Browsing by Subject "Oil-shales"
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Item Chemical and thermal effects on wellbore stability of shale formations(2002) Yu, Mengjiao; Chenevert, Martin E.; Sharma, Mukul M.A new three-dimensional wellbore stability model is presented that takes into account thermal stresses and the flux of both water and solutes from drilling fluids (muds) into and out of shale formations. Mechanical stresses around a wellbore placed at any arbitrary orientation in a 3-dimensional stress field are coupled with changes in temperature and pore pressure due to water and solute fluxes. The radial and azimuthal variation in the stress distribution and the “failure index” are computed to check for wellbore failure. This model accounts for the hindered diffusion of solutes as well as the osmotically driven flow of water into the shale. The model for the first time allows a user to study the role of solute properties on wellbore stability. Results from the model show that a maximum or minimum in pore pressure can be obtained within a shale. This leads to wellbore failure not always at the wellbore wall as is most commonly assumed but to failure at some distance inside the shale. Since the fluxes of water and solute, and temperature, are time dependent, a clearly time dependent wellbore failure is observed. The time to wellbore failure is shown to be related to the rate of solute and water invasion. Comparisons with experiments conducted with a variety of solutes on different shales show excellent agreement with model results. It is shown in this study that the solutes present in the mud play an important role in determining not only the water activity but also in controlling the alteration of pore pressures in shales. To account for this phenomenon a model is presented to compute the flux of both water and solutes into or out of shales. The relative magnitudes of these fluxes control the changes in pore pressure in the shale when it is exposed to the mud. The effect of the molecular size of the solute, the permeability of the shale and its membrane efficiency are some of the key parameters that are shown to determine the magnitude of the osmotic contribution to pore pressure. A range of behavior is observed if the solute is changed while the water activity is maintained constant. This clearly indicates the importance of the solute flux in controlling the pore pressure in shales. Critical mud weights are obtained by inspecting the stability of the wellbore wall and the entire near wellbore region. Pore pressures at different time and position are investigated and presented to explain the model results. It is shown in this study that the critical mud weights are strongly time dependent. The effects of permeability, membrane efficiency of shale, solute diffusion coefficient, mud activity and temperature changes are presented in this work. The collapse and fracture effects of cooling and heating the formations are also presented. A powerful simulation tool has been developed which can be used to perform thorough investigations of the wellbore stability problem. A user-friendly interface has been developed to ease usage.Item Environmental effects on quality of leachate from simulated in situ retorted oil shale(Texas Tech University, 1980-08) Barber, Donald RayNot availableItem Ground Water Leaching of In Situ Retorted Oil Shale(Texas Tech University, 1977-12) Watts, Jackie CarrolNot Available.Item Item The role of bacteria in the deposition and early diagenesis of the Posidonienschiefer, a Jurassic oil shale in southern Germany(1988-05) Hiebert, Franz Kunkel; Folk, Robert L.The Jurassic (Toarcian) Posidonienschiefer of southern Germany is famous for its well preserved vertebrate fossils and its high organic content. The majority of the Posidonienschiefer (10 meters thick in the study area) consists of the Bituminous Shale, a fossiliferous laminated illite claystone. Two thin (30-40 mm) clayey pyritic biomicrosparites, the Upper and Lower Schlacken, interrupt the Bituminous Shale. Geologists who have studied the Posidonienschiefer disagree about the exact nature of its depositional environment. The argument centers on the interpretation of an impoverished benthic fauna and whether or not the water column directly above the sediment-water interface was anoxic or normally oxygenated. Kauffman (1981) proposed that an algal/fungal mat located at or near the sediment/water interface marked the boundary between aerobic and anaerobic conditions during deposition. The purpose of my research was to investigate the geologic conditions during deposition and early diagenesis of the Bituminous Shale and the Schlacken and to search for evidence of microbial activity. A detailed petrologic investigation of these two lithologies found no evidence of an algal/fungal mat, but did reveal the important contribution of microbial activity in the formation of pyrite and calcite cement. The Bituminous Shale was deposited in a low-energy tropical seaway. The upper water-column supported a diverse marine fauna. The aerobic/anaerobic boundary in the water column may have been located several millimeters above the sediment/water interface. Pore waters of the ocean-floor mud were dysaerobic to anaerobic. Occasional oxygenation events allowed opportunistic benthic organisms to colonize the sea-floor. Compaction of the Bituminous Shale occured prior to cementation of original porosity. Framboidal pyrite was formed during sulfidic diagenesis under anaerobic, but open, sediment/pore water conditions. Euhedral pyrite formed later as communication between pores became restricted during sediment compaction. The skeletal grains of the Schlacken formed as a winnowed lag deposit of Bituminous Shale sediment. During the early stages of sulfidic diagenesis the winnowed beds were rapidly cemented in a concretion-like sheet. Early cementation preserved delicate algal spores and clay fabric. Fossil bacteria were discovered in the calcite cement of the Schlacken by modified petrographic techniques, and confirmed with the scanning electron microscope. Experiments in which live bacteria were gradually entrapped in halite produced a crystal fabric identical to that of the fossiliferous calcite cement of the Schlacken. The microbial production of bicarbonate and ammonia during sulfidic diagenesis played a significant role in altering local geochemical conditions in the Schlacken sediment and initiated the precipitation of calcite cements. Fossil bacteria in the cements of the Schlacken are direct evidence of the presence and entrapment of bacteria during cementation, but do not conclusively prove their active role in the formation of calcite.