Characteristics of High Permeability Streak of Mishrif Formation in B Oilfield and Its Influence on Waterflood Development

Heterogeneity in carbonate reservoir is commonly driven by a combination of deposition and diagenesis, which may result in the occurrence of high permeability streaks in the carbonate reservoir. Porosity type such as fractures and diagenetic overprints control the productivity of depositional facies. These high-permeability streaks, also referred to as thief zones, have the potential to cause premature water breakthrough in production wells and result in uneven displacement around water injection wells. This study identified and characterized high permeability streaks through conventional petrophysical tests integrated with formation micros-canner image (FMI), nuclear magnetic resonance (NMR) and production data in B oilfiled. The reservoirs with good porosity are throughout subzone B21. Multistage centrifugation nuclear magnetic experiments indicate good reservoir on top has larger pores, comparing to other parts with good porosity. Pore network of good reservoir which is located on top is irregular and well-connected, while the pore system of the rest good reservoirs are relatively regular and poorly-connected. Integrated analysis shows that the high permeability streak was recognized below the top of B21, contributing more than 80% of the total production, while it is not developed in other layers. The permeability of thief zone exceeds 2 darcies as a result of the intergranular dissolved pores developed in the reservoir. Wireline log response of high permeability streak is very low gamma, high resistivity and high AC values. The horizontal distribution of high permeability streaks is closely related to the paleostructure during the depositional processes. Grainstone is distributed on the palaeohigh, with dissolution pores well-developed, in some cases, having a good effect on permeability. Production data show the thickness of high permeability streak decides whether saw productivity boosted or water breakthrough - when it is thinner than 1.5m, only productivity would be affected; conversely, when it is thicker than 1.5m, water breakthrough tends to be observed.