Effect of hybrid water-based mud on the improvement of wellbore stability: Kolosh Formation in Iraqi Kurdistan Region

Most important aspect of drilling clay-rich formations is preventing the hydration and dispersion. Oil based drilling muds (OBM) have historically been the first choice for drilling these kinds of formations, however the usage of the OBM is currently forbidden in drilling operation due to environmental issues. As an alternative, many inhibitive water-based muds were proposed. This paper describes the experimental work carried out on ten shale samples to evaluate stabilization of shale and clay-fluid interactions. After mineralogical analysis of shale samples, swelling and hot-rolling dispersion test were conducted using fresh water, polyamine, sodium silicate and oil base mud. Mineralogical analysis results showed that the cutting samples of 3, 4, 5, 6 and 7 have the highest clay content. Cation exchange capacity (CEC) results indicated that the shale sample with high concentration of smectite recorded the highest CEC value as it has the ability to absorb water into its inter-layers and exchange cations. When shale samples were tested in linear Swell meter, it is expressed the lower swelling percentage for oil base mud and maximum for fresh water. However, it was found out that both sodium silicate and polyamine mud systems yield the same performances. Furthermore, dispersion results showed that the shale recovery percentage increased from 16.3% to 56.2% when sodium silicate mud was used instead of polyamine mud. For the fresh water and oil base mud, results confirmed findings of linear swelling meter test, which is the lowest recovery for the fresh water (6.7%), and maximum recovery for the OBM (109.9%). The improved swelling inhibition suggested that sodium silicate mud could be effectively used to control wellbore instability while drilling through the Kolosh Formation .