Mechanical Analysis Of Casing Damage Well In Restoration And Fracture Process And Its Application

Along with oilfield development, casing damage problem becomes more and more serious around the world, directly affecting oil production. A series of restoration techniques and detection techniques are developed for treatment of casing damage well after decades of research at home and abroad, but reproduction cycle casing damage wells after restoration cannot achieve expected effect, especially for casings after mechanical reshaping without theoretical study or engineering case of fracturing. Therefore, based on overall analysis on present status of casing damage well, cause and mechanism, types and detection techniques, this paper focuses on mechanics property of casing, cement mantle and formation in the process of restoration and fracturing of casing damage wells, presenting theoretical basis for design and field test of restoration and fracturing. Finally a series of restoration and fracturing combination technique for casing damage wells is established for Daqing Oilfield, breaking through the limit that fracturing cannot be performed after restoration of casing damage well. This study is carried out through the following five parts:First, aiming at present working condition of casing and cement mantle before casing damage, stress and strain model is established, destruction criteria for casing and cement mantle is determined, and computational formula of ultimate external pressure for casing under axial tensile force and external biaxial stress is derived, providing a computational method to evaluate casing intensity and cement mantle destructiveness.Second, aiming at downhole working condition after casing damage, and based on finite element theory, ABAQUS—engineering simulation software with powerful function is applied to study destruction range of cement mantle and formation after casing damage. Study shows that when casing leaps cement mantle is completely destroyed within 1m range of casing damage, but when formation glides and deforms, cement mantle is wholly destroyed within 0.5-0.8m range of casing damage, and rock formation around 1m-1.5m range of cement mantle will suffer damage.Third, aiming at working condition in the process of pulverizing and reshaping, mechanics model of pulverizing and reshaping is set up, concept of minimal drill pressure is proposed, and computational formula is also derived. Damage criteria for first and second cementing interfaces are established according to strain coordinating condition. On the basis of damage mechanics theory of cement materials, residual bearing capacity of first and second cementing interfaces is calculated respectively. Computation cases verify the accuracy of theoretical method.Fourth, aiming at working condition of impact and reshaping, and based on impact kinetics and propagation theory of stress wave, mechanic model of impact and reshaping system is set up, damping factor for propagation of stress wave through casing and cement mantle is derived, and stress distribution of cementmantle is precisely described. Through combining catadioptric law of stress wave on cementing interface of casing and cement mantle with damage theory, damage criteria for cement mantle is established, and computational formula of plastic deformation for casing under impacting force is derived. Average error between computational solution and measured result for 5 wells is 2.45%, proving the validity of mechanic model of impact and reshaping system.Fifth, finite element model used to analyze force from casing damage point to fractured interval is set up. Lower limit distance from casing damage point to fractured interval is 0.8m in unprotected condition and 0.3m in protected condition respectively, providing a theoretical basis for successful fracturing and prevention of secondary casing damage.After a small scale experiment including 7 wells, another 55 wells field test is performed to prove the research result. Through correlation test of well diameter before and after fracturing, as well as correlation test of cement bonding before and after swelling, reshaping and fracturing, validity and practicability of research result is proved, with favorable economic benefit of 6461×104 Yuan and good social benefit. So this research has wide prospect for popularization and application.