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A. Klaetsch - One of the best experts on this subject based on the ideXlab platform.
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Compaction bands and the formation of slot-shaped breakouts in St. Peter sandstone
Special Publications, 2016Co-Authors: B. Haimson, A. KlaetschAbstract:Abstract: Drilling of vertical wellbores in sandstone often results in stress-induced failed zones called breakouts. In the laboratory, miniature Drilling under simulated far-field crustal stress con-ditions above a certain threshold produces breakouts that can be studied to enhance our under-standing of deformation and failure mechanisms in rock. We report on Drilling experiments in St. Peter sandstone, an Ordovician aeolian rock consisting of well-rounded bimodal quartz grains held together by sutured contacts. Breakouts here are slot-shaped and oriented at right angles to the far-field maximum stress direction (sH), resembling emptied compaction bands. We distinguish between two porosity ranges in this rock. In the high-porosity variety (16–22%), grains are bonded through narrow sutures. In the vicinity of the borehole at points aligned with the far-field least horizontal principal stress (sh) direction, where the maximum com-pressive stress concentrates, grain sutures sever at relatively low stress levels, leading to intergra-nular cracking. Debonded intact grains repack and produce a reduced-porosity narrow compaction band. Loose grains in the band abutting the borehole wall are then flushed out, assisted by the Circulating Drilling Fluid. This intensifies the stress concentration ahead of the breakout tip, advan-cing the compaction band and lengthening the slot-shaped breakout. The process continue
Bezalel C Haimson - One of the best experts on this subject based on the ideXlab platform.
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micromechanisms of borehole instability leading to breakouts in rocks
International Journal of Rock Mechanics and Mining Sciences, 2007Co-Authors: Bezalel C HaimsonAbstract:This paper reviews the different borehole breakout failure micromechanisms observed during a multiyear laboratory research effort at the University of Wisconsin. Vertical borehole Drilling experiments were conducted in a variety of granites, limestones, and sandstones under a wide range of pre-existing stress fields. Test samples that developed breakouts during Drilling were analyzed under optical and scanning electron microscopes to establish the micromechanics of failure. All rocks tested, except for the quartz-rich sandstones, develop dog-eared breakouts along the minimum horizontal far-field stress springline, even though the grain-scale mechanisms leading to the final appearance may differ considerably. The common denominator is the incipient failure in the form of dilatant microcracking in the zones of the highest compressive stress concentration around the borehole. Dependent on rock type, these microcracks could be tensile or shear openings, extending inter- or intra-granularly. A type of failure not hitherto recognized was discovered in quartz-rich sandstones, which develop tabular slot-shaped breakouts that maintain a constant very narrow width over an extensive length, resulting in a fracture-like appearance. Such breakouts are the result of a largely non-dilatant micromechanism consisting of localized grain debonding and repacking leading to the formation of an apparent reduced-porosity compaction band along the minimum horizontal far-field stress springline. Breakouts are produced by the removal, with the help of the Circulating Drilling Fluid, of loose grains and grain fragments that were debonded in the process of compaction band forming.
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borehole breakouts in berea sandstone reveal a new fracture mechanism
Pure and Applied Geophysics, 2003Co-Authors: Bezalel C HaimsonAbstract:Vertical Drilling experiments in high-porosity (22% and 25%) Berea sandstone subjected to critical true triaxial far-field stresses, in which ay (maximum horizontal stress) > a, (vertical stress) > ah (least horizontal stress), revealed a new and non-dilatant failure mechanism that results in thin and very long tabular borehole breakouts that have the appearance of fractures, and which counterintuitively develop orthogonally to a y .These breakouts are fundamentally different from those induced in crystalline rocks, as well as limestones and medium-porosity Berea sandstone. Breakouts in these rocks are typically dog-eared in shape, a result of dilatant multi-cracking tangential to the hole and subparallel to the maximum far-field horizontal stress a y , followed by progressive buckling and shearing of detached rock flakes created by the cracks. In the high-porosity sandstone a narrow layer of grains compacted normal to a n is observed just ahead of the breakout tip. This layer is nearly identical to “compaction bands” observed in the field. It is suggested that when a critical tangential stress concentration is reached along the a y , spring line at the borehole wall, grain bonding breaks down and a compaction band is formed normal to ay Debonded loose grains are expelled into the borehole, assisted by the Circulating Drilling Fluid. As the breakout tip advances, the stress concentration ahead of it persists or may even increase, extending the compaction band, which in turn leads to breakout lengthening
