SPE/IADC 79846Imaging unstable wellbores while drilling Stephen Edwards, Bruce Matsutsuyu, Steve Willson, BP America Copyright 2003, SPE/IADC Drilling Conference This paper was prepared for presentation at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 19–21 February 2003. This paper was selected for presentation by an SPE/IADC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers or the International Association of Drilling Contractors and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the SPE, IADC, their officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers or the International Association of Drilling Contractors is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract This paper presents a case study of borehole instability from 4 wellbores on the Gulf of Mexico (GOM) shelf, offshore Louisiana. Logging while drilling (LWD) borehole images are combined with observations of cavings and modeling of borehole shear failure in order to diagnose the mechanisms of instability and thus select the appropriate remedial action. It is observed that instability due to shear failure of intact rock (borehole breakout) can be suppressed by increasing mud weight. However, where pre-existing planes of weakness such as bedding planes and fractures dominate the mechanism of instability, mud weight increases do not necessarily lead to a more stable hole and can in fact further destabilize the wellbore. Introduction Despite considerable effort from the drilling, subsurface and geomechanics communities, many oil wells continue to suffer from wellbore instability problems during drilling. Although instability is quite common, in the majority of cases a considerable amount of uncertainty exists around exactly where, when and why the instability occurred. Unfortunately, it is almost axiomatic that logs will not be run in an unstable wellbore. Direct measurements of the borehole shape and condition which can be obtained from caliper and image logs are therefore rarely acquired in the wellbores where (from a geomechanics point of view) they would be most valuable. Modeling and cavings analysis alone, can leave considerable uncertainty as to the location and to some extent the mechanism of failure. An exception to the axiom can be where LWD image data is acquired. It is still unlikely that LWD imaging tools would be run in a well where significant instability was expected. However, LWD is often acquired in wells that turn out to be less stable than anticipated. In these cases a rare glimpse of the unstable wellbore wall in the early stages of collapse may be captured. This is very useful information, which would normally remain the secret of the well. Mechanisms of wellbore instability Mechanism of mechanical wellbore instability can be grouped in two main classes. 1. Instability due to failure of intact rock (rock which is unbroken and isotropic in strength) 2. Instability due to failure of rock containing pre-existence planes of weakness (bedding planes, fractures, cleavage). Rock containing pre-existing weaknesses such as bedding or cleavage may be intact in the sense that it is unbroken. However, for the sake of this discussion intact is defined as above. The majority of quantitative wellbore stability studies since the 1979 paper by Bradley1 have modeled the wellbore wall as intact rock subject to the stresses imposed from the far-field and the wellbore fluid. This type of failure gives rise to symmetrical breakouts in the wellbore walls. Breakouts can be stabilized by increasing the mud weight, or may stabilize after reaching a certain size under favorable combinations of stress and strength. Breakouts are quite often observed in image and multi-arm caliper log data and are clearly a common cause of wellbore instability. Other mechanisms of instability where pre-existing weaknesses are present do not necessarily stabilize with time or with increased mud weight. Instability due to such mechanisms is therefore rarely calipered or imaged, making the exact location and mechanism of instability uncertain. Consideration of wellbore instability due to pre-existing weaknesses in oil wells is for the most part relatively recent 2,3,4,5,6 . Evidence of these mechanisms came from observations such as correlations of trouble time with wellbore trajectory and the existence of pre-existing fracture planes, bedding planes and cleavage in cavings. Types of wellbore instability associated with pre-existing weaknesses can be grouped into two classes. T. SPE/IADC 79846 (A) Failure due to the existence of “impermeable” pre-existing weaknesses.g. Some time was spent in fishing attempts (while continued instability was experienced) before deciding to set a cement plug