Gilsonite works as a borehole stabilizer through a mechanism that most laboratory tests cannot detect. It needs both temperature and pressure to become malleable and extrude into shale micro-fractures — so an additive tested at ambient conditions looks inert, while the same material stabilises a hole at 180 °F downhole. That gap between bench results and field performance is why the material was undervalued for years, and why downhole simulation testing changed the picture.
RAHA Gilsonite Co. supplies borehole stabilising grades across the softening point range, with the measured softening point stated on the batch certificate of analysis. The technical background below draws on published industry research; the specification detail is on our specification page.
What causes borehole instability
Before selecting an additive it is worth being precise about the failure mode, because gilsonite addresses some causes and not others. Instability has three distinct origins.
| Category | Mechanism | Does gilsonite address it? |
|---|---|---|
| Mechanical | Borehole erosion from high annular velocity, adverse hydraulic stress from high annular pressure, collapse from swab and surge pressures caused by excessive wall cake, erosion from drill string movement | Partly — the film on the wall resists erosion; a thinner cake reduces swab and surge |
| Chemical | Hydration, dispersion and disintegration of shale through clay interaction with mud filtrate | Partly — by sealing, it limits filtrate reaching the clay, but it does not inhibit swelling chemically |
| Physical | Spalling and rock burst from pressure imbalance, fracture and slippage along bedding planes in hard brittle shale, collapse of fractured shale above deviated holes | Yes — this is where it works best, bonding the matrix across micro-fractures and bedding planes |
The distinction that decides the additive. Chemical inhibitors and encapsulators work on swelling clay. Gilsonite works mechanically, by physically sealing and plugging. If the cuttings show chemical alteration, an inhibitor is the answer. If they do not — and the problem is tectonic stress, bedding plane slippage or erosion — a chemical inhibitor will not fix it, and an asphaltite will.
How gilsonite stabilises a borehole
The proposed mechanism has been consistent in the literature for decades, and core examination under high-powered microscopy supports it.
Gilsonite is added to the mud system before the problem shale is encountered. As the bit penetrates the formation, the material reaches the newly exposed face. Under sufficient temperature and pressure it becomes malleable and extrudes by plastic flow into pore spaces, micro-fractures and bedding planes.
Two things follow from that. Inside the rock, the extruded material bonds the matrix — it mends or holds the clay platelets together, preventing further disintegration and slippage. At the face, the particle plates onto the borehole wall and forms a tough film that strengthens the wall against the erosional effect of circulating fluid.
Why solubility works against stabilisation
This is the counter-intuitive part, and it is the reason sulfonated products behave differently.
| Additive type | Behaviour at the core face