ناپایداری دهانه چاه در ماسه سنگ های متخلخل Berea

Borehole instability in high-porosity Berea sandstone and factors affecting dimensions and shape of fracture-like breakouts
Abstract
Stress-induced breakouts in vertical boreholes are failure zones caused by excessive compressive stress concentration at theborehole wall along the springline of the least horizontal far-field stress. Wellbores are sometimes drilled into aquifers or oilreservoirs that are weak, poorly consolidated, and highly porous sandstone formations, which are often conducive to breakoutformation. Breakouts are an expression of borehole instability and a potential source of sand production. On the other hand, thebreakout phenomenon can be used advantageously in obtaining an estimate of the in situ stress condition. The averageorientation of breakouts, as identified by borehole geophysical logging, is a reliable indicator of in situ stress directions. It hasalso been suggested that breakout dimensions could potentially be used as indicators of in situ stress magnitudes. The researchreported here has concentrated on the unique type of breakouts observed for the first time in high-porosity Berea sandstone.Drilling experiments in rock blocks subjected to critical far-field true triaxial stress regimes, simulating in situ conditions,induced breakouts that were unlike the ‘dog-ear’ ones previously observed in granites, limestones, and low-porosity sandstones.The newly observed breakouts were thin, tabular, and very long, resembling fractures that counterintuitively extendedperpendicular to the maximum principal stress. We found that a narrow zone ahead of a fracture-like breakout tip underwentapparent localized grain debonding and compaction. In the field, such zones have been termed ‘compaction bands’, and are asource of concern because in oil fields and aquifers they constitute curtains of low permeability that can impede the normal flowof oil or water. In order to determine whether a correlation exists between fracture-like breakouts and in situ stress, weconducted several series of tests in which the minimum horizontal and vertical stresses were held constant and the maximumhorizontal stress was increased from test to test. These tests showed strong dependence of the breakout length on far-field stress,signaling that potentially the ability to assess fracture-like breakout length in the field could be used to estimate in situ stressmagnitudes in conjunction with other indicators. Another series of tests revealed that breakout length increased substantiallywhen borehole diameter was enlarged. This result suggested that in the field, where wellbore size is considerably larger,fracture-like breakout could extend to sizable distances, creating a sand production hazard. Two series of tests, one to evaluatethe effect of drill-bit penetration rate, and the other to verify the drilling-fluid flow rate effect on breakout formation anddimensions yielded inconsistent results and showed no unique trends. Remarkably, fracture-like breakouts maintained aconsistent narrow width of about 5–10 grain diameters, irrespective of the test conditions. This characteristic supports thesuggestion that fracture-like breakouts are emptied compaction bands