基于核磁共振的天然气驱储集层孔喉动用下限

Lower Limits of Pore Throat Producing in Natural Gas Drive Reservoirs Based on Nuclear Magnetic Resonance

依据核磁共振原理,推导岩心核磁共振弛豫时间与孔喉半径的关系,结合岩心高压压汞孔喉半径分布,利用数值差值和最小二乘法,将核磁共振T2谱转换为孔喉半径分布。在此基础上,利用岩心驱替过程中含油饱和度和核磁共振信号幅度同步逐渐降低的规律,分析不同注气速度、注气压力和位置下不同孔喉半径的核磁共振信号幅度变化,确定不同开发方式下的孔喉动用下限。研究表明,微米级孔隙中的原油是水驱采收率的主要贡献者,低注气速度下孔喉动用下限为0.04μm,高注气速度下孔喉动用下限为0.01μm;注气压力由5.0 MPa升高到17.0 MPa时,孔喉动用下限从0.05μm下降到0.01μm;注气压力较高时,岩心整体孔喉动用较均匀;随着注气压力的升高,主要动用半径小于0.01μm孔隙内的剩余油。

Based on the principle of nuclear magnetic resonance(NMR),the relationship between NMR relaxation time and pore throat radius of core sample was derived. Combining with the pore throat radius distribution obtained from high-pressure mercury intrusion test,the NMR T2spectrum was converted into the pore throat radius distribution by using the numerical difference and the least square methods.Then,according to the law that the oil saturation and NMR signal amplitude gradually decrease synchronously during core displacement process,the changes in the NMR signal amplitude of different pore throat radii under different gas injection rates,gas injection pressures and positions were analyzed,and the lower pore throat producing limits for different development patterns were determined. The results show that the crude oil in micron-sized pores is the main contributor to water flooding recovery. The minimum pore throat producing limit is0.04 μm at low gas injection rate and 0.01 μm at high gas injection rate. When the gas injection pressure increases from 5.0 MPa to 17.0MPa,the lower pore throat producing limit decreases from 0.05 μm to 0.01 μm. When the gas injection pressure is high,the overall pore throat producing degree of the core is relatively uniform. With the increase of gas injection pressure,the remaining oil in the pores with the radius less than 0.01 μm is mainly produced.