperture and permeability increases first and then decreases with the increase of fracture angle.Based on the above experimental rules and theoretical analysis,a pressure-sensitive equation considering fracture direction was established,and the calculated results agree well with the experimental data and the semi-analytical permeability model.This pressure-sensitive equation is applied to analyze the influence of a single group of fractures on the principal value of permeability tensor.The results show that the deformation of a single group of fractures only affects the principal value of permeability.Secondly,the influence of fracture angle,fracture interval and elastic parameters on the principal value of permeability with several groups of fractures are analyzed with the results of physical simulation and numerical calculation of several groups of fractures in different fracture directions.Fractures at different angles having different change rates of fracture aperture will change the relative magnitude of permeability of two groups of fractures.Fractures at different angles having different degrees of deformation under the same pressure conditions will make the direction of the principal value of permeability rotate,causing the injected water to change direction in the process of water injection development and making it difficult for the original well pattern to achieve the expected development effect.Then,based on the newly established pressure-sensitive equation considering fracture direction,a productivity model of area well pattern of ultra-low permeability fractured reservoir is derived and established.The effects of different fracture directions,different injection-production pressures and other parameters on the productivity of five-spot,anti-seven-spot,anti-nine-spot and rhombus anti-nine-spot well network were analyzed.The results show that the maximum productivity occurs when fracture angle is 45°,and rhombus anti-nine-spot well network has the highest productivity.The higher change rate fracture aperture is,the smaller productivity the corresponding well pattern has.Under the same well pattern conditions,the stronger pressure sensitivity is,the smaller limit well spacing is,indicating that the pressure sensitivity makes it more difficult to develop ultra-low permeability fractured reservoirs and requires infill well pattern development.Finally,using the pressure-sensitive equation considering fracture direction,this paper establishes a mathematical model for three-phase seepage of oil,gas and water considering fracture pressure sensitivity in different directions,develops a fracture pressure sensitive numerical simulation software module,and studies the influence of fracture direction on seepage field of ultra-low permeability fractured pressure sensitive reservoirs.The results show that the well pattern unit has the highest recovery percent when fracture angle is 45°,and the principle of adjustment of injection-production system is to achieve anti-nine-spot well network area injection in fractured pressure sensitive reservoirs.According to the above research results,combining with the effect of the permeability of the fracture anisotropic reservoir on the damage and recombination of the well pattern,there are two development and adjustment ideas for the change of injected water flow direction caused by fracture directional pressure sensitivity in ultra-low permeability fractured reservoirs.1)In order to make the angle between the direction of principal value of the maximum permeability and the direction of well array be within the range that can satisfy the injection-production relationship of original well pattern unit without being destroyed,it is necessary to maintain reasonable formation pressure during water flooding development process.2)The rotation phenomenon of the direction of permeability principal value caused by fracture pressure sensitivity can be used to adjust formation pressure so that the injected water flows in a favorable direction.The research results of this paper develop the theory of seepage flow in fracture anisotropic pressure sensitive media,which can provide a theoretical basis for productivity prediction and modification of injection well pattern in the ultra-low permeability fractured pressure-sensitive reservoirs.