Study On The Mechanism And Damage Effect Of CO₂ Boiling Liquid Expanding Vapor Explosion In CO₂ Flooding

As a very important geological storage measure in carbon capture and storage technology, CO2 flooding to enhance oil recovery is now widely applicated, which can both sequester CO2 and improve crude oil production. But during the highly concentrated CO2 injection process with high pressure, the severe boiling liquid expanding vapor explosion(BLEVE) accident is entirely possible if the integrity of injection pipe is destroyed. Then domino accidents may be generated by the consequent shock wave and projectiles, toxicosis, asphyxia and frostbite accidents may be caused by the leak and dispersion of CO2 and so as blowout, which would bring about serious threat to people’s life and property. Therefore, effectively evaluating the CO2 BLEVE risk in CO2 flooding and proposing prevention and control measures are important in CO2-EOR safe production.The mechanism and damage effect of CO2 BLEVE in CO2 flooding were studied in this paper. Based on the theoretical analysis of CO2 BLEVE thermodynamic process, a small experimental system was designed and built, a large amount of tests were carried out to research on the typical process of CO2 BLEVE and the influencing factors, series of field tests were put up on exploiting the damage effect of CO2 BLEVE to the injection pipes. Experiments were also finished to test the propagation of blast wave after the CO2 injection pipe rupture in BLEVE mode.1) Based on the basic properties of CO2 and theories of heat trasnsfer and bubble thermokinetics. The thermodynamic process of CO2 BLEVE was divided into five stages, the initial rapid release of gas, the instability of gas-liquid mixture, the nucleation and boiling of the superheat liquid, the generation and development of bubbles and the release of two-phase flow from the pressure vessel. Some typical parameters were theoretically described in each stage, which fitted in with the experimental phenomena2) A small experimental system was designed and built and the CO2 BLEVE process and critical conditions were experimentally studied. The results showed that there was a repressurization period for the top dynamic pressure sensor at the beginning of the rapid phase change of CO2 after the vessel ruptured and an overpressure spike was generated, which was the typical charateristic of BLEVE. Once the vessel failed and the stability of two-phase was destroyed, the microscopic movement of the fluid included the up and down swell of two-phase layer formed at the gas-liquid interface and the movement of homogeneous and heterogeneous nucleation bubbles towarded to the center of the container. Finally, the two-phase fluid continued spraying with white mist and a large number of ice particles appeared in the late period.3) From the perspective of the explosion response pattern and the fluid initial state, the influcences of relief size, initial pressure, initial temperature and the heat stratification were experimentally researched. The results showed that suitable values of relief size and filling ratio were needed to form a BLEVE senario, the latter was controlled by initial pressure in this paper. Within the scope of experiments in this study, the most advantaged conditions for BLEVE is the relief size of 5 mm~10mm and the filling ratio of 85% with initial pressure 8 MPa. Furthermore, the increament of initial temperature woud promote the formation and the degree of BLEVE. The higher the level of heat stratification was, the lower the BLEVE would be. All the experimental data concluded that the supercritical CO2 applied in CO2 flooding had great risk in BLEVE and would be more serious as the initial pressure and temperature increased.4) Scaled field tests were also conducted to explore the damage effect of CO2 BLEVE to the P110 steel injection pipe used in CCS-EOR. The results showed that a distinct BLEVE phenomena was found after the corroded injection pipe failed under CO2 internal pressure and the BLEVE risk increased as the failure pressure droped. Injection pipes with longitudinal corrosion groove failed along the defect line with ductile fracture and then changed to brittle rupture when the fracture extended to the both ends, the failure pressure decreased with the increment of defect length and depth, while defect depth played a more important role. Pipes with general corrosion defect failed with tear fracture and the failure pressure is smaller than that with longitudinal groove. The axial tensile load applied to defected pipe didn’t change the rupture pattern but reduced the degree of burst as well as the failure pressure, which meant a more serious state of BLEVE.5) The blast wave generated after the rupture of CO2 injection pipe in the way of BLEVE was also experimentally tested. The results showed that blast wave generated after CO2 BLEVE was resulted from both the gas expansion and the liquid boiling. Pipes ruptured with bigger failure pressures got larger blast wave peak overpressures and decayed faster with the increasing distances.