| In recent years,the demand for oil resources is increasing year by year.Safe exploitation of offshore oil and gas resources with abundant reserves is an important way to develop oil resources.However,the temperature in the well gradually rises with the increase of cementing depth,resulting in the accumulation of closed loop annular pressure,and there is a risk of casing damage,lost circulation and blowout.At present,in engineering applications,compressible foam is added to the periphery of the casing or nitrogen is injected into the spacer fluid to reduce the closed loop annular pressure.However,the compressible foam has low strength and is easy to break and fail during the process of pumping the spacer fluid;When injecting nitrogen,it is difficult to further reduce the closed loop annular pressure when the amount of nitrogen injected exceeds 20%.In this paper,materials such as paraffin were used to coat the surface of porous materials such as ceramsite,and a wrapping material(WM)with temperature sensitivity and pressure reduction function was prepared.The influence of coating process on the average thickness and damage resistance of the coating was studied,and the pressure reduction performance of the WM in the spacer fluid was evaluated.On this basis,the prepared WM were matched with elastic materials,hollow particles and other materials to prepare several groups of heated compressible elastic spacer fluids,and the influence of the proportion of various heated compressible elastic materials on the pressure reduction performance was studied.Then,the additives such as suspending agent,surfactant agent were selected,and the effects of the dosage of suspending agent and weighting agent on the rheology,suspension and anti-water loss performance of the spacer fluid was studied,the composition of the heated compressible elastic spacer fluid was determined.Under different well conditions,the pressure reduction performance of the spacer fluid and the compatibility with drilling fluid and cement slurry were evaluated,verifying its applicability in deep-water oil and gas wells.The specific work is as follows:(1)The pore structure of volcanic rock,zeolite and ceramsite was observed and analyzed by micro fluorescence method.According to the pore size and distribution,ceramsite was selected as the support material in the wrapping material,and the optimal size range of ceramsite was 3～4mm.Three kinds of polymer materials,including polycaprolactone,EVA emulsion and paraffin,were used to prepare WM from ceramsite.According to the homogeneity,dispersion of WM and the thickness of polymer material,paraffin wax was selected as the coating layer of WM,and polyethylene wax(PYW)was used as modifier to further improve the thickness of paraffin wax on the surface of ceramsite.The results showed that with the increased of PYW content,stirring time and stirring rate,the average coating thickness(AT)increased gradually,and the stirring damage resistance of WM also increased at the same time.The optimal conditions of wrapping test were as follows:PYW content was 7%～10%,stirring time was 3～4min,and stirring rate was 500～600r/min.(2)Three kinds of heated compressible elastic materials,WM,elastic materials and hollow particles,were added into the special ratio of spacer fluid to study the influence of these materials on the closed loop annular pressure at different temperatures.The results showed that WM could effectively reduce the annular pressure at 60℃;When the content of WM was 14%,the closed loop annular pressure decreased by 24.6%at 90℃.By comparing the reduction of closed loop annular pressure with three kinds of elastic materials,styrene butadiene block copolymer(SEBS),nitrile rubber powder(NBR)and styrene butadiene rubber powder(SBR),it was found that SBR had the best effect.When the temperature reached 90℃,the FBS could significantly reduce the pressure by 29.6%.It was found that the optimum ratio of WM,SBR and FBS was 3:2:3 when the content of heated compressible elastic material was 24%of water mass.Under this ratio,the annular pressure of the spacer fluid 90℃was only 5410 psi,which was 32.1%lower than that of the control group.(3)The effects of various suspending agents and surfactant agents on the rheology,suspension stability,water loss and flushing efficiency of the spacer fluid were studied.The results showed that xanthan gum(XC)and PC-S32S were the best suspending agents,and the best ratio of them was 2:3;When Tween80 and AOS were used as surfactant agents,the best ratio was 3:2.Under the condition of optimizing the content of heated compressible elastic material and surfactant agent,the heated compressible elastic spacer fluid with density between 1.1g/cm~3and 1.6g/cm~3was prepared by adjusting the content of suspending agent and barite.The results showed that the fluidity index n of each spacer fluid was in the range of0.5～0.8 at 25℃and 90℃,and the consistency coefficient K was within 1 Pa·s~n,which had good rheological property;In the height direction,the density difference of each part of the spacer fluid was within 0.06g/cm~3,which had excellent suspensibility;The anti-water loss met the requirement of less than 100ml.(4)The effect of heated compressible elastic spacer fluid on reducing the closed loop annular pressure in different well conditions was simulated.At 90℃,when the initial pressure increased from 290 psi to 2900 psi,the decrease of the closed loop annular pressure increased from 32.1%to 47.3%;When the heating rate in the closed loop annular increased from 0.5℃/min to 2.5℃/min,the pressure reduction range of the spacer fluid at 90℃ranged from 31.4%to 32.1%,indicating that it had excellent pressure reduction performance under different initial pressure and heating rate.Through the spacer fluid cycle heating and cooling test,it was found that the pressure reduction range was 33.3%～53.9%at the set temperature,which showed that the performance of spacer fluid was stable in the process of multiple temperature rose and fell.When the spacer fluid was mixed with drilling fluid and cement slurry in different proportions,the plastic viscosity,dynamic shear force and consistency coefficient of the mixed slurry decreased,and the mixed slurry had no obvious thickening,flocculation and early setting,which fully indicated that the spacer fluid had good compatibility with drilling fluid and cement slurry.It was found that the setting time of cement paste could be prolonged and the compressive strength of cement paste could be reduced by using the spacer fluid,but the anti-water loss of cement paste could be improved.When the spacer fluid accounted for 30%of the mixed fluid volume,the compressive strength of cement paste decreased by 67.3%,which met the requirement of less than 70%in the industry. |
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