A joint industry project was established to study barite sag mechanisms and to develop field guidelines to manage the consequences. A simple empirical model was developed to compare sag potential for a wide range of fluid types. In the study, physical properties of the mud, wellbore conditions, and characteristics of the weighting material were shown to have a large influence on sag behavior. The study also included direct measurements of the properties of settled weight-material beds. These results provide new insight into the mechanisms of barite sag and how best to manage problems in the field.Data from the tests clearly demonstrate that the parameters affecting sag are interrelated and seldom act in isolation. For all muds tested, the highest sag occurred at low annular velocities over angles from 60 to 75°. Drillpipe rotation was particularly beneficial in minimizing barite settlement. Rotation also assisted in re-distributing barite deposits formed on the low side of the hole.The improved understanding of the mechanisms of barite sag enabled development of practical field guidelines. Case history studies presented in the paper demonstrate how the results of the work together with better field monitoring have been successfully applied to manage the effects of barite sag in high-pressure/hightemperature and extended-reach drilling operations.Test Fluids. The 20 test fluids represented a variety of mud types, formulations, weights, suppliers, and geographical sources. One of the project goals was to select muds from active directional wells ͑Ͼ30°͒ using weighted muds ͑Ͼ12 lbm/gal͒ in order to provide immediate feedback to operations. No other stipulations were made on field muds. Some laboratory-modified and laboratory-prepared fluids were also tested. Physical properties of all test fluids are listed in Tables 1 and 2. Mechanical-Parameter Results. Fig. 3 shows results for Mud 6 using the standard protocol at four inclinations ͑45, 60, 75, and 90°͒. Circulating fluid density change ͑corrected to 120°F͒ is plotted vs. time. All fluids tested responded similarly although the magnitudes varied.Most, if not all, of the sag-bed formation occurred at low flow rates and no rotation. Initial pipe rotation to 75 rpm consistently had the greatest effect on removing the beds. Doubling the rotary speed and then the annular velocity helped as expected, but to a much lesser extent.Time-segment 3 in Fig. 3, during which the sag was greatest, most clearly demonstrates the effects of angle. For Mud 6, the order of decreasing sag severity was 60, 45, 75, and 90°. While each individual mud tested exhibited slightly different behavior, the angle at which the maximum sag occurred was consistently in the range 60 to 75°. This is consistent with previously reported data. 3Fluid-Parameter Results. Rheology, density, weight material, and chemical treatments were the key fluid parameters investigated. These parameters emerged from an earlier study as the most significant variables affecting sag that can easily be controlled in the fie...

A joint industry project was established to study barite sag mechanisms and to develop field guidelines to manage the consequences. A simple empirical model was developed to compare sag potential for a wide range of fluid types. In the study, physical properties of the mud, wellbore conditions, and characteristics of the weighting material were shown to have a large influence on sag behaviour. The study also included direct measurements of the properties of settled weight-material beds. These results provide new insight into the mechanisms of barite sag and how best to manage problems in the field. Data from the tests clearly demonstrate that the parameters affecting sag are interrelated and seldom act in isolation. For all muds tested, the highest sag occurred at low annular velocities over angles from 60-75. Drill-pipe rotation was particularly beneficial in minimising barite settlement. Rotation also assisted in re-distributing barite deposits formed on the low side of the hole. The improved understanding of the mechanisms of barite sag enabled development of practical field guidelines. Case history studies presented in the paper demonstrate how the results of the work together with better field monitoring have been successfully applied to manage the effects of barite sag in HP/HT and extended-reach drilling operations. P. 89

Summary Two gas well collapses showed a peculiar failure mechanism. Excessive fluid levels and temperature variations during the ultimate drilling phase and later workovers induced microannuli and cementation damage. Abnormal fluid pressures from deeper gas or water bearing layers channeled through damaged cementation and increased the pore pressure in uphole faults or bedding joints. Thus the effective normal stress on faults was released, inducing shear displacements and casing deformation. Weakened casings were further collapsed by later fluid level variations. It was then suspected that this mechanism also occured in open holes during drilling. This was shown to be so with BHTV images showing borehole lateral shifts. Although never mentioned in the past in the domain of borehole stability, this mechanism explains some poorly understood drilling incidents such as tight-holes, problems to RIH or POOH, abnormal torques; in spite of no indications of instability such as cavings. This mechanism can be mitigated by taking the necessary precautions during drilling and by the adequate selection of mud characteristics. Introduction The major drilling problems identified until now are usually attributed to the failure of the intact rock material (sometimes called "rock matrix") around the hole. Borehole stability is evaluated usually by comparing the rock strength to the assumed stress distribution around the hole coming from in situ stresses, drilling and mud characteristics, and rock behaviour. An extensive literature is available on this topic. During completion when running in with tubing for either oil or gas production, some damage is sometimes observed which prohibits either testing or production. In the case of large ground deformations such as in North Sea subsidence, it is obvious that the casing cannot stand large ground deformations and deforms accordingly. Several methods have been proposed to account for this deformation. In the case of no noticeable deformation at the surface, the origin of these casing collapses is less clear. Most of the time they are considered as the consequence of casing wear during subsequent drilling phases, or corrosion, or inadequate installation procedures. Sometimes observations made during drilling or completion invalidate this explanation.

It is usually accepted that wellbore instabilities are caused by either or both an excessive stress concentration at the borehole wall and the chemical reactivity of the formation. Assuming good hole cleaning, common cures for such instabilities are therefore mud density increase and/or change of the mud system. However, even though these methods have proved highly successfull when drilling through intact formations, the same may not stand when it comes to highly fractured rocks. The purpose of this paper is to describe the studies performed in order to allow the safe drilling of a particularly troublesome, highly fractured formation which had led previously to several successive side tracks.The efficiency of the different attempts at solving the problem are analysed in the light of the various side tracks. The core taken during one of these side tracks was analysed, showed that the formation was highly fractured and chemically inert, and provided many parameters for the modelling of the problem which was performed by a discrete element programme. Further modelling was performed on the mud hydraulics. Both theoretical and field data show that density increase has, in such a case, a negative role on borehole stability while filtrate reduction and mud rheology enhancement are highly positive.

It is usually accepted that wellbore instabilities are caused by either or both an excessive stress concentration at the borehole wall and the chemical reactivity of the formation. Assuming good hole cleaning, common cures for such instabilities are therefore mud density increase and/or change of the mud system. However, even though these methods have proved highly successfull when drilling through intact formations, the same may not stand when it comes to highly fractured rocks. The purpose of this paper is to describe the studies performed in order to allow the safe drilling of a particularly troublesome, highly fractured formation which had led previously to several successive side tracks.The efficiency of the different attempts at solving the problem are analysed in the light of the various side tracks. The core taken during one of these side tracks was analysed, showed that the formation was highly fractured and chemically inert, and provided many parameters for the modelling of the problem which was performed by a discrete element programme. Further modelling was performed on the mud hydraulics. Both theoretical and field data show that density increase has, in such a case, a negative role on borehole stability while filtrate reduction and mud rheology enhancement are highly positive.