An Investigation On The Evolution Of In-Situ Stress Before Refracturing In The Injection-Production Well Group

At present,many domestic oil wells have entered the late stage of production,the moisture content of oil wells has increased,and the output has dropped sharply.Most of the oilfields are low-permeability oil and gas reservoirs,possessing poor porosity,permeability and oil saturation conditions,and hydraulic fracturing technology is required to achieve economic exploitation.However,due to the influence of technical and geological conditions,fractures created by hydraulic fracturing will disabled after a certain number of years of production.Refracturing is an effective retrofit measure that creates new fractures laterally in the primary fracture,extracts the remaining oil,and restores production.The stress field before and after fracturing is one of the most important factors affecting direction of fracture propagation and morphology of new fractures.The magnitude and direction of the three-way stress determines not only the direction of the fracture,but also the shape of the fracture.On-site observations found that the mechanical effect induced stress field generated by fracturing and the pore elastic effect induced stress field generated by production will have a redirective effect on the in situ stress field,so it is necessary to study the influence of different measures on the stress field of the well group under the conditions of the well group.The research on the stress field of refracturing is mostly focused on a single production well or injection well,and most of the research models established are simple homogeneous models that cannot well reflect the real underground situation.According to the typical diamond-shaped anti-nine-point well network layout scheme in Huaqing area,combined with the on-site data,a four-dimensional geomechanical model that can better reflect the geology and engineering conditions of the well group is established,and the changes in the in-situ stress field after production and fracturing under the conditions of the well group are quantitatively analyzed through theoretical analysis and numerical simulation.The main achievements and recognitions are as follows:(1)Based on the variation function analysis in geostatistics,based on the field logging data,the initial four-dimensional geomechanical model before refracturing of the injection and production well group,including nine wells,was established by combining stochastic modeling and deterministic modeling.(2)The effect of induced stress field generated by mechanical effect and pore elastic effect on in situ stress field was studied.Based on fracture mechanics theory,the induced stresses generated by rock opening and crushing during fracturing are studied,and the induced stresses generated by reservoir physical properties changes and rock deformation during production are studied based on fluid-structure interaction theory and elastic mechanics theory.The changes in the in-situ stress field during the development of the local layer of the well group are quantitatively characterized by cloud maps.(3)The water injection will obviously change the size and direction of the stress field around the water injection well;the change in the size and direction of the stress field caused by fracturing is obviously more drastic than the change caused by production;with the progress of production,the stress change range of the well group will increase first and then decrease,and there is an inflection point for the change of the stress range.(4)The analysis of the stress field of the well group was carried out with fracturing timing.By comparing the production volume,it was determined that the in-situ stress field reversal range of the well group model after three years of production was determined,which was the best time for refracturing.