特低渗透油藏CO_2非混相驱极限井距计算方法

极限井距计算方法对于特低渗透油藏确定合理井网密度具有重要意义。考虑CO_2对原油的降黏作用及油相启动压力梯度的变化,建立了一维两相渗流数学模型,并应用特征线法、试算法和迭代法进行求解,推导出特低渗透油藏CO_2非混相驱极限井距计算方法。同时开发了极限井距计算软件进行井距实例计算,绘制了理论图版。结果表明:CO_2对于原油的降黏作用可达到60%以上,油相黏度自注气端至采出端逐渐增大;生产压差为10~20 MPa,注气速度分别为10、15、20、25、30 t/d的极限井距均比未考虑油相黏度及油相启动压力梯度变化的极限井距大;极限井距随生产压差增大而增大,随注气速度、地层原油黏度、CO_2黏度增大而减小;生产压差相同时,注采两端压力越大,极限井距越大,油相黏度及油相启动压力梯度的下降幅度越大。

CALCULATING METHOD OF THE CRITICAL WELL SPACING FOR CO2 IMMISCIBLE FLOODING IN ULTRA-LOW PERMEABILITY OIL RESERVOIRS

The computing method of the critical well spacing possesses the vital significance for determining the reasonable well spacing density in ultra-low permeability reservoirs.A one-dimensional two-phase mathematical model was established considering the visbreaking action of CO2 in the crude oil and the changes of the threshold pressure gradient,and moreover with the help of the solutions by characteristic linear method,trial-and-error method and iterative method,the calculating method was deduced for the critical well spacing of CO2 immiscible flooding in ultra-low permeability reservoirs.At the same time,the calculating software for the critical well spacing was developed and furthermore practiced,and then the theoretical board chart was drawn as well.The results show that when the visbreaking action is more than 60％,the viscosity of the oil phase gradually increases from the gas injected end to produced one;the critical well spacing is greater for the ones with 10-20 MPa production pressure differential and gas injection rates are 10,15,20,25,30 t/d respectively comparing with the ones whose oil phase viscosities and threshold pressure gradients are not considered;with the rise of the producing pressure differential,the critical well spacing is enlarged as well,while it is reduced with the increases of the gas injection rate,reservoir oil viscosity and CO2 viscosity;when the producing pressure differential is same,the greater the pressures at both ends,the larger the critical well spacing will be,so are the reduced degree for the oil viscosity and starting pressure gradient.