Research On The Safety And Reliability Of Service Casing In Marine Gas Reservoir Wells

Since the founding of People's Republic of China,we completed the first venture of the oil industry due to the establishment of marine facies hydrocarbon accumulations and successful exploration,and basically achieved self-sufficiency when the annual crude oil production exceeded 100 million tons in 1978.With the rapid development of national economy,China's demand for oil and gas resources is increasing,since 1993,China has became a net importer in crude oil,and the gap increased year by year,with the external dependence climbing to 60%in 2015.With the severe situation that the consumption of oil and gas growing rapidly and the reserve and production of continental oil and gas are difficult to significantly increase,our country launched the second venture of the oil industry which aiming at marine carbonate rock reservoirs in the south.With the proposing of the second venture of the oil industry,marine oil and gas exploration showing great momentum of rapid development,especially the discovery of Puguang and Anyue(Longwangmiao Group)large monolith marine carbonate gas reservoir,which preliminary demonstrates China's tremendous exploration potential of marine gas and provides an effective measure to improve domestic energy consumption structure and reduce the external dependence.However,due to the particularity of Chinese geological structure in marine strata,the exploitation of marine gas is also facing unprecedented challenges,as a tool to protect borehole wall,seal off different pressure zones and to maintain circulation passage for drilling fluid during the process of drilling and exploitation,well casings are prone to damage during service,which can cause severe downhole accidents and even security incidents,with huge economic losses,environmental damage or casualties in marine gas reservoir exploration and development,therefore,security and reliability study of casing serving in marine gas reservoir well will become an unavoidable difficulties in the process of long-term,efficient exploitation of marine gas reservoir resources.This paper is aimed at the particularity of marine gas reservoir,combineing theoretical analysis,simulation and experimental data comparison,the safety of casing serving in the condition of corrosive,variable loading and stochastic factor are systematically investigated on the basis of the elastic-plastic mechanics,rock mechanics,the finite element method and reliability theory for the first time,the analysis of the specific process and results are as follows:(1)Developing the casing material selection method for marine gas reservoir well based on the investigation on casing selection standards and specifications under acidic corrosive environment used internally by domestic and international oil companies,and the corrosive casing strength was investated based on casing corrosion model and stress concentration theory;(2)Based on field geological data and core sample creep test data,a three-dimensional computational model coupled with salt rock interlayer,cement and casing is established,besides,the safety variation of casing serving in salt rock interlayer in marine gas field is studied using the computational model;(3)In this paper,the new perspective of using hole enlargement while drilling technology to improve the safety of casing serving in salt rock interlayer in marine gas reservoir well is proposed,and the hole enlargement size design method based on additional load and equivalent resistance strength is developed;(4)Based on TSUST(twin shear unified strength theory)and ISO/TR 10400 collapse model,a new mathematical model of HC(high collapse)OCTG(oil country tubular goods)collapse strength is developed,furthermore,the collapse strength formulas for casing(API casing and HC caing)serving in marine gas reservoir well are proposed in this paper.(5)Analyzing the shortcomings of traditional casing design methods,the casing reliability design method are presented to avoid risks and economic losses by controlling the failure probability at a predetermined level.This study provides scientific basis for material selection,design and safety reliability evaluation of casing in marine gas reservoir well.