and re-drill this wellbore from approximately 10. Hole conditions appeared to be worst below around 12.. Mud weight increase is seen to suppress the breakouts but increase instability on pre-existing weaknesses.5 and then to 10.700 ft MD. The LWD tools were recovered and the memory data downloaded. A detail of the well trajectories is shown in figure 2. S.400 ft MD. cracks. The following case study contains examples of instability due to both shear failure of intact rock and failure due to preexisting weaknesses. increase in mud weight will tend to further support the wellbore wall. An Azimuth Density Neutron (ADN) tool was included in the LWD suite. (B) Failure due to the existence of “preferentially permeable” planes of pre-existing weaknesses. Tight hole was noted tripping back in and large pieces of shale (cavings) were seen over the shakers. Two distinct cavings types are linked to the 2 mechanisms. B. Both modes of failure are imaged with an LWD tool. The emerging use of LWD imaging tools provides such a possibility. down dip (i. bedding and cleavage planes. Wellbore 1 An original hole was drilled in 1993. An example of this type might be where a single set of bedding planes is intersected. tight spots were encountered and washing/reaming was required on trips. Where the mud and filtrate preferentially enters pre-existing planes of weakness. After more tight spots and over-pull while tripping out. The first indications of instability occurred after drilling to approximately 12.. a variety of shapes and sizes of cavings were seen. rather like a pile of rubble or the material seen in brittle/semi-brittle fault zones. This difficulty in modeling. Figure 3 shows the ADN image from wellbore 2 from 13000 to 13100 ft MD. it was observed that the lower part of the BHA was left in the hole. a high photo-electric factor and low .5 ppg in the equivalent section to that described below for the wellbores 2-4. where wellbore instability has occurred and mud instead of formation is adjacent to the tool. This well is labeled wellbore 1 in figure 1. While drilling ahead to 13. S. subparallel to the dip of the bedding.2 EDWARDS.g. the rock having been effectively rubbelized. such as thinly bedded shale. In the case where the pre-existing weakness are not preferentially permeable. increasing the mud weight does not add support to the wellbore wall and may increase instability. bedding and fractures – intersect) are probably more likely to be permeable than the single plane of weakness (e. the BHA was unable to ream past 12.300 ft MD. It was drilled at an angle of 50 degrees with mud weight of between 10 and 11.M. MATSUTSUYU. or perpendicular to the strike of the bedding).7 ppg in response to the hole conditions. From the available descriptions however. which did not appear to improve and cavings continued to be seen on the shakers. The study looks at 4 wellbores: an original hole and 3 sidetracks.56 degrees arc) to create a density image. is likely to be particularly susceptible to becoming rubbelized where it is affected by faulting. WILLSON. These pre-existing weaknesses could be a combination of fractures. Thus. The figure is from a standard field log ADN presentation.400 ft MD.3 ppg at this stage.e. No cavings were kept or recorded in detail. The relative angle between the wellbore and bedding in the plane of the wellbore (the attack angle) is therefore 18 degrees. LWD Borehole Images in wellbore 2. Wellbore 1 did not encounter any significant instability. The geological setting of the wells is shown in figure 1. just bedding planes).407 ft MD. Wellbore 2 In 2002. The 8 ½” diameter section of wellbore 2 was drilled from approximately 7100 MD to 13. Naturally fissile rock. wellbore 1 was sidetracked.427 MD (12. The mud weight was increased in stages throughout the hole from 10 and 10. In an extreme case. The mud weight was increased from 10 to 10. Density measurements are binned into 16 sectors (each sector covers a 22. This sidetrack (wellbore 2 in Figure 2) was drilled at 60 degrees. In brief. The data displayed (from left to right) is as follows: 1st track shows the rotational speed of the tool in RPM 2nd and 3 rd tracks show the raw photo-electric absorption factor data 4th track is the azimuthal photo-electric factor image 5th track is the azimuthal density image and gamma ray 6th and 7th tracks are the raw density data Drilling mud has a higher photo-electric absorption factor and a lower density than the formation. GOM shelf case study This paper presents a wellbore stability case study from the GOM shelf. The bedding angle is 12 degrees. Details of the ADN tool are available from Anadrill. Networks of pre-existing weakness (such as where two sets of