Factors influencing the productivity and technology optimization of horizontal wells for moderately deep coalbed methane in the northern Zhengzhuang block

Objective Hydraulic fracturing of vertical wells was adopted during the early production of moderately deep coalbed methane (CBM) of the northern Zhengzhuang block in the Qinshui Basin, generally yielding low productivity and inefficiency. In contrast, single-casing fracturing of horizontal wells has increased the single-well production to 10‒50 times that of vertical wells, establishing horizontal wells as the dominant development well type. However, the horizontal wells differ greatly in CBM production. This study aims to determine the factors influencing the productivity of horizontal wells using staged fracturing for moderately deep CBM in the northern Zhengzhuang block and improve the production performance. Methods To this end, based on the production practice using horizontal wells in the northern Zhengzhuang block, combined with geological characteristics and engineering parameters, this study analyzed the dominant factors governing the productivity of horizontal wells for moderately deep CBM and proposed targeted suggestions for efficient production of horizontal wells. The results indicate that the productivity of horizontal wells using single-casing fracturing is influenced by both geological and engineering factors. Results and Conclusions The effects of geological conditions are as follows: (1) Moderately deep reservoirs exhibit significantly higher gas saturation than shallow reservoirs, generally boasting high resource enrichment. (2) Horizontal wells with high productivity are primarily distributed in gentle areas with low structural curvature. (3) There is a positive correlation between the number of perforation sections in primary coal seams and the gas production performance of horizontal wells. (4) The horizontal principal stress differences in the study area range from 8 to 16 MPa and increase with burial depth, posing challenges in the formation of complex fracture networks. This is the cause of the low gas production performance in the early stage. The effects of engineering conditions on the productivity of horizontal wells include: (1) The optimal gas production performance occurred when the included angle between the orientation of the wellbore trajectory and the azimuth of the maximum horizontal principal stress varied from 60° to 90°, with the average stable gas production rate of horizontal wells reaching up to 9700 m³/d. (2) Longer horizontal sections of horizontal wells were associated with higher stable gas production of coal seams. (3) The pumping bridge plug and clustering perforation fracturing technology yielded significantly higher gas production performance of horizontal wells than tubing fracturing, with the stable gas production increasing significantly with the fracturing scale. Among fracturing parameters, the fracturing fluid injection rate produced more significant controlling effects on fracture stimulation. Specifically, the stable gas production rate of a horizontal well was less than 2000 m³/d when the injection rate was less than 7 m³/min, increased gradually when the injection rate increased to 8‒10 m³/min, remained stably between 10000 and 12000 m³/d when the injection rate remained at 10‒12 m³/min, and exceeded 18000 m³/d when the injection rate increased to 16‒18 m³/min. Finally, based on optimal geological parameters such as gas-bearing properties, structural curvature, coal structure, and in-situ stress, along with optimal engineering parameters like horizontal section length, fracturing section number, the fracturing fluid volume and proppant volume for single-stage fracturing, fracturing fluid injection rate, and proppant concentration, this study analyzed the dominant factors influencing the productivity of horizontal wells for moderately deep CBM using grey relational analysis. The results indicate that coal structure and fracturing scale are the primary factors influencing the productivity of horizontal wells. Therefore, the principal methods for enhancing the productivity of horizontal wells through hydraulic fracturing for moderately deep coal seams in the study area include increasing the drilling rate and placement efficiency of primary coal seams, as well as further enhancing the fracturing fluid injection rate and fracturing scale.