Study On Gas Invasion Mechanism And Wellbore Pressure During Well Shut-in In Deepwater Drilling

Due to the low temperature and high pressure,complex geological conditions,frequent typhoons in the deepwater of South China Sea,preventing and controlling kicks or blowouts become to be an urgent major issue for deepwater oil and gas resources exploitation.Herein,it is of great importance for preventing and controlling kicks or blowouts under the conditions of well shut-in by revealing the mechanisms of gas invasion and gas bubble growth and migration from the point of gas invasion characteristics,and obtaining the wellbore pressure prediction method.The formation type determines how the gas intrudes into the wellbore.For vertical fractured formations,a physical model of flat fracture is proposed for gravity displacement gas invasion,the basic principle of gravity displacement gas invasion is revealed,and the pressure window of gravity displacement gas invasion is obtained.The control equations of gas and liquid phase in a fracture and its solution method are established.The laws of gravity displacement gas invasion are revealed from the perspectives of gas-liquid interface,pressure and velocity distributions in the fracture,and the effects of bottom hole pressure,fracture size and surface tension on the gravity displacement gas invasion are analyzed.For permeable formations,considering the diffusion and negative pressure difference gas invasion,and the mass transfer of gas in the filtrate area,inside and outside cake area of the filter cake,the seepage-diffusion coupling model of gas invasion and its solution method are established.The gas invasion laws under the seepage-diffusion mechanism are discussed by introducing the seepage-diffusion ratio factor.Only when the factor is small,the contribution of diffusion gas invasion to the total amount of gas invasion will be more obvious.Under the condition of example,when the factor is less than 1%,the contribution of diffusion gas invasion is greater than that of seepage gas invasion.Based on the experimental simulation method,the bubble growth and migration behaviors in wellbore during the well shut-in in deepwater are studied.During the bubble growth process,the bubble volume is reduced and the time is prolonged while it leaves the nozzle with the increase of pressure under the same conditions.During the bubble rise process,the degree and cycle of the bubble deformation decreases,the transform of the bubble motion from linear to helical speeds up and the time of the bubble accelerated rising decrease with the increase of pressure under the same conditions.Therefore,the pressure increase is conducive to the stability of the bubble rising.Based on the experimental data,the prediction method of bubble initial equivalent diameter and the bubble rising velocity correlation based on drag coefficient are proposed by comprehensive analysis of the bubble mechanical characteristics during its growth and rise process,which provides support for predicting accurately the gas rising velocity in non-Newtonian fluid in deepwater wellbore.The gaseous bubble will change into hydrate under the conditions of low temperature and high pressure in deepwater and then form bubble covered with hydrate film(HF-bubble).The dynamic behavior of HF-bubble in deepwater wellbore is revealed by a simulation experiment.The hydrate film growth on the stationary bubble surface is axial symmetric,and there will be shrinking effect.However,its growth on the rising bubble surface often shows asymmetric.Compared with the pure gaseous bubbles under the same conditions,all of the swirling amplitude,the swirling frequency and the rising velocity of the HF-bubble are decreased during the rising process.Based on the experimental data analysis,a new correlation for the aspect ratio of the HF-bubble is proposed,a prediction method for the grown thickness of the hydrate film is obtained,and a new correlation of the HF-bubble rising velocity based on drag coefficient is established,which solves the problem that the existing model cannot be applied directly to the hydrated film bubble.For the well shut-in condition in deepwater drilling,considering the gas expansibility,drilling fluid compressibility,wellbore elasticity and drilling fluid filter loss,based on the different gas invasion models and gas migration models,the prediction model of wellbore pressure for the negative pressure difference gas invasion and the gravity displacement gas invasion and its solution method are established.The effects of the formation pressure,reservoir opening thickness,formation permeability and filter cake permeability on the bottom hole pressure are analyzed.Furthermore,the gas migration laws in wellbore during well shut-in are revealed,and the hydrate phase change has a significant inhibitory effect on the gas rising,and on this bisis the plates of operation safety period to evade typhoon for various formation pore diameters,drilling fluid densities and viscosities are obtained,which can provide guidance for the operation safety during well opening in deepwater.The results show that the larger the formation pore diameter and the drilling fluid density are,and the smaller the drilling fluid viscosity and the well depth are,the smaller the operation safety period will be.According to the evading typhoon time,the operators can determine whether there is a storm trap gas trap by these plates.