Applications of strongly inhibitive silicate-based drilling fluids in troublesome shale formations in Sudan

For several decades, wells drilled in Sudan Block 6 have experienced serious hole-instability problem related to drilling fluids due to the highly reactive and dispersive shales existing in the Aradeiba and Abu Gabra formations. The problem included washed out hole sections combined with tight hole, as well as serious sloughing shale. Frequent wiper trips were frequently required and logging of the wells was not usually successful.Previously, several conventional inhibitive water-based drilling fluids such as KCl/polymer, KCl/lime/polymer, and KCl/PHPA have been used in this area, but with only marginal improvements in hole stability and drilling performance. Recently, a newly formulated KCl/sodium silicate system, which overcome the fatal drawbacks of the silicate-based mud systems used previously and characterized by the perfect rheological properties and filtration control, was developed and applied to provide the necessary inhibitive character. The first trial well was drilled with this kind of drilling fluid in Sudan Block 6 and the following benefits were observed: (1) Excellent integrity exhibited by drilled cuttings for shale formations; (2) Stable borehole kept with lower mud weight; (3) Reduced wiper trips; (4) In-gauge borehole shown from caliper logs.Later on, five more wells were drilled with the KCl/sodium silicate drilling fluids in this area. It was found that this kind of drilling fluid could fully meet the demands of drilling operations in this area. Case studies were presented in this paper in terms of wiper trip, inhibitive character of cuttings, hole conditions and mud weight. This newly formulated silicate-based drilling fluid is currently applied in Block 3/7 and Block 6, Sudan widely.