Fracture Initiation Mechanism Of Fractured-vuggy Carbonate Fracture

The production of carbonate reservoir has a great contribution to world oil and gas production.Compared with ordinary sandstone reservoirs,the matrix permeability of carbonate reservoirs is low with cracks and holes developed.Fracture and holes are the main reservoir space.As a result,carbonate reservoirs are generally difficult to meet production capacity requirements,and fracturing acidizing technology is a common stimulation technique.The first consideration for fracturing and acid fracturing is the prediction of fracture pressure in order to make a reasonable fracturing acidification program,ensuring the fracturing effect,safety and economy.To prediction of fracture pressure to the crack-hole type carbonate rocks,the existing literature does not consider the effect of karst cave and fracture on the stress near wellbore,furthermore,the influence on fracture pressure and fracture azimuth.In this paper,by using the analytical continuation method and the Cauchy integral method in complex function method,combined with Schwarz alternating method,to discuss and analyze the circumferential stress distribution under the effects of cracks and curve,including fracture pressure and fracture azimuth and its influencing factors.The paper mainly carried out the following work:(1)Establishing the physical model of arbitrary shape single hole in crack-hole type carbonate rocks.Using the analytic continuation method to solve the problem,and obtain the analytic solution of the stress distribution at any point around the hole.Further,to simplify to circular single hole,and elliptical single hole.The analytic solution of the stress distribution at any point around the single-hole round hole,and single-hole elliptical hole in the crack-hole type carbonate rocks is obtained.(2)Using the analytic continuation method,the Cauchy integral method in the complex function method combined with Schwarz alternating method to solve the complex stress field of the ellipsoidal-cave-circular-wellbore system.The stress distribution solution at any point of the composite stress field of the ellipsoid-cave-circular-wellbore system is obtained.To adopt the related geological parameters of Longnv temple structure in Longwang temple group in Sichuan basin.Through the programming,the stress distribution solution is obtained under the influence of the cave and comparing with the stress distribution solution under the influence of no cave.The influence factors of stress distribution solution are also analyzed.(3)Based on the tensional fracture criterion.The fracture azimuth and corresponding fracture pressure are obtained by numerical method.The fracture pressure and the fracture azimuth are compared with the solution without holes.The field data were fitted on site.It is found that the fracture pressure can be well fitted.(4)The influence of parameter variation on the wellbore wall stress is analyzed from the four parameters:the size of the cave,the center distance,the main stress and the angle of between the center line of the hole and wellbore and the x-axis.The effect of the cave on the fracture pressure is obtained.