| The Yangquan mining area is rich in coal bed methane resources,with a resource volume of 457.49 billion cubic meters below 2000 m.However,the geological conditions of coal reservoirs in the mining area are complex,and low-pressure and low-temperature reservoirs are developed.The development technology of coal bed methane is difficult,and the development effect is poor.This article is mainly based on the exploration and development data of coal bed methane in Yangquan mining area,comprehensively and deeply analyzes the characteristics of coal reservoirs in the study area,reveals the distribution law of low-pressure and low-temperature characteristics and geological genesis mechanism of coal reservoirs,evaluates the development characteristics of coalbed methane in the study area and analyzes the reasons for low production,and develops the principle of high-pressure nitrogen water-based composite fracturing adaptability development technology for low-pressure and low-temperature reservoirs.The main research results of the paper are as follows:(1)Through the analysis of coal reservoir pressure,gas pressure and the spatial variation characteristics of apparent reservoir pressure,it is concluded that the coal reservoir in the study area is a low-pressure reservoir,and the formation of its low reservoir pressure characteristics is mainly affected by factors such as formation uplift and erosion,crustal stress state,collapse column and fault activity,groundwater flow field and coal series hydrostatic pressure.Among them,the combined effects of strata uplift,erosion,and unloading,as well as the groundwater flow field in the Niangziguan spring area,the static water pressure of coal measures,and interlayer hydraulic transport are the main controlling factors for the low-pressure coal reservoirs in the study area.The continuous uplift and erosion of the formation will cause the expansion effect of the coal bearing formation skeleton and the volume expansion effect of the fluid in the pores,leading to a decrease in the pore fluid pressure of the coal bearing formation and a decrease in the reservoir pressure;After the formation uplift,the burial depth of the coal bearing rock series becomes shallow,the surface precipitation infiltrates into the coal bearing strata,and the supply of the coal bearing strata is weak,and the surface evaporation is far greater than the precipitation.The water power of each aquifer moves from the high to the low interlayer to the Niangziguan Spring Group,the leakage point of the drainage area at the lower position,and their combined action results in the low pressure of the coal reservoir.(2)The well temperature logging data in Yangquan mining area reflects that the geothermal gradient is mainly low anomaly and normal low.It is concluded that the coal reservoirs in the study area are low-temperature reservoirs,and the characteristics of low-temperature reservoirs are mainly affected by factors such as strata uplift and erosion,structural basement undulation,formation lithology thermal conductivity and surface water compensation,runoff and seepage,and groundwater runoff migration activities.Among them,the low temperature plane distribution of geothermal field is mainly controlled by the uplift and denudation of strata,and the continuous uplift and denudation of coal bearing strata are destroyed;After the increase of burial depth,the thermal conductivity of coal and rock improves,leading to an increase in ground temperature and a rise and fall in local ground temperature field in the study area caused by the fluctuation of the geological structure basement.Both of these will cause erosion and damage to the overlying shallow loose strata caused by the uplift of the strata,resulting in a significant decrease in the thermal conductivity of the coal bearing strata and a decrease in the temperature of the coal reservoir,The vertical difference caused by the thermal conductivity of the coal bearing strata and the horizontal difference caused by the fluctuation of the stratigraphic tectonic base have far less effect on the change of the geothermal field than the denudation caused by the uplift of the strata;The long-term recharge,runoff,and seepage of shallow surface cold water in the research area migrate to the underground aquifer.When the runoff flows from the shallow high level to the deep low level,it continuously carries away the heat of the surrounding rock,leading to a decrease in ground temperature and geothermal gradient.After the shallow groundwater is migrated to the deep and enriched,the cooling effect of groundwater is weakened and the heating effect of deep surrounding rock is enhanced,which will compensate for the heat loss caused by the denudation of stratum uplift to a certain extent.However,the heat lost from the surface of the overlying strata in the study area is far more than the heat supplied by the heating of deep surrounding rock,so the geothermal field of coal bearing strata has not increased significantly,which also indicates that the surface cold water compensation The control effect of runoff and seepage and groundwater runoff movement on the geothermal field in the study area is weaker than that of strata uplift and denudation.(3)Based on the emission data of the 3 # and 15 # coal seams,the current development characteristics of coal bed methane in the research area were analyzed,and it is believed that the low pressure and low temperature of coal reservoirs have an important impact on their coal bed methane production.Under low reservoir pressure,the internal fluid migration energy of coal reservoirs is insufficient,and the pressure reduction space is limited,resulting in difficulty in producing gas and water in coal reservoirs;At low reservoir temperatures,the desorption behavior of coal bed methane is inhibited,resulting in low desorption efficiency of coal bed methane;In addition,after coal reservoir fracturing,the fracturing water is prone to invade the pores and fractures,hindering the desorption,diffusion,and seepage of coal bed methane,and also inhibiting methane desorption.Based on the adaptability of horizontal well development technology under low pressure and low temperature geological conditions,the feasibility of high-pressure nitrogen water based composite fracturing technology for horizontal wells has been demonstrated.The backflow of high-pressure nitrogen gas after nitrogen water based composite fracturing in XJ-1L horizontal wells can promote the backflow of fracturing fluid and coal powder.Compared with active water pressure fracturing coal bed methane wells,the rate of liquid column decline during the fracturing process has been slowed down by 2.7 to 34.8 times,reducing the damage of water-based fracturing to coal reservoirs,Improved the power of coal seam water production and increased water production;After nitrogen water based composite fracturing of XJ-1L horizontal well,the desorption pressure increased by 2.7 times,which is equivalent to a reservoir warming of about 13 ℃.This effectively verifies the "energy enhancing protection effect" on low-pressure reservoirs and the relative "warming effect" on low-temperature reservoirs,providing technical support for the development engineering practice of coal bed methane in similar reservoirs in China.This dissertation contains 95 figures,31 tables and 209 references. |
PDF Full Text Request