Abstract:
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.