| Acid pressure is one of the main measures to increase production in oil fields of low-permeability-tight reservoirs,and the fracture initiation location and spreading mode caused by acid pressure conditions are the most important factors affecting the fracturing effect Due to the limitations of existing simulation experimental methods,reliable conclusions cannot be drawn by simulation experiments based on comprehensive consideration of all factors.Targeting on the issues of unclear fracture propagation behavior of low-permeability-tight oil reservoirs under three-directional confining pressure,pore pressure,temperature and other formation conditions,this paper prepares a full-diameter core wrapped with a new high-strength anti-porous pressure material and conducts core-scale three-dimensional fracturing physical simulation experiment.The paper also selects an acid system that suits the low-permeability-tight reservoir of the Yanchang Formation in the Ordos Basin,studies and develops a new acid-and temperature-resistant thickeners,and conducts numerical simulation on fracturing,etc.The crack initiation spot,expansion law,formation behavior,acid-rock reaction,reduction of fracture pressure,and increase of reservoir seepage capacity under acid fracturing conditions are also studied.The main conclusions are as follows:(1)Taking nano-silica,carbon nanotubes,and trifluoropropyl trimethylsiloxane(TFPTMS)coupling agent as raw materials,an ellipsoidal nano-silica-carbon nanotube composite structure material with diameter of 10 microns and length of 4～12 microns is prepared through dehydration and condensation reaction.This material is further compounded with the ultrafine portland cement material to form a new high-strength anti-pore pressure material.The mechanical performance test indicates that the material can withstand 15 MPa pore pressure.Through hydrogen nuclear magnetic resonance spectroscopy(1H-NMR),infrared spectroscopy(FT-IR),X-ray diffraction(XRD),scanning electron microscope(SEM),X-ray photoelectron spectroscopy(XPS)and other analysis on the structure and performance of the material,it is proved the target material has been successfully prepared.The performance of the new high-strength pore pressure resistant material was determined by physical adsorption and desorption(BET),thermogravimetry(TG-DTG),isothermal calorimetry,freeze-thaw damage test,electrochemical corrosion test,contact angle test and dielectric constant-resistivity analysis and other characterization methods.The results show that the size of the material can fill the voids of cement hydration products,and further promote the next step of hydration of Ca(OH)2 in cement-based materials,thereby improving its physical-chemical properties,so as to meet the requirements of the experiment.(2)In order to select the acid fracturing fluid that suits the core of the Yanchang Formation in the Ordos Basin,the adaptability test of the acid fluid was carried out for the core,and the optimal ratio combination for the acid fluid was obtained by calculating the mass loss rate and conducting mechanical performance test.Using physical adsorption and desorption(BET),scanning electron microscopy(SEM),X-ray diffraction(XRD),inductively coupled plasma emission spectroscopy(ICP-OES)and other characterization analysis methods,the behavior characterizations are analyzed.The results indicate that the optimal acid fluid system for acid fracturing fluid that suits the core of Yanchang Formation in Ordos Basin is a combination of 2.5%hydrochloric acid and 6%acetic acid.It is also found that when using citric acid alone,calcium carbonate whiskers can generate easily,causing side effect on the acid rock reaction.(3)Through free radical polymerization,two acid-resistant and temperature-resistant thickeners AMPS-AM-DMDAAC and DMC:AM:DMDAAC are synthesized;after optimizing the experimental conditions,it can be concluded that the optimal polymerization conditions of AMPS-AM-DMDAAC are:temperature:45℃,monomer mass fraction:27%,mass ratio of AMPS:AM:DMDAAC:1:3:1,amount of initiator:0.6%,directing agent:0.15%,crosslinking agent:0.3%,and urea:1%;the apparent viscosity measured under room temperature is 135 mPa·s.the optimal polymerization conditions of DMC-AM-DMDAAC are:temperature:65℃,monomer mass fraction:27%,mass ratio of AMPS:AM:DMDAAC:1:3:1,amount of initiator:0.6%,directing agent:0.15%,crosslinking agent:0.3%,and urea:1%;the apparent viscosity measured under room temperature is 122 mPa·s.The acid resistance experiment indicates that after AMPS-AM-DMDAAC mixed with 20%HCl,the apparent viscosity was 45 mPa s,after DMC-AM-DMDAAC mixed with 20%HCl.the apparent viscosity was 31 mPa-s,proving its excellent acid resistance;The temperature resistance experiment indicates that at 120℃,the apparent viscosity of AMPS-AM-DMDAAC is 99.7 mPa·s,and the apparent viscosity of DMC-AM-DMDAAC is 32.5 mPa·s,proving its excellent temperature resistance.However,the thermal stability of DMC-AM-DMDAAC is not as good as AMPS-AM-DMDAAC polymer.Both gels can be made into crystals or solutions,which can effectively alleviate the loss of acid liquid during acid fracturing,and then deepen the migration distance and greatly improve the efficiency of acid fracturing.By comparing the performance of the two thickeners,this study finally adopted the AMPS-AM-DMDAAC thickener for physical simulation experiments.(4)Through using high-strength anti-pore pressure materials to cover the full-diameter core in three dimensions,the full-diameter core-scale three-dimensional fracturing physical simulation experiment is carried out Ordinary hydraulic fracturing,acid fracturing and acid fracturing with thickener are carried out respectively.The basic laws of the initiation and extension of hydraulic fractures in the Yanchang Formation core in the Ordos Basin are obtained:pure hydraulic fracturing only caused one main fracture;after the acid-rock reaction between the acid liquid and the core,the static pressure of the fracture decreases,and one main fracture with 2 to 3 micro-fractures are obtained;in the group of acid fracturing with thickeners,it is clearly seen that on the basis of one main fracture,multiple micro-fractures of different planes are produced,showing a volumetric fracturing mode.(5)A three-dimensional model is established to simulate the three fracturing processes and study the three-dimensional fracture morphology.The initiation and spreading of fractures during hydraulic fracturing of the Yanchang Formation in the Ordos Basin are simulated.It can be told from the different stress distribution maps when the core ruptures under the action of pore pressure that when there is a pressure difference in the three-dimensional confining pressure,that pure hydraulic fracturing should only exist one stress concentration,which generates the main fracture;the acid fracturing has 2～3 stress concentrations,with a narrow spreading surface on concentrated area;in the case of acid fracturing with thickeners,the stress distribution map shows a relatively obvious stress concentration accompanied by a number of micro-concentrations,the concentration area spreads widely and presents an irregular planar dispersion.The simulation results are also consistent with the results obtained by physical simulation experimentIn this paper,for studying the fracture initiation position and propagation law under acid fracturing conditions,the full-diameter core is wrapped with the prepared high-strength anti-porous pressure new material in three dimensions.Then,by using the acid solution,and temperature-resistant and acid-resistant thickener selected by prior experiments,the experiment is carried out after 24 hours of pre-injection of acid solution(simulating the borehole condition),with the help of full-diameter core-scale three-dimensional fracturing physical simulation equipment.Numerical simulation experiments are used to further confirm the reliability of physical simulation experiments.The final results of the experiment indicate that the simulation experiment has reached a volumetric fracturing mode with the full-diameter core is dominated by 1 to 2 main fractures and supplemented by multiple micro-fractures after the fracturing experiment.These results hold certain significance in guiding on-site fracturing constructions. |
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