Mitigating Surface Tension Effects in Multi‐Coalbed Methane Reservoirs: Case Study of Liupanshui Field

Water locking effect caused by surface tension severely restricts coalbed methane (CBM) productivity in multilayer reservoirs. The Songhe demonstration area features main target coal seams with overpressure, high gas content, and high gas saturation, presenting favorable CBM development conditions. However, due to the combined effects of low porosity, low permeability, low initial water saturation, and complex pore–fracture structure, severe surface tension effect is prone to occur during gas–water two‐phase flow (TPF; with water as the dominant phase) in CBM development, hindering coal reservoir pressure reduction by drainage, desorption gas seepage, and output. Using an integrated laboratory‐field method combining scanning electron microscopy, mercury intrusion porosimetry, and production data analysis, this study analyzes the temporal and spatial dimensions of surface tension effect formation during drainage. It is found that the main reasons for the intensification of surface tension effect under commingled production conditions are: the presence of free gas in un‐dissorbed coal reservoirs; the growth of bubbles in gas–water TPF zones; the enlargement of liquid droplets in water–gas TPF (with gas as the dominant phase) zones; the expansion of the planar range of water–gas TPF zones. Innovative drainage control measures targeting key stages are proposed with quantitative constraints: In the pressure‐limited slow drainage stage (Phase III) and pressure‐controlled gas production stage (Phase IV), the casing pressure (CP) is controlled within a preset upper limit (e.g., 1.5 MPa in GP‐6 well), and the fluctuation ranges of daily gas and water production are limited to less than 100 and 4 m 3 /day, respectively, with the fluctuation frequency less than 50 times/day, so as to avoid “gas lock” damage caused by large bubbles around the well‐bore. In the pressure‐reducing and production‐increasing stage (Phase V), the bottom hole flow pressure is slowly reduced to realize a steady increase in gas production (e.g., GP‐7 well increased from 170 to 750 m 3 /day), avoiding large fluctuations in gas and water production rates and ensuring a smooth transition from gas–water TPF to water–gas TPF. In the pressure‐controlled stable production stage (Phase VI), the principle of “continuous, slow, and stable” drainage is followed and measures such as selecting hydraulic hollow rodless pump and timely anticirculation pump washing are adopted to avoid discontinuous drainage, thus, preventing “water lock” damage in near‐well‐bore zones. These stage‐specific and quantified control strategies effectively mitigate the surface tension effect, providing important practical guidance for improving the development efficiency of multilayer superimposed CBM reservoirs.