Tests performed in the O-G-S Downhole Simulation Cell (DSC) using cores of various age and hardness in a Bore-Plate®- treated mud system indicate that Bore-Plate(a patented blend of specially-treated Gilsonite grades and other additives) can reduce the effects of drilling fluid filtrate invasion. In water based mud systems, filtrate invasion can destabilize the shales and cause many problems such as sloughing shale, borehole enlargement, stuck pipe, and excess bridging during trips. Bore-Plate, used as a pretreatment suggested at 5.0-6.0 pounds per barrel (ppb) in any water-based mud system can reduce or eliminate these problems. The following three case histories illustrate the use of Bore-Plate not only as a shale stabilizer but also as a lubricant to reduce torque and drag .
In the South Pass area, offshore Louisiana, Bore-Plate was used to reduce torque and drag. Normally wells in this area are deviated approximately 30° from vertical and are drilled with a conventional lignosulfonate mud. On well A, conventional and bead-type lubricants were used for torque and drag reduction. On well B, 3-4 ppb of Bore-Plate were added at approximately 10,600 feet measured depth. By comparison, the torque on well B was reduced on an average from 1400 foot-pounds to 900 foot-pounds, a 36% reduction, and drag from 60,000 pounds to 30,000 pounds (a 50% reduction). Caliper log comparisons from the two wells indicated substantial improvement with the use of Bore-Plate.
Another well was drilled in the Eugene Island area of offshore Louisiana. While this area is not known for borehole instability, hole enlargement, however, is a problem. Prior to the introduction of Bore-Plate, hole enlargement averaged about 50% in the 12.25 inch hole. At casing point, 4% Bore-Plate, suspended in a water-based solution, was added to a conventional water-based solution. Hole enlargement in the 8.5 inch hole was reduced to an average of 15%.
On a third well, in Evanston, Wyoming, borehole instability was encountered. The shale sections are noemally hard and brittle, normally Cretaceous or older in age. The area is in the thrust fault zone known as the hinge line. In this part of the field, hole instability is frequently encountered. On this well, excessive torque and drag, bridges after trips, and reaming problems continued to increase. An inhibitive, KOH-gypsum mud system was being used and chemical alteration was not indicated from the cuttings. An on-site analysis by Chevron employees suggested that tectonic stresses probably caused the borehole instability problem. A hole cleaning program was initiated by first viscosifying the mud by increasing the YP from 7 to 15 lbs/ft2. Secondly, 5 pound per barrel of Bore-Plate was added to the system. Torque and drag were reduced substantially. Bridging and reaming after trips were no longer encountered. Within two days, hole stabilization was achieved and drilling continued without further instability problems. On other wells drilled in troublesome shale areas, mainly in the older shale rocks, Bore-Plate in concentrations of 5-6 ppb has been used in various mud systems, including polymer muds and salt-treated systems.
These well histories confirm the results of the DSC testing of Bore-Plate conducted by Chevron. First Bore-Plate, used in sufficient concentrations, provides borehole stability, reduces hole enlargement significantly, and reduces torque and drag. Secondly, Bore-Plate should be used as a pretreatment to seal off filtrate invasion as the virgin shale is drilled. The additive plugs microfractures, bedding planes, and pore spaces and deposits a thin film on the borehole wall which mitigates borehole erosion. It has been proposed that this film deposits an impermeable membrane which does not allow either filtrate invasion or cation exchange. In test run on the Downhole Simulation Cell, neither filtrate invasion nor cation exchange was observed if treated prior to the drilling of the shale section. However, Bore-Plate can be used successfully even after borehole instability has occurred and will seal off the microfractures and pore spaces of the shale.