Papers by Mimonitu Opuwari
Pore pressure prediction of some selected wells from the Southern Pletmos Basin, offshore South Africa
South African Journal of Geology, 2016
An accurate pore pressure prediction is vital in any risk management plan during the exploration,... more An accurate pore pressure prediction is vital in any risk management plan during the exploration, exploitation and development of wells in the petroleum industry. It is therefore important to understand the factors controlling these pore pressures namely, sediment compaction, overburden, lithology characteristics, hydrocarbon generation, tectonic stress, thermodynamic effects, osmosis and clay mineral transformation, which are all influenced by physical, geological, geochemical and mechanical processes. In this study we present the results of data selected from three wells: Ga-N1, Ga-W1 and Ga-AA1, which were all drilled within the Southern Pletmos sub-basin, offshore South Africa. The pore pressure in the wells was predicted using the Ben Eaton method by creating a depth dependent Normal Compaction Trend (NCT), using resistivity and sonic wireline logs. 2D seismic data were used to develop depth imaging of the pre-drilling pore pressure predictions; this was done using seismic interval velocity data and pressure data to obtain a reflection tomography extraction grid map. Detailed depth plots of overburden gradient (OBG), the Effective Stress (ES), Fracture Gradient (FG), Fracture Pressure (FP), Pore Pressure Gradient (PPG), and the Predicted Pore Pressure (PPP) were thus derived for the three wells. The overburden density varies from 2.09 gm/cc to 2.24 gm/cc between the wells, while the PPP changes from 3,405 psi to 5,062 psi within the selected reservoir intervals. The results show that most of the reservoir intervals of the three wells experience normal pore pressures (between 3000 and 4000 psi), except the reservoir intervals between depths 1868.73 m to 1875.40 m and 1880.3 m to 1887.3 m, respectively, in well Ga-W1, where significant overpressure in the pores exceeding 4000 psi was demonstrated (normal pore pressures are between 3000 and 4000 psi). The tomography extraction grid map that was generated from 2D seismic data of the wells was used to delineate the depth imaging of the pore pressure, in both overpressure and normal pressure formations before drilling, yielded good results.
Shale-gas potential from Cretaceous succession in South Africa’s Orange Basin: Insights from integrated geochemical evaluations
Marine Georesources & Geotechnology
Journal of Petroleum Exploration and Production Technology
The study attempts to address the knowledge gap in the Bredasdorp Basin Offshore South Africa by ... more The study attempts to address the knowledge gap in the Bredasdorp Basin Offshore South Africa by using newly acquired seismic data with an enhanced resolution, integrating core and well log to provide a solution. The main objective of this study was a volumetric sandstone reservoir characterization of the 13At1 and 10At1 sandstones deposited in the upper shallow marine environment. The results reveal four facies grouped as facies 1(claystone), facies 2(intercalation of claystone and sandstone, facies 3(medium sandstone), and facies 4 fine-medium sandstone grain deposited in a deep marine environment. Facies 3, the medium-grained sandstone, has the best reservoir quality rock, while facies 1, which is predominantly claystone, has the least rock quality. The study has produced a calculated volume of gas in the 10At1 sand (upper and lower sand is 30.01 billion cubic feet (bcf)) higher than that of the 13At1 sand (27.22 bcf of gas). Improved seismic resolution enhanced the accuracy of r...
Journal of Petroleum Exploration and Production Technology
Aptian to Campanian sediments from the Western offshore to Central Orange Basin were studied by i... more Aptian to Campanian sediments from the Western offshore to Central Orange Basin were studied by integrating molecular geochemistry, inorganic and isotopic studies to recognize their geochemical characteristics via the reconstruction of the Orange basin’s paleoweathering, paleosalinity, paleovegetation, paleoclimate, and tectonic records. Molecular analyses of both aliphatic and aromatic compounds reveal an input dominantly from a marine source. The source rocks accumulated in a reduced, anoxic, saline water column. Based on various biomarker proxies and vitrinite reflectance data, some samples are thermally mature to produce petroleum, while others are not. According to the V/Ni ratio, samples from the Orange Basin in South Africa are mainly anoxic, with only a few samples ranging from suboxic to anoxic. This is congruent with biomarker and isotope analyses that further indicate the presence of marine-derived source rocks with some terrestrial remains generating hydrocarbons. The in...
