Hole Stability Through Mud Technology An Organized Approach

This paper is intended to have a twofold purpose. While the data cited refer to the specific problem of borehole instability as affected by the drilling fluid, it is also intended that the approach taken toward alleviation of the borehole instability problem through mud technology in this case is applicable to other drilling problems as well. Borehole instability serves as an example of a drilling problem to illustrate how drilling problems may be approached in an organized manner by way of the drilling mud. The varied drilling problems that are susceptible to alleviation in part through the drilling fluid, may be approached systematically by considering the drilling problem in terms of the fundamental characteristics of drilling fluids. These fundamental characteristics are stated in Fig. 1. More detail could be added to the criteria listed. Solids content could be listed in addition to weight, for example. Nevertheless, if a given drilling problem is considered carefully in terms of the criteria listed, it will be found that the analysis thus carried out will be accurate and reasonably thorough, insofar as the problem in question is subject to solution through the drilling fluid. Futhermore, in addition to serving as a guide for the application of mud technology, these same criteria point to areas in which improvements in presently existing technology may be sought. In the text to follow, the problem of borehole instability will be analyzed in terms of the weight, rheology, filtrate and other characteristics of the drilling fluid. Of the various rocks that are penetrated in the course of drilling a well, the rock most likely to be unstable is shale. Both sandstones and carbonate rocks may be unstable when subjected to tectonic stresses or when the hydrostatic mud pressure is lower than the pressure on the fluids in the rocks, particularly when the permeability is low. But the instability problem with shale is compounded by the extraordinary manner in which this rock is affected by wetting with water.