Compaction bands induced by borehole drilling

Drilling experiments in rock blocks subjected to pre-existing true triaxial far-field stresses simulating real in situ conditions often result in localized failure around the created borehole, which brings about the formation of borehole breakouts. In weakly bonded quartz-rich porous sandstones breakouts take the form of narrow tabular (slot-like) openings extending along a plane perpendicular to the maximum applied-stress direction. Scanning electron microscopes images of failed boreholes strongly suggest that these breakouts are compaction bands that have been emptied to different extents. The bands form as a result of the stress concentration accompanying the creation of the borehole. The evacuation of the compaction bands is brought about by the circulating drilling fluid flushing out debonded and often fragmented grains from within these bands (Haimson and co-workers, 2003-2007). The objective of this paper is to predict the conditions under which compaction bands are formed around boreholes. To this end, a new analytical model is formulated that enables prediction of the stress field around emptied and filled compaction bands, the various factors affecting the breakouts lengths, and their final length. Good agreement of the developed analytical model with experimental results obtained by Haimson and co-workers (Haimson and Klaetsch in Rock physics and geomechanics in the study of reservoirs and repositories, vol 284, pp 89-105, 2007; Haimson and Kovachich in Eng Geol 69: 219-231, 2003; Klaetsch and Haimson in Mining and tunneling innovation and opportunity, University of Toronto press, pp 13651371, 2002; Sheets and Haimson in Proceedings, paper ARMA/NARMS 04-484, 2004) is demonstrated. The presented study is of practical relevance: boreholes are often drilled deep into weak porous sandstone formations for the purpose of extracting oil and gas, and the question of borehole stability is crucial. In addition, borehole breakouts are often used to estimate the state of stress in the Earth's crust, and our new formulation will help improve these estimates.