Hydrocarbon Generation Potential of the Albian to Turonian Lower Post-Rift Succession Orange Basin, South Africa
82nd EAGE Annual Conference & Exhibition, 2021
Journal of Petroleum Exploration and Production Technology, 2018
This case study involves the unique application of density correction software applied to density... more This case study involves the unique application of density correction software applied to density data, prior to the estimation of geopressure gradients. The K-R gas field was discovered in 1983 about 50 km west, off the F-A gas field offshore South Africa. During exploration; gas discoveries were made in well KR-1 and KR-8, potentially commercial gas and encouraging oil flow rates in well KR-2, KR-4 and KR-6, a dry well for KR-5 and a dry well with encouraging oil shows in KR-3. The aim of this study is to create a model that evaluates the geomechanical behaviour of the upper shallow marine reservoir (USM) of the Bredasdorp Basin, South Africa and provide a safe drilling mud window for future work in the area. The K-R field has a strong NW-SE fault trend, resulting in a maximum horizontal stress orientation of 125°, determined from structural depth maps. All geopressure gradients were modelled using the drillworks @ software at the top (TUSM) and bottom (BUSM) of the reservoir. The Eaton method, that can predict pore pressure from either velocity or resistivity, was used to calculate both pore pressure and fracture gradient and then calibrated using "real" data from well completion and driller's reports. The pore pressure and fracture gradient are what set the upper and lower mud weight limits. These values range between 8.46 and 9.60 ppg and 10.12-15.33 ppg, respectively. The rock mechanical properties (Friction angle, cohesive strength and uniaxial compressive strength) were empirically derived and show a similar trend for all wells. The drilling mud window becomes more constricted at depths below 2600 m, to the TD of the well.
Minerals
The present study uses core data to group reservoirs of a gas field in the Bredasdorp Basin offsh... more The present study uses core data to group reservoirs of a gas field in the Bredasdorp Basin offshore South Africa into flow zones. One hundred and sixty-eight core porosity and permeability data were used to establish reservoir zones from the flow zone indicator (FZI) and Winland’s methods. Storage and flow capacities were determined from the stratigraphy-modified Lorenz plot (SMLP) method. The effects of the mineralogy on the flow zones were established from mineralogy composition analyses using quantitative X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Results reveal five flow zones grouped as high, moderate, low, very low, and tight reservoir rocks. The high flow zone is the best reservoir quality rock and has porosity and permeability values ranging from 12 to 20% and 100 to 1000 mD. The high and moderate zones contribute more than 60% of each well’s flow capacities. The moderate and low flow zone extends laterally to all the wells. The tight flow zone is an im...
Geomechanical characterization of CO2 storage sites: A case study from a nearly depleted gas field in the Bredasdorp Basin, South Africa
Journal of Natural Gas Science and Engineering
The objective of this study was to assess the concentration of trace elements in acid mine draina... more The objective of this study was to assess the concentration of trace elements in acid mine drainage (AMD) from Odagbo coal mine. Composite AMD samples were collected from active and abandoned mining pits and were analysed for lead, nickel, cobalt, chromium, mercury, zinc, arsenic and iron using Atomic Absorption Spectroscopy (AAS). Comparisons were made between the trace elements and environmentally acceptable quality standard (EQS) for heavy metal discharges from mines using student’s t-test. The mean concentrations of these elements were lead (0.10 mg/l), nickel (0.49 mg/l), cobalt (0.88 mg/l), chromium (0.55 mg/l), cadmium (0.19 mg/l), arsenic (0.01 mg/l) and iron (5.80 mg/l). There were significant differences between the means of lead, mercury, cadmium, arsenic and EQS for heavy metal discharges from mines (P 0.05). Cobalt, iron, nickel and chromium were the dominant trace elements in the AMD. Further studies are required to determine the influence of AMD on surface water and s...
