The Effect Of Gas Phase Change Of Wellbore Oil On Well Control Safety

When the overflow of supercritical acidic gas intrude into the wellbore during drilling, with the temperature and pressure drop in the process of the rise, it will be transformed into gas in critical point. The change of phase will lead to the change of the volume of acidic gas into the wellbore, which makes the pressure of the liquid column of the wellbore have a change, and it will lead to the security problems of well control. Therefore, the study of the law of oil and gas phase change and analysis of the influence of phase change on well control safety are important to ensure the well control safety of acid gas reservoirs.Based on the multiphase flow in wellbore, the dynamic distribution of the phase change of the supercritical fluid in the wellbore and the annular space of the well bore in the overflow, this paper forms a set of dynamic analysis method in killing well of supercritical overflow, and then matches a complete set of effective engineering measures of well control safety.The main results of this paper are as follows:(1) The critical temperature and critical pressure can be used to determine whether the fluid in the reservoir is in a supercritical state. For single component of reservoir fluid, it can directly reference the critical value table to determine. For multicomponent of reservoir fluid, it will use the pseudo critical parameters to calculate the multicomponent critical value, and use the pressure and temperature of each component to determine. In the high temperature and high pressure reservoir, the reservoir fluid is mostly in the supercritical state.(2) After the reservoir fluid in supercritical state intrude into the wellbore, with the temperature and pressure less than the critical value, the phase change will result in the change of volume. The change degree is determined by the distance between the temperature and pressure of the supercritical and the critical point. The more it approach the critical point, the greater supercritical reservoir fluid temperature and pressure drop path directly into the gas area will change than early entering the liquid zone and then the gas zone.(3) In this paper, the vertical pipe flow is divided for four flow patterns of bubble flow, slug flow, disturbance flow and annular flow by Hasan and Kabir method, and the calculation formula of the flow pattern is given. The calculation of the Z-factor is used by the Dranchuk-Abou-Kassem method (D-A-K method), and solved by Newton iteration method.(4) Based on the known ground observation parameters, such as shut vertical pressure, closed well casing pressure and initial discharge flow, this paper employs the equation of state of real gas, and the numerical calculation method of setting the initial value, dividing micro segment, piece wising calculation, overlying the test value, and retrieving the initial state of the wellbore of after the overflow and before the killing well.By calculating the height of initial overflow and the distribution of gas-bearing ratio,the initial state of the wellbore was demonstrated successfully.(5) In this paper, according to the known initial state of the wellbore, the killing well of the flow, the height of the overflow, the gas-bearing ratio,the pressure distribution of the well bore and the pressure of the wellbore are calculated by using the numerical calculation method of setting the initial value, dividing micro segment and piece wising calculation. It provides a reliable numeric basis for the implementation of casing pressure control in the process of killing well.(6) Three overflow conditions of the killing well are calculated in this paper. The results show that under the condition of killing well, due to the casing pressure, the overflow of the supercritical state in the process of rising in wellbore is difficult to appear the sudden change of the volume caused by the phase change. The more the component of acidic, the closer to the critical point of the volume expansion, the greater the rate of the change of the volume expansion, the shorter time of the appearance of maximum casing pressure. It shows that the precise control of casing pressure during the killing well is the key to eliminate the hidden danger of well control.(7) The pressure back method and the closed circulation method are effective measures to assure the well control safety of the supercritical overflow. Under the condition of the fractured reservoir, the pressure back method can be used. On the contrary, the closed circulation method can be used. In this paper, the design of the calculation of construction parameters and the construction procedure are regulated. It proposed the specific scheme of closed circulation system. There are two main functions:to prevent the leakage of acid gas and safe burning of acid gas.The research results can provide a strong theoretical support for the construction work, and provide a complete set of the engineering measures of well control security for the field operation so as to reduce the serious consequences caused by the process of drilling, and improve the safety of drilling.