Comprehensive identification of CO2 nonhydrocarbon gas layers by relative CO2 content and apparent porosity calculated from well logs: A case study in the Baiyun deep-water area in the eastern South China Sea

Abstract

The accurate identification of [Formula: see text] nonhydrocarbon gas layers plays an important role in the exploration and development of natural gas reservoirs. The logging response characteristics of layers containing nonhydrocarbon gas and hydrocarbon gas are very similar because there are gas fluids in both kinds of layers, which leads to the uncertainty in the interpretation results of gas types. We have developed a comprehensive method for the identification of [Formula: see text] nonhydrocarbon gas layers by using [Formula: see text]-relative content and apparent porosity. First, based on the formation component bulk volume model of the coexisting reservoir of the [Formula: see text] nonhydrocarbon gas layers and the hydrocarbon gas layers, we develop an optimization algorithm to quantitatively calculate the [Formula: see text] relative content in the reservoir. Second, by analyzing and determining the logging response value of the [Formula: see text] nonhydrocarbon gas under the formation temperature and pressure conditions, we establish a qualitative identification chart using the difference between the calculated value of the apparent porosity of [Formula: see text] and the hydrocarbon gas. Finally, we were able to accurately identify the [Formula: see text] nonhydrocarbon gas layers by comprehensively analyzing the calculation results of the [Formula: see text] relative content in the reservoir and the identification results of the [Formula: see text] apparent porosity identification chart. We apply the above workflow in the Baiyun deep-water area in the eastern South China Sea, and it shows high efficiency in the identification of [Formula: see text] nonhydrocarbon gas layers.