Pressure drop expansion law of high rank coalbed methane reservoir and its application in well pattern optimization

In order to clarify the law of reservoir pressure drop expansion,a pressure drop expansion model considering the same flow of gas and water is established,and the factors affecting the pressure drop expansion velocity and the shape of pressure drop funnel are analyzed.The results show that the pressure drop expansion of high coal rank coal reservoir is affected by fracturing,and the pressure drop expansion in parallel to the horizontal maximum principal stress direction is obviously double due to the influence of fracturing fracture,and the pressure drop longitudinal expansion velocity is faster.The influence of the elastic energy of methane desorption expansion on the pressure drop expansion of CBM wells cannot be ignored.The pressure expansion of reservoir boundary has a “double peak” change before and after the pressure drops to the critical desorption.Under the same drainage system,the expansion speed and shape of the pressure drop funnel are affected by hydrodynamic conditions,permeability,solution ratio and bound water saturation.The more active the hydrodynamic conditions are,the lower the permeability is,the smaller the geo solution ratio is,and the greater the irreducible water saturation is,the more unfavorable the lateral and vertical propagation of pressure drop is.When developing reservoirs with low gas saturation,it is necessary to extend the drainage period and expand the sweep range of pressure drop funnel before desorption,so as to obtain higher desorption efficiency after desorption and obtain longer stable production time.By analyzing the measured data of the inter-well formation pressure monitoring wells deployed in the development well pattern of Fanzhuang and Zhengzhuang blocks,there are two stages of reservoir pressure drop expansion:cooperative pressure reduction and cooperative desorption.In order to achieve a cooperative production increase,the coal reservoir pressure must be reduced below the desorption pressure within the range of two wells.The maximum well spacing that reduces the reservoir pressure between wells to below desorption pressure is defined as the “critical well spacing for synergistic production increase”.The well distance is controlled by permeability and stress field,and the permeability and stress field are controlled.The higher the permeability,the greater the critical well spacing,and the smaller the angle with the direction of the maximum horizontal principal stress,the greater the critical well spacing.In the process of coalbed methane development,it is necessary to consider the difference of “critical well spacing for synergetic stimulation” in different directions,and carry out differentiated well pattern and well spacing design,so as to maximize the utilization of resources and enhance oil recovery.