Standards Development

Scope

This standard specifies a method for determining the adsorbed gas content and desorbed gas volume of dry-basis and equilibrium moisture shale samples by using the static volumetric method. This standard specifies the principle of the method, apparatus and materials, sample preparation, test procedures, data processing, rounding of test results, precision requirements, and test report. This standard is applicable to the determination of the adsorbed gas content of shale samples under reservoir conditions. It is also applicable to the determination of the desorbed gas volume as free gas converted from adsorbed gas due to formation pressure reduction during shale gas production.

Purpose

Shale gas reservoirs are mainly distributed in North America, China, Latin America, South Africa, and Australia. According to the U.S. Energy Information Administration, 95 basins in 46 countries around the world are expected to extract shale gas, with technically recoverable shale gas reserves of 214 × 1012 m³.

Currently, shale gas production accounts for more than 21% of the world’s total natural gas production and continues to increase.

The evaluation of shale gas-bearing properties is a key step for the efficient and large-scale development of shale gas reservoirs. Its purpose is to accurately measure and assess the volume and occurrence state of natural gas in shale formations, with gas content being the core indicator. Shale gas exists mainly as free gas in larger pores and fractures, and as adsorbed gas within the shale matrix. The proportion of adsorbed gas in the total gas content generally ranges from 20% to 85%, making it an important component of shale gas reserves and production sources. When submitting shale gas reserves, evaluating reservoirs, or formulating reservoir development plans, the adsorbed gas content needs to be evaluated separately. Especially in shale gas reserve calculations, the adsorbed gas reserve is determined by the volumetric method, while the free gas reserve is determined by the compressibility method. The total geological reserve is the sum of both. When the volumetric method is used to calculate the geological reserve of adsorbed gas, in addition to gas-bearing area, effective thickness, and density, one of the key parameters is the adsorbed gas content. The adsorbed gas content of shale must be determined experimentally, and the most widely used method is the static volumetric method. In this method, a shale sample is placed in a sealed container and maintained at reservoir temperature. The unoccupied void volume within the container is first measured, after which natural gas of known pressure and volume is introduced. Once adsorption equilibrium is reached on the sample surface, the gas pressure in the container is measured again, and the volume of adsorbed gas at equilibrium under various pressure conditions is calculated using the gas state equation.

This method is widely adopted globally to determine the adsorbed gas content of shale. The Society of Petroleum Engineers recognizes its reliability and accuracy and recommends it for shale gas reserve calculations. It is also a widely used method in China, Canada, France, Korea, and the United Kingdom for shale gas assessment. The experimental results obtained using this method have been widely applied in shale gas fields around the world, supporting the efficient development of shale gas, and the method continues to receive increasing research attention. According to publicly available data, the number of papers based on this method has risen from about 50 in 2000 to over 300 by 2024. Experimental systems for applying the static volumetric method to shale gas reservoirs have been continuously developed, improved, and commercialized. These developments demonstrate that the static volumetric method for determining shale adsorbed gas content is mature, well-founded, and in high demand, providing a solid basis for developing an international standard.

At present, however, there is no unified international standard for determining shale adsorbed gas content by the static volumetric method. Differences in procedures used across countries and regions lead to inconsistencies in accuracy, reliability, and methodological standardization, making it difficult to evaluate or compare experimental data on a common basis. This situation hinders the implementation of international shale gas exploration, development, and research projects, and impedes efforts to promote efficient shale gas production and the global supply of affordable, cleaner energy. In recent years, China has conducted extensive research on the static volumetric method for determining shale adsorbed gas content and achieved the following outcomes: 1) The helium expansion method was used to correct the dead volume followed by vacuum evacuation to remove any other gases that might be adsorbed on the shale surface and affect the results. Comparison tests showed that this approach increased the measured adsorbed gas content by about 10%. It not only improves the measurement accuracy but also the repeatability and reliability of the experiment. 2) A modified Langmuir equation that takes into account the density of adsorbed gas was used to process the experimental data. This converted the excess adsorbed gas content that is directly measured into a more realistic value of absolute adsorbed gas content. As a result, the abnormal drop in adsorbed gas content seen above 20 MPa was eliminated, eliminating the systematic error caused by the conventional Langmuir model at high pressures. This also reduced the effect of different pressure ranges on the test results. 3) The method’s applicability has been expanded to determine desorbed gas content as free gas converted from the adsorbed gas as formation pressure declines during shale gas production.

Repeatability and comparison tests on the same shale samples were conducted by seven laboratories from three countries, including PetroChina, Sinopec, CNOOC, RWTH Aachen University (Germany), and Newcastle University (UK). The results showed that the relative error between laboratories was less than 4.2%, confirming the validity of the method and the accuracy of the data. This experimental method has already been applied by PetroChina, Sinopec, CNOOC, and other organizations to analyze over 10,000 shale samples from more than 300 shale gas wells, supporting the evaluation of over one trillion cubic metres of natural gas reserves and enabling efficient shale gas development. Based on such work, China has developed a national standard titled “Determination of isothermal adsorption/desorption of methane in shale — Part 1: Static volumetric method” (GB/T 35210.1-2023). Experts from shale gas producing countries such as the United States, Canada, and Saudi Arabia have expressed strong interest in this method and their willingness to participate in an international standard working group. The technical foundation, expert support, and industry need for establishing an international standard on the static volumetric method for determining shale adsorbed gas content are now fully in place.

“Determination of adsorbed gas content using static volumetric method” is the second part of a threepart series on shale gas content measurement. The first part is Determination of total gas content using desorption method”. The third part is “Determination of adsorbed gas content using gravimetric method”. Together, these three standards form a comprehensive technical framework for measuring shale gas content, serving as a key technical reference and guideline for evaluating shale gas-bearing properties.

Therefore, developing the above experimental method for determining the adsorbed gas content in shale by the static volumetric method into the international standard Natural gas upstream area — Determination of shale gas content — Part 2: Determination of adsorbed gas content using static volumetric method, and establishing a standardized experimental method for determining adsorbed gas content by the static volumetric method, will not only ensure the accuracy and reliability of experimental results, but also provide guidance for the efficient exploration and development of shale gas reservoirs worldwide. It will help relevant stakeholders (such as oil companies, commercial laboratory testing institutions, and research organizations) optimize exploration decisions and reduce business risks, thereby contributing to mitigating global energy shortages while reducing the environmental impact of the oil and gas industry.