| AT block is located in the northern part of Hexiwu buried hill belt in Langgu depression.The main pay formation are Fengfeng Formation and Shangmajiagou Formation of Ordovician.The reservoir is buried deep(4700?5800m)and has high temperature(170?180?).The VES volumetric acid fracturing pattern with large-scale(fluid volume>1000m3)and high injection rate(>8.0m3/min)has been adopted and a breakthrough in exploration has been made.The conductivity of acid etching fracture is one of the key factors affecting the effect of acid fracturing.Under the condition of large-scale and high-rate for acid injection,the mechanism of VES acid etching fracture conductivity is seldom studied,which affects the pertinence of acid fracturing treatment design.As a result,Well AT-1,3 and 4 yield high oil and gas production and Well AT-2 yields low production.Therefore,experimental study on etching morphology and conductivity characteristics of VES acid has been carried out to clarify the regulation law of conductivity of VES acid etching fractures,which is of great significance to support and guide the optimization design of acid fracturing treatment in AT block.A complete set of experimental methods for conductivity of acid etching fractures was established around the self-developed "test system of acid fracture conductivity",including the experimental design method considering matching of geological and engineering parameters and 2-D fractal dimension characterization method of fractures based on fractal theory.The rheological properties and acid-rock reaction rate of VES acid and gelling acid and gelling acid were compared experimentally,so as to reveal the reasons for the difference of etching morphology and fracture conductivity between VES acid and gelling acid.The effects of mineral composition,acid concentration,acid injection rate and acid injection time on the fracture morphology and conductivity characteristics of VES acid etching were studied experimentally,thus,the formation mechanism and regulation mechanism of VES acid etching morphology and conductivity in AT block were clarified.Research shows that:(1)In fresh acid state,VES acid viscosity is lower than gelling acid and acid-rock reaction rate is higher,so the dissolution is larger,while the residual acid state is opposite;Because the flow stability of VES acid in fracture is lower than that of gelling acid,VES acid etching fracture surface shows "dot" etching characteristics,while the surface of gelling acid etching is characterized by "channel" etching morphology,thus the conductivity of the former is lower than that of the latter under medium and high closure pressure.(2)Under the loading of closure pressure,the fractal dimension and average fracture width of acid etching self-supporting fracture in Fengfeng Formation decrease rapidly;The conductivity of acid etching fractures after sanding is stable within 10.35-34.5 MPa.The maximum closure pressure that acid etching fracture could withstand(i.e.the closure pressure when the conductivity of the acid etching fracture approaches zero)increases from 27.6 MPa to 62.1 MPa.(3)Increasing acid concentration,acid injection rate and prolonging acid injection time can increase fractal dimension and average fracture width of fracture space in Shangmajiagou Formation,the stable stage of conductivity is prolonged by 24.15 MPa,and the conductivity is increased by more than 200?m2·cm in the stable stage.The maximum closure pressure that acid etching fracture can withstand increased by 13.8 MPa.(4)"dot" etching morphology has fewer contact points on the fracture surface,and the fracture compression is rapid under the loading of closure pressure,and the retainability of its conductivity is poor,while "channel" etching morphology has the opposite effect.Under the same etching morphology,the change rate of fractal dimension and average fracture width with closure pressure determines the stability of the conductivity of acid etching fractures.Based on the experimental knowledge,with the aim of stability of conductivity under medium and high closure pressure(>10.35 MPa),it is suggested that the acidizing with sand adding process should be adopted in Fengfeng formation reservoir to solve the problem of poor stability of conductivity under large-scale and high injection rate conditions;and the reservoir of Shangmajiagou formation should increase acid injection rate and acid injection time so as to improve the stability of conductivity.The application of optimization suggestion in acid fracturing construction has achieved good production. |
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