Standards Development

Scope

This document specifies the principle, equipment and facilities, materials and tools, coring procedures, collection and recording of samples and related data, and safety requirements for the tripping-out pressure-retained coring method for gas wells.

This document is applicable to the tripping-out pressure-retained coring operations for gas wells.

Purpose

Natural gas is one of the primary clean energy sources. Global demand for natural gas continues to grow annually, particularly in unconventional and offshore reserves, with an annual growth rate exceeding 5% for unconventional natural gas sources such as shale gas, coalbed methane, and tight gas. The large-scale and commercial development of unconventional natural gas will support global efforts to improve energy structures, reduce greenhouse gas emissions, and improve environmental quality. Currently, unconventional natural gas reserves are abundant and widely distributed worldwide. According to a recent report by the International Energy Agency (IEA), the global unconventional natural gas sector is entering a golden era of development, with global shale gas production alone expected to reach 1.6 trillion cubic meters by 2035, accounting for nearly two-thirds of the incremental natural gas supply at that time and maintaining an upward trend.

In the upstream exploration and development of natural gas, core sampling is the primary means of obtaining key reservoir parameters such as porosity, permeability, saturation, brittleness index and gas content. The quality of these data directly affects the accuracy of reserve evaluation and the feasibility of development plans. Currently, conventional coring methods are mainly used in gas wells to obtain core samples for reservoir gas-bearing evaluation. Studies have shown that, during conventional coring, shale gas losses can account for 40 % to 80 % of the total gas content, and the accurate determination of gas loss has become a critical challenge in determining the total gas content of the reservoir. Unlike conventional coring methods, the tripping-out pressure-retained coring method has been rapidly developed and field-proven in recent years. Using sealing devices, this method completely encloses the core within a high-pressure-resistant barrel, isolating it from the wellbore environment and preventing gas loss and escape during tripping out. Combined with on-site testing technology after the core is retrieved, this method enables accurate determination of reservoir gas content and resolves the difficulty of quantifying gas losses in gas well coring. It is therefore an important technical means for accurately evaluating reservoir properties and resource volume.

Research institutions in countries such as the United States, Japan, Germany, and the United Kingdom have conducted extensive studies on pressure-retained coring technology, confirming its effectiveness in evaluating unconventional gas reservoirs. The results and practical value of this technology have been widely recognized internationally, providing reference and guidance for global coring operations. In China, this method has been applied in more than 100 wells in unconventional gas fields covering coalbed methane and shale gas. It has been used to provide deep coal-gas coring services for companies such as Schlumberger and Arrow Energy (Australia). Accurate gas content data have been obtained through field testing, gas collection, and component analysis. The test results of gas-bearing properties are significantly better than those from conventional coring, with an average core recovery of 92%, a pressure-retained coring success rate of 85%, a maximum coring depth exceeding 5 000 m, and a maximum formation pressure reaching 100 MPa. However, despite the extensive field verification data supporting its effectiveness, the lack of a unified technical specification has resulted in differences in operating standards and data interpretation among regions, limiting the broader application of the technology. The unification of the tripping-out pressure-retained coring method will help resolve this issue, provide consistent operational guidance for global applications, and promote the standardization and harmonization of the technology. This will bring greater transparency and consistency to the service market, facilitate fair competition among service providers in different regions, and improve overall industry service quality. Meanwhile, Chinese higher education institutes, oil and gas enterprises, and international organizations have carried out multiple technical exchanges and research collaborations to continuously optimize the tripping-out pressure-retained coring method.

A number of high-quality scientific papers have been published, ensuring the method’s effectiveness and the accuracy of its data. These efforts have laid a solid foundation for the development of an ISO technical specification and provided strong support for the detailed evaluation of unconventional hydrocarbon reservoir properties and resource potential.

The development of the technical specification Natural gas upstream area — Core sampling — Tripping-out pressure-retained coring aims to provide a unified method for coring operations in unconventional gas reservoirs worldwide. It plays a positive role in improving coring success rate, ensuring the accuracy of gas content determination, and reducing uncertainty in reserve estimation. It also supports reliable evaluation of reservoir properties and resource potential. Through standardized operations, this method will drive advancement in unconventional natural gas technologies, promote international adoption of coring practices, improve exploration and development efficiency, and provide technical support for efficient utilization of clean energy and sustainable development of global energy resources.