For many years, Gilsonite had been recognized by the industry as a solution to shale sloughing, hole enlargement, and borehole instability. In the early 1980s, at the request of American Gilsonite Company, owned by Chevron Corporation, the Chevron Drilling Fluids at the Drilling Technology Center, investigated how Gilsonite works under downhole conditions. A review of the testing equipment available at that time revealed there was no equipment that could accurately evaluate the action of Gilsonite and asphalts under downhole conditions. It was only until the mid-1980's the O-G-S Laboratory developed this type of equipment . The equipment could measure the instability of shales caused by filtrate invasion under downhole pressures and temperatures by drilling a borehole through the shale and circulating heated drilling fluid for up 50 hours.
Gilsonite plug off the core bore
The initial tests performed in the Downhole Simulation Cell (DSC) on a Pierre Shale core indicated that Gilsonite could plug off the core bore spaces and microfractures and deposit a thin film on the surface which would not allow any filtrate invasion . Tests showed no softening of the shale, no cation exchange, and no erosion of the borehole. A similar core sample was run using the same mud system without the Gilsonite and indicated a 45% washout and filtrate invasion. Using this success, the study was continued looking at the temperature limitations of low and high softening point Gilsonites, and evaluating various surfactants, and additives to permit easy of mixing in fluid systems.
With the success of the study, American Gilsonite invented a new Gilsonite mixture which could perform excellently in all environments. The new product, Bore-Plate , was developed by blending several softening point Gilsonite resins, adding a chemical for easy dispersion in any mud system, and treating the Gilsonite with a strong surfactant that provided excellent wettability and rewettability. The new product not only worked well to stabilize sloughing shales, but was also found to greatly reduce borehole washout, prevent differential pressure sticking, provide some lubricity in shale and sand zones, and is effective as an ecologically sound drilling fluid additive for use in environmentally sensitive areas.
A field trial of this invention, Bore-Plate, was run on an offshore well in the Gulf of Mexico. The field trial was very successful with the well reaching its total depth of 8400 feet with no problems. After surface casing was set at 4500 feet, the mud system was broken over to a lignite/polymer mud containing 2 ppg of the Bore-Plate additive. No mixing or coalescing problems were observed while adding the mixture to the system. Additional concentrations were maintained at a level of 2.5-3.0 ppb. Mud weights ranged from 9.7 to 10.7 ppg. Hole angle averaged 21 degrees. In this directional well, compared to surrounding wells, no excessive torque or drag were experienced.
Offset well caliper logs were compared with the well using Bore-Plate, wells using asphalts or blown asphalts, and other commercial uintaites. The borehole on the well using Bore-Plate experienced 15% washout compared to the 30-40% washout of the surrounding wells using other materials. Additional tests performed in the DSC using younger or Gulf Coast shales indicated that 5.0-6.0 ppb of Bore-Plate is necessary for proper treatment. Gulf Coast shales because of their quick reaction with water will have some filtrate invasion, compared to the older shales, before the Bore-Plate film can be deposited, however laboratory and field tests indicate that Bore-Plate is better than other water-based additives. Bore-Plate has been found to be effective in any water-based mud system, be it either fresh or salt water, polymer or deflocculated, low weight or high weight systems. It is recommended that Bore-Plate be added as a pretreatment of 5-6ppb prior to drilling into the troubled shale interval for best results.