The succesful drilling of the continental deep borehole (KTB) will be dependent on knowledge of the causes of borehole instability. In addition to secondary KTB research and development projects in this field the drilling of the KTB pilot borehole, a series of overlapping caliper logs was run at intervals of only a few days. This enabled the study of borehole diameter changes on a time dependent basis. The caliper log data were processed using a statistical method to elaborate characteristic parameters of the borehole geometry in 25 m sections. Important characterization variables are borehole roughness, elongation and ellipticity. The time dependence of these parameters was correlated to changes in the circulation. The data shows the successful application in the KTB pilot borehole of increased use of the complex bond-forming viscosifier Dehydrill in the fluid circulation to practically stop all borehole enlargement at all depths. This factor shows the primary influence of circulation conditioning on borehole stability. Over virtually the entire borehole it was observed that enlargement took place in the direction of dip of foliation, which is undoubtedly due to the presence of hydraulic permeability along paths forming preferentially in the direction of steeply dipping foliation.
A large number of overlapping caliper logs run at relatively short time intervals during drilling of the KTB pilot borehole make statements of a previously unknown degree of accuracy possible when describing the time dependent behaviour of borehole geometry. Literature on the subject of borehole stability (CHEATHAM, 1984, GUENOT, 1987- see also summary by BORM, 1988) makes It clear that the majority of authors used stationary models in their interpretation of borehole instabilities. The initial logging data acquired from the KTB pilot borehole made it soon apparent (KESSELS, 1988) that borehole instability in this case displays a clear dependence upon time, a fact confirmed in the subsequent borehole logs. It is then necessary to include non-steady-state mechanisms in a realistic description of borehole instability. In addition to these effects caused by inhomogenity of the structure (KESSELS, 1987) other observable effects are due to chemical interaction with the drilling fluid and the drilling activity itself, both non-steady state processes. When comparing caliper logs taken at different times it should be borne in mind that technical reasons will cause a four-arm caliper log to supply differing data on a second run through the same borehole geometry than on a first, because the orientation of the tool in the borehole cannot be fixed unequivocally. In general it is, therefore, assumed that one pair of arms will tend to run in the direction of the longitudinal axis of borehole ellipticity, as shown in figure 1. That this assumption is justifiable is confirmed by a large number of caliper log runs in which, by employing the relative bearing angle (RB), it is possible to show that the caliper log maintains a stable position in an enlarged section, whereas, in a smooth section of the borehole, cable tension leads to axial rotation (KESSELS, 1988).