B. Haimson - One of the best experts on this subject based on the ideXlab platform.
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Compaction bands and the formation of slot-shaped breakouts in St. Peter sandstone
Special Publications, 2016Co-Authors: B. Haimson, A. KlaetschAbstract:Abstract: Drilling of vertical wellbores in sandstone often results in stress-induced failed zones called breakouts. In the laboratory, miniature Drilling under simulated far-field crustal stress con-ditions above a certain threshold produces breakouts that can be studied to enhance our under-standing of deformation and failure mechanisms in rock. We report on Drilling experiments in St. Peter sandstone, an Ordovician aeolian rock consisting of well-rounded bimodal quartz grains held together by sutured contacts. Breakouts here are slot-shaped and oriented at right angles to the far-field maximum stress direction (sH), resembling emptied compaction bands. We distinguish between two porosity ranges in this rock. In the high-porosity variety (16–22%), grains are bonded through narrow sutures. In the vicinity of the borehole at points aligned with the far-field least horizontal principal stress (sh) direction, where the maximum com-pressive stress concentrates, grain sutures sever at relatively low stress levels, leading to intergra-nular cracking. Debonded intact grains repack and produce a reduced-porosity narrow compaction band. Loose grains in the band abutting the borehole wall are then flushed out, assisted by the Circulating Drilling Fluid. This intensifies the stress concentration ahead of the breakout tip, advan-cing the compaction band and lengthening the slot-shaped breakout. The process continue
Haimson, Bezalel C. - One of the best experts on this subject based on the ideXlab platform.
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DEFORMATION AND FRACTURE OF POORLY CONSOLIDATED MEDIA - Borehole Failure Mechanisms in High-Porosity Sandstone
University of Wisconsin-Madison, 2005Co-Authors: Haimson, Bezalel C.Abstract:We investigated failure mechanisms around boreholes and the formation of borehole breakouts in high-porosity sandstone, with particular interest to grain-scale micromechanics of failure leading to the hitherto unrecognized fracture-like borehole breakouts and apparent compaction band formation in poorly consolidated granular materials. We also looked at a variety of Drilling-related factors that contribute to the type, size and shape of borehole breakouts. The objective was to assess their effect on the ability to establish correlations between breakout geometry and in situ stress magnitudes, as well as on borehole stability prediction, and hydrocarbon/water extraction in general. We identified two classes of medium to high porosity (12-30%) sandstones, arkosic, consisting of 50-70% quartz and 15 to 50% feldspar, and quartz-rich sandstones, in which quartz grain contents varied from 90 to 100%. In arkose sandstones critical far-field stress magnitudes induced compressive failure around boreholes in the form of V-shaped (dog-eared) breakouts, the result of dilatant intra-and trans-granular microcracking subparallel to both the maximum horizontal far-field stress and to the borehole wall. On the other hand, boreholes in quartz-rich sandstones failed by developing fracture-like breakouts. These are long and very narrow (several grain diameters) tabular failure zones perpendicular to the maximum stress. Evidence provided mainly by SEM observations suggests a failure process initiated by localized grain-bond loosening along the least horizontal far-field stress springline, the packing of these grains into a lower porosity compaction band resembling those discovered in Navajo and Aztec sandstones, and the emptying of the loosened grains by the Circulating Drilling Fluid starting from the borehole wall. Although the immediate several grain layers at the breakout tip often contain some cracked or even crushed grains, the failure mechanism enabled by the formation of the compaction band is largely non-dilatant, a major departure from the dilatant mechanism observed in Tablerock sandstone. The experimental results suggest that unlike our previous assertion, the strength of grain bonding and the mineral composition, rather than the porosity, are major factors in the formation of compaction bands and the ensuing fracture-like breakouts. Some breakout dimensions in all rocks were correlatable to the far-field principal stresses, and could potentially be used (in conjunction with other information) as indicators of their magnitudes. However, we found that several factors can significantly influence breakout geometry. Larger boreholes and increased Drilling-Fluid flow rates produce longer fracture-like breakouts, suggesting that breakouts in field-scale wellbores could reach considerable lengths. On the other hand, increased Drilling-Fluid weight and increased drill-bit penetration rate resulted in a decrease in breakout length. These results indicate that breakout growth can be controlled to some degree by manipulating Drilling variables. Realizing how Drilling variables impact borehole breakout formation is important in understanding the process by which breakouts form and their potential use as indicators of the far-field in situ stress magnitudes and as sources of sand production. As our research indicates, the final breakout size and mechanism of formation can be a function of several variables and conditions, meaning there is still much to be understood about this phenomenon
Agnar Aamodt - One of the best experts on this subject based on the ideXlab platform.
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Improved Efficiency of Oil Well Drilling through Case Based Reasoning
2000Co-Authors: Paal Skalle, Jostein Sveen, Agnar AamodtAbstract:A system that applies a method of knowledge-intensive case-based reasoning, for repair and prevention of unwanted events in the domain of offshore oil well Drilling, has been developed in cooperation with an oil company. From several reoccurring problems during oil well Drilling the problem of "lost circulation", i.e. loss of Circulating Drilling Fluid into the geological formation, was picked out as a pilot problem. An extensive general knowledge model was developed for the domain of oil well Drilling. About fifty different cases were created on the basis of information from one North Sea operator. When the completed CBR-system was tested against a new case, five cases with descending similarity were selected by the tool. In an informal evaluation, the two best fitting cases proved to give the operator valuable advise on how to go about solving the new case. Introduction Drilling of oil wells is an expensive operation, costing around 150 000 US $ pr. day, and any loss of time caused..