Wells included in this study are in the 9,500 to 12,500 ft range. The section is characterized by a long sand-shale sequence of recent to Miocene age formations from the surface to about 9,500 ft. Below, a predominately Oligiocene shale section of about 1,500 ft long is encountered prior to penetrating the normally pressured objective, the Camerina sand. It is this long shale section that has the tendency to slough and cause hole problems. The formations dip in excess of 50° and it is believed that the high dip angle is at least partially responsible for the heaving problems since equivalent age formations of normal dips have been drilled in other areas without problems. The temporary solution to the problem was raising the mud weight, to 12.5 pounds per gallon (ppg) opposite a 9.0 ppg equivalent formation. This contributed to increase drilling costs because of low penetration rates
Amoco's Tulsa Research group was asked to find a solution to the shale problem. One solution to the problem would be to add a material to the mud before the problem shale was penetrated to retard filtrate invasion along the microfractures and bedding planes. The research group indicated that Gilsonite , a naturally occurring mineral, dispersed throughout the mud system as fine particles, would extrude by a plastic-flow mechanism into the pores, bedding planes and microfractures. Once in place, it would reduce fluid invasion and bond the matrix of the formation to prevent sloughing. Looking at the well temperatures, the lab recommended a softening point Gilsonite around 338°F along with a special wetting agent they designed to prevent balling of the product. The research laboratory indicated that Gilsonite was superior from both a temperature and purity standpoint than the blown asphalts. The recognition by the lab of the formation sensitivity to Gilsonite softening point and the development of a good surfactant was the initial work on Gilsonite investigation which later led to the development of Bore-Plate , a blended Gilsonite with a strong surfactant.
As a result of the research center's recommendation, Gilsonite was used successfully in the mud system without encountering any problems of sloughing shale. In 1970, a decision was made to begin lowering the mud weights from 12.5 ppg in 0.5 ppg increments to determine if the shale would remain stable and determine what mud weights would be required using atreatment of 5.0 pounds per barrel (ppb) Gilsonite. 14 wells were successfully drilled with mud weights as low as 10.5 ppg with no serious problems resulting from shale sloughing. As a result, drilling penetrations were increased and borehole problems relating to shales were minimized.
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