Calculation of the wellbore temperature and pressure distribution during supercritical CO_2 fracturing flowback process

摘要

Supercritical CO2 fracturing is a new type of waterless technology developed in recent years. When the supercritical CO2 flows back after fracturing, the formation water is lifted by the upward movement of CO2. Since the pressure gradually decreases along the wellbore and the temperature also decreases due to the CO2 expansion, it is easy to result in the formation of hydrate under the conditions of high pressure and low temperature. In order to prevent the clogging of the wellbore due to CO2 hydrate, the wellbore temperature and pressure need to be predicted and regulated. Based on the Span-Wagner CO2 gas state equation and the Fenghour gas transport equation, combined with the classical wellbore flow heat transfer model, a supercritical CO2 fracturing flowback wellbore flow model with heat source and sink considered is developed, the dual coupling solution of axial and radial borehole is realized by iterating the pressure and temperature and coupling of tubing-annulus-formation. The results show that the wellbore pressure and temperature both decrease from the bottom to the top of the well, which is similar to the flow in oil and gas production. The parameters such as discharge output, tubing size, formation temperature gradient and pressure gradient have a great influence on the wellbore temperature and pressure. The reduction of the discharge output and the increase of the tubing size can effectively keep the wellbore temperature high and reduce the risk of CO2 hydrate formation. The discharge time has almost no effect on the wellbore pressure, and only slightly affects the wellbore temperature. The results can provide guidance for the study of supercritical CO2 fracturing flowback. (C) 2019 Elsevier Ltd. All rights reserved.