New Laboratory Tests Evaluate the Effectiveness of Gilsonite as a Borehole StabilizerOnly recently have new laboratory techniques been developed to evaluate Gilsonite under simulated downhole conditions. Innovative procedures using a newly-built downhole simulation cell tested Gilsonite under temperatures and pressures similar to drilling conditions in which the product would normally be used. These innovative tests indicate that borehole enlargement was minimized using Gilsonite while substantial enlargement was measured using the same drilling fluid system without Gilsonite.
Gilsonite theoryAs stated previously, Gilsonite is classified as an asphaltite and is a relatively pure hydrocarbon without significant amount of mineral impurities. Gilsonite used for oilfield purposes is mined from an area around Bonanza, Utah, and has a specific gravity of 1.05 with a softening point ranging from 370°F to 400°F, although a lower softening point (330°F) material is available. It has a low acid value, a zero iodine number, and is soluble in aromatic and aliphatic hydrocarbons. It is processed and ground to where 80% passes through an 80 mesh screen with approximately 5% being retained on a 100-mesh screen and 30% on a 200-meshscreen.(6,10)
For many years Gilsonite and other asphaltic - type products have been used in water - base drilling fluids as an additive to assist in borehole stabilization. It has been well d ocumented that these additives can minimize hole collapse in formations containing water - sensitive, sloughing shales.
The causes of borehole instability are numerous. The reasons for the instability can be mechanical, chemical, or physical in nature. Th e mechanical problems include borehole erosion by high annular velocities, adverse hydraulic stresses due to high annular pressures, hole collapse from high swab and surge pressures due to excessive wall cake, and stressed erosion due to drill string movem ent. Chemical alteration problems include hydration, dispersion, and disintegration of shales due to the interaction of clays with mud filtrate.
Physical instability problems include the spalling and rock bursts of shales due to subnormal pressure or over pressure relationships of hydrostatic and formation pressures. Fracture and slippage along bedding planes of hard, brittle shales, and the collapse of fractured shales above deviated holes are also physical problems encountered while drilling troublesome s hales. This problem also occurs in non - deviated holes while drilling over pressured shales.
Borehole instability problems are often referred to as sloughing, heaving, spalling, or over - pressured shales, mud balls, mud rings, and many other descriptive name s. There are many solutions to this problem. The use of additives to inhibit or partially inhibit the swelling of clay has been well documented .
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