Casing Damage Mechanism Of Thermal Recovery Wells

Heavy oil thermal recovery has greatly enhanced the production of oilfield,but the injected high temperature steam makes casing withstand great thermal stress and thereby reduces the service life of casing.Casing damage is a major problem in the process of thermal recovery of unconsolidated sandstone reservoir.Casing damage will decrease the producing degree of oil reserves,resulting in increased plane contradiction and affected stable production and EOR of oilfield.Moreover,casing damage will cause huge economic losses to the oilfield.It costs a lot of money every year to drill new wells and repair damaged wells,which destroys the economic benefits.Therefore,the study on casing damage in thermal recovery wells in this dissertation will provide theoretical reference for prevention and control of casing damage in thermal recovery wells.Since production in 2010,Zhong 32 well area of Xinjiang Fengcheng oilfield has encountered 86 casing damage wells in 3-4 years.Casing damage forms includes casing shrinkage,casing rupture and casing failure.This dissertation carried out casing damage mechanism study in Zhong 32 well area of Fengcheng oilfield,and the main achievements and results are as follows:(1)In this dissertation,firstly,the research status of casing failure in thermal recovery wells at home and abroad is analyzed.Casing damage wells data is collected and the main form of casing damage is analyzed.The effects of damage location,casing size,injection parameters,cementing quality and sand production on casing damage of thermal recovery wells are considered,which provides field data for this dissertation.(2)According to analysis results of casing damage wells data,the thermal-structure coupling finite element model of casing-cement sheath-formation is established using ANSYS software.Temperature field,stress field and casing strength are studied in detail.It is found that the thermal stress of casing has exceeds the yield strength of N80 casing material at the first steam cycle,which provides a theoretical basis for casing material selection.(3)In view of casing thread failure phenomenon,study on residual tension of casing is carried out to investigate the effect of steam injection cycle on residual tension.This study provides theoretical data for the control of injection cycle and casing safety.(4)Based on the analysis on field geological conditions,three layers model of sandstone-mudstone-sandstone is established to analyze the thermal stress of casing and cement sheath at different injection cycle.Due to the difference of thermal parameters of sandstone and mudstone,the distribution of thermal stress in casing and formation is uneven,which will lead to casing damage.(5)To consider the effect of unqualified cementing quality on casing stress,a finite element model of cement sheath with circumference defect is established.The results indicate that stress and displacement of casing with cement defect are both greater than that without cement defect.Meanwhile,Von Mises stress and strain of casing with cement defect is obtained,to provide theoretical data for the relation between cementing quality and casing damage.