Study On The Safe Discharge Pressure Of Gas Drilling

During gas drilling, annulus blockage might be caused by the wellbore instability that possibly results from gas production, water production or improper construction. With the blockage, pressure in the annulus agglomerates due to the injection or formation of gas. When the pressure agglomerates beyond a certain extent, high-pressure gas breaks the debris-clogging segment, forms high-speed airflow, carries debris to the wellhead, and causes shock and erosion to the wellhead. In order to guarantee wellhead safety, it is necessary to know the shock strength and the erosion rate at the wellhead under such circumstances. Hence, in this paper we explore safety issues of gas drilling under such special conditions. The conducted research work and the obtained conclusions are listed below.(1) We couple the formation seepage model, the gas production model, the state equation and the Darcy equation, and then establish a computational model for the pressure at the bottom hole. Through the computational model, we obtain a pattern of the variance in pressure beneath the clogging segment as a function of time following the annulus blockage: the pressure at the bottom grows rapidly immediately after the blockage and slowly afterwards, and finally gets stabilized when the gas production equals that of the gas leakage in the clogging segment.(2) Through experiments, we study the pattern of dense gas-solid two-phase flow at the moment when the pressure breaks debris. Using experimental data, we quantitatively analyze the relationship among the blockage pressure, the debris mass, and the debris velocity. We conclude that the velocity of debris particles quasi-linearly grows with blockage pressure and drops when the total mass of debris particles increases.(3) Through simulations, we investigate the pattern of the high-pressure high-velocity gas-solid two-phase flow flowing from the bottom to the wellhead after the pressure breakage, and obtain the relationship between the blockage pressure and the flowing velocity. We build a simulation model for both the four-direction wellhead and the wellhead with a rotary blowout preventer. We compare the flow field of the high-pressure high-velocity gas-solid two-phase flow at both kinds of wellhead, and find that under the same conditions, both the flow velocity and pressure at the four-direction wellhead are respectively lower than those at the controllable rotating wellhead. By changing the boundary conditions, we are able to conclude that the four-direction wellhead has a stronger capability to relieve pressure when the pressure at the bottom of the well rises.(4) We propose a computational method to calculate the force at the wellhead based on the formation condition and the engineering condition, and provide a theoretical basis for safe construction after blockage. On the basis of this paper, we can quantitatively analyze the pressure needed for relieving the blockage and provide guidance for blockage relieving after field sticking.