Development of Economical Gas-tight Casing Connection for Low Pressure Gas Field Application

Abstract

Abstract Changqing Sulige gas field in China covers a large area of low pressure (reservoir pressure of less than 35 MPa) and low permeability reservoir. It was noted that the production rate per well has been low. Therefore, to increase the production efficiency, it was necessary to increase the number of wells to be drilled. As a result, casing connections became a large portion of the total cost of the field development. A low cost gas-tight casing connection would help to reduce the development cost in Sulige gas field. Primarily due to high cost, premium connections with dedicated shoulder and seal do not satisfy the economical development requirement. API long round threaded casing connections do not allow breakout after makeup with special thread compound (Catts101). Therefore, it is necessary to develop an economical casing connection with fast makeup feature and large bending and tensile load carrying capacity. Based on the finite element analysis, to improve the gas-tight seal performance with the API long round thread connection and buttress connection, a new type of casing connection was developed. The new casing connection has modified radii of the crest and root of the thread with wider ridge than groove to ensure full contact of the load flank and stab flank of the internal and external threads upon makeup. In addition, the thread crest and root are separate to prevent galling. The internal thread on the box has varying taper at the makeup location to enable gas-tight seal upon makeup with the last thread on the pin. Nonlinear finite element analysis of this new connection design (139.7×9.17mm N80) showed the new design with a clearance of crest and root with surface coating on box at the engaged gas-tight seal position, combined with the make-up and tensile load carrying capacity of 80% material yield, can meet low pressure (60 MPa) gas seal. This new casing connection design was verified with full-scale testing to ensure the successful application in Sulige gas field. Contrary to other premium connections, this new casing connection has no dedicated seal and shoulder regions and relies on the last thread for sealing. Such design has several advantages, such as low cost, low material consumption, easy for QC, no requirement for special thread compound and easy for make-break. It satisfies the requirement of economical development of low pressure and low permeable gas field.