Utilizing Borehole Stability Modeling to Optimize Mud Weight Window for Drilling in Shale Formations in an HPHT well

Abstract

Det norske oljeselskap ASA is planning to drill an HPHT well on PL341Stirby during the 3rd quarter of 2010. Several operators have experienced challenging formations related to borehole stability when drilling in this area during the last 3 decades.

The aim of this master thesis was to investigate drilling experiences in Stirby’s offset wells, and by use of offset data utilize borehole stability modeling to make recommendations towards an optimized drilling procedure for the Stirby well in terms of a safe mud weight window and borehole stability for the 17 ½“ section. The formations evaluated are chosen from troublesome shale formations from the offset analysis.

In the simulations performed, wellbore data from Stirby has been assembled with log data from offset wells to better predict a stable mud weight window to avoid possible borehole instabilities. The obvious limitation to the simulations performed in this thesis is missing core data for the formations evaluated. Cores may help operators to provide important formation input data to PSI, to better predict a stable mud weight window, to be able to quantify the possibility of borehole instabilities when drilling in these troublesome shale formations. The uncertainties related to missing core data to provide more accurate formation input data should be assessed more thoroughly in the future.

The scope of work in this master thesis was to make final recommendations towards the safe mud weight window to ensure borehole stability when drilling through the troublesome shale formations in the Rogaland Group for well 24/12-6S, Stirby. Due to the diversity in the results given by the simulations in PSI it is hard to make a final and precise conclusion. Based on offset analysis, borehole stability software and drilling experiences from the former operators, this master thesis concludes that the 24/12-6S, Stirby well is in risk of experiencing borehole instabilities related to sloughing shales in the Balder, Sele and Lista Formations within the 17 ½” section.