Adjusting Bentonite Drilling Fluid for Adverse Conditions
Eleven months were needed to obtain the special permits required to undertake this project. All necessary precautions were taken to secure the drill site for containment of the drilling fluids and proper disposal of the spoils removed for the bore path. Special provisions were in place in the event that we would encounter inadvertent returns (frack-out) into Mission Bay.
High-yield bentonite drilling fluids were chosen to be used in a 4,000-gallon mud recycling unit. A question that faced the contractor was how the bentonite would perform in the sandy, silty soils that were saturated with saltwater. The bentonite drilling fluid was mixed to a 70-second viscosity with fresh water after the pH level had been adjusted to 10+ using soda ash. The first 80 feet to 100 feet went perfectly, with little or no changes in our drilling fluid pH, viscosity or gel strength. Between 120 feet and 150 feet, we noticed the viscosity of the drilling fluid returned from the pick-up pit to the mud recycling unit had dropped from 70 to 60, our gel strength was noticeably reduced and the pH had dropped to 8.5. We immediately adjusted the pH back to 10+, added bentonite to bring us back to a 70 viscosity, and added a natural cellulosic pac polymer, which further increased our viscosity to 80.
The purpose of the pac polymer was to help stabilize our bentonite from the adverse effect of the salty soil conditions -- which were under the bay at a depth of 50 feet -- and to tighten our filter cake to hold back any migrating waters and assist in gel strength. After drilling from 150 feet to 180 feet out, our fluids were again returned to the cleaning unit and tested. This time the viscosity had dropped form 80 to 75, there was little or no change in our gel strength and the pH had dropped to 9.5. We added fresh water to our mixing system, adjusted our pH to 10+, added bentonite until we hit a 70 viscosity and added a more pac polymer to reach a viscosity of 85.
We again drilled out another 30 feet with no problems and great returns, and we tested our fluids as they were being pumped to the cleaning system. This time we noticed only a slight change in our viscosity from 85 to 83 and no change in our gel strength, and our pH was at 9.8-10. We added a small amount of water, adjusted our pH, and added bentonite and pac polymer to increase the viscosity back to 85. We then drilled another 30 feet without any problems and tested our mud. Our mud had changed slightly as in the previous 30 feet. Drilling continued until we reached our exit point 1,500 feet out without any problems with the drilling fluid or inadvertent returns. The hole was reamed to 24 inches and the pipes were safely pulled in.
Had it not been for a closely monitored drilling fluids program, the viscosity, gel strength and filter cake properties of our bentonite would have been lost by the adverse salty soil conditions. The addition of the pac polymer -- along with maintaining a high pH -- enhanced our bentonite, which allowed more stability and a successfully completed project.