| dc.description.abstract | Shale makes up more than 70% of drilled formations, and about 90 % of all wellbore stability problems occur when drilling in shale. Drilling with water based mud in deep waters and high deviation, extended reach drilling for enhanced oil recovery make the conditions even worse.The high expenses due to drilling problems in shale have led to increased focus on wellbore stability analysis in recent years.
The wellbore instabilities are experienced as time-delayed failure, which can not be explained completely with existing borehole stability models. More work is therefore needed to reveal the time effects in the stability analysis. It has been shown that temperature has a weakening effect on shale strength. However, no thorough laboratory testing has been performed to explain the phenomena satisfyingly. The temperature dependence is interesting both in terms of strength results from laboratory testing and in-situ temperature changes caused by circulating drilling fluid.
Triaxial testing at elevated temperatures is time-consuming and expensive and limited test material is available. Performing strength measurements at various temperatures on small samples is therefore an attractive alternative. Small samples accommodate faster, more available and less expensive tests. The shale puncher is developed by SINTEF Petroleum Research, to allow for shear strength measurements on small samples.
In this work, a new procedure and new equipment have been developed to allow for punch tests at various temperatures. The procedure includes normalization in brine to obtain chemical equilibrium prior to punching and to avoid capillary effects biasing the results.Modifications on equipment accommodate temperature control and accurate temperature measurements.
A pilot study has been performed to evaluate the applicability of shale punch measurements for the investigation of temperature dependent shale strength. The current results are promising and indicate a weakening effect of temperature, which is supported by available data from triaxial tests on shale and other rocks at elevated temperature. The results indicate that the specific layered structure of shale, with bound water in interlayer spaces, is especially sensitive to temperature at moderate temperatures
The observed temperature dependence implies that laboratory measurements of shale strength should be performed under downhole temperature, or be corrected for in-situ temperature. For the latter purpose, work is needed to investigate if satisfyingly accurate correlations exist.Furthermore, when drilling long sections, the upper strata are heated and consequentlyweakened in time. Therefore, the temperature dependence of shale strength may at least be acontributing factor for time-delayed failure. Measures to reduce heating in the upper strata should therefore be considered to mitigate the weakening effect, when drilling long sections. | nb_NO |
