Reservoir Dynamics And Interlayer Interference Quantification During Methane Co-Production From Superimposed CBM System

The western Guizhou and eastern Yunnan region has emerged as a new frontier for coalbed methane(CBM)exploration and development in China,characterized by significant development of "coal seam groups".Although individual coal seams exhibit relatively minimal thickness,their significant cumulative thickness suggests that multi-layer co-production serves as the most efficient method for CBM extraction.Intriguingly,within the superimposed gas-bearing system,an increase in coal seam thickness or in the number of operational CBM wells corresponds with a decrease in gas production,a phenomenon illustrating substantial interlayer interference in co-production wells.To address this challenge,the present study focuses on the Tucheng and Laochang blocks,employing physical simulation experiments and reservoir modeling techniques based on the interlayer interference characteristics of on-site CBM co-production wells.This research investigates the dynamic process of reservoir development in CBM co-production wells,illuminating the occurrence and development patterns of various forms of interlayer interference,quantifying the identification of interlayer interference intensity,and revealing the mechanism of interlayer interference.The results yield the following insights:The occurrence of interlayer interference during co-production and its impact on production performance have been investigated,the dynamic balance process of reservoir energy and production performance control mechanisms in co-production wells has been elucidated.In the early stage of co-production,fluids from different layers mix and interfere with one another in the wellbore due to reservoir pressure differences,leading to significant fluctuations in the initial production performance of co-production wells and the incompatibility of the co-production system.As the peak gas production approaches,interlayer interference gradually decreases,compatibility reaches a turning point,and gas production stabilizes during the later stages of production,potentially resulting in a slight increase in cumulative production.Moreover,fluid backflow during co-production can cause reservoir permeability damage behaviors,such as water blockage and gas blockage,which may occur in the early stage of co-production and during the exposure of the production layer.This reservoir damage affects production performance throughout the entire co-production process,leading to substantial gas production attenuation.The material balance process and co-production performance control mechanism in CBM wells have been elucidated.Building upon the commingled well flow theory and the constant flow rate internal boundary condition,the mechanism of layered water production regulation in wellbores under varying reservoir water-rich conditions is revealed.When the difference in water-richness is minimal,the water production from the upper and lower reservoirs is balanced,resulting in weak interlayer interference.However,when artificial fracturing establishes communication with aquifers,the water-richness of certain reservoirs significantly increases,thereby obstructing the water production process of other layers and leading to imbalanced water production.This discrepancy in water-richness serves as the key factor in the decline of productivity.The interlayer interference,primarily governed by the aquifer,affects production performance with varying degrees of intensity,considering factors such as reservoir pressure coefficient,aquifer thickness,aquifer permeability,coal seam thickness,artificial fracture permeability,in-situ coal permeability,and gas content.A quantitative identification system for interlayer interference has been established and practically applied.Utilizing fluid backflow volume and reduced coal seam water production as indicators,the interlayer interference intensity in the early and middle-late stages of co-production is quantitatively identified.Reservoir pressure difference is the root cause of fluid backflow formation and the most critical factor affecting interlayer interference;followed by coal seam thickness and permeability,which affect fluid backflow volume by influencing the coal seam’s water production index;gas content,Langmuir volume,and Langmuir pressure indirectly affect fluid backflow volume by influencing two-phase seepage.An increase in the thickness and permeability of the aquifer connected to the coal seam will result in a decrease in coal seam water production.In the later stage of drainage,the aquifer gradually becomes the primary source of water production in co-production wells,and suitably increasing drainage intensity can effectively reduce the occurrence of such interference.The dissertation contains a total of 132 figures,22 tables,and 190 references.