Several challenges are experienced with wellbores when drilling in shale above and within reservoirs. These are especially prevalent in deviated wells and wells through depleted intervals caused by some years of production.
Drilling the well to total depth of the wellbore section might be easy. However, in some cases depending upon formation properties and drilling conditions it is a big challenge to come out of the hole and subsequently run casing or liner. It is of great importance to take the right actions regarding recommended downhole parameters like mud circulation rate, RPM on drillstring and pulling speed.
Some of the wellbore aspects to discuss are: unstable hole, small margins between mudweight and collapse pressure, what is the right technique to use with the drillstring, flow rates and finally oil based mud versus water based mud. In case of a marginal downhole situation, it would be beneficial to test out some effects in a laboratory beforehand.
Such questions were discussed and became an issue within a joint industry project on borehole stability in shales run at SINTEF Petroleum Research (SPR). A modified test cell was designed and used with the purpose of systematically testing these effects in the laboratory.
This paper describes how this equipment was designed and tested in order to achieve reliable laboratory results as well as results from initial work on outcrop shale. The main objective was to a certain extent to visualize some effects in a laboratory and conclude with some good recommendations regarding drilling procedures on the rig offshore. Thus for testing some of the down hole effects, a long period of qualification tests was gone through. This was important before real tests could be done on shale at simulated downhole condition. Testing was performed on hollow cylinder samples.
The initial results show that it is possible to run such laboratory simulations. It is possible to a certain extent to visualize some of the effects with varying flowrates and RPM on a drillstring with stabilizer. The laboratory tests so far suggest that high RPM on a drillstring can have destabilizing effects on a wellbore through a section with shale that has been exposed to very close to collapse conditions, calling for optimized drilling procedures in situ.