Use of Mixed-Metal Oxide Water-Based Drilling Fluid System Increased Drilling Performance and Eliminated Mud Losses

Abstract

In a deep gas drilling project, the 22-in section across shallow fractured carbonates is drilled using an unweighted clay-water system incorporating up to 50-lbm/bbl bentonite. The main challenges comprise lost circulation, tight hole, and low penetration rates due to high clay content and lack of inhibition, resulting in geological complications and affecting the well delivery time. To seal off the large fractures in the lower-cretaceous limestones, the new drilling fluid was engineered with high thixotropic characteristics presenting a flat, shear-thinning rheological profile with low plastic viscosity, high yield point and flat gel strengths. The selection of candidate wells was supported by offset wells analysis considering drilling performance, penetration rate and footage achieved, and the likelihood of encountering losses. Fine-tuning of the fluid rheology was performed to effectively account for the probability of losses on each well and a fit-for-purpose drilling fluid formulation was designed. This innovative technology combining mixed-metal oxide with premium bentonite was run in a series of wells as a substitute to the previously used system. Due to its superior viscosity at low shear rates the fluid successfully prevented losses by gelling up in the interstices of the highly fractured limestone intervals. In addition, the fluid delivered higher drilling performance across the abrasive sandstone-clay intercalations and the hard carbonates toward the bottom of the section. By maintaining full circulation all way through and therefore avoiding the expenses associated with blind drilling and pumping mud cap, the initiative resulted in considerably lowering the fluid cost in this section. Significant operation time savings were also achieved by drilling the section faster to the intended casing point in a minimum number of runs. Enhanced wellbore condition that allowed the drill string to trip out on elevators instead of back-reaming also contributed to saving rig time. The casing could be run to bottom and cemented trouble free in one stage with cement returns to surface thus precluding the cost of stage collar tool in most of the wells. This paper unveils the facets of this versatile water-base fluid that was introduced as a solution to prevent losses and address poor drilling performance.