| The carbonate reservoirs in Tahe Oilfield are deep low grade and matrix density but have complex natural fractures and caveswith "Red,blue,weak,small" seismic reflection characteristics.During the development process,the acid fracturing stimulation is needed to connect the sweet point..With the development and progress of technology,horizontal well staged acid fracturing application gradually increased,but is given priority to general acid pressure of open hole section.It will cause excessive transformation of reservoir-developed intervals,the effect of acid fracturing is poor on poor reservoir intervals,and unable to form high conductive flow acid fracture.In order to improve the acid fracturing overall effect,large-scale horizontal well section acid fracturing technology was studied in this paper.Through the indoor experiment,numerical simulation and theoretical analysis,the ground stress field distribution of the typical block in tahe oilfield was studied and developed high temperature acid system suitable for target block,optimized the corresponding fluid loss and diversion agent additive material,optimized design of acid fracturing technology and make successful application in the oil field.Some conclusions were listed as following.(1)By using the acoustic velocity test,Kaiser effect,the value and the direction of formation pressure in Tahe Oilfield were obtained.Then the geological model was set up to modeling the pressure distribution by the Abaqus software and the relative error value is less than 15%.The existence of the cave relieves the pressure and caused the change of stress redirection.The orientation of maximum principal stress near the faults is vertical.At the cross,branches,tips and inflection point position of the fault,stress concentration effect generated.(2)Based on acidizing experiments on carbonate samples in gelled acid,the factors which influence the ultrasonic velocity and mechanical property can be obtained,and the effects of different type of acidizing fluid is studied.The reaction rate of the acid treatment is tested and two dynamic equations of acidizing fluid reaction are obtained.The study found the increased acid injection rate can only extend dynamic acid corrosion fracture width,the effective distance of acid etching can not be increase.Through the rock plate diversion experiment,the flow conductivity of gelled acid was analyzed.According to the classic Nierode-Kruk relationship,the flow conductivity prediction model can be written as WKf=3.707Ct·e(-0.0424?).(3)As the main polymer,Acrylamide was compounded with the anionic monomer containing sulfonate group and then be made to solid powder.Then the compounded Acrylamide was initiated by redox system,crosslink acid thickening agent was produced.By mixing the thickening agent and the crosslink agent in different ratios,the optimum formula was given which can keep viscosity at 50 mPa.s after shearing 1 hour by keeping temperature at 150 ? and shearing velocity of 170 s-1.The delayed crosslinking test and temperature and shearing resistance tests showed that the formed acid had good performance.(4)In accordance with the pressure records gained from in-situ fracturing process,compared with the operating friction value and calculation value,the ultimate operating friction model can be written as ?Pf= 3.189 105D-4.8Q1.8L;(5)Temperature calculation model was set up near wellbore zones and inside the bore.Results showed that the cooling effect of fracturing liquid was limited near the bore.From 0.4 ~ 0.5 hours after fracturing construction the temperature outside this distance equaled to the formation temperature.With the injection time increasing,the temperature inside the bore was closer to the fluid temperature near the surface.The fluid cooling effect was related to the injection fluid volume near the borehole.(6)The filtration experiment revealed that the combination fluid of large and small particle balls and biodegradable fibers has better plugging effects in sealing the corrosion cracks.The large balls filled the big hole and the small balls filled the pore throat.The fibers sealed the cracks and twined around the mineral grains.All these effects provided a stable support and formed a displacement pressure bigger than 10 MPa which changed the fractures direction.(7)Fracture extension model was established to study the hydric fracture evolution after they encountered the natural cracks and caves.Natural fracture Angle,and the size of the cave and horizontal principal stress difference are considered as the dominant factors affecting acid fracture.Then based on the production forecast model,the wellbore-reservoir coupling model was set up to optimize the segment design in horizontal well,and determined there are 4 stages of the horizontal well acid fracturing The completion method,completion procedure and combined drill string were also optimized.(8)By the numerical modeling,the acid fracture length,flow conductivity and completion procedures were optimized.The optimal fracture length was 120m;the flow conductivity should surpass 300 mD.m;the injection fluid volume in single acid fracturing process was about 600m~3;the pre-flush rate was controlled to 50-60%;and the fracture fluid discharge was less than 6.0m~3/min. |
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