Multi-stage Throttling Reduces The Risk Of Annular Pressure In High Temperature And High Pressure Gas Wells

With further exploration and development of natural gas, wellbore integrity issues have attracted more and more attention. Especially in the production process of gas well with high temperature and high pressure, the problem of sustainable casing pressure (SCP) becomes more and more outstanding, poses a serious threat to safety and attracts the attention of experts and scholars. The change of temperature in annular is a critical factor leading to annular pressure. High temperature will also aggravate the annular casing corrosion in sour gas fields, which may lead to environment pollution and casualty. Therefore, it is necessary to take certain measures to decrease annular temperature so that the annular pressure can be reduced effectively to improve wellbore integrity and extend the production life of gas well.This paper presents a new method to control pressure in annular, namely in high temperature and high pressure gas well, down-hole choke is installed at a suitable position in the bottom hole to reduce annulus pressure caused by temperature increase in the wellbore. For high temperature and high pressure gas wells, down-hole choke is not capable of lowering the temperature and the pressure at the wellhead to an ideal value, so multistage throttle technology may need to be used. The down-hole throttling mechanism is analyzed and comparative analysis of the down-hole throttle and surface throttle is made. Based on theories of heat transfer and fluid dynamics, laws of conservation of momentum and conservation of energy, models of temperature drop and pressure for down-hole throttling are used to establish calculation models of the pressure and temperature distribution in the wellbore for down-hole throttling in gas wells with high temperature and high pressure and sensitivity analysis of wellbore pressure and temperature distribution is also conducted. According to the theory of heat transfer, calculation model for forecasting the annular pressure and temperature is developed. Based on calculation models of temperature and pressure drop for throttling and mechanism of annular pressure caused by thermal expansion, nodal system analysis method of gas wells is used to determine the number of down-hole chokes on the basis of the temperature, the pressure at the wellhead to be achieved and allowable annulus pressure. At last, a domestic gas well which applies the multistage throttling technology to reduce the annulus pressure is calculated.