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GEO 2008
- Conference date: 03 Mar 2008 - 05 Mar 2008
- Location: Manama, Bahrain
- Published: 03 January 2008
381 - 385 of 385 results
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Fracture characterization using borehole acoustic reflection: theoretical modeling and field data applications
More LessCharacterization of borehole fractures is important because they provide conduits for reservoir fluid flow. Borehole Stoneley waves have been used as a means for fracture analyses, which however, are a difficult task to interpret because borehole changes (e.g. washout) and bed boundaries also cause reflections. We used theoretical modeling to characterize fracture-induced reflections. Fractures were modeled as localized, highly permeable structures embedded in a varying borehole environment (e.g. washout). The modeling led to an important result; namely that permeable fractures/structures generally cause Stoneley wave reflectivity to increase towards low frequencies. This result was used to provide a dual-frequency Stoneley reflection method for fracture characterization. The reflection data were processed in two different frequency ranges (e.g., 0–1,000 Hertz and 1,000–2,000 Hertz). Fracture-induced reflections were characterized by low-frequency reflectivity, which is significantly higher than high-frequency reflectively. This method has been applied to fractured reservoirs with promising results. The fractures measured by the acoustic method agreed well with those from borehole image data. The presentation proposes a refined acoustic reflection method for fracture characterization. It demonstrates the practicality and robustness of the proposed technique in field data applications.
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Formation evaluation in Palaeozoic gas reservoirs of the South Rub’ Al-Khali Basin, Saudi Arabia
More LessAcquiring accurate subsurface data forms a key objective for the South Rub Al-Khali Company in its currently ongoing exploration drilling campaign in the remote and challenging environment of the South Rub’ Al-Khali Desert. Exploring for non-associated gas in Paleozoic formations brings significant well data gathering challenges. These include logistics and planning, hole conditions, depths and associated temperatures and pressures. The combination of often poor reservoir properties and deep invasion can result in impaired reservoirs that cannot be sampled or tested with conclusive results. In order to maximise the chance of obtaining a petrophysical evaluation with conclusive results in terms of reservoir quality and fluid content, an extensive dataset has been acquired during the exploration campaign. The data set comprises cores, ditch cuttings, compositional analysis of the various gas show components, and surface sampling of any shows. Petrophysical data include logging-while-drilling data, open-hole electric wireline
logs, pressure measurements, sampling and multiple offset VSP’s. Data obtained while drilling was repeated on electric wireline to spot invasion effects as a result of the mud system used. The presentation will focus on the data gathering strategy and planning and how this has been catered for in the well design. Lessons from the first well were captured in a structured review process leading to cost savings in subsequent wells without impacting the quality of the dataset acquired. Integration of the extensive data acquired has lead to a better understanding of the various play elements resulting in a robust risking of all exploration prospects.
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Advances in marine seismic acquisition
By Mundy BrinkThree recent developments in marine seismic acquisition will be presented. The first is solid hydrophone cables and Sercel’s new navigation system. Due to their construction, the solid hydrophone cables better preserve low-frequency seismic signals, while the signal-to-noise ratio at the higher frequencies is improved by the dense spacing of the individual hydrophones. With these superior characteristics the recorded seismic data not only provide a better structural image of the subsurface geology, they also allow for a more reliable computation of rock parameters and pore fluids. The combination with Sercel’s new Nautilus System for integrated navigation and streamer positioning helps to more reliably record weak 4-D signals in reservoir monitor surveys. The solid cables also reduce risks to the environment, since they are not filled with oil. The second development is wideazimuth acquisition, which yields a better image of complex geological structures. Some of the geophysical and operational experience will be presented. It allows for true 3-D processing like 3-D attenuation of multiples and multiple diffractions and derivation of fractures and their orientations. An efficient dual depth acquisition geometry is being introduced yielding extra enhancement of the lower frequencies. The third development is the application of feasibility studies to determine the optimal acquisition geometry for meeting the survey’s objectives. This can only be established after analysis of available data and modelling, done by specialists in different fields of geoscience. It may include towed hydrophone cables or 4C seabed nodes or cables. Some examples will be shown.
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Issues in uncertainty estimation for time-to-depth conversion
More LessGenerating a structure map from depth conversion of a time seismic horizon is a standard practice in seismic interpretation. For most methods, however, a depth uncertainty estimate is not provided. Geostatistical methods are the exception in this regard. Some commonly used approaches such as cokriging the depth markers from well control and kriging with external drift, provide uncertainty estimates through the kriging variance, where the time horizon is incorporated as soft information. A shortcoming of these methods is that no accommodation is made for including time-horizon uncertainty. We provide a simple approach to remedy this by using kriging to interpolate average velocity, which we then use to convert the time-horizon to depth. We obtain a depth uncertainty by combining the average velocity uncertainty (obtained from kriging variance) with the time-horizon uncertainty, using standard methods from the theory of propagation of errors. Another issue for all of the above methods is: how good an estimator of local error is the kriging variance? We address this with a data example, where we blind-test our results with some wells that were held out of the analysis. We also introduce an attribute that we call a gridding sensitivity indicator, which gives another
uncertainty measure in addition to kriging variance. This attribute extends the concept of cross-validation to a map view by displaying the standard deviation of all of the maps generated by the leave-one-out method. We also discuss other alternatives for local error estimation such as conditional simulation.
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Water movement controls in an Early Cretaceous reservoir: An integrated analysis from a large offshore field, Abu Dhabi, United Arab Emirates
A Thamama carbonate reservoir in a major offshore field has experienced early water breakthrough with rapidly increasing water cuts. Concern over this production challenge and its impact on water handling, wellbore completion, sweep efficiency and ultimate recovery prompted a study to improve the understanding of the geologic controls over reservoir quality variations, how to properly capture them in static and dynamic reservoir models and mitigate their impact on reservoir performance. A multi-disciplinary, integrated study of relevant geoscience and engineering data has been undertaken to address this issue. The study focused on several geological components including depositional facies, diagenesis and tectonic factors. A deepening upward succession of
carbonate ramp facies have been identified and characterized from examination of slabbed core, thin sections and SCAL data from over 35 wells in the field. Diagenetic overprints have impacted the original porosity and permeability resulting in inter-beds of moderately to highly porous zones, which exhibit significant variations in permeability. The diagenetic overprint has, in part, been correlated to major faults zones in the field. These faults and diagenetic effects have resulted in the formation of fractures within the reservoir. Cased-hole log and production data has been instrumental in the identification of the spatial and stratigraphic distribution and timing of water movement. Results indicate that water breakthrough is controlled by variation in permeability due to depositional texture and diagenetic overprints and fracture enhancement on the original reservoir character. These variations are stratigraphically controlled near the top and base of the reservoir, resulting in a potential lack of sweep and recovery from the middle portion.
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