Case Study of Secondary Pore System and Bimodal Reservoir Rocks of Permian-Triassic Khuff Gas Reservoirs in an Offshore Field, Abu Dhabi

Abstract

Abstract Sweet gas accumulations in Permian-Triassic Khuff reservoirs are key players to meet growing gas demand in Abu Dhabi. In the study green field, thickness of seven Khuff reservoir units ranges between 100 ft to 500 ft with high variation in reservoir quality and productivity. Besides the pervasive faults and fractures related to salt-dome tectonics, complex pore systems play a major role in reservoir heterogeneity that impacts fluid distribution and flow behavior. The first whole well section cores recovered from the Upper Khuff in the study field were integrated with log data to establish a sequence framework and lithofacies scheme. Routine core analysis (RCA), thin section description and mercury injection capillary pressure (MICP) results were used to define pore systems and rock types at the core scale. Afterwards integrated with petrophysical log evaluation and well test results, secondary pore system with bimodal rock types were differentiated and distributed in sequence frame at well and field scale. The layer-cake sequence of Khuff Formation is composed of aggradation cycles of lagoonal and tidal to supra-tidal deposition across the study field. Primary reservoir rocks were developed in grainy facies with intergranular macro pores from shoals and moderate-energy lagoon settings. During later diagenesis, non-selective dissolution, dolomitization and late-stage cementation in these facies resulted in tortuous pore throat and low-connected secondary micro-meso pores. In low content of these secondary pores, it retains unimodal reservoir rocks dominant by macro-meso pores. With increasing complexity in these secondary pores, it develops bimodal reservoir rocks where low-connected to isolated micro vugs form the second peak on pore size distribution and shows a stair-step on saturation-pressure curve. On the other hand, increase of the meso pores leads to a gradient change in a wide range of pore size without obvious second peak, it tends to develop multimodal reservoir rocks that can be simplified by pseudo-unimodal pressure curve with a larger transition zone. Because of the isolated pores, some reservoirs of the secondary pore system are characterized by high irreducible water saturation (Swirr) and featured as low-resistivity pay zones, where high gas rates were achieved in high water saturation (Sw) intervals without water recovery. For these reservoirs, permeability estimation from cores and logs is biased by connected macro pores, high reservoir quality index (RQI) is reserved in high Sw. But water saturated in isolated secondary pores is immovable. As such secondary pore system can be differentiated from unimodal reservoirs on J-Function with high J values and high Sw high above contact. With the seismic data constrain and statistics analysis, secondary pore reservoirs can be stochastically modelled. And its contribution to reserve can be quantified. In summary, this case study provides a method to mimic the process and impact of secondary porosities in Khuff reservoir from core to field scale.