Numerical study on the effectiveness of directional well with multiple hydraulic slots for enhanced gas recovery in deep coal seam

Abstract

Coalbed methane production failures through hydraulic fracturing in the Linxing gas field in China have encountered significant setbacks, hindering efforts to enhance the recovery of deep CBM reserves in the region. To address this challenge, we investigate the potential of directional wells with multiple hydraulic slots as a stimulation technology for deep coalbed methane extraction. This study presents two numerical models to evaluate the effectiveness of this technology. The first model is a continuum damage numerical model based on the Material Point Method, which simulates large deformations and complex contact behaviors induced by hydraulic slotting in the deep coal seam. The second model is a wellbore-pore coupling model to simulate the gas extraction process. We verified both numerical models against theoretical and experimental results. We conducted a case study in the Linxing gas field using these models. The investigation assessed stress relief, gas desorption, and permeability enhancement around a large deformed hydraulic slot, as well as the gas extraction performance of a directional well with multiple slots. The results indicate that (1) The Linxing gas recovery failure may be attributed to unsuccessful hydraulic fracturing based on the history matching analysis of field production; (2) Hydraulic slotting causes gradual compaction of the slot until full closure, and results in a rapid stress drop exceeding 7 MPa in the elliptical zone surrounding the slot, which is the large plastic damage zone; (3) The enhanced permeability in plastic zone exceeds ten times the natural coal permeability, leading to total gas desorption of 556 m3 from elastic and plastic zones; (4) Performance optimization can be achieved through reducing slot length and increasing slot width when using directional well with multiple slots. In summary, this research demonstrates the efficiency of directional wells with multiple slots in enhancing gas recovery from deep coal seams.