Browsing by Subject "Invasion"
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Item Development and application of a 3D equation-of-state compositional fluid-flow simulator in cylindrical coordinates for near-wellbore phenomena(2011-12) Abdollah Pour, Roohollah; Torres-Verdín, Carlos; Sepehrnoori, Kamy, 1951-; Delshad, Mojdeh; Demkowicz, Leszek; Johns, Russell T.Well logs and formation testers are routinely used for detection and quantification of hydrocarbon reserves. Overbalanced drilling causes invasion of mud filtrate into permeable rocks, hence radial displacement of in-situ saturating fluids away from the wellbore. The spatial distribution of fluids in the near-wellbore region remains affected by a multitude of petrophysical and fluid factors originating from the process of mud-filtrate invasion. Consequently, depending on the type of drilling mud (e.g. water- and oil-base muds) and the influence of mud filtrate, well logs and formation-tester measurements are sensitive to a combination of in-situ (original) fluids and mud filtrate in addition to petrophysical properties of the invaded formations. This behavior can often impair the reliable assessment of hydrocarbon saturation and formation storage/mobility. The effect of mud-filtrate invasion on well logs and formation-tester measurements acquired in vertical wells has been extensively documented in the past. Much work is still needed to understand and quantify the influence of mud-filtrate invasion on well logs acquired in horizontal and deviated wells, where the spatial distribution of fluids in the near-wellbore region is not axial-symmetric in general, and can be appreciably affected by gravity segregation, permeability anisotropy, capillary pressure, and flow barriers. This dissertation develops a general algorithm to simulate the process of mud-filtrate invasion in vertical and deviated wells for drilling conditions that involve water- and oil-base mud. The algorithm is formulated in cylindrical coordinates to take advantage of the geometrical embedding imposed by the wellbore in the spatial distribution of fluids within invaded formations. In addition, the algorithm reproduces the formation of mudcake due to invasion in permeable formations and allows the simulation of pressure and fractional flow-rate measurements acquired with dual-packer and point-probe formation testers after the onset of invasion. An equation-of-state (EOS) formulation is invoked to simulate invasion with both water- and oil-base muds into rock formations saturated with water, oil, gas, or stable combinations of the three fluids. The algorithm also allows the simulation of physical dispersion, fluid miscibility, and wettability alteration. Discretized fluid flow equations are solved with an implicit pressure and explicit concentration (IMPEC) scheme. Thermodynamic equilibrium and mass balance, together with volume constraint equations govern the time-space evolution of molar and fluid-phase concentrations. Calculations of pressure-volume-temperature (PVT) properties of the hydrocarbon phase are performed with Peng-Robinson's equation of state. A full-tensor permeability formulation is implemented with mass balance equations to accurately model fluid flow behavior in horizontal and deviated wells. The simulator is rigorously and successfully verified with both analytical solutions and commercial simulators. Numerical simulations performed over a wide range of fluid and petrophysical conditions confirm the strong influence that well deviation angle can have on the spatial distribution of fluid saturation resulting from invasion, especially in the vicinity of flow barriers. Analysis on the effect of physical dispersion on the radial distribution of salt concentration shows that electrical resistivity logs could be greatly affected by salt dispersivity when the invading fluid has lower salinity than in-situ water. The effect of emulsifiers and oil-wetting agents present in oil-base mud was studied to quantify wettability alteration and changes in residual water saturation. It was found that wettability alteration releases a fraction of otherwise irreducible water during invasion and this causes electrical resistivity logs to exhibit an abnormal trend from shallow- to deep-sensing apparent resistivity. Simulation of formation-tester measurements acquired in deviated wells indicates that (i) invasion increases the pressure drop during both drawdown and buildup regimes, (ii) bed-boundary effects increase as the wellbore deviation angle increases, and (iii) a probe facing upward around the perimeter of the wellbore achieves the fastest fluid clean-up when the density of invading fluid is larger than that of in-situ fluid.Item The effects of habitat loss and fragmentation caused by woody plant encroachment on native plant diversity and on an invasive grass(2010-05) Alofs, Karen Marie; Fowler, Norma L.; Leibold, Mathew; Parmesan, Camille; Keitt, Timothy; Young, KennethHabitat loss, habitat fragmentation and species invasions have been recognized as three of the leading threats to biodiversity. I examined the effects of habitat loss and fragmentation on native and invasive plants in central Texas. During the last century, the density and abundance of woody plants has been increasing in the savannas of eastern Edwards Plateau. This