A cohesive approach at estimating water saturation in a low-resistivity pay carbonate reservoir and its validation

Journal of Petroleum Exploration and Production Technology, Feb 2017

Carbonate reservoir characterization and fluid quantification seem more challenging than those of sandstone reservoirs. The intricacy in the estimation of accurate hydrocarbon saturation is owed to their complex and heterogeneous pore structures, and mineralogy. Traditionally, resistivity-based logs are used to identify pay intervals based on the resistivity contrast between reservoir fluids. However, few pay intervals show reservoir fluids of similar resistivity which weaken reliance on the hydrocarbon saturation quantified from logs taken from such intervals. The potential of such intervals is sometimes neglected. In this case, the studied reservoir showed low resistivity. High water saturation was estimated, while downhole fluid analysis identified mobile oil, and the formation produced dry or nearly dry oil. Because of the complexity of Low-resitivity pay (LRP) reservoirs, its cause should be determined a prior to applying a solution. Several reasons were identified to be responsible for this phenomenon from the integration of thin section, nuclear magnetic resonance (NMR) and mercury injection capillary pressure (MICP) data—among which were the presence of microporosity, fractures, paramagnetic minerals, and deep conductive borehole mud invasion. In this paper, we integrated various information coming from geology (e.g., thin section, X-ray diffraction (XRD)), formation pressure and well production tests, NMR, MICP, and Dean–Stark data. We discussed the observed variations in quantifying water saturation in LRP interval and their related discrepancies. The nonresistivity-based methods, used in this study, are Sigma log, capillary pressure-based (MICP, centrifuge, and porous plate), and Dean–Stark measurements. The successful integration of these saturation estimation methods captured the uncertainty and improved our understanding of the reservoir properties. This enhanced our capability to develop a robust and reliable saturation model. This model was validated with data acquired from a newly drilled appraisal well, which affirmed a deeper free water level as compared to the previous prognosis, hence an oil pool extension. Further analysis confirmed that the major causes of LRP in the studied reservoir were the presence of microporosity and high saline mud invasion. The integration of data from these various sources added confidence to the estimation of water saturation in the studied reservoir and thus improved reserves estimation and generated reservoir simulation for accurate history matching, production forecasting, and optimized field development plan.

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A cohesive approach at estimating water saturation in a low-resistivity pay carbonate reservoir and its validation

A cohesive approach at estimating water saturation in a low-resistivity pay carbonate reservoir and its validation Adedapo Awolayo 0 1 Ayham Ashqar 0 1 Miho Uchida 0 1 Andi Ahmad Salahuddin 0 1 Saheed Olawale Olayiwola 0 1 0 Abu Dhabi Company for Onshore Petroleum Operations Ltd. (ADCO) , P.O.Box 270, Abu Dhabi, UAE 1 Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary , 2500 University Drive NW, Calgary, AB T2N 1N4 , Canada Carbonate reservoir characterization and fluid quantification seem more challenging than those of sandstone reservoirs. The intricacy in the estimation of accurate hydrocarbon saturation is owed to their complex and heterogeneous pore structures, and mineralogy. Traditionally, resistivity-based logs are used to identify pay intervals based on the resistivity contrast between reservoir fluids. However, few pay intervals show reservoir fluids of similar resistivity which weaken reliance on the hydrocarbon saturation quantified from logs taken from such intervals. The potential of such intervals is sometimes neglected. In this case, the studied reservoir showed low resistivity. High water saturation was estimated, while downhole fluid analysis identified mobile oil, and the formation produced dry or nearly dry oil. Because of the complexity of Lowresitivity pay (LRP) reservoirs, its cause should be determined a prior to applying a solution. Several reasons were identified to be responsible for this phenomenon from the integration of thin section, nuclear magnetic resonance (NMR) and mercury injection capillary pressure (MICP) data-among which were the presence of microporosity, fractures, paramagnetic minerals, and deep conductive borehole mud invasion. In this paper, we integrated various information coming from geology (e.g., thin section, X-ray diffraction (XRD)), formation pressure and well production tests, NMR, MICP, and Dean-Stark data. We discussed the observed variations in quantifying water saturation in LRP interval and their related discrepancies. The nonresistivitybased methods, used in this study, are Sigma log, capillary pressure-based (MICP, centrifuge, and porous plate), and Dean-Stark measurements. The successful integration of these saturation estimation methods captured the uncertainty and improved our understanding of the reservoir properties. This enhanced our capability to develop a robust and reliable saturation model. This model was validated with data acquired from a newly drilled appraisal well, which affirmed a deeper free water level as compared to the previous prognosis, hence an oil pool extension. Further analysis confirmed that the major causes of LRP in the studied reservoir were the presence of microporosity and high saline mud invasion. The integration of data from these various sources added confidence to the estimation of water saturation in the studied reservoir and thus improved reserves estimation and generated reservoir simulation for accurate history matching, production forecasting, and optimized field development plan. Water saturation; Carbonate reservoirs; Low- resistivity pay; Core analysis; Field development - LRP reservoir was first discovered in a sandstone reservoir within the Gulf of Mexico (Boyd et al. 1995) and has progressively been at the frontline of several industrial projects that involves deep water exploration and brownfield development. It has been described as hydrocarbonbearing zone that appears as water interval based on openhole resistivity measurements. Meanwhile, such intervals show strong hydrocarbon on mud logs and produce hydrocarbon as either gas or oil with little or no water cut from core studies, pressure and production tests (Pittman 1971; Keith and Pittman 1983; Worthington 2000). Additionally, the LRP intervals are commonly identified with high water saturation, which makes such intervals of low interest to the extent that they are discarded as attractive to appraise, particularly when oil prices are low. Typically, LRP zones are characterized by formation interval, with moderate to high porosities, showing extremely low resistivity that are often less than 3 X-m and most frequently encountered in areas with saline formation water (Griffiths et al. 2006; Obeidi et al. 2010; Worthington 2000; Farouk et al. 2014; Uchida et al. 2015). While Boyd et al. (1995) proposed that the resistivity range is between 0.5 and 5 X-m, several other researchers, like Zhao et al. (2000), stated that LRP can be identified by the ratio of the pay zone to the water-bearing zone and this ratio is considered to be in the range of 2. LRP occurs in both clastics and carbonates, while in carbonates, it has been reported to be as a result of either or a combination of deep high saline mud invasion, presence of conductive minerals, presence of microporosity, and anisotropic effect due to drilling high angle wells within thin reservoirs (Griffiths et al. 2006; Obeidi et al. 2010; Chu (...truncated)


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Adedapo Awolayo, Ayham Ashqar, Miho Uchida, Andi Ahmad Salahuddin, Saheed Olawale Olayiwola. A cohesive approach at estimating water saturation in a low-resistivity pay carbonate reservoir and its validation, Journal of Petroleum Exploration and Production Technology, 2017, pp. 1-21, DOI: 10.1007/s13202-017-0318-2