Research On Quantitative Production System For Coal Bed Methane Double Layer Combined Production

Multi-layer combined mining is an ideal form of development for CBM with thin thickness and multiple layers.The production system has a profound impact on the final production capacity.The current research on multi-layer drainage and extraction systems mainly focuses on qualitative analysis or providing empirical measures through historical fitting of production data.It is difficult to accurately formulate drainage and extraction plans through quantitative methods before production,and it is also difficult to accurately adjust the bottom hole flow pressure of two reservoirs during production,resulting in unreasonable drainage and extraction systems and poor joint production results.This article first analyzes the similarities and differences between single-layer mining and double-layer combined mining,as well as the influencing factors.Through theoretical research,the main controlling factors of the mining system were obtained.At the same time,it analyzes the difficulties of quantitative mining scheduling and provides countermeasures.Research suggests that among the main controlling factors of dual layer combined mining,permeability is the most important control factor of pressure drop propagation,followed by porosity,shear compression coefficient,temporary storage ratio,and desorption characteristics.Secondly,a pressure drop model for the reservoir section was established.Based on the theoretical derivation method,the relationship between the coal seam hydraulic conductivity coefficient and the pressure propagation distance was established.The porosity including the shear compression coefficient was used as the intermediate parameter,and the dynamic changes in pore permeability were considered.A quantitative relationship model and water production model between the bottom hole flow pressure and the pressure drop funnel during the single-phase drainage stage were established.Based on the changes in gas saturation distribution as a node,the pressure drop propagation process is divided into multiple combinations of propagation forms.A pressure drop composite propagation model is established,which varies in both time and space dimensions.On this basis,a quantitative relationship model between bottom hole flow pressure and pressure drop funnel,desorption funnel,and water production gas model is established based on the relationship between water saturation and gas water relative permeability,as well as single point pressure of the reservoir.Using the constructed model,it can be concluded that:(1)in the single-phase drainage stage,the radius of the pressure drop funnel considering stress sensitive damage is smaller.When the total pressure drop is constant,the more times the bottom hole flow pressure decreases,the smaller the permeability damage,and the farther the pressure drop funnel expands.(2)In the co-production stage of gas and water,the stress sensitive damage is lower than the positive effect of permeability caused by matrix shrinkage effect.The calculated desorption funnel radius and cumulative gas production are consistent with the results obtained from commercial software using pseudo pressure calculations.The third,a wellbore pressure drop propagation model was established,dividing the two reservoir sections into multiple small layers.The variable mass formulas considering radial inflow in the reservoir sections were derived for pure water flow,pure air flow,bubbly flow,slug flow,and annular flow,respectively.Combined with the constant mass formula for non-reservoir sections,quantitative models for bottom hole flow pressure and wellhead parameters in three production stages were established.Using the constructed model,it was found that the variable mass model considering radial inflow has an average error of 7.35% lower than the traditional constant mass model in the gas water co-production stage.At the same time,the maximum difference in bottom hole flow pressure between the upper and lower reservoirs is 49%,indicating that the bottom hole flow pressure of the lower reservoir cannot be used to represent the bottom hole flow pressure of the two reservoirs.Finally,based on the sorting of the main control factors and the division of production stages in double-layer combined production,the optimal bottom hole flow pressure reduction plan was determined using the reservoir pressure drop propagation model,and the wellbore pressure drop propagation model was used to determine the measures for adjusting the actual bottom hole flow pressure of the two reservoirs using the wellhead parameters.As a result,a quantitative production scheduling system for double-layer combined production was established.According to the established quantitative drainage and extraction system,(1)economic drainage time and optimal pressure reduction law can be quantitatively given before production;(2)In production,the actual control method to bottom hole flow pressure can be quantitatively provided.Through experimental verification,the quantitative extraction method in this article has a cumulative water and gas production that are 9.42% and 6.20% higher than conventional methods,respectively.Through on-site examples,it was verified that the cumulative gas production obtained by the model in this article is 8.45% higher than the actual value on site.This article quantifies the production scheduling system,which can develop the optimal bottom hole flow pressure reduction plan and control method based on the characteristics of the reservoir itself,achieve the goals of optimal pressure drop propagation and maximum production capacity,and provide technical support and theoretical guidance for the development of multi-layer coalbed methane co-production.