| Shale gas resources in our country are very huge.Investment on shale gas extraction is enormous;however,the output is relatively small.One of the important reasons is that nanopores develop well in shale matrix,and working fluid,under the effect of capillary force,filtrates into shale reservoirs,and reacts with clay minerals,resulting in wellbore destabilizing,reservoir damage and even other problems.Currently,oil based drilling fluids are most used in shale reservoir drilling work;however,oil based drilling fluids cost highly,cause damage to environments,and are difficult to be flowback after filtrating into shale reservoirs which results in severe damage.In order to decrease working fluid filtrating into shale reservoirs,the technical method of sealing micro and nano pores temporarily using soluble nano particles is determined.In this thesis,taking outcropping shale from Longmaxi group as research object,physical property parameters,mineral composition,pore diameter distribution and its contribution ratio to permeability were tested.Hydrophobic nano CaCO3 was optimized as nano additives and Cetyl trimethyl ammonium Chloride was optimized as dispersing agent.And then nano drilling fluids were prepared.Test results indicate that with the increase of nano particle concentration and Cetyl trimethyl ammonium Chloride concentration,apparent viscosity,plastic viscosity and yield point increase first and then decrease,and reach the maximum when concentration of nano particle is 2%.Besides,API fluid loss decreases with the increase of nano particle concentration and Cetyl trimethyl ammonium Chloride concentration.This academic dissertation put forward that testing shale matrix permeability based on pressure transmission test and then evaluating influencing of factors on sealing effect and seal removing effect.Sealing shale matrix using nano drilling fluid test results indicate that: when nano particle concentration is 1%,sealing effect is good;and core permeability rapidly decreases with time and then tends to be flatten;under the condition of 2MPa pressure difference and after 30 minutes' corrosion by 15% HCl,core permeability recovery rate reaches 83.91%~87.50%,indicating good seal removing effect.Besides,the effect of capillary force and pressure difference on drilling fluid and nano particles' entering into matrix pores was researched by tests.Results indicate that when the water saturation in shale core is small,capillary force plays the main role,and with the increase of water saturation,the effect of pressure difference increases.During nano particles' sealing shale matrix pores,as for pores with similar diameter with nano particles,nano particles seal pores in throat-stuck method;and as for pores with larger diameter than nano particles,nano particles seal pores in bridging method.Sealing about 1?m equivalent width fractures using nano drilling fluid tests indicate that: when nano particle concentration is 4%,sealing effect is good;and core permeability rapidly decreases with time and then tends to be flatten;in seal removing tests by 15%HCl corrosion under the condition of 2MPa pressure difference,permeability recovery rate increases with time and reaches 68.09%~74.19% of original permeability after 1 hour's corrosion;what's more,increasing flowback pressure can help improve permeability recovery rate further,and when pressure difference reaches 8MPa,permeability recovery rate suddenly increases to 85.11%,indicating good seal removing effect.During sealing micro fractures using nano fluid,particles with larger diameter and nano particles accumulate at the head of micro fractures,and nano particles accumulate at the space among larger particles,which can make the sealing zone tighter and decrease fracture permeability.Besides,large amounts of nano particles and few particles with relatively larger diameter but smaller than fracture width can enter into fractures and decrease micro permeability further. |
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