Dynamic Leak-off Experiment And Model Study Of Fractured Carbonate Reservoir

Leak-off acid flows into large pore throat selectively in acidizing and acid fracturing,so pore throat enlarged forms wormhole where acid-rock reaction happens,as a consequence,wormhole increases leak-off depth and area.On the one hand,wormhole can connect flow channels;on the other hand,severe leak-off forms short acid etching fractures which cannot connect fracture-cave systems far away from wellbore.Therefore,studying the enlargement regulation of wormhole in carbonate is important for us to understand the leak-off mechanism in acid fracturing.This paper studied fractured carbonate reservoir,conducted leak-off behavioralstudy of different fluids in core with fractures using dynamic leak-off analyzer in reservoir condition.The fluids includestandard brine,slickwater and gelled acid,in-situ acid at the concentrations of 20%,15%,10%,5%(common ion effect is considered).The achievements of this paper as follows:(1)This paper analyzes acid leak-off mechanism in matrix and fracture,and different influence factors including reservoir properties,experimental conditions in laboratory,parameters and technologies of the treatment on leak-off mechanism.(2)Use artificial split cores to conduct acid leak-off experiment instead of natural fractured cores,and demonstrate the feasibility.(3)Use original designed dynamic acid leak-off analyzer and matching evaluation methods,and first come up with the digitalization method of fracture surfaces to study wormhole quantitatively.(4)Experimental results show the relationship between the ratio of acid leak-off rate to initial fracture conductivity and the initial acid leak-off concentration is exponential,vc/(kf.w)=aebJ,a,b are coefficients related to high viscosity acids,a = 0.0115×μ0.1095,b=0.2538×e0.0023μ.(5)Establish the acid leak-off model of wormhole in fractured carbonate reservoir,and present the shape of wormhole is conical.Divide the leak-off course into two parts when the fresh acid becomes residual acid at the front end.The earlier stage is wormhole growth and enlargement because of acid-rock reaction;the later stage is wormhole enlargement because of acid-rock reaction and residual acid leak-off into fractures.Methods and models for calculating acid filter loss of two stages are established.(6)Simulate the change rule of acid concentration at the front end of wormhole and the change rule of fracture length with the variation of acid concentration in acid fracturing.Simulated results conform to realistic situation,and confirm the validity of the mathematical model in this paper furthermore.