KEM-13 Drilling in carbonate stringers occurring in salt formations

Stringers are layers or fragments of rock consisting of minerals other than halite (salt) that are embedded in salt formations. Wells in the Netherlands must typically be drilled through the Zechstein salt formation, in order to reach the target hydrocarbon reservoir. Pore fluid pressures in a stringer are often anomalously high (“overpressure”), which may lead to loss of drilling fluids into the formation, or flow of gas or brine from the stringer into the borehole. Both types of events may affect well control and drilling safety.

The research request aims for an inventory of current state of the art knowledge and insights concerning type of fluids (water/brine, oil, gases) contained in “stringers’ or floater’ encountered in Salt formations in The Netherlands and the pressures to be expected for various stringers and stringer depths in relation to depth and location of the salt formations. The inventory consists of a literature search and request for stringer risk mangement from mining companies, a stringer risk mapping for The Netherlands and recommendation for best practices for managing stringer drilling risks.

FULL RESEARCH REQUEST

The KEM-13 research project “Stringers in Salt as a Drilling Risk"started in 2017. Intermediate results have been reported in several meeting with clients and the sector. The final report consists of an extensive powerpoint presentation.

RESEARCH PROJECT FINAL REPORT

The project concluded that most of the potentially problematic stringers are identifiable through proper interpretation of seismic surveys. However, there is no way to predict, a priori, the amount of overpressure to expect. Based on analysis of previously drilled wells, it was suggested that the best option for planning a well trajectory is to avoid folded, tilted stringers, and to avoid areas where the base Zechstein is faulted, or areas below the flanks of folded top Zechstein.

The KEM scientific expert panel evaluated the KEM-13 project (no separate report available).

The geological discussion of stringers in the Zechstein formation is considered to be of good quality. The statistical analysis of observed drilling events is not very rigorous or quantitative. For example, some of the text seems to be contradictory, or at the least, insufficiently clear. The amount of work produced in this project is commensurate with the time and cost of the project. Overall, it can be concluded that the project met its stated goals.

The project concluded that most of the potentially problematic stringers are identifiable through proper interpretation of seismic surveys. However, there is no way to predict, a priori, the amount of overpressure to expect. Based on analysis of previously drilled wells, it was - rightly - suggested that the best option for planning a well trajectory is to avoid folded, tilted stringers, and to avoid areas where the base Zechstein is faulted, or areas below the flanks of folded top Zechstein.