Study On Heat Transfer Characteristics In Wellbore During Cryogenic Fracturing

Economical and high efficiency stimulation technology is important for the development of deep mineral resources.High temperature and deep buried depth lead to strong diagenesis,forming rock with low porosity and permeability.High initiation pressure and poor fracturing result are always occured during a sitimulation in deep formation.Cryogenic fracturing technology employs liquid nitrogen(LN2)as working fluid,facilitate cracking the tight and hot formation by inducing thermal stress in it,and forming interconnected fracture network over a large volume.Higher temperature difference between fracturing fluid and target formation would lead to better stimulation result.It means cryognic fracturing technology is suitable for Hot Dry Rock formation.This research focus on the fluid flow and heat transfer during the transportation of cryogenic nitrongen in fracturing string.The numerical simulation and experimental study have been carried to investigate the transient pressure and temperature distribution in wellbore during cryoigenic fracturing,and the pumping program and some key parameters are optimized.The detailed results are as follows:An experimental system simulating the heat and mass transfer of LN2 in fracturing string under high pressure and high Reynolds number(Re)has been developed.The heat transfer intensity was measured with steady state method.The effects of temperature,pressure,mass flow rate and heat flux on heat transfer was obtained.Comparing the prediction accuracy of the existing heat transfer correlations to the experimental results,the ource of error was analyzed.A novel heat transfer prediction equation was correlated with non-dimensional analysis method.A high accuracy correlation for the prediction of heat transfer intensity under fracturing conditions was obtained.With the new correlation,a transient numerical model for the calculation of temperature and pressure in wellbore is developed.The heat and mass transfer during cryogenic fracturing,inject cryogenic nitrogen in fracturing tubing and nitrogen in annulus,is simulated.The effects of well depth,thermal gradient,injection mass flow rates on temperature and pressure distributions were analyzed.The results indicate that the injection rate in annulus have little effects on temperature.The increase of well depth will increase the lowest available fluid temperature.Increase the LN2 injection rate can reduce the downhole fluid temperature effectively,but it increases the injection pressure dramatically as well.It can be concluded that the co-injection cryogenic fracturing model can not transport the cryogenic nitrongen to the bottom effectively.Through analyzing the heat flux in wellbore,it is found that the thermal capacity of fracturing string and low heat insulation in annulus under high pressure are major parameters affecting the wellbore temperature distribution.A novel fracturing model,named precooling of fracturing string and partial isolate fracturing,was presented.The precooling efficiency of fracturing string can be improved by reduce the wellhead pressure during precooling process.Blocking the annulus with packer near the perforated intervals improves the thermal insulation efficiency of annulus,and improves the transport efficiency of LN2.A natural convective heat transfer model is established to calculate the temperature distribution in annulus during cryogenic fracturing with the novel fracturing model.The consumption of LN2 for the precooling of fracturing string and the downhole fluid temperature are calculated.The results show that the novel fracturing model can transport the cryogenic nitrogen to the bottom with high efficiency,and the available downhole working fluid temperature can be reduced lower than-180?.The consumption of LN2 for the precooling of fracturing string increases with the increase of well depth.The available downhole working fluid temperature decreases with the increase of well depth and thermal gradient.Take the development of Hot Dry Rock(HDR)resources with Enhanced Geothermal System(EGS)technology as example,the effects of volume fracture networks induced by cryogenic fracturing in tight formation is evaluated.The interconnected fractures created by thermal stress contribute to prolong the economic life of EGS.Implement cryogenic fracturing in production well can bring in better stimulation result than in injection well.Cryogenic fracturing is an effective way to settle the flow channeling problem during the development of HDR with EGS technology,and to improve the overall development efficiency of HDR geothermal resources.This research investigated the transient temperature and pressure fields in wellbore during cryogenic fracturing systematically.The newly proposed fracturing model can improve the transportation efficiency of LN2 in fracturing string effectively.The results are expected to offer a solid foundation for the investigation and field application of cryogenic fracturing technology.