The Evaluation Method Of Casing Annulus Seal Integrity For Weekly Consolidated Formations

Weakly consolidated formation has been forming from special environment and sedimentary process,which widely exists in shallow formation of deep water.It has special physical and mechanical properties,such as poor cementation,low strength,and softening characteristics.Weakly consolidated formation has poor protective effect on cement sheath and exists feeble strength on cementation interface,which results in cement crack and annulus under the hydraulic stress or compaction stress surrounding and influences the cement ring integrity and performance seriously.Therefore,in order to predict and analyze the integrity of the sealing system,realize the risk identification of oil and gas well in the process of life,ensure long-term stability of cement sheath in weakly consolidated formation,it is necessary to establish an evaluation method of casing annulus seal integrity for weekly consolidated formations.This paper solves several scientific problems around the evaluation method of casing annulus seal integrity for weekly consolidated formations,First of all,according to the properties of weakly consolidated stratigraphic characteristics,this paper raises weak strain softening model of weekly consolidated formations,establishes the casing-cement-weekly consolidated formation mechanical analysis model and deduces the assembly stress distribution and deformation characteristics of the parts.And it utilizes the dynamics simulation software of ABAQUS,to research the stress distribution of the cement sheath with the influence of the initial stress,the cement and formation mechanics parameters under different casing pressure.What's more,this paper analyzes the cement hardening shrinkage process of the influence of initial stress.The convergence-confinement method is applied to this paper and set the initial position of cement ring and casing as well as the initial stress of the first interface,considering the cement slurry in the process of hardening,suffering the influence of the surrounding convergent pressure gradually.It establishes casing-cement sheath convergence-constraint curve and the casing-cement interface sealing integrity evaluation method,to judge the cement sheath shear failure and tensile failure as well as micro-clearance,according to the balanceable position of the interfaceThe convergence-confinement method is applied to this paper and sets the cement sheath and the weakly consolidated formation at the elastic state as the integrity standard,considering the influence of the formation characteristics,the wellbore and the cement sheath size,cement sheath mechanical parameters,to establish the evaluation method of the cement sheath-weakly consolidated formation and set the quantitative relation between the interface compressive strength of the cement sheath and the characteristic parameters of weekly consolidated formation.The results of the study provide theoretical supports for the research on the integrity of cementing isolation in deep water.