weakness . This type of rock mass could be referred to as a rubble zone. This bias may be partly addressed if more direct observations of failure on pre-existing planes of weakness are made.8 ppg in response to signs of instability (tight hole and cavings). On tripping back in the hole. Quantitatively including pre-existing weaknesses in the wellbore stability models is difficult although recently encouraging progress has been made for the case of a single set of pre-existing non permeable weaknesses 7. The mud weight was raised again to 10. the body of rock may actually be made up of many discrete rock fragments with no cohesion between them. coupled with a lack of direct downhole observations. has tended to bias the industry towards thinking in terms of shear failure of intact rock.232 TVD).e. the tool measures density as a function of azimuth around the hole. it may also have experienced some swabbing which would tend to induce instability. The bedding planes in figure 5 may not be so weak as in figure 3. The image in Figure 3 is oriented relative to the borehole geometry. time restrictions and difficulty in retrieving the data has prevented any time-lapse study. When studying Figures 3 and 5.540 ft MD was therefore not logged until approximately 2 days after it was drilled. For display purposes. An example from Okland and Cook4 is shown in Figure 4. there is some element of roof collapse apparent in this section. In this section. This is possible. The photo-electric factor is considerably more sensitive to the mud/formation contrast than the density.9. In the case of Figure 5. A simple analytical model of shear failure in intact rock indicates that such failure would be not be unexpected in this hole with the relatively low mud weight (10 ppg) used to drill this interval. The photo-electric factor image is saturated at the high end in much of the failed section making details difficult to discern. The interval 12. The sides of the hole in a 60 degree well where the maximum stress is vertical may be expected to exhibit breakouts. Logging in the time domain. Imaged modes of instability The nature of the instability can be best seen in the photoelectric factor image in Figure 3. the sides of the hole are seen to be intact (compare to Figure 4). another factor that should be kept in mind is time. It would be extremely interesting however to examine these and other sections at a latter time. Figure 3 is presented at a compressed scale such that the sinusoid amplitude is low. the density image is more revealing. Instability dominated by failure on weak bedding planes in fissile shale. It can be seen that the failed area is largely delimited by the bedding planes and is located mainly on the wellbore roof. In most of the section shown in Figure 3.540 ft MD and then a trip was made to test equipment. However. where the wellbore is close to parallel with the bedding planes has been observed in the laboratory.440 to 12. and partly from the wellbore floor. This is discussed further (below) with reference to the failed section shown in Figure 5. For most petrophysical applications to is desirable to obtain log data acquired as soon after the well is drilled as possible in order to measure undisturbed or uninvaded formation properties.SPE/IADC 79846 IMAGING UNSTABLE WELLBORES WHILE DRILLING density are recorded. In the case of figure 3. this correlates to dark areas on the photo-electric factor image and light areas on the density image. about 5 hours circulating time occurred between drilling the interval and logging it. it is instructive to see the state of the hole both directly after being drilled and at later times. the cylindrical hole is cut down the axis of the hole (along the top) and “unwrapped”. Thus. the mode of failure appears to be different to that in Figure 3. That means that each time the well is deepened. In a normal faulting environment (where the maximum principal stress is vertical) such as this part of the GOM shelf. Wellbore 2 summary Two modes of instability are apparent in the ADN images: . In Figure 3. even though the angle between the wellbore and bedding is only approximately 18 degrees. The top of the hole therefore lies on the far left and right-hand sides of the unwrapped image and the bottom of the hole (being 180 degrees from the top of the hole) lies in the center of the image. Unfortunately in this case. the well was drilled to 12. Bedding planes can be seen as low amplitude sinusoids in Figure 3. The angle between wellbore and bedding is roughly the same in figures 3 and 5. In a couple of places however (e. The ADN tool is about 100 ft behind the bit. The dark areas are where the wellbore wall has failed and the material