Yonghai Gao

Prediction of gas hydrate formation region in the wellbore of deepwater drilling

Abstract It is essential to consider the thermodynamics, temperature, and pressure conditions of gas hydrate formation in order to predict the gas hydrate formation region in the wellbore of deepwater drilling. Multiphase flow governing equations (including continuity equation and momentum conservation equation), temperature field equations in annulus and drill pipe, and hydrate formation thermodynamics equation are established based on the characteristics of deepwater drilling. The definite conditions, discrete methods, and iterative steps are given to solve the equations under different working conditions. Through examples of calculation, the impacts of relevant parameters on gas hydrate formation region during drilling, off drilling, and well killing process are discussed. The result shows that the hydrate formation region is getting small when circulation volume becomes great, inhibitor concentration becomes high, downtime becomes short, or choke line size becomes large. And the result also indicates that it is more efficient to consider multifactors to suppress hydrate formation. The parameters can be optimized based on the method.

Gas kick during carbonate reservoirs drilling and its risk assessment

Abstract The gas kick characteristics during carbonate reservoir drilling were analyzed taking carbonate reservoirs in Shunnan area of Tazhong as an example, a quantitative evaluation method for gas kick risk was established, and then a case was simulated. Looking into reservoir space characteristics and gas invasion mechanisms of carbonate reservoirs in Shunnan area based on drilling geologic data, it is found that the reservoirs are rich in fractures, pores and caves, and the gas invades into wellbore through gas-liquid replacement and differential pressure. By fully considering the gas invasion mechanism, gas migration law and wellbore temperature-pressure field, and introducing the gas volume fraction density function, a quantitative evaluation method for gas kick risk and a method for ranking well control risk were established. The case study shows that the wellhead back pressure method can be used to tell the mode of gas invasion; pore-fracture-cave and acid gas characteristics in carbonate reservoirs have stronger effect on the gas volume fraction in wellbore, and may cause hidden and severe gas kick; the effects of well depth, well diameter, drilling fluid density, drilling fluid displacement, drilling fluid viscosity and drilling rate on gas kick risks weaken in that order.