Heat Transfer Characteristics In Cryogenic Quenching Of Rock Surface With Liquid Nitrogen

The rock breaking assisted by liquid nitrogen has potential in enhancing the effi-ciency of drilling&completion in unconventional or deep geothermal reservoirs.Inten-sive heat transfer is accompanied with the contact between liquid nitrogen and rocks.As a result,the rock surface is subject to large temperature gradients and thermal stress.Based on the background of liquid nitrogen assisted drilling&fracturing,this thesis sys-tematically investigated the heat transfer characteristics between liquid nitrogen and rock surfaces.The study focused on the intrinsic properties of rock material and their influence on the cryogenic quenching of liquid nitrogen.The accompanied thermal stress within rocks was also simulated and analyzed based on heat transfer results.We first modified the conventional inverse heat transfer algorithm in the literature,in order to compute the rock surface heat flux during liquid nitrogen cooling with a better accuracy.The least square method was introduced into the conventional space marching technique to improve the compatibility of algorithm to noises in data.It is shown that the present method is accurate and efficient in calculating the wall temperature and heat flux on rock surface.Comparing to other methods,the present one is more flexible in selecting the future time steps,increasing the inversion accuracy by more than 90 folds.Cryogenic quenching experiments were conducted with three types of rock samples,namely sandstone,shale and granite.Different surface morphologies were applied on rock surfaces through sandpaper polishing,sand particle coating,groove structuring and crude oil covering.The quenching results show that the LFP temperature of rock is 130K higher than that of typical metals.This is attributed to the low thermal conductivity and porous micro structure of rock surface.Moreover,the sand particle coating and grooves served to further enhance quenching heat transfer by destabilizing the vapor film and promoting solid-liquid contact during film boiling.Visualization study of liquid nitrogen quenching was performed and the associated heat transfer dynamics were captured by the developed numerical model.It shows that liquid nitrogen firstly wetted the two ends of cylindrical sample,then the two wetting fronts approach each other towards the middle part of sample.The heat transfer intensity at the lower wetting front was found to be 2.7-4.2 times larger than that at the upper wetting front.Film boiling heat flux of vertical surface was twice that of horizontal surface.The maximum heat flux of vertical surface is 1.4-2.2 times that of horizontal surfaces.Furthermore,the heat transfer of liquid nitro-gen jet on rock surface was experimentally investigated.Experimental parameters cover a range of 1.4-2.4MPa for jet pressure,3-5cm for stand-off distance and 3×10~5-4×10~5for jet Reynolds number at nozzle exit.The results showed that film boiling was absent at the stagnation zone of rock surface;liquid nitrogen wetted the stagnation point instan-taneously after jet initialization.The maximum heat flux of liquid nitrogen jet was2.4×10~5W/m~2 which was essentially larger than that in quenching tests with stagnant liq-uid nitrogen pool.Under present experimental setup,jet pressure and stand-off distance have no effect on the maximum heat flux of liquid nitrogen jet,yet they influence the propagation of wetting front appreciably.An 80%improvement of front velocity was seen with the largest jet pressure and smallest stand-off distance.The thermal stress distribution within rocks under cryogenic quenching was studied by numerical simulation.We developed the thermal-mechanical analysis model and treated the heat transfer experimental results as boundary conditions of the model.Tem-perature fields and thermal stress were calculated accordingly and results showed that the film boiling could suppress the thermal stress considerably.Before and after film boiling,the local thermal stress at rock surface increased by three folds.For vertical cylindrical samples,the maximum thermal stress was located at the wetting front position.The max-imum thermal stress was about 6 MPa during liquid nitrogen jet and it was larger than the thermal stress in the case of stagnant liquid nitrogen pool.This work serves as an attempt to provide some theoretical basis for the field appli-cation of liquid nitrogen drilling&completion technology.It revealed the influencing mechanism of rock material's properties on the quenching heat transfer and could poten-tially guide the surface modification design for heat transfer enhancement.