has been removed. It is called time-lapse logging 8. Where wellbore stability is the topic of interest. This interval can be seen from the gamma ray to be a shale section which might be expected to be somewhat fissile. The field log used for this study is a depth based “drilling log”. A more silty rock is likely to be less fissile than a shale.g. It could be that one mode of failure triggers or interacts with another mode of failure. therefore the well is typically logged approximately 1 hour after being drilled. The influence of the bedding planes on the geometry of the hole failure indicates that they are acting as pre-exiting planes of weakness. From their experiments and field data they suggest that the bedding plane splitting or roof collapse mode of failure occurs where the angle of attack is less than 10 to 20 degrees. 3 The density image reads beyond mud in the immediate vicinity of the tool to show the lowest densities (the failed section) to be on the sides of the hole. as this is where the maximum stress concentration would form. On the trip out of hole. the log data over the new depth is spliced to the bottom of the previous log. Any planar feature that the borehole intersects will appear in the image as sinusoidal lines running from one side to the other. breakouts in intact rock would be expected to form on the sides of a hole deviated at 60 degrees. It is converted to depth for presentation. When the wellbore intersects planar features at low angles. which may have been initiated by breakouts on the side of the hole. the amplitude of the sinusoid is usually high. Although most of the failure in Figure 5 is on the sides of the hole. it also clearly extends in some places to the roof of the hole along bedding planes. at approximately 13075 ft MD) failure is seen to extend from the roof of the hole down to the sides of the hole. We could therefore describe this instability as predominantly a roof collapse mechanism. In figure 5. The location of much of the failed zones in Figure 3 is therefore probably controlled by the stress concentration rather than the pre-existing planes of weakness. The lithology in the failed section of figure 5 can also be seen (from the gamma ray) to be more silty than that in Figure 2. The measurements are originally recorded in the time domain. On the other hand. Either the bedding planes themselves are preferentially permeable. The imaged sections of unstable hole were both recorded some time after having been drilled. S. The higher mud weight in wellbore 3 is likely to have Figure 7 shows the resistivity data from wellbore 2 and 3. S. multiple packing-off events occurred even after further increases in MW (to 11. particularly in the interval below 12. or there is an additional set of cracks and fractures which provides the permeability. In a shale however. The LWD resistivity tool measures many resistivities at different spacings and frequencies. the only likely mechanism of significant invasion is via some set of network of pre-existing permeable planes of weakness. Figure 1 shows the general structural setting of the wells and figure 2 shows two mappable faults in close proximity to the wells. Some of the curve separation is probably due to anisotropy in the sections drilled at low attack angle to bedding. they may be well below the resolution of these images. wellbore 3 was drilled at exactly the same azimuth and deviation (60 degrees towards the east). In a shale. the apparent “invasion” is greater in wellbore 3. Wellbore 3 was kicked off on the top of the cement plug. These were the dominant type of caving in this wellbore. A variety of cavings were reported but not collected or recorded. which also resulted in lost circulation.M. as the BHA was lost. Although no record of cavings from wellbore 2 exists. Wellbore 3 Wellbore 2 was abandoned due to the instability problem encountered and a cement plug set at 10700 ft MD. anecdotal information from the mud engineers and shaker hands indicates that there was more of a mixture of cavings type in wellbore 2. However. all resistivities read essentially the intrinsic resistivity of the formation such that all resistivity curves more or less stack on top of each other when plotted.. However. if the mud is able to penetrate along pre-existing weaknesses the effect of the mud pressure will be to destabilize the wellbore wall rather than support it.9). Any curve separation indicates some abnormality. the higher mud weight in wellbore 3 seems to have worsened the pre-existing weakness mode of failure. the key observation is the comparison of the curves from wellbore 2 and wellbore 3. curve separation is caused by either invasion into a fracture network (or other path