Browsing by Author "Smith, George"

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    Open Access
    Analysis of effect of using estimated shear wave data as compared to measured compressional and shear wave sonic log
    (2010) Oghenekohwo, Felix Onovughe; Smith, George
    This study is aimed at developing a workflow, and ultimately a model, for quantitative interpretation of sonic and seismic data. Measured data collected at the point of logging can be fraught with errors that can lead to wrong interpretation. One of such data is the shear wave velocity which in most cases is collected with the compressional wave velocity. The measured shear wave velocity log may contain errors that are due to drilling conditions, mud invasion etc. It may also contain cycle skips and might contain a lot of missing data and information. It is because of the poor quality of this type of log that has often made well log analysis companies and log interpreters neglect the measured shear wave log and subsequently generate or create an estimated shear wave log which they use for interpretation and modelling to check how the amplitudes vary with increasing offsets, among other uses.The workflow presented in this study considers the effect of working with the measured data, a reprocessed shear wave log and a locally estimated shear wave log. Specific correction procedures for invasion of the logs was done and synthetic seismograms were created for each type after correction for comparison to a 3D seismic data. The results of this study suggest that oil based mud invasion can cause significant problems to sonic logs especially the shear wave log. It also suggests that, if a shear wave log is of low or bad quality, a reprocessed shear wave log would be better for interpretation and modeling rather than a locally calibrated shear wave log or an estimated shear wave log using global predictions. The conclusion is evident from the synthetics generated using the measured shear wave data and the estimated shear wave data.
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    Open Access
    Applicability of rock physics models in conjunction with seismic inverted data to characterize a low poro-perm gas-bearing sandstone reservoir for well location optimization, Bredasdorp Basin, SA
    (2015) Adrian, Jorge Isaac; Smith, George
    The primary focus of this dissertation is to develop a predictive rock physics theory that establishes relations between rock properties and the observed seismic and to present the results of different seismic characterization techniques to interpret a tight gas sand reservoir off the south coast of South Africa using as input rock physics analysis and inverted seismic outcomes. To perform the aims and goals of this study a workflow that involves the execution of three main processes was implemented: (1) rock physics modelling, (2) a simultaneous seismic inversion, and (3) seismic reservoir characterization techniques. First, a rock physics model was generated as a bridge between the seismic observables (density, Vp and Vs) and reservoir parameters such as fluid content, porosity and mineralogy. In situ and perturbational log - derived forward modelling was performed. Both in situ and perturbational forward modelling were used to generate synthetic seismic gathers, which were used to study the AVA attribute responses. Overall, the effect of fluid fill on this tight gas sand seismically is modest compared with the effect of porosity changes. Second, there follows a detailed description of a workflow implemented to simultaneously invert P and S pre - stack seismic data. The derived elastic properties (acoustic impedance, Vp/Vs and density) were then used in combination with the rock physics analysis to characterize seismically the reservoir. The predicted acoustic impedance and Vp/Vs volumes show a good tie with the log data. However, the density outcome was of limited quality compared with the two mentioned above. Finally, using outcomes from rock physic s analysis and/or inverted data, four seismic techniques to characterize the reservoir were conducted. The techniques involved are: (1) AVO cross - plotting to generate a good facies property based on AVO attributes (intercept - gradient) and rock physics in the area of study , (2) rock physics templates (RPTs) to compute discrete rock property volumes (litho - Sw, litho - porosity) using a collection of curves that cover all possible "what if" lithology - fluid content - porosity scenarios for the reservoir and the inverted data, (3) a lithological classification to calculate litho - facies probability volumes based on a litho - facies classification using petrophysical cut - off s , multivariate probability functions (PDFs) and inverted data, and (4) an extended elastic impedance (EEI) inversion to derive rock property volumes (Vclay, porosity) based on AVO attributes (intercept, gradient). Despite differences in the input and theory behind each technique, all outcomes share parallels in the distribution of good and poor facies or reservoir and non - reservoir zones.