Journal of African Earth Sciences, 2019
This study outlines the different methodologies used to identify and classify different rock unit... more This study outlines the different methodologies used to identify and classify different rock units in the sandstone reservoirs of the Orange Basin in South Africa into flow units. The flow units are defined on the basis of conventional core porosity and the permeability of the reservoir occurring in the fluvial and shallow marine depositional environments within the lower Cretaceous period. A total of six facies were identified from the core data, and they were corroborated with gamma ray log data .Facies A1, A2, A3, and A4 are the main reservoir units with generally fine-to medium-grained sands, which dominantly comprise quartz grains. Facies A5 mainly comprises silts with lenses of fine-grained sands, while Facies A6 is claystone laminated with minor burrows filled with shale and occasional siltstone presence. The porosity and permeability were used to compute the permeability/porosity relationship,the rock quality index, pore spaces, storage and flow capacity values, and to indicate the flow zone. The highest porosity and permeability values correspond to facies A1 and A2, which are represented by mega pore spaces, and they displayed high flow capacity values and moderate to high storage capacities. The lowest porosity and permeability
Transactions of the Royal Society of South Africa, 2016
The objective of this study was to determine the concentration of trace elements in soils of Kean... more The objective of this study was to determine the concentration of trace elements in soils of Keana-Awe brine-fields. Composite soil samples were randomly collected at a depth of 0-15 cm and were analysed for molybdenum, zinc, arsenic, lead, cobalt, chromium, copper, barium and nickel using Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). Quantification of the degree of soil contamination by these trace elements was carried out using the enrichment factor (EF) and the geo-accumulation index (Igeo). The data were subjected to principal component analysis (PCA). The average concentrations were 1.56 ppm molybdenum, 1116.42 ppm zinc, 23.80 ppm arsenic, 71.40 ppm lead, 17.64 ppm cobalt, 237.35 ppm chromium, 24.16 ppm copper, 254.67 ppm barium and 143.71 ppm nickel. Cobalt, nickel and chromium showed positive loadings in component 1 with a total variance of 29.56%. Zinc, copper and lead showed positive loadings in component 2 with a total variance of 18.79%, while copper showed negative loading in component 3 with a total variance of 14.79%. Considering the concentration of trace elements in the soils and statistical analyses, we conclude that soils of the study area were severely enriched in molybdenum, cobalt, chromium, copper, barium, nickel, while arsenic and zinc are in excessive concentrations in the soils. These trace elements could have originated from geogenic and anthropogenic sources.
Natural Resources Research, 2020
The overarching aim of this study is to use core measurements of porosity and permeability in thr... more The overarching aim of this study is to use core measurements of porosity and permeability in three wells (MO1, MO2, and MO3) to generate a scheme of sandstone reservoir zonation for identification of flow units in the E-M gas field of the Western Bredasdorp Basin Offshore in South Africa. The evaluation method began by establishing rock types within a geological framework that allowed the definition of five facies, grouped as facies A, B, C, D, and E. Facies A was recognized as the best petrophysical rock type. In contrast, facies E was recognized as impervious rock. The results of independent reservoir classification methods were integrated to identify flow zones that yielded positive results. The results ultimately culminated in a zonation scheme for the Basin. Twelve flow zones were identified and were broadly classified as high, moderate, low, very low, and tight zones. The high zone was characterized by pore throat radius of ‡ 10 lm, flow zone index (FZI) of ‡ 5.0 lm, and flow unit efficiency (FUE) of ‡ 0.8. In contrast, very low efficiency zones had pore throat radius and FZI of < 2 lm, and FUE of £ 0.2. The high-efficiency zones were comparable to facies A and the tight zone to facies E. Facie C provided sand-sand contacts that allowed flow between the zones. One high, two moderate, four low, and five very low efficiency zones were identified. The plot of FUE can be compared directly with flowmeter logs. The results obtained from this study will serve as an input parameter for reservoir studies in the western Bredasdorp Basin.
Geomechanical characterization of CO2 storage sites: A case study from a nearly depleted gas field in the Bredasdorp Basin, South Africa
Journal of Natural Gas Science and Engineering, 2020
Abstract Geomechanical analysis and integrity assessment of hydrocarbon reservoirs upon depletion... more Abstract Geomechanical analysis and integrity assessment of hydrocarbon reservoirs upon depletion and injection are crucial to ensure that CO2 storage projects can be safely implemented. The Bredasdorp basin in South Africa has a great potential for CO2 storage given its hugely available exploration data. However, there has not been any geomechanical characterization carried out on this basin to determine its integrity issues. The aim of this study is to provide a guideline as to how geomechanical analysis of depleted fields can be done for a safe CO2 sequestration practice. The results obtained from the geomechanical model constructed for the depth of 2570 m indicated that the magnitude of the principal vertical, minimum and maximum horizontal stresses in the field are respectively 57 MPa, 41 MPa and 42–46 MPa, indicating the presence of a normal faulting regime in the caprock and the reservoir. However, according to the pore pressure-stress coupling assessment, this normal faulting is much severe in compartment C3 of the reservoir. Fault reactivation and fracture stability were also investigated after depletion and it was found that faults in the compartments C1 and C2 are stable after depletion. However, normal faults (FNS8 and FNS9) in compartment C3 dipping SW were critically stressed and may be reactivated without a proper injection planning. Fractures in compartment C3 were also critically stressed, highlighting a great potential of leakage from this compartment upon injection. It was also revealed that the sustainable maximum fluid pressure of 25 MPa would not induce any fractures in the reservoir during CO2 storage.
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Papers by Mimonitu Opuwari