of preferential permeability). The faulting occurred late in the geological history and is related to movement of the salt shown in figure 1. After more than a day of fighting the battle between instability and lost returns (and losing on both sides) the drillstring became irretrievably stuck. Figure 6 presents a sample of cavings recovered from wellbore 3. However. tool eccentricity or hole enlargement.500 ft MD. the mud weight in wellbore 3 was increased to 11. Curve separation in a permeable rock is normal.4 EDWARDS. essentially interacting with the bedding planes to creating a network of pre-existing weaknesses. In much of the section showing curve separation we know the hole to be in gauge from the ADN tool. Fortunately. We know from the ADN image in wellbore 2 that 2 modes of instability were occurring. Normally. In a “normal” shale. as the drilling fluid is expected to invade to some extent and alter the resistivity. the ADN images from this section were not retrieved.. MATSUTSUYU.T. After drilling to 13. The Many cavings were observed while drilling wellbore 3. Tool eccentricity isn’t a likely cause in an 8 ½” hole. The LWD resistivity provides additional evidence that the rock has some kind of additional permeability. After steering about 75 feet away from wellbore 2. There is no clear evidence of cracks or fractures in the ADN images. ?? Breakout on the sides of the hole – also in a section close to bedding parallel but more silty. . The observation that increased mud weight in wellbore 3 increased instability implies that the mud filtrate was penetrating the planes of weakness and destabilizing them. B. This was an important piece of information for the post-mortem diagnosis. In the interval shown in Figure 3.7 ppg. which would not be found in a “normal” (unfractured) shale. increasing mud weight should add support to the wellbore wall and prevent collapse. Only real-time LWD data was recovered. Failure dominated by planes of pre-existing weakness on the other hand give rise to cavings which are delimited mainly by the planes of pre-existing weakness and therefore tend to be blocky to tabular and characteristically have parallel sides. Although the sediments penetrated by these wells are clearly not highly disturbed. Breakout of intact rock gives rise to angular cavings. The greater invasion with the higher mud weight supports the idea that the mud is penetrating the formation. anisotropy. ?? Roof collapse dominated by failure on bedding planes in a section drilled close to parallel with bedding in a shale. Because of the instability experienced in wellbore 2. WILLSON. However. Unfortunately. Other parts of the wellbore may also have been seen to be unstable had they also been logged some time after drilling. they may have experienced some degree of faulting/fracturing in response to nearby fault movement. This is 1ppg more than the maximum mud weight used in wellbore 2. even though it was logged directly after being drilled as opposed to several hours after in wellbore 2 (the resistivity tool is much closer to the bit than the ADN tool so the time effect is not so great). in this case they were saved and photographed using a digital camera at the wellsite. the cavings in Figure 6 are the type of cavings that might be expected from a zone dominated by failure of bedding planes and roof collapse. We would therefore expect a variety of cavings from wellbore 2. where the higher mud weight was used. SPE/IADC 79846 suppressed the shear failure on the sides of the hole. However.400 ft MD. the stability problems in wellbore 3 were seen to be worse than in wellbore 2. ” Transactions of the ASME Volume 101.. Charlez.. A combination of LWD imaging. Although the details are beyond the scope of this paper. P. Min. International Journal of Rock Mech. Cavings were predominantly blocky to tabular in wellbore 3 indicating failure associated with planes of pre-existing weakness. 2. The cavings are predominantly angular. Summary of wellbore 3 Like wellbore 2. Abstracts. There are no obviously blocky or tabular cavings indicating the bedding plane failure mode was not occurring extensively in this borehole. The existence of preexisting fractures may have also played a role in the geometry of the cavings. Increasing mud weight appeared to prevent the shear failure mode but worsen the roof collapse mode. December 1979. however. M. cavings analysis and modeling is required to unambiguously diagnose the location and mode of wellbore instability.J. Silman. Baroudi.. Figure 9 attempts to summarize the geological setting. Wellbore 4 Wellbore 4 was drilled after careful examination of all the data available from wellbore 2 and 3 (presented above). This mud penetration