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    Open Access
    A geo-cellular model of a multiphase hydrocarbon field incorporating fault-seal analysis : Bredasdorp Basin, South Africa
    (2015) El Saadi, Omar; Smith, George
    Geological 3D static modelling has become an integral tool during the appraisal and developmental stages of a hydrocarbon field lifecycle. The 3D model becomes the basis upon which reservoir heterogeneity and characterisation are understood, hydrocarbon volumetrics are calculated and field development plans are designed. Reservoir compartmentalisation and fault-seal analysis is also an industry topic which has drawn much interest. Having a 3D model allows for fault-seal analyses to be carried out and evaluated using the statistically distributed reservoir properties. This study incorporates the building of a 3D geo-cellular reservoir model with a fault seal analysis of the E-S field, which is located on the north flank of the Bredasdorp Basin. The reservoir model was built using geostatistical methods to populate the several reservoir parameters into the model to calculate a hydrocarbon volume. In addition, a fault-seal analysis was carried out in order to investigate the phenomenon of having an oil accumulation separated from a gas accumulation either side of a fault. The facies modelling was carried out using the object modelling technique, in order to produce a model which is geologically plausible. Most of the remaining reservoir parameters were modelled using a variogram except in the case of water saturation, which was modelled using a J function equation. The volumetrics were assigned per fault block. Using a recovery factor of 75% for gas and 11% for oil, the calculated total recoverable hydrocarbons were 12.6 Bscf and 1.3 MMbbl respectively. The fault-seal analysis showed that the faults separating two of the fault blocks are not completely sealing. All the calculated fault properties supported this view, with the Shale Gouge Ratio (SGR) and threshold pressure relationship indicating a high likelihood for leakage across parts the faults. Pressure data from Repeat Formation Tests (RFT) however, indicates that the hydrocarbon accumulations in both blocks are isolated from each other. This contradiction has informed the recommendation to drill a highly deviated or short horizontal well which will cross the fault and intersect both blocks, and to complete the well using a sliding sleeve, thus providing the flexibility needed in order to manage multi-phase flow.
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    Investigation into potential gas hydrate and gas zones off the South African coastline
    (2014) Smith, Bronwyn Kate; Smith, George
    Gas and petroleum products are important to modern life and, as peak oil is reached, the search is on for alternative fuel sources. A natural gas hydrate, also known as a clathrate, is formed when a gas molecule (such as methane) is trapped in a lattice of ice. Once considered oilfield nuisances, they are now being considered as an alternative fuel source. I asked whether any indications of hydrates, and gas, were present off the South African coastline within Block 2. Two hundred and sixty (260) pre-processed seismic lines and eighteen (18) well reports were provided by the Petroleum Agency of South Africa (PASA) for review and study. Within these, evidence of gas was abundantly clear. The presence of gas, and thus a gas source, is a good indicator that - should the other formation conditions be present – hydrates could occur in this area within the Gas Hydrate Stability Zone (GHSZ). Unfortunately, no bottom simulating reflectors (BSRs) - the clearest indicator of gas hydrates - were found. These findings do not, however, confirm the absence of gas hydrates as where there is gas, there maybe hydrates. The field of hydrate research is still new in terms of technology and practical applications, and the means to extract and produce hydrates is still expensive. However, in the drive for more sources of power to supply a growing demand, the South African government has already drafted a plan to develop infrastructure for future gas market developments. When developed, this infrastructure could potentially make use of the gas found within Block 2 and its surrounds and, as the technology to detect and extract methane hydrates becomes more mature (and associated costs to extract and produce it drop), it may prove to be a valuable additional future resource as well.
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    Using rock physics to determine uncertainties in pore-fluid and lithology-estimates from seismic attributes in the Bredasdorp Basin, offshore South Africa
    (2006) Tönsing, Tobias R; Smith, George
    This study is aimed at developing a workflow for quantitative seismic interpretation. The workflow generated probability maps of various facies and pore-fluid by combining seismic attributes and wireline log data through rock physics relationships and supervised statistical classification. The workflow was developed mainly for hydrocarbon exploration, but could be used for other purposes, provided the target is seismically detectible. Any prior regional geological knowledge is built into the workflow, by extending the training date appropriately. The workflow aims to maximize the extraction of quantitative geological parameters from data that are most commonly acquired for hydrocarbon exploration, namely seismic and wireline log data. The workflow is presented using 3D seismic data from the Bredasdorp Basin offshore South Africa's south-coast. Wireline log data from the E-BX1 borehole are also used in the study, as well as regional geological interpretations. The study focused on the siliciclastic Aptian "13B" sequence, which was encountered at a depth of 2500 m below sea level at borehole E-BX1. Two massive 13B sandstone units were encountered at E-BX1. The lower unit is 50 m, and the upper 20 m thick. Both are water wet. The results of this study suggest that there are two oil accumulations at the 13X level around E-BX1. This is indicated by the high probability predicted for oil-bearing sandstone in these two areas.