Pressure-bearing characteristics of pressure-preserved controllers under deep in-situ temperature and pressure conditions

Deep in-situ pressure-preserved coring is an important method for the reserves evaluation of deep flow resources, such as coalbed methane and shale gas. The sealing performance and pressure-bearing capacity during the coring are significantly affected by temperature and pressure. However, existing studies scarcely incorporate the influence of the deep in-situ environment (i.e., temperature and pressure) on the ultimate pressure-bearing capacity and pressure-preserved effects of pressure-preserved controllers (PRCs). Focusing on the key issues related to the pressure-bearing performance of PRCs in a deep environment, this study conducted a kinematic analysis of PRCs. This study also performed high-temperature tensile tests of 304 stainless steel, determining its mechanical properties. Based on the self-developed laboratory simulation test platform and numerical simulation methods for pressure-preserved coring, this study obtained the structural deformations and stress distribution of PRCs under the simulated deep in-situ environment. Accordingly, this study proposed a failure control strategy for PRCs. The results are as follows: (1) The yield strength of 304 stainless steel showed a downward trend as the temperature increased. Its average values were 556.42 MPa, 536.26 MPa, 513.22 MPa, 511.00 MPa, and 489.88 MPa at 25℃, 50℃, 100℃, 150℃, and 200℃, respectively. (2) The internal reason for PRC failure was determined. For the valve cover of a PRC, its equivalent stress was concentrated in the central part of its bottom, and its structural deformations were dominated by the contraction from both wings of its short axis toward its inside. As a result, the pressure intensity of the sealing contact reduced, making it difficult to form effective sealing. (3) To improve the pressure-bearing capacity of PRCs, it is recommended that material properties should be optimized (i.e., by selecting materials with a high elastic modulus) to reduce the deformation of PRCs or that the deformations on the weak side of the valve cover should be controlled by installing a limit structure in the valve seat to make the PRCs become tighter under higher pressure. This study can be used as a reference for the structural optimization and capability enhancement of deep in-situ PRCs.