Borehole instability, in most of the cases, is a direct reflection of earth's in situ stress state. It is well known that the stress distribution around the wellbore induces deformation depending on many factors ranging from wellbore pressure history and rock strength to the trajectory orientation. A stress direction map is generated for the GoS from observations of borehole breakout detected in multi-arm-caliper logs and other log data base, viz., electrical Images and sonic logs. In vertical wells, the maximum tangential stress around borehole can produce breakouts and their orientation indicates the direction of minimum in situ horizontal stress (Sh). In the case of deviated wells, a stress-tensor diagram defines Sh direction with reasonable accuracy, provided wells cover wide range of deviation angle and azimuth The current study indicates that Sh in GoS is aligned along two major trends. The main NNE - SSW trend, with average orientation of N10degE, exists in most of the region. The second trend is aligned NE - SW and observed locally at the central eastern and south-western part of GoS, with an average orientation of N50degE. Most studies of the structural and tectonic history of the GoS have identified two age significant orientations for this extensional rift. The early to middle Miocene rifting, responded to a Sh direction of N55–60degE (rift-climax). The younger stress fields of the Late Miocene and Pliocene times rotated progressively counterclockwise to a N15-25degE direction that persisted into early-late Pleistocene time. The dominant in situ stress orientation trend, identified in this study, therefore, is mainly controlled by this younger stress field of the GoS rifting. In situ stress directions have strong impact in drilling high angle wells in GoS. Proper placement of well trajectory with respect to in situ stress reduces instability in drilling. The paper exhibits example of directional sensitivity of well trajectory and successful drilling campaign based on the developed stress map. Introduction Since the global power scenario changes with increasing demand for oil, more and more complex trajectory wells, highly deviated and horizontal, are being drilled in the areas with minimum knowledge with scanty or barely minimum previously drilled data. Enhancing the production to its maximum level is the reason, though the drilling uncertainty is being pushed to a limit that causes unexpected drilling problems resulting in high NPT; the expenditure goes beyond expectation involving multiple side tracks or abandonment of the well in worst case. In all the brown fields those are now being developed; the original data quality is very poor or inadequate to extract meaningful results that could be used to formulate a drilling program, one of this being the stress orientation of the area. This is a significant and most important input to forecast the drillability of the well. And in most of the cases the wells are being drilled without the proper knowledge of stress pattern of the area. The demand of an in-situ stress map, therefore, is extremely important while drilling a deviated high angle well. The situation becomes exceedingly critical if the drilling is being carried out in a tectonically active region involving multiple faults and variable degree of displacement of the adjoining structures. While mentioning a stress map we are mainly concerned about the directionality of the minimum stress across the region.
Combining logging while drilling (LWD) azimuthal density images from real-time measurements with wireline electrical borehole imaging logs minimizes risks, maximizes data acquisition, and creates a better understanding of reservoir characteristics than using either of the tools alone. The azimuthal density images provide structural images in real time, but not fine sedimentary details. Meanwhile, wireline electrical borehole images fill in detail in stratigraphic dip analysis. Comparing the median grain size derived from core sieve analysis with the pseudo median grain size derived from the image resistivity distribution of wireline electrical borehole imaging data confirmed that the results are comparable and wireline electrical borehole image can substitute core analysis in some extent to extract some textural information from sands.
Computing clay volume using elemental neutron capture spectroscopy logs in combination with a multimineral solver for the complex, shaly sand reservoirs of the Nile Delta reservoir improved accuracy over using the mineral fractions output from the spectroscopy model alone. It was also found that the aluminium log from direct aluminium yield measurement leads to a better clay volume estimation, as opposed to using the aluminium log from the aluminium emulator algorithm. Combining the spectroscopy data with borehole image data generated a high-resolution lithofacies column that provides an accurate stratigraphic interpretation. Applying cutoffs to generate a high-resolution sand count enabled us to sort the reservoir units from the poorest to the best quality sands and improved our understanding of the distribution of the best reservoir quality in the well. This approach provides a unique solution to characterize thinly bedded reservoirs in wells drilled with oil-based mud.
An increasing number of deviated wells are being drilled to maximize production and hydrocarbon recovery in the mature reservoirs of the Gulf of Suez (GoS). Successfully drilling a high-angle well in a tectonically disturbed and structurally complex area like the GoS is very challenging, especially in depleted reservoirs. Selecting the optimal mud weight is absolutely essential. Stress orientation and magnitude also have a major impact on wellbore stability.The region poses significant drilling challenges that vary widely from reactive shale and salt creep to stress-related instability. From the findings of multiple wellbore stability projects we conducted in the GoS, we review the dominant mechanisms of wellbore instability in the GoS. We provide a summary of the failure mitigation measures and an overview of stress magnitude and orientation in the region, demonstrating how it impacts the knowledge of the most stable drilling direction.Understanding the main causes of rock failure in the GoS resulted in improved drilling efficiency and reduced drilling costs. We show an example, where a new, nearly horizontal (86º) well was successfully drilled through the Asl formation with less than half a day of non productive time during the entire drilling process.We conclude that acquisition of new, high-quality data would considerably reduce the uncertainty surrounding drilling complex wells in the area and reduce their cost. IntroductionThe Gulf of Suez (GoS) is a mature hydrocarbon region of Egypt. Most of the wells were drilled several decades ago, but, to optimize the hydrocarbon recovery, operators are now drilling increasingly deviated wells. This has led to a rise in the severity of borehole instability events, adding to the complexity of drilling in the area. In this tectonically active area, numerous drilling problems are present. They range from salt creep to mud losses in natural fracture networks or subseismic fault zones, without mentioning borehole failure linked to formation/fluid interactions. Highly depleted reservoirs are encountered, with sometimes a high uncertainty in the current pore pressure, leading to high uncertainty on the fracture gradient or the mud loss gradient.The available data is, however, usually scarce because most of the fields were developed a long time ago. Essential data like sonic compressional and shear logs can be missing. Mechanical properties from core are rarely available to calibrate the rock mechanical properties. Little direct information is available about stress magnitudes, as leakoff tests (LOT), extended LOT (XLOT), and hydraulic fracturing are rarely conducted.Wellbore stability analysis can, however, deliver crucial information to mitigate failure in complex new well trajectories. A systematic approach in analysing the causes of borehole instability is necessary to forecast the risks associated with drilling new wells and to recommend mitigation measures. First, the evidence for formation and borehole failure needs to be gathered and analyzed from drilling reports. Th...
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