Study On The Integrity Of The Wellbore Of Depleted Oil And Gas Reservoirs

Underground gas storage is an important component of natural gas distribution networks. Its integrity directly affects the safe and stable operation about overall distribution system. Integrity management and assessment of underground gas storage aims to conduct risk identification and evaluation on field devices of gas storage in dynamic changes, formulate the corresponding risk control measures, and continuously improve the operational environment for gas storage. So that contingency occurrence probability is reduced and the operation risk is minimized to an acceptable level.Therefore, on the basis of traditional natural gas well safety management, this paper puts forward the concept and main contents of natural gas storage integrity management and assessment.the main objects are as follows:Firstly, on the basis of fitness for purpose, an evaluation method of integrity assessment is proposed for the completion strings. A mechanical analysis is made for casing on the condition of gas injection and gas production in underground gas storage on the depleted hydrocarbon reservoirs. Considering that the casing is easy to be damaged because of forced injection and forced production and corrosion by corrosive medium, residual intensity computational modules are established for casing with defects, which provide theoretical support for casing reliability analysis.A three dimensional calculation model of casing-cement-formation system and the structure function of casing based on Von Mises yield criterion are established. Stochastic characters of geometrical size, load and material parameters of the casing system are analyzed, which are applied to the safety analysis. Then, failure probability and sensitive factors are calculated based on Monte Carlo method.Through studying the situation of annulus pressure in gas wells and collecting annulus pressure field test data, the most dangerous situation of annulus pressure is that caused by gas channeling which may continue to exist permanently after being bleed-off. And according to field operation, the main reasons causing annulus pressure include failures of oil and casing strings. Combined with operation characteristics and medium parameters for the depleted storage, the formulas for allowable working pressure are put forward. The calculation example has been performed. And combing the actual production, relevant recommendations are put forward for gas storage management with SCP.The model of casing-cement-formation system under non-uniform in-situ stress for injection and production wells is established through the method of undetermined coefficients. The model is solved by the MATLAB. Combined with the corresponding failure criterion, the method to analyze the cement sheath failure for injection and production wells of the depleted gas storage by investigating several common failure modes home and abroad. Then the elastic-plastic numerical simulation of CCFS is proceeded using ANSYS software. The distribution laws of stress is given on inner and outer walls of cement sheath.