Study On Sand Carrying Critical Velocity And Gas-water Transport Characteristics Of Gas Wells Under Advanced Discharge Measures Of Shallow Gas

Shallow gas is widely distributed in the Hangzhou Bay area of ??my country.For projects that encounter shallow gas geological disasters,the main disaster prevention measures in engineering practice are to set up advanced exhaust wells in the pre-construction stage for artificially controlled degassing.In-depth understanding of the critical flow rate of sand-carrying wells and gas-water migration properties in gas reservoirs during the exhaust process,and how to take controlled measures,reasonably control the exhaust rate,optimize the well spacing of exhaust wells,and evaluate the exhaust effect,etc.has important guiding significance.In this paper,taking the engineering geological disasters in the typical shallow gas-rich region of Hangzhou Bay as the background,based on the indoor gas-carrying sand-carrying critical velocity measurement test and the numerical simulation of gas-water migration in the gas reservoir under controlled venting,the advance and controlled venting of shallow gas is analyzed.Research on air measures.The research contents and the main results obtained are as follows:(1)The basic physical properties of the undisturbed gas storage sand were analyzed by means of geotechnical tests,and the undisturbed sand was determined to be medium-density sand;SWCC predicts the original gas reservoir pressure and the unsaturated water and gas permeability coefficients of the reservoir.(2)Aiming at the problem of sand blasting in gas wells in the process of shallow gas discharge,a set of test equipment for measuring the critical flow rate of gas carrying sand was designed,and the reliability of the test equipment was tested;and its influencing factors.The results show that the essential reason for the change of sand particle motion state is the change of its stress state;the critical flow velocity increases gradually with the increase of sand particle density,and the flow velocity-pressure drop curve will show an obvious peak;the critical flow velocity varies with the sand particle layer.The thickness increases gradually.During the controlled venting of shallow gas,the wellhead flow rate should be controlled not to exceed the critical flow rate of sand-carrying,so as to provide conditions for the controlled discharge of shallow gas.(3)Shallow gas reservoir with controlled venting process The water cone state and height in the gas reservoir are the main factors affecting the venting effect of venting wells.Wellhead flow velocity is the most significant factor affecting the water cone state and height;the greater the wellhead flow rate,the sharper the water cone state;the lower the wellhead flow rate,the smoother the water cone state;within the same exhaust time,the larger the water cone swept area,the larger the discharge cone.The better the gas effect;on the contrary,the worse the exhaust effect;the water cone theory proves the rationality of the wellhead controlled exhaust measures.(4)Based on the study of water and gas migration characteristics in the gas reservoir during the controlled degassing process,the optimal layout method of degassing well spacing and its influencing factors are analyzed,and a well spacing optimization method is given.The results show that the well spacing of the engineering advanced venting wells can be reasonably optimized for the longest well flooding time and the smallest remaining gas reservoir thickness;the original gas reservoir pressure is the most significant factor affecting the optimal placement of venting wells.For projects with small gas reservoir pressure coefficient,it is recommended to develop horizontal exhaust well layout technology to improve the exhaust effect.In the actual exhaust project,a small exhaust rate should be controlled under the conditions allowed by the construction period to avoid premature well flooding and carry out the formation sediment,so as to achieve the purpose of controlled exhaust.In addition,the setting of the well spacing of the advanced exhaust wells in the project should also consider the safe and reasonable distance from the building(structure)to achieve the purpose of high efficiency,safety and avoiding investment waste.