Simulation And Geological-mathematical Models Of CBM Co-production Interference

The area of western Guizhou and eastern Yunnan is a reserve base for CBM development under construction in China.CBM resources are mainly discovered in the Longtan formation and Changxing formation of the upper Permian.However,the unique geological characteristics such as multiple layers and superposed gas-bearing systems cause significant interlayer interference during CBM co-production and bring great challenges to the efficient and economic development of CBM resources.In response to this problem,three pilot areas of Tucheng,Enhong and Laochang were selected as the study objects.Based on exploration data of CBM reservoirs and production data of CBM wells,the controlling factors of co-production interference were determined.Through physical experiments and numerical simulation of CBM co-production,the mechanism of reservoirs combination optimization for CBM co-production was analyzed,and then a set of geological and mathematical models for CBM co-production were constructed.In this work,we obtained the following innovative understandings.1)The geological conditions which constrain the effects of CBM co-production are revealed.The relationship between physical characteristics of CBM reservoirs and draining dynamics of 20 CBM wells in the study area is analyzed and the essential requirements for high yield are revealed,i.e.sufficient gas content,high efficiency for energy transfer,excellent porosity and permeability,and rational geometric features.The gas-bearing characteristics are the basis for a high co-production productivity and the groundwater hydrogeology is the key to a high yield.The magnitude of interlayer interference occurring in co-production jointly depends on the permeability ratio,the fluid pressure difference and the interval distance.Among three controlling geological factors,permeability ratio directly affects the gas productivity from each seam co-produced and the fluid pressure difference poses an impact on the drainage and depressurization.The interval distance shows a double-edged effect of promoting and inhibiting CBM desorption and the latter prevails.2)The quantified thresholds of geological conditions favorable for CBM co-production are proposed.The physical experiments simulating CBM co-production in twin seams elucidate how the controlling geological factors jointly work on the distribution of production contribution and interlayer interference.According to results of physical experiments,the permeability ratio determines the production contribution and the logarithmic models can describe the changes.The interlayer interference of the top seam depends on the fluid pressure difference,and that of the bottom seam is determined by the fluid pressure difference and the interval distance together,through which the former weakens and the latter strengths the interference.The results of numerical simulation show that the fluid pressure difference and the interval distance jointly determine the pressure distribution inside reservoirs and the reversed flow in borehole.It is suggested that CBM co-production should be implemented in coal seams belonging to the same fluid pressure system and the permeability ratio should be within 5 times.The recommended interval distance of CBM co-production is within 100 m and 50 m in western Guizhou and eastern Yunnan,respectively.3)A set of geological-mathematic models for CBM co-production are constructed.8scenarios of CBM co-production are concluded based on the conditions of desorption sequence,coal seams exposure and fluid pressure.These scenarios are classified into 3categories according to their drainage effects,with class I scenarios having top priority for CBM co-production,class II scenarios given proper considerations of reservoirs combination and class III scenarios not recommended for reservoirs combination.Based on the geological-mathematic models,the working mechanism of the permeability ratio,the fluid pressure difference and the interval distance on production contribution distribution and interlayer interference is analyzed from the perspective of engineering compatibility.The reservoirs combination schemes of two typical wells in the study area are evaluated and optimized based on the identification results of the proposed models and the conclusion accounting for the different production of CBM co-production wells.This thesis contains 107 figures,30 tables and 274 references.