process, known as woody plant encroachment, not only reduces the amount of open herbaceous habitat but also fragments that habitat creating smaller and more isolated patches. In three studies, I investigated the consequences of this habitat loss and fragmentation for plants which do not occur under the cover of woody plants including native grasses and forbs and the invasive Eurasian bunchgrass, Bothriochloa ischaemum (King Ranch Bluestem). In the first study, I show that woody plant encroachment reduces native herbaceous species richness (the number of species in a given area). Using a collection of historical aerial photographs, I demonstrate that current native herbaceous species richness was most strongly related to recent habitat amount, but to the degree of habitat fragmentation at least 50 years ago. In a second study, I show that the presence of B. ischaemum was negatively related to the degree of fragmentation in the surrounding landscape. Finally, I found that B. ischaemum had higher rates of germination and growth in experimental plots where the species commonly lost with woody plant encroachment were removed than in unmanipulated control plots. Together, this work suggests that woody plant encroachment is directly slowing the spread of an invasive species while indirectly facilitating its establishment.Item Inversion-based petrophysical interpretation of multi-detector logging-while-drilling sigma measurements(2014-05) Ortega, Edwin Yamid; Torres-Verdín, CarlosPulsed-neutron borehole measurements involve a physical process in which a source emits energetic neutrons that lose energy upon collisions with formation nuclei, and are eventually captured by a nucleus to form a heavier, excited state. The excited nucleus decays to its ground state by the emission of gamma rays. Both thermal-neutron and gamma-ray populations decay with time at a rate defined by Sigma, which is a nuclear property that quantifies a material’s ability to capture thermal neutrons. The large contrast in Sigma between hydrocarbon and salty connate water enables calculations of water saturation directly from pulsed-neutron measurements. Sigma logs have proven useful in the assessment of thinly bedded formations because they exhibit a small volume of investigation, and have been deemed superior to resistivity logs in the petrophysical evaluation of carbonate formations. The recognized potential of Sigma logs in formation evaluation initiated the development of multi-detector Logging-While-Drilling (LWD) Sigma measurements. These measurements are acquired using one thermal-neutron and two gamma-ray detectors at different spacings from the source. Such a design is aimed at providing distinct radial depths of investigation to detect filtrate invasion in the near-wellbore zone. Despite their formation-evaluation potential, multi-detector time-decay measurements commonly remain affected by invasion, shoulder-bed, and well-deviation effects. The purpose of this dissertation is to develop a fast-forward simulation method to reproduce multi-detector time decays and combine the method with inversion techniques to improve the petrophysical interpretation of LWD Sigma measurements. First-order perturbation theory and a library of pre-calculated Monte Carlo detector-specific sensitivity functions and time decays are used to numerically simulate borehole Sigma measurements in realistic logging environments. The new simulation method is one hundred thousand times faster than rigorous Monte Carlo calculations and remains within two capture units of disparity. Next, the fast-forward simulation method is embedded within inversion algorithms to estimate layer-by-layer radial length of invasion and formation Sigma corrected for shallow invasion, shoulder-bed, and well-deviation effects. Both fast-forward and inverse modeling algorithms are benchmarked against laboratory and synthetic time decays. The improvement of formation Sigma obtained with inversion-based interpretation leads to an improvement in the estimation of Sigma-derived water saturation. Likewise, the estimated radial length of invasion is combined with neutron and density measurements to correct the latter for invasion effects. Results indicate that the inversion-based interpretation method is well suited for the evaluation of high-porosity formations invaded by salty mud filtrate. Inversion-based interpretation of field LWD time decays enables the estimation of lower values of water saturation when compared to conventional Sigma interpretation or resistivity methods. Estimated values of water saturation are as much as fifty percent lower than predicted by conventional interpretation of Sigma logs in the case of measurements affected by shoulder-bed effects, and as much as one hundred percent lower than predicted by the conventional interpretation method for measurements additionally affected by salty filtrate invasion. The key attributes of the combined petrophysical interpretation of multi-detector Sigma, neutron, and density measurements developed in this dissertation are that it explicitly enforces the physics of all nuclear measurements, honors the pressure and temperature dependency of reservoir fluid nuclear properties, and takes into account a-priori information such as mud-filtrate salinity, connate-water salinity, and bed-boundary locations.