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An Integrated Approach to Evaluate Carbonate Pore Structures of Early Triassic Reservoir, Puguang Gas Field

Abstract

Carbonate rocks have complicated rock textures and petrophysical behaviors resulted from depositional rock fabrics or diagenetic modification. It is crucial to include the characterization of pore structure into the conventional formation evaluation for better management of reservoir performances. Core and log measurements are affected by internal pore structures and can be integrated to differentiate them. In this study, evaluation of measurements from mercury injection capillary pressure (MICP) and common well logs are integrated to understand the pore structure variation. Five zones with distinct pore systems are identified and compared for the early Triassic rock, after evaluation of pore throat size, capillary pressure, pore throat tortuosity, irreducible water saturation and Leverett J-function. Within the five zones, pore types changed from moldic pore dominant to, sucrostic macro-intercrystalline, mixed moldic and intercrystalline (moldic dominated), meso-intercrystalline, and micro-intercrystalline shallowing upwards. Reservoir performance is the best in Zone 2 followed by Zone 1, Zone 4, and Zone 3. Multi-evaluation of resistivity, velocity, neutron-density logs for pore type prediction agrees with the observations from core analysis. Methods applied include calculating the differences between shallow and deep resistivity, and between neutron and density; evaluating cementation factor (m) from Archie's law, frame flexibility factor (?µ) from Sun's rock physics model, and S factor from Castagna's modified Whyllie's equation. It is found that Zone 1 (dominated by moldic pores) and Zone 2 (dominated by macro-intercrystalline pores) can be differentiated by resistivity, neutron-density, m, ?µ and S factor by a value of 0.34, 40, 2.4, 4 and 1.04 respectively. However, m and ?µ are more effective to separate all four reservoir zones with different pore systems. It is found that when m>2.4, ?µ <4, it's dominated by moldic pores; when 1.6 µ <8, it's dominated by meso to macro intercrystalline pores; when m <1.6 and ?µ >8, it's dominated by micro intercrystalline pores. The results of pore structure evaluation can be used to predict shear wave velocity and extract other petrophysical properties such as permeability, for better reservoir description.