Economy and Reliability Selection of Production Casing Thread Connections For Low pressure and Low permeability Gas Field

Abstract

Abstract At present it is widely accepted that if gas reservoir pressure is more than 28 MPa, premium connections in production casing should be selected. However, the price of premium connections is about 1.5 to 2 times than traditional API round thread connections. Therefore, economy and reliability selection of casing thread connections type is one of effective measures to reduce development costs in low –potential gas field. Economy and reliability selection of casing thread connections type in sulige gas field (in China,Well depth at 3550 m, Gas reservoir pressure 35 MPa) was study. Through ϕ 139.7×9.17/7.72 mm N80/J55 LC casing gas-tight test under complex loads and statistical casing full-scale test data in past were analyzed, a minimum safety factor for casing string gas-tight design was determined by casing strength margin and reliability analysis method. Gas-tight reliability effect of load variability on casing was validated by Monte Carlo method. Two sets of casing string were successfully designed and applied to over 800 wells in the gas field, and all wells were reported running well until now. Over 4 hundred million Yuan(RMB) investment was reduced in developing these wells. Following conclusions were achieved by practice: Casing string of production using API long round thread and CATTS101 compound instead of premium connection was economical and feasible for low-pressure and low-permeability gas field development. Main effect factors on gas-tight with standoff and pitch were discoverer in full-scale test, which was different from the current standards on taper in evaluation of gas-tight performance. By statistics of full-scale test data in past, the casing string of design and check is a reliability method with advantage of economic and universal applicability.