Multiphase Flow Regularity of Wellbore in Acid Natural Gas-invading Period

Abstract

Currently, unbalanced drilling technique is widely applied in the exploration and development process of acid oil and gas reservoir. However, the acid natural gas that invades under high-temperature and high-pressure conditions can easily result in underground failures, such as well kick and blowout. Thus, a calculation model for the multiphase flow of a wellbore that is invaded by acid natural gas was established in this study based on the principle and method of gas-liquid two-phase flow to reveal the multiphase flow regularity inside a wellbore after the invasion of acid natural gas, considering the phase change and solubility of acid gas under high temperature and high pressure. Meanwhile, a solution for the model was provided. Finally, in the case of the H75-29-9 gas well conditions in Jilin Oilfield, China, the change rule of physical properties and the change features of parameters, such as mud pit increment and pressure drop in the bottom hole after acid natural gas (methane and hydrogen sulfide) penetrated the wellbore, were obtained through an analog calculation. Results indicate that the H 2 S solubility inside the wellbore is greater than the CH 4 solubility. Moreover, H 2 S is 135 times more soluble than CH 4 in the bottom hole. The density of acid natural gas mutates near the wellhead. High H 2 S content implies that the mutation location is near the wellhead. The density mutation location of pure H 2 S mutates 400 m away from the wellhead, whereas the density of a 50% H 2 S mutates 900 m away from the wellhead. Within the same overflow period, a high H 2 S content implies a small pressure drop value in the bottom hole, mud pit increment, and shut-in casing pressure, and small volume fractions of gaseous phases at the wellhead and in the center of the wellbore. Backpressure and the gas-invaded quantity at the wellhead have great influences on pressure in the bottom hole, that is, a great backpressure at the wellhead implies a low pressure drop in the bottom hole. Moreover, less gasinvaded flow quantity implies a small pressure drop in the bottom hole. This study provides a theoretical basis for wellbore pressure control and well-killing construction under complicated conditions.