Advanced description of pressure drop funnel is crucial in coalbed methane (CBM) production because of dewatering and depressurization methods. Improving the precision of the pressure drop funnel description facilitates obtaining the actual production status and productivity potential, both pivotal for responsible development plans. The study presents a semianalytical model that integrates pressure profiles and material balance equations, incorporating inner and outer boundary conditions, and dynamic reservoir characteristics. The pressure propagation characteristics in undersaturated coal reservoirs are described during the production life of CBM wells, and the model is validated using two wells with different production characteristics. The results indicate that the effect of water saturation on the expansion of the drainage radius surpasses that of the desorption radius, demonstrating a more precise prediction of the production boundary when dynamic water saturation is considered. Additionally, a rapid drop rate of bottomhole flowing pressure triggers simultaneous propagation of the drainage and desorption radii, resulting in a smaller production boundary and fewer well-controlled resources. Conversely, an appropriate production strategy results in a larger drainage radius and lower boundary pressure before massive gas desorption, thereby facilitating efficient propagation of the pressure drop funnel. Moreover, the pressure drop funnel characterized by the model can compute the dynamic CBM resources and recovery efficiency of a single well, providing a valuable basis for assessing productivity potential. In summary, this model offers a time-saving and practical tool for describing the dynamic pressure drop funnel in various CBM production stages and promoting efficient development for undersaturated CBM reservoirs.

Aquifer leakage recharge poses a prevalent challenge in coalbed methane (CBM) development, severely impeding its efficient production. This study focuses on the Sanjiao Block, located on the eastern margin of the Ordos Basin, and provides a comprehensive evaluation of the impact of aquifer leakage recharge on CBM well productivity, employing hydrochemical characteristics and numerical simulation. Fisher's discriminant analysis reveals a close association between external water sources for CBM wells in the Taiyuan Formation and the hydrodynamic environment: in the western stagnant zone, hydrochemical characteristics resemble those from the Shanxi Formation; in the eastern strong runoff zone, water from the Taiyuan Formation directly contributes to CBM well development; in the central weak runoff zone, hydrochemical characteristics suggest mixed water sources from the Shanxi Formation and Taiyuan Formation. The numerical simulation employs orthogonal experiments to quantify the sensitivity of aquifer geological parameters to CBM production, ranked from strong to weak for aquifer types, leakage channel properties, and aquifer physical properties. The fundamental reason for disparities in the efficacy of leakage recharge lies in categorizing aquifers into finite and infinite recharges based on their water supply capacity. The properties of leakage channels, including the location and scale, manifest as effects on the magnitude and shape of gas production characteristics in infinite and finite recharge aquifers, respectively. Furthermore, a discriminant flowchart of CBM production is presented, delineating the production characteristics of CBM wells under the influence of aquifer leakage recharge into six patterns and illustrating their distribution in the study area. This discriminant process provides scientific guidance for analyzing CBM production characteristics and evaluating potential under the influence of aquifer leakage recharge.

At the same structural position within the northern part of Block K, the coalbed methane (CBM) wells in the 3^# and 15^# coal seams exhibit marked productivity disparities. This study integrates a correlation analysis of engineering parameters with dynamic diagnosis of production indicator curves to examine two dimensions: static engineering compatibility and dynamic seepage interference. The results show that the degree of matching between engineering parameters and geological conditions is a key factor controlling the productivity variation. The 3^# coal seam has favorable engineering performance, and boosting fracturing scale and drainage rate both help promote gas output. Compared with the 3^# coal seam, the 15^# coal seam has a notable deficiency in engineering compatibility: its effective horizontal section length and coal encounter rate decrease by 8.4% and 5%, respectively, while sidetracking frequency increases approximately fivefold; moreover, enlarging fracturing scale does not bring better gas results. Production indicator curves were established based on the relationship between cumulative water production per unit effective horizontal length and production pressure difference and were classified into upward-convex and downward-concave types. The resulting chart for exogenous water interference enables quantitative characterization of key metrics, including theoretical ultimate gas production, interference level, and gas production efficiency. The theoretical ultimate gas production of the 3^# coal seam reaches 69.454 m³/(d·m), over ten times that of the 15^# coal seam, but the critical interference threshold of the 3^# seam is lower than that of the 15^# seam. Most 3^# wells are stably located in the non-interference zone, indicating high production capacity and strong sensitivity to exogenous water interference. For the 15^# seam, the production indicator curves of most wells deviate toward the interference zone at an early production stage, reflecting strong exogenous water leakage recharge. Therefore, for this block, CBM development must shift from the traditional approach of blindly enlarging stimulation scale to geological compatibility and recognition of exogenous water interference.

Hydraulic fracturing is a widely used stimulation technology in coalbed methane (CBM) fields. However, the coal reservoir damage caused by high-pressure hydraulic fracturing seriously affects the production effects, and the mechanism is not clear. Therefore, based on high-pressure water injection (HPWI), Fourier transform infrared spectroscopy (FTIR), and contact angle tests, the effects of HPWI on surface chemical properties and wettability of different rank coals were studied. The FTIR results show that surface functional groups of different rank coals have changed to varying degrees after HPWI. After HPWI, the content of Ash in Shaqu and Yonghong coal decreases by 2.29% and 27.91%, while it increases by 297.87% in Shaping coal. The C–O bond content in Shaping and Yonghong coal decreases by 6.32% and 15.19%, while the C–O bond content in Shaqu coal increases by 50.96%. The content of C=O in Shaping and Yonghong coal increases by 2.44% and 27.84%, respectively. The R₂CH₂ contents increase by 19.75% and 12.5% in Shaping and Shaqu coal, while decreasing by 6.48% in Yonghong coal. The RCH₃ content increases by 21.11% in Yonghong coal, while it decreases by 19.09% and 24.01% in Shaping and Shaqu coal. The content of cyclic associated hydroxy–hydrogen bond decreases by 41.25%, 63.92% and 65.86% in Shaping, Shaqu, and Yonghong coals, and the content of free hydroxyl group increases by 57.92%, 58.42%, and 93.71%. The f_ar^c of coal remains almost unchanged, the DOC increases by 20.21%, 126.77% and 0.24% in Shaping, Shaqu, and Yonghong coals, and the I decreases by 16.67% and 51.46% in Shaping and Yonghong coals, indicating that the ordering of coal becomes better, and the content of methylene carbon in the form of long straight chain increases after HPWI. The complexity and differences of changes in functional groups are mainly due to differences in coal structures caused by coalification. The contact angle tests show that the wetting contact angle of different rank coals decreased by 2.30% to 14.50%, revealing that the hydrophilicity of coals increases after HPWI. The decline rate of wetting angles in medium and high-rank coals was significantly higher than that of low-rank coal. This phenomenon discovered that the increase in hydrophilic functional groups caused by HPWI action leads to an increase in the hydrophilicity of coal samples, which is not conducive to the drainage efficiency in CBM development.