Study On Temperature And Pressure Field Model Of Supercritical Carbon Dioxide Fracturing Wellbore

Shale gas resources occupy a pivotal position in the future oil and gas exploration and development, and hydraulic fracturing technology is more widely used with the vigorous development of shale gas. The drawback of excessive dependence on water of hydraulic fracturing technology has become increasingly prominent with the large scale fracturing needed for shale gas development. Thus waterless fracturing technology is in the ascendant, and supercritical carbon dioxide becomes the ideal medium of waterless fracturing with its unique physical and chemical properties. Accurate distribution of wellbore temperature and pressure is the basic for the calculation of temperature and pressure in the fracture, but the conventional model can’t describe the interaction among temperature, pressure and thermal physical parameters during the process of supercritical carbon dioxide fracturing. Therefore, it is necessary to establish a unsteady model coupling temperature and pressure to describe the physical process, thus providing some support for subsequent development of supercritical carbon dioxide fracturing.The thermal physical parameters of carbon dioxide was calculated by Span-Wagner model and Vesovic model, then the characteristics of flow and heat transfer and the rule of phase distribution during the process of supercritical carbon dioxide fracturing was analyzed. The contrasts calculated by with vertical heat transfer model and without vertical heat transfer model indicate that vertical heat transfer has a little effect on wellbore temperature distribution, so it can be ignored within the scope of project. On the basis to simplify the model, an unsteady coupling model with shifty physical parameters is established. Staggered grid method and double iteration procedure of temperature and velocity are used for numerical calculation. By analyzing the results, the following conclusions and understandings are achieved in this thesis:The contrasts calculated by with vertical heat transfer model and without vertical heat transfer model indicate that vertical heat transfer has little effect on wellbore temperature distribution, and can be ignored within the scope of project. On the basis of this conclusion to simplify the model, an unsteady coupling model with shifty physical parameters is established. Staggered grid method and double iteration procedure of temperature and velocity are used for difference discrete numerical solution. By analyzing the results, the following conclusions are achieved:(1) The thermal physical parameters of carbon dioxide change with temperature and pressure. Therefore, they could not be treated as constants during the process of fracturing.(2) The unsteady coupling model with shifty physical parameters established in this thesis overcomes the shortcomings of conventional models and can describe the intercoupling among temperature, pressure and thermal physical parameters. Besides, it can represent the unsteady process of both flow and heat transfer.(3) Carbon dioxide can reach supercritical state in bottom hole in the present treatment condition. Injection temperature and ground temperature gradient have a strong influence on bottom hole temperature and a weak effect on bottom hole pressure. Injection pressure and tubing roughness have a strong influence on bottom hole pressure and a weak effect on bottom hole temperature. Injection rate and tubing size have a significant effect on both bottom hole temperature and pressure. Furthermore, bottom hole temperature and pressure are almost not affected by lithology.(4) The temperature difference caused by Joule-Thomson Effect can be ignored during supercritical carbon dioxide fracturing process. The heat caused by friction can’t be ignored unless under a small friction gradient.(5) According to the curves of limited depth and critical depth, whether a reservoir is suitable for supercritical carbon dioxide fracturing and its corresponding treatment parameters can be estimated.(6) This model can be applicable for wellbore temperature distribution forecast of hydraulic fracturing, drilling and water(steam) injection operation with a little correction.