In the process of exploration and development of coalbed methane (CBM), it covers multiple key links such as geological exploration, geophysical exploration, drilling, mud logging, well logging, and so on. These links work closely together to jointly promote CBM from underground exploration to surface production. Among them, well logging technology, as an important means to discover coal seams and CBM resources, is often vividly compared to the "eyes" of geologists, playing an indispensable role in the entire exploration and development system.
I. Characteristics of Coalbed Methane Reservoirs
Coal seams are solid combustible minerals formed from the remains of plants through complex biochemical and geological processes. China has a rich variety of geological conditions, resulting in diverse coal reservoirs and CBM accumulation environments. The thickness of coal seams varies greatly, ranging from thin coal seams of less than 1 meter to extra-thick coal seams as thick as 130 meters. Generally, they are characterized by shallow burial depth, low hardness, and ease of fracturing and exploitation. Coal reservoirs have a unique dual-porosity structure, consisting of matrix pores, micro-pores, and a natural fracture network composed of cleats and micro-fractures. However, their properties of being brittle, having low porosity and permeability, strong heterogeneity, and being prone to hole enlargement make the logging response characteristics of gas content in coal seams not significant, posing great challenges to logging interpretation and evaluation.
As an associated product of coal, CBM is stored in the pores of coal seams. Its occurrence state is similar to that of shale gas and has the characteristics of self-generation and self-storage. It exists in three forms of hydrocarbon gases: adsorbed state on the surface of coal matrix particles, free state in coal cleats and pores, and dissolved state in water (with extremely low content), and is in a dynamic equilibrium. The production of CBM is a dynamic equilibrium process of desorption, diffusion, and seepage: when the fluid pressure in the coal seam decreases, the adsorbed CBM is desorbed and transformed into free CBM. The free CBM enters the fractures in the formation through diffusion or seepage in the coal matrix, and finally, the free CBM in the fractures reaches the wellbore through seepage and is produced. The gas content of CBM is affected by various factors such as geological structure, coalification degree, occurrence conditions of coal seams, and characteristics of roof and floor rocks. Well logging technology comprehensively applies multidisciplinary knowledge such as geology, rock mechanics, mathematics, artificial intelligence, and big data to finely depict the coal seam from the microscopic nano-pore structure to the macroscopic micro-pore structure, thus realizing the "diagnosis" of CBM.
II. The Origin and Development of the "Magic Eye"
Well logging technology is a high-precision and sophisticated technology developed based on physical processes such as sound, electricity, nuclear, and magnetic fields. Special instruments are sent hundreds or even thousands of meters deep underground, and various parameter signals reflecting the rock formations and the fluids in them are obtained through measurement. Interpretation and evaluation personnel use these signals to explore CBM resources underground and achieve accurate coal and gas exploration, which is the origin of well logging being called the "magic eye".
Since the 1920s, well logging technology has gone through four development stages: analog well logging, digital well logging, numerical control well logging, and imaging well logging. In the analog well logging stage, the data collection method was single. Analog recording was used, and the data was manually drawn on the drawing. It was mainly used for qualitatively identifying oil and gas layers. In the digital well logging stage, digital recording and computer processing of the data were introduced. The well logging methods and items were continuously improved, and quantitative evaluation and comprehensive analysis of oil and gas were realized. The technology was extended to application fields, and the well logging technology began to expand into fields such as geology and reservoir engineering. In the numerical control well logging stage, thanks to the rapid development of computer and microelectronics technology, well logging instruments and data collection items became mature. The data collection and processing technology achieved a major breakthrough, meeting the evaluation requirements of complex lithology reservoirs such as fractured reservoirs and thin interbedded layers. In the imaging well logging stage, that is, the current stage, the imaging collection has realized a comprehensive measurement of the formation. The data collection ability has been greatly improved, and the processed results present a 3D image around the wellbore, which is more accurate and intuitive. The new generation of well logging equipment demonstrates the high-precision and sophisticated level of modern technology. In the National "13th Five-Year Plan" Scientific and Technological Innovation Achievement Exhibition in 2021, it was exhibited together with the high-speed maglev train with a speed of 600 kilometers per hour. Modern well logging interpretation and evaluation methods have broken through traditional limitations and extended to fields such as geological structure, testing, fracturing and reconstruction, and engineering evaluation, playing a key role in the exploration and development of complex and hidden oil and gas reservoirs.
At present, the well logging collection and methods for CBM vertical wells mainly draw on oil and gas well logging technology. After decades of development, a series of CBM well logging technologies such as electrical logging, acoustic logging, and radioactive logging have been formed, providing strong support for CBM exploration.
III. The Role of the "Magic Eye"
By using parameters such as natural gamma ray, spontaneous potential, acoustic wave, density, neutron, lateral or induction resistivity in conventional well logging, the logging response characteristics of "two lows and three highs" of CBM reservoirs can be determined, that is, low density, low natural gamma ray, high neutron, high acoustic transit time, and high resistivity. Through continuous research and development, a set of CBM well logging interpretation methods and procedures has been formed. The innovative concept of the "CBM system" has been proposed, regarding the coal seam, its roof and floor, and the upper and lower strata as a whole for systematic research. For the development of CBM in the middle and shallow layers, the "Comprehensive Evaluation Technology System of CBM System Well Logging" has been formed, which includes 5 categories and 16 characteristic technical items, aiming to solve the four-property evaluation problems of CBM, namely "reservoir property, gas-bearing property, water-bearing property, and fracturability".
In practical applications, according to different geological conditions and exploration and development stages, well logging technologies need to be flexibly selected to obtain accurate and comprehensive reservoir parameters. For example, when evaluating CBM reservoirs based on the rock physics volume model, considering the differences in components between CBM and oil and gas reservoirs, a coal seam rock physics volume model considering organic components is constructed by referring to coal geology. The industrial components of the coal seam are divided into fixed carbon, volatile matter, ash, and moisture. Using information such as gamma ray, density, and acoustic wave in well logging, interpretation models for each component are established respectively to achieve quantitative calculation of the components of the coal seam. However, due to the superposition of multiple factors affecting the characteristics of CBM reservoirs, the response of well logging information shows a non-linear pattern, and the conventional interpretation and evaluation methods for oil and gas are no longer applicable.
With the development of the CBM industry, new generation well logging instruments and evaluation methods have emerged continuously. Electrical imaging well logging can clearly reflect the characteristics around the wellbore, accurately divide the gangue interlayers in the coal seam, analyze the development of the roof, floor, and fractures, and realize the quantitative identification of the coal body structure. Nuclear magnetic resonance (NMR) well logging can accurately calculate the porosity of the coal seam and analyze the fluid components and multi-scale pore structure. Array acoustic well logging can identify the coal seam, evaluate the rock mechanical properties and acoustic anisotropy of the coal seam, and provide reference data for the fracturing construction design. These new technologies have made up for the deficiencies of conventional well logging technology and realized the fine evaluation of coal seams.
However, well logging technology still has limitations in the exploration and development of CBM. The brittleness of coal seams is prone to cause wellbore collapse and caving, resulting in distortion of well logging collection signals and affecting the authenticity of well logging curves, and in severe cases, the collection cannot be completed. The longitudinal resolution of conventional well logging instruments is insufficient, making it difficult to accurately divide the thickness of thin coal seams and identify the gangue interlayers. In the calculation of gas content, the classic Archie formula used to calculate saturation by resistivity in conventional oil and gas cannot be applied due to the complex electrical property differences of different coal ranks in coal seams. In soft formation coal seams, the amplitudes of shear waves and Stoneley waves in the array acoustic wave decay rapidly, and it is a practical problem for researchers to accurately and effectively extract the wave train information.
At present, the CBM well logging evaluation technology and methods cannot fully meet the technical requirements of the rapid development of the CBM industry and the exploration and development of deep coal rocks. As the exploration and development of CBM advance towards deep coal rock gas, people have a more comprehensive and in-depth demand for information about coal seams. Well logging technology needs to be further researched and innovated to better serve the development of the CBM industry.
