柴达木盆地西部地区下干柴沟组湖相白云岩晶间孔型储层物性下限的确定

Determination of petrophysical property cutoffs of lacustrine dolomite intercrystalline pore reservoir in the Xiaganchaigou Formation,western Qaidam Basin

物性下限是储层有效性评价及资源量计算的重要参数。以柴达木盆地英西地区古近系致密湖相碳酸盐岩为研究对象,在高压压汞测试及场发射扫描电镜分析确定纳米级白云石晶间孔孔喉分布范围的基础上,提出了一种确定储层物性下限的实验方法,该方法的核心是通过核磁共振多次离心测试来达到分离可动流体与束缚流体的目的,将可动流体与束缚流体分离点对应的孔喉半径作为可动流体分布孔喉下限,通过孔喉半径与物性的拟合关系确定储层物性下限,此外,通过毛细管曲线法、分布函数法及测井解释结论对核磁共振离心方法确定的物性下限进行了验证。研究结果表明,通过压汞及扫描电镜确定的白云石晶间孔孔喉分布范围为40~300 nm,蒸馏水饱和后核磁共振测试确定的样品孔径分布范围为50~300 nm;可动流体分离点对应的可动流体分布孔喉下限为47 nm,通过核磁共振离心测试确定的孔隙度下限为3.29%,渗透率下限为0.02 mD,该结果相对压汞曲线法确定的物性下限偏小,这是由于两种方法采用的测试流体不同造成的,该结果与分布函数法确定的物性下限一致,与测井解释结论的符合程度达85%,确定的物性下限可作为一种评价低渗储层有效性的依据。

The petrophysical property cutoffs are important parameters for the evaluation of reservoir effectiveness and resources calculation. Focusing on the Paleogene tight lacustrine carbonate rocks in the Yingxi area of Qaidam Basin, based on determining the distribution range of nano-scale dolomite intercrystalline pore throat by high pressure mercury injection test and field emission scanning electron microscopy analysis, this study proposes an experimental method to determine the petrophysical property cutoffs of reservoirs. The core of the method is to separate the movable fluid from the irreducible fluid through multiple centrifugal tests under nuclear magnetic resonance(NMR);the pore throat radius corresponding to the separation point of movable fluid is taken as the pore throat radius cutoffs for the movable fluid distribution. The petrophysical property cutoffs of reservoirs are determined by the fitting relationship between the pore throat radius and physical properties. In addition, the petrophysical property cutoffs determined by the NMR-based centrifugal method is verified by the capillary curve method, distribution function method and log interpretation conclusions. The research results show that the distribution range of dolomite intercrystalline pore throat determined by mercury injection test and scanning electron microscope is 40 to 300 nm, and that of pore size of the sample determined by NMR test after saturated distilled water is 50 to 300 nm. The separation point of movable fluid corresponds to the pore throat radius cutoffs of 47 nm for the movable fluid distribution, the porosity cutoffs determined by NMR-based centrifugal test is 3.29%, and the permeability cutoff is 0.02 mD. These results are smaller compared with the petrophysical property cutoffs determined by mercury injection curve method. It is because two different test fluids were used in the two methods. The result is consistent with the petrophysical property cutoffs determined by the distribution function method, and the degree of coincidence with the log interpretation conclusions is 85%. The determined petrophysical property cutoffs can be used as a basis for evaluating the effectiveness of low-permeability reservoirs.