Reservoir characterization and baseline rock physics investigation for CO2 storage in the Bredasdorp Basin, offshore South Africa
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Date
2024
Authors
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Publisher
University of Fort Hare
Abstract
Reservoir characterisation and baseline rock physics investigation for CO2 storage in the Bredasdorp Basin, offshore South Africa was extensively executed in 3 wells, namely E-E1, E S3, and E-W2. This study aimed to provide a qualitative and quantitative description of Lower Cretaceous clastic rocks (upper shallow marine sandstones) to explore their potential viability for CO2 sequestration in the Bredasdorp Basin based on their petrographic, petrophysical and rock physical characteristics. Geophysical wireline logs, conventional core analysis, core logging and petrographic studies, i.e., thin sections and Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS), were utilized to determine lithology, fluid type, porosity, and permeability. The reservoirs were identified at ranging depths of 2336 m to 3528.5 m, with corresponding temperature and pressure ranging from 102.38 – 132.66°C and 26.95 – 36.29 MPa - meeting critical CO2 storage criteria. Clay volume, porosity, permeability, water saturation, and pore throat radius were quantitatively determined. Clay volumes ranged between 19.55 % and 27.19 %, confirming the shaly sandstone nature of the reservoirs. Water saturation varied from 51.78 % to 67.94 %, impacting the storage potential. Pore throat radius was examined using Winland’s technique and ranged from 1 – 2.4 µm, with the best reservoir quality found in lithofacies A and B (mesoporous). In contrast, microporous lithofacies E, F, and G were deemed non-reservoir zones due to their limited storage capacity. Reservoir quality across the wells was significantly impacted by diagenesis. In E-E1, porosity and permeability were reduced due to the presence of anhydrite or marialite and smectite. Concurrently, E-S3 experienced occlusion from illite and smectite while E-W2 exhibited concave-convex (long) contact diagenesis with the presence of kaolinite, further restricting reservoir potential. Average porosity ranged from 6.89 – 10.88 %, with permeability-range of 0.34 – 5.35 mD. The complementary acoustic properties of the reservoirs show a bulk density of 2.49 – 2.55 g/cc, with shear and compressional velocities between 3743.43 – 4792.12 m/s, respectively. Fluid substitution evaluation minimal changes in Vp when brine was replaced with oil and gas, however, significant decreases were observed when replaced by CO2. CO2 substitution notably reduced density in all wells except E-W2, which remained unaffected due to its micro- and nanopores. The results from the studied reservoir properties showed that the Upper Shallow Marine sandstones, E-S3 is the most prospective site for CO2 storage, even though its quality may be compromised by its significant water saturation and excess glauconite. E-E1 and E-W2 displayed some unfavourable properties such as high-velocity values, compaction (long contact) and smectite and kaolinite making them unsuitable for CO2 geological storage.
Description
Master of Science in Geology (MSc (Geology))
Keywords
Carbon sequestration -- South Africa, Reservoirs -- South Africa
Citation
Vandala, S. (2024) Reservoir characterization and baseline rock physics investigation for CO₂ storage in the Bredasdorp Basin, offshore South Africa. MSc (Geology) dissertation. Alice, South Africa: University of Fort Hare.