Research On Random Time-Variant Reliability Of High-Pressure Gas Well Casing String

In the process of sour gas reservoir development, sulfur dioxide and other acidic gases usually cause corrosion and damage in oil lines, casing pipes and drilling apparatus. The casing pipe should have the characteristics of high pressure resistance, corrosion resistance and long lifetime. The research on casing safety and reliability is illustrated in this paper. The major contents include high pressure casing failure, corrosion caused strength decrease, inhomogeneous stress distribution in casing thread and mathematical model of gas leakage rate through casing sealing surface. Firstly, pressure–temperature interaction and distribution along the well are calculated with a dual iterative algorithm and heat transfer theory. The corresponding calculation model of the corrosion rate is used according to the different CO2/H2S ratio. Secondly, the physical-mechanical model and the deformation-compatibility equation are established for casing connection, and then the inhomogeneous stress distribution in casing thread is obtained. The critical loads of thread slipping, rupture and shear failure are calculated with the failure mechanism of casing connection. Thirdly, the different roughness sealing surface profile is simulated with Monte Carlo method and studied by using fractal dimension method. Based on the application of the contact mechanics and the flow characteristic, the theoretical relationship between the contacting surface gas leakage and their superficial micro-geometry is deduced. The leakage rate formula for non-API casing connection is obtained with the analysis. Finally, considering the random and time-variant properties of the casing strength and load, a casing reliability calculation model for high-pressure gas well is proposed in this paper. The load changing with time mainly includes the gas pressure in pipe and the surrounding rock pressure. The strength randomness is decided by the probability distribution of the casing performance parameters. The casing strength reduction is caused by thickness decreasing and its changing with time is researched based on the CO2/H2S corrosion rate. Owing to the axial differences along the casing well, it is difficult to calculate the casing reliability as a whole. This paper proposes a new method to divide the casing into several computing units. The method comprehensively considered the geological conditions, casing program and cementing, oil and gas production and stimulation treatments, earth stress and casing corrosion. The method is easy to interface with the traditional calculation and consistent with the actual engineering practice. It is also easy to be programmed by using computer. The software named“Safety and Reliability Assessment of High-Pressure Gas Well Casing String”is developed with the Visual Basic 6.0 as platform. The database technology and the modularization design are used to process data and calculate the casing failure probability. With the software, the preliminary safety assessment is obtained by inputting the actual working parameters of high-pressure gas well. The calculation results of the casing reliability can provide guidance for the operator.