Drilling problems are frequently encountered throughout the Tertiary section in the North Sea. The problems are mainly to be associated with smectitic shales and tuffaceous sediments. Typical problems are drill tool sticking and lost circulation. The causes of borehole instability can be both of chemical and mechanical nature. Possible causes are fracturing of the formation, borehole collapse and creep or swelling in the borehole wall. Borehole instability take place even with inhibitive oil-based mud systems, so part of the problem is clearly of a mechanical nature. In the paper, we present an experimental investigation of mechanical strength and elastic parameters of shales at stress levels corresponding to depths of wellbore instabilities. Four different clays were selected as analogue materials to those encountered in North Sea Tertiary formations. The pre-failure, peak strength and post failure behaviour of these shale materials have been studied.
Borehole instability has been a significant contributor to unproductive drilling time during the history of drilling for hydrocarbons. As a consequence, drilling costs have been unacceptably high. This fact has led the oil drilling industry to allocate increasing resources on trying to understand, alleviate and solve the problem of borehole instability. While drilling, unstable boreholes cause a variety of problems. The problems are most likely caused by fracturing of the formation, borehole collapse, large deformations and creep in the borehole wall and destructive rock-fluid interactions between the formation and the mud-system. These destructions result in excessive washing and reaming, hole "packing off", lost circulation and stuck pipe, and in the worst case losing the hole. Observations of drilling problems on the Statfjord field indicate that initial tight hole sections after some time may develop into an oversized borehole. This situation is probably due to further collapse in the borehole Wall. For the purpose of keeping the wellbore mechanically stable, increased mud weight has proven to enhance the situation. Using conventional methods for estimating pore pressures, it has been shown that troublesome shales very often are "overpressured", i.e. its pore pressure has attained a higher pressure level experienced at an earlier stage in the rocks stress history. This means that these shales normally are drilled "underbalanced". Another factor that has proven to reduce borehole instability is to minimize the sail angle through the particular hole sections of troublesome shales. Optimization of casing setting depths with respect to geology/ lithology, formation strength and pore pressure, will also be a contribution to reduced borehole instability problems during drilling. As of today, oil based muds are regarded to be the most inhibitive mud with respect to highly reactive clays, which often is dominant in troublesome shales. Use of oil based mud will reduce the destructive chemical effects of interactions between the rock and the mud. The result is that the wellbore will be closer to the gauge size, which in turn contributes to optimum hole cleaning and improved drilling conditions. To be able to predict potentially instable formations, it is of vital importance to know certain rock parameters.