appears to have been detected from the resistivity tool. Colombia”. SPE 30464 . “Mechanisms of borehole instability in heavily fractured rock media”.5 PPG. Alsen.. the dominant influence of the bedding planes is not seen. 1995. As the hole has a lower deviation angle than wellbores 2 and 3. the mud weight used in this section was still relatively low for the insitu stress and strength conditions. It is a key tool for any attempts at real-time wellbore stability. Although the volume of cavings was noticeable (no Conclusions Two modes of instability can occur in the same hole at the same time but require different treatments.9 through these zones of suspected fracturing/invasion would be very instructive but has not been performed in this case. There is enormous potential to improve our understanding of wellbore instability from further LWD imaging studies and in particular from time-lapse LWD imaging. As a secondary precaution against destabilizing the pre-existing planes of weakness.8 ohmm. Harkness. 5 quantitative estimate was made) the instability was not sufficient to impact drilling. The main conclusion drawn from the above observations is that the majority of instability in wellbores 2 and 3 is due to the low attack angle to bedding coupled with penetration of mud into pre-existing weaknesses (either just bedding planes or a network of bedding planes and fractures). H. “Failure of inclined boreholes. leading to the loss of the BHA. N. Figure 8 shows cavings recovered from the shakers while drilling wellbore 4.B. packing-off or significant tight spots were experienced. Although this risked allowing the breakout mode of failure to occur. Plumb. No stuck pipe. “An integrated approach to evaluating and managing wellbore stability in the Cusiana field.B. which is downloaded from the memory. Last. In this example the two modes observed were: Shear failure of intact rock at the point of maximum stress concentration on the borehole wall. instability worsened. Sci. Having lost the BHA in wellbore 3.. This implies that mud was preferentially penetrating the bedding planes or other pre-existing planes of weakness (such as fractures). K. R. Bradley. W. 1992 (29) 457 – 467. Normal shale resistivity in this interval is approximately 0. LWD imaging offers a rare glimpse at a severely unstable wellbore. J.. Wellbore 3 used a higher mud weight than wellbore 2. the simple analytical model of shear failure in intact rock mentioned above was again run to predict the mud weight required to prevent breakouts in this wellbore. Roof collapse due to splitting of weak (fissile shale) bedding planes where the angle between the wellbore and the bedding planes was approximately 18 degrees. evidence that the breakout mode of failure was indeed active in wellbore 4 was obtained from the cavings.SPE/IADC 79846 IMAGING UNSTABLE WELLBORES WHILE DRILLING data from wellbore 3 has been depth shifted to match that of wellbore 2... 3. Santarelli. the mud weight was also lowered back to 10. R. However. The trajectory of wellbore 4 is shown in Figure 2. Dahen. consistent with the shear failure of intact rock or breakout mode. wellbore 3 was drilled at a low angle of attack to bedding. However. D. and the observation of breakouts is again not unexpected. The data from wellbore 3 is realtime data and is therefore more coarsely sampled than the data from wellbore 2. Real-time LWD resistivity indicates increased invasion (compared to wellbore 2) of oil based mud into the unstable shale sections. They are noticeably different to those recovered from wellbore 3. However. The key change in plan was therefore to increase the attack angle. Mclean. this was thought preferable to risking the more catastrophic bedding plane failure. A wellbore stability study for this well had not previously been conducted. and Geochem. location and modes of failure seen in wellbore 1 to 4. less mud weight is required to prevent breakout.. no LWD tools were run in wellbore 4. Time-lapse resistivity 8. Refrences 1. F. “ Accidental Geomechanics: Capturing in situ stresses from mud losses encountered while drilling” SPE/IADC 79846 . “How to Diagnose Drilling Induced Fractures in Wells Drilled with Oil-Based Muds with RealTime Resistivity and Pressure Measurements” SPE/IADC 67742.. J.. Moos. Okland. “Bedding related borehole instability in high angle wells”.. Tan. 9. 7...M.L. Standifird. C. 6. T. I. D.T. D. MATSUTSUYU.. Li. Detournay.M. S. A. “Drilling in South America: A wellbore stability approach for complex geolgic conditions”.. X.. B.M...D. Zoback. S. M. Willson.R.. “The impact of mud infiltration on wellbore stability in fractured rock masses”.P.. Rezmer Cooper. Chen. Gille YE. J.R. Bratton. Bratton. Q. Oilrock 2002. SPE 53940. 