Improving the Wellbore Stability While Drilling Long Open Hole Shale Intervals in Tahe Oil Field

Abstract

Abstract In order to accelerate the drilling speed and reduce the drilling cost, the well structures were simplified in Tahe oil field in recent years. As a result, the length of open hole section increased dramatically, even as high as 4700m for some wells, and different formations including active shales, gypsum, and limestone were encountered in the drilling process. Drilling in such long interval of open hole undertook the risks of cavings, stuck pipe, hole enlargement, lost circulation and so on because of the complex formation and changing pressure gradients. Therefore, highly inhibitive water-based drilling fluids were essential to improve the wellbore stability of the challenging sections. To understand the mechanisms of shale-drilling fluid interactions when exposed to water-based drilling fluids, the morphology and mineralogy of shale samples were characterized via scanning electron microscope (SEM) and X-ray analysis. Meanwhile, inhibitive evaluation methods such as swelling test, dispersion test and cation exchange capacity (CEC) test were carried out to analyze the interaction potential of shale samples from the open hole intervals. The results indicated that for the upper interbedded sandstone and shale formation, impressive content of clay minerals (mainly smectite) and weak bonding were contributed to the high potential of swelling and dispersion. While for the deep brittle shale formations, the content of clay mineral (mainly illite and illite/smectite) was also relatively high, and micro-fractures were observed, which provided the access of water invasion. After analyzing the wellbore instability mechanism of the open hole interval, different drilling fluid strategies were proposed according to the reactivity of formations. For shallow formation, polyamine polymer water-based drilling fluid was selected to suppress the hydration and dispersion of reactive shales. While for the deep formation, polyamine and sulfonated polymer water-based drilling fluid was optimized, in which polyamine as shale hydration inhibitor, sulfonated asphalt and superfine calcium carbonate as microfracture sealing agent were used in combination to realize the chemical and physical stability of brittle shales. The optimized inhibitive water-based drilling fluids in laboratory were successfully transferred to the application in the long open hole intervals of Tahe oil field. The field application in this area resulted in reduced cost, improved gauge hole and reduction in total NPT. No complicated problems occurred in the field trial. The application indicated that elaborately designed drilling fluids reached satisfying results.