Determiniation Method For Gas Reserve And Critical Desorption Pressure Of Coalbed Methane By Water Effect

With the development of conventional reservoirs enter to mid-late period,it's important to develop the unconventional reservoirs such as coalbed methane and shale gas.Where coalbed methane occupies an important place because the proven reserves of coalbed methane has been 256.1×1012m3 in the world,and the production is over700×10~8m~3 in 2011.But there is still a lot of problem in the development of coalbed methane,for example,most coalbed methane reservoirs'production are far fewer than expected,lots of coalbed methane reservoirs have been developed in American,but production is still dominated by San Juan Basin and Black Warrior Basin,which production is exceeds 80%of total production in American.One reason for these phenomena is that even the research of the micro-mechanism has been developed,while in the calculation of coalbed methane,the old method which is based on the?A guide for Determining Coalbed Gas Content?in 1995 is still used.Which doesn't considered the effect of liquid in coal's pore.This paper start from the formation of coalbed methane reservoir and characteristics of thermal evolution,considered both pore characteristics and fluid generation in thermal evolution,summarize the law of gas-water distribution,and built mathematical formulas of the rate of single phase fluid flow.Furthermore,evaluate the key parameters influenced by gas-water distribution in the calculation of geological and recoverable reserves.Then analysis the mechanism of the influencing this parameters by gas-water distribution,and establish the mathematical model of the relationship between this parameters and gas-water distribution.Finally,analysis the error and a new method of the critical desorption pressure.The key conclusions are as follows.1. Give the distribution characteristics of gas and water in coal reservoirs.Considered the pore characteristics and fluid generation in thermal evolution,the main pore of adsorption is full of gas phase,while the primary pore is full of water in the begin of the thermal evolution,with the thermal evolution degree improves,water in part of the primary pore will be displacement by formed methane,which make primary pore become full of gas or two-phase,suppose the gas is one-way flow from main pore of adsorption to the primary pore,the rate of single phase fluid flow can be calculated.2. Quantified the effect of gas-water distribution to the geological reserves.Gas-water distribution has little effect on adsorbed reserves,while to the free gas,the gas-water distribution will increase the pressure of gas because of the capillary,consider of this,the free gas reserves will increase more than 2 times when the initial formation pressure is 4MPa and 20%gas saturation.3. Quantified the effect of gas-water distribution to the recoverable reserves.Gas-water distribution influence recoverable reserves by desorption curve and gas permeability.Gas-water distribution will change the desorption from solid-gas interface adsorption to solid-water,which is much difficult to desorption,also it will increase flow resistance and change the flow passage by capillary.Consider of this,the water in matrix will significant influence the recoverable reserves,the recoverable reserves will decrease 16.67%while the water saturation in matrix increase 10%,and the recoverable reserves will decrease 75.87%while the water saturation in matrix increase 50%.4. Bulid a new method of the critical desorption pressure consider the relationship of critical desorption pressure and the pressure when the casing pressure is not zero.Between the pressure when the casing pressure is not zero and critical desorption pressure,there are pressure overcoming the capillary and make sure the gas saturation of gas is greater than critical flow saturation by relative permeability curve.Which can be used to calculate the critical desorption pressure.The error will be decreased by using both the new method and the definition of critical desorption pressure to calculated.This paper quantified the relationship between gas-water distribution and free gas reserves,desorption curve and gas permeability,laid the foundation of calculate the geological and recoverable reserves more accurate.Improved the theory of coalbed methane evaluation and development.