4. 8. 1999. M. Willson.M. Eurock..T. C.J. WILLSON. SPE 78238. Desroches. Crook. “Development of an orthotropic 3D elastoplastic material model for shale” Oilrocks 2002.6 EDWARDS. SPE 78241. W. S. SPE 47285 5.. J.. McFayden.. Edwards. S.B. Cook. S. Yu.. 1998. M. T. 5# PRESSURE SALT SALT 5000’ Figure 1. Regional West-East seismic line showing the general structural setting of the discussed GOM shelf wellbores 1-4. .SPE/IADC 79846 IMAGING UNSTABLE WELLBORES WHILE DRILLING 7 REGIONAL WEST-EAST SEISMIC LINE THROUGH THE SUBJECT WELLBORES 1-4 SHOWING THE RELATIONSHIP TO MAJOR STRUCTURAL ELEMENTS WEST EAST LINE SUBJECT WELLS TOP OF 12. T. S. SPE/IADC 79846 DETAIL SEISMIC LINE PARALLEL TO WELLS SHOWING THE RELATIONSHIP OF BORE HOLE ANGLE AND BED DIP (Some Vertical Exaggeration) EAST WEST Shallower Target Target Anomaly Bed Dips ~12 Degrees Fault Wells 2&3: 60 Degrees Fault Well 4: 36 Degrees Well 1: 50 Degrees 1000’ Figure 2. WILLSON. The angle between wellbore and bedding (for the bottom secti ons of wellbores 2 and 3 is 18 degrees.M.8 EDWARDS. B. S.. Detail of structural setting of wellbores 1 -4. .. MATSUTSUYU. SPE/IADC 79846 13000 MD IMAGING UNSTABLE WELLBORES WHILE DRILLING Bottom of Hole Bedding Plane 13100 MD Top of Hole Figure 3. ADN image from wellbore 2 showing wellbore instability on the roof and floor of the wellbore predominantly related to bedding planes in this section. See text for explanation of data in the display. Angle between the wellbore and bedding planes is 18 degrees. 9 . S. Bedding plane orientation Figure 4. Original hole diameter is 10mm..10 EDWARDS. B.. SPE/IADC 79846 . WILLSON. The large cross cutting cracks (running from one side of the sample to the other) are thought to be pre-existing cracks roughly parallel to bedding. Montage of scanning electron microscope image of a laboratory hollow cylinder test in a fissile Jurassic North Sea Shale showing catastrophic hole collapse dominated by failure of bedding planes.M.T. MATSUTSUYU. S. Taken from Okland and Cook4. ADN image from wellbore 2 showing failure predominantly on the sides of the hole (clearest in the density image). The failure on the sides of the hole appears to be shear failure (breakout) of “intact” rock rather than being dominated by the bedding planes. the bedding does clearly influence the failure. It may be that breakout on the sides of the hole and bedding plane failure on the roof of the hole are inter-related in this section. it is more silty (note gamma ray) and therefore perhaps less fissile than the interval in Figure 3. See text for explanation of data in the display. .SPE/IADC 79846 IMAGING UNSTABLE WELLBORES WHILE DRILLING 11 Bottom of Hole 12500 ft 12550 ft Breakouts on side of hole Top of Hole Figure 5. In some places however. Although angle between wellbore and bedding is only 18 degrees in this section and bedding planes can still be seen. . The predominant cavings type in wellbore 3 were blocky to tabular shales as shown here. MATSUTSUYU.T. WILLSON.M. S. SPE/IADC 79846 . S. B. Figure 6..12 EDWARDS. They are characterized by parallel bounding surfaces. which by analogy to Figure 4 and from examination of Figure 3 are probably bedding planes. Higher resistivities in the shaley sections in wellbore 3 may indicates greater invasion of synthetic oil based mud.1 0.5 150 2. 13100 -200 -250 -300 13200 .SPE/IADC 79846 IMAGING UNSTABLE WELLBORES WHILE DRILLING 13 2.9 WB2_LWD_Res_P40H WB2_LWD_Res_A40L 0.7 -50 1.1 50 1.5 -100 1.9 0 1.5 12800 12900 13000 Figure 7. Comparison of LWD resistivity in wellbores 2 and 3 (depth shifted to match).7 WB3 LWD_Res_P40H_RT WB3 LWD_Res_A40L_RT WB2_LWD_GR WB3_LWD_GR_RT 0.3 -150 1.3 100 2. B. Figure 8. WILLSON.M. These cavings are predominantly angular with bounding surfaces at less than 90 degrees to each other. The presence of some other set of pre-existing fractures is also apparent form the shape of some of the cavings. S.T. None of the blocky – tabular cavings were seen in wellbore 4.14 EDWARDS. MATSUTSUYU. These are likely to be caused by shear failure of “intact” rock.. SPE/IADC 79846 . Cavings from wellbore 4 shown here were noticeably different to those from wellbore 3. S.. instability mechanism and resulting cavings in wellbores 1 to 4. Wellbore 4 . Summary of structural setting.SPE/IADC 79846 IMAGING UNSTABLE WELLBORES WHILE DRILLING 15 Shear failure on sides of hole produces angular cavings Wellbore 1 Wellbores 2 and 3 Possible fractures related to Fault Trend B ed D ip = 1 2 Deg rees X100 Roof collapse on bedding planes produces blocky tabular cavings Figure 9. The ADN image shown here with wellbore 4 is actually from wellbore 2 (see text) but the type of failure is interpreted to be the same.