Simulation Study On Stress-Strain And Casing Damage Mechanism Based On Ground Surface Deformation In Sazhong Oilfield,Daqing

Daqing Sazhong oilfield has entered the development stage of extra-high water cut period after more than 60 years of development.The casing damage problem that has plagued the oil field development for a long time is becoming more and more prominent,which seriously affects the production of oil.Formation deformation is caused by high-pressure water injection and unbalanced of injection-production.Formation deformation is the fundamental cause of casing damage.Therefore,it is important to study the correlation law between the injection-production information,formation deformation and casing damage in the process of oilfield development to prevent and delay the development of casing damage.In this paper,the following results are obtained through the simulation research on stress-strain and casing damage mechanism based on ground surface deformation in Sazhong oilfield,Daqing.The DS-INSAR technology was applied to surface deformation monitoring.The surface deformation field is obtained.The surface deformation history and characteristics of Sazhong oilfield are studied and the main factors affecting surface deformation are analyzed.The production of the oilfield is closely related to the surface deformation.The changes of water injection and oil production will affect surface deformation.Water injection will lead to surface lifting and uplift,while stopping water injection(or excessive oil production)will lead to surface decline and subsidence.There is a correlation between surface deformation and casing damage.Casing damage mainly occurs in the surface uplift area and the transition zone between uplift and subsidence.The transition period between surface uplift and subsidence is the main time of casing damage.A fluid-solid coupled seepage mathematical model was established to simulate the characteristics of formation stress changes and analyze the dynamics factors affecting the formation stress.The change of injection-production volume causes the disturbance of formation stress.When the injection-production intensity increase,the variation amplitude of formation stress will increase.and the whole regional stress field presents the disturbance phenomenon of different amplitude.The stress field in the whole region shows the perturbation phenomenon with different magnitudes.Under single well injection-production conditions,the absolute value of the maximum principal stress tends to decrease as the flow pressure at the bottom of the well decreases.Under multi-well injection-production conditions,the maximum principal stress variation pattern is exactly the same as that under single-well conditions,The superposition effect of multi-well injection-production can cause a larger change in the principal stress.In this paper,the mechanism of casing damage is analyzed.The mathematical model of casing damage is established and the characteristics of formation strain and displacement that cause casing damage are simulated.When the pressure of the underlying formation(dip angle 5°)increases by 1 MPa,the displacement increment of the overlying shale of SⅠis is 7.9mm,6.8mm for SⅡand 7.6mm for PⅠ.The overlying shale displacement of SⅠis is the largest.According to the current water injection conditions and rock occurrence,the horizontal displacement of the overlying strata of SⅠ,SⅡand PⅠis greater than the vertical displacement.The difference is about 11.4times.For horizontal strata,the horizontal slip of the overlying mudstone is much smaller than the vertical slip.The threshold values of casing damage control factors were calibrated.The formation pressure limit is 10.76 MPa in the Sazhong oilfield study area.The maximum allowable injection pressure limit is 18.49 MPa(the wellhead limit is 9.76 MPa).The injection-production ratio threshold is 1.18(about 6 months of safe production period),and the optimal injection-production ratio is 1 to 1.05.The dynamic simulation of oilfield production shows that as long as the injection production ratio is maintained at1～1.05 and the injection pressure does not exceed the limit value,the formation pressure will not exceed the maximum allowable pressure of 10.76 MPa and casing damage will not occur.Based on the results of formation strain simulation,the surface characteristics of casing damage threshold were analyzed.The threshold value of surface displacement that causes casing damage is determined as 11.0 mm.The casing is in the critical state of damage when the surface displacement is between 7.4 mm and 11.1 mm.The casing damage will occur when the surface displacement is greater than 11.1mm.The high-precision surface deformation information obtained by DS-INSAR technology was used to map the surface displacement contours in the study area.The high-risk areas for casing damage occurrence were identified.It provides technical support for rapid monitoring and early warning of casing damage over a large area.It provides technical support for large area rapid monitoring and early warning of casing damage.The measures to control casing damage in different periods of surface uplift(or subsidence)and the development programs to prevent casing damage and protect casing are proposed.The maximum injection pressure should not exceed 18.49 MPa in the Sazhong field.The formation pressure should be close to the original saturation pressure of 8.93 Mpa.The injection intensity should be 6.52 m~3/(d·m).The production intensity of oil well should be 2.17 m~3/(d·m).The injection-production ratio should be kept at 1 to 1.05.This development plan can maximize the fluid production of the wells and at the same time protect the casing from damage.