Study On Gas Invasion Mechanism And Sealing System Of Fractured Gas Reservoir On North Slope Of Tazhong Basin

The Ordovician reservoir in the northern slope of Tazhong Basin is characterized by fracture and cavity development,frequent gas invasion and leakage during drilling,fast gas-liquid displacement,long cycle exhaust time and high material loss,which seriously restrict the development process of this area.Therefore,it is necessary to carry out targeted research to reveal the mechanism of the coexistence of underground leakage and gas invasion,so as to provide theoretical basis for the follow-up research.At the same time,a plugging slurry system with good plugging performance is constructed to provide technical support and reference for field plugging operation.Based on the data of field logging and drilling,the situation of gas invasion and leakage in the field is analyzed.According to the fracture morphology,a one-dimensional natural fracture drilling fluid leakage model is established.Under different conditions,the peak leakage velocity ranges from 10-100L/s,the stable leakage velocity ranges from 0.5-4L/s and the leakage length reaches 30 m.The leakage mechanism under different pressure difference,drilling fluid performance and fracture width conditions is analyzed.It is found that compared with the reference group,the influence of drilling fluid flow pattern index on the leakage is the largest(up to 10 times).The influence of dynamic shear force is small(0.3 times),and the influence of seam width and seam height on leakage is medium(5 times).Under the condition of opening pressure,the variation of fracture width is power function.Combining the drilling fluid and plugging material used in the field,the gas invasion situation after drilling cracks is simulated.When the drilling fluid has not passed the fracture,the maximum gas invasion velocity of vertical fracture reaches 2L/min under the conditions of maximum invasion pressure difference and maximum fracture width;when the drilling fluid forms a certain plugging layer in the fracture,the wider the fracture in a certain range,the stronger the plugging pressure-bearing capacity,reaching 1.12MPa;when the gas breaks through the plugging layer,the gas invasion velocity will reach a peak value of 2.36L/min.A high temperature and high pressure two-phase flow model was established by Ansys Fluent finite volume simulation software.The process of gas invasion under field conditions was simulated,and the mechanism of displacement gas invasion and negative pressure gas invasion was revealed.The stable value of gas-liquid displacement gas invasion velocity is0.1kg/s-0.6kg/s,and the influence of fracture width and fracture height on gas-liquid displacement gas invasion is the greatest,followed by pressure difference and drilling fluid density;the stable value of negative pressure invasive gas invasion is 12kg/s-50kg/s,and the pressure difference,fracture height and fracture width have the greatest influence on the flow velocity of negative pressure invasion;the dynamic pressure fluctuation near the fracture and the change of stress on the wall near the fracture are easy to promote natural fracture.And induce fracture development,aggravate leakage,easily lead to wellbore instability and collapse.By introducing some new plugging materials,a plugging system with good positive and reverse pressure-bearing capacity was constructed,and the plugging pressure-bearing capacity values under experimental conditions were obtained.The optimum formula of plugging slurry obtained by orthogonal test has good plugging and pressure-bearing effect.The positive pressure is 10.2 MPa and the reverse pressure is 3.2 MPa.Compared with the field plugging slurry system,the positive pressure and reverse pressure-bearing capacity are increased by 39%and 73%,respectively,which has better sealing and gas-blocking effect.