As an important unconventional natural gas resource, the efficient development of coalbed methane is of great significance for optimizing the energy structure. However, the multiphase water present in the coal seam pore fracture system, like an invisible shackle, profoundly constrains itMethane'sAdsorption, desorption, transport and productionThe entire process has become a long-term "water lock" dilemma faced by the industry. To solve this problem, precision is crucialIdentify and quantify the occurrence state of waterIt is the primary prerequisite. In recent years, low field nuclear magnetic resonance technology, with its unique advantages, is becoming a new generation of perspective eyes that illuminate the microscopic world of coal seams and guide efficient development of coalbed methane.

Pore water: the "dual blade sword" for coalbed methane development

The water in coal seams is not uniformly distributed, but exists in complex pore networks in various forms such as adsorbed and free states. Its impact on coalbed methane development is multifaceted and profound: firstly, the presence of pore water directly occupies reservoir space, resulting in a reduction in the volume that can accommodate free methane under in-situ conditions. Secondly, the more crucial factor is the competitive adsorption effect - water molecules compete with methane molecules for adsorption sites on the surface of coal matrix, directly reducing the methane adsorption capacity of coal seams. In addition, the "water lock effect" will hinder the contact between methane gas and coal matrix, further inhibiting methane desorption. From a transport perspective, pore water significantly increases the resistance to gas flow, reducing the permeability of coal seams and the diffusion ability of methane. These factors work together to ultimately constrain the productivity of coalbed methane wells. Therefore, clarifying the distribution, phase, and dynamic changes of pore water is the theoretical foundation for optimizing drainage and gas production processes and improving recovery rates.

The working principle of low field nuclear magnetic resonance technology: the "radar" for detecting pore fluids

Low field nuclear magnetic resonance technology can reveal the mysteries of water, and its core principle lies in detecting the relaxation characteristics of hydrogen nuclei (protons) in fluids. This technology typically operates at magnetic field strengths below 0.5 Tesla. When coal samples are placed in a magnetic field, hydrogen nuclei in water molecules undergo energy level transitions; After removing external stimuli, these hydrogen nuclei will gradually return to equilibrium, a process called relaxation, and release detectable signals.

The relaxation rate of water in different states of occurrence is significantly different. Adsorbed water confined to the surface of tiny pores or narrow throats, in close contact with solid particles, has an extremely fast relaxation rate; Free water that exists in the center of larger pores or cracks is weakly bound and has a slower relaxation rate. By analyzing the received nuclear magnetic resonance signals and their relaxation time distribution, researchers can non destructively and quantitatively distinguish the water content in pores of different sizes in coal samples, and even visually "see" the distribution and transport path of water inside the coal body through imaging. This ability makes it an ideal tool for studying the occurrence state of pore water.

Technical Application: From Static Characterization to Dynamic Simulation

The application of low field nuclear magnetic resonance technology in the field of coalbed methane has moved from static physical property analysis to complex dynamic geological process simulation, mainly reflected in the following aspects:

Fine characterization of pore structure and water distribution: Research can accurately determine the complex pore system composed of adsorption pores, permeation pores, and migration pores in coal, and clarify the proportion of each part. This helps to determine the main storage space for water. For example, studies have shown that water phase is difficult to enter micropores with pore sizes below about 20 nanometers under capillary resistance, which explains why these pores are usually dominated by adsorbed gas.

Revealing the laws of gas water transport and competition: Through real-time nuclear magnetic resonance monitoring, the dynamic process of gas driven water or water self-priming can be intuitively studied. Experiments have found that in the process of gas flooding, gas preferentially displaces free water in the center of large pores, while residual water is trapped in narrow throats and blind ends of pores. This directly reveals the main source of production water and the difficulty in reducing residual water saturation.

Simulating geological conditions and evaluating development measures: The advanced low field nuclear magnetic resonance system can be coupled with true triaxial loading, seepage experiments, etc., to simulate the influence of underground stress changes on pore fracture structure and gas water seepage. For example, studying how changes in confining pressure lead to pore compression or rebound, in order to optimize the depressurization and extraction plan. There are also studies using this technology to evaluate the effectiveness of hydraulic measures (such as spontaneous imbibition) in improving coal seam diversion capacity.

Unique advantages compared to traditional methods

Compared with traditional research methods, low field nuclear magnetic resonance technology exhibits multiple advantages:

Non destructive testing: It does not damage the sample structure and can perform multiple and continuous tests on the same coal sample to obtain dynamic evolution data.

Comprehensive and fast: Multiple information such as porosity, pore size distribution, and fluid saturation can be obtained simultaneously in one test, and the speed is much faster than traditional splicing characterization methods such as mercury intrusion porosimetry and adsorption.

Intuitive and Accurate: Not only can it perform quantitative analysis, but it can also be visualized through magnetic resonance imaging with high spatial resolution. Compared to methods such as scanning electron microscopy that can only observe surface morphology, nuclear magnetic resonance can detect the overall information inside the sample.

Strong adaptability: The equipment is relatively compact compared to high field nuclear magnetic resonance, with low maintenance costs, making it easy to build a comprehensive testing platform that combines with geological engineering in a laboratory environment.

The occurrence state of pore water in coal seams is the invisible hand that controls the efficiency of coalbed methane development. Low field nuclear magnetic resonance technology has opened up a window for us to directly observe the gas water game process in coalbed methane reservoirs through its powerful functions of non-destructive, quantitative, and visualization. From clarifying the micro mechanism of action to guiding macro engineering practice, this technology is continuously driving the development of coalbed methane towards a more precise and efficient direction. With the further integration of this technology with artificial intelligence and big data analysis, its potential in unconventional energy exploration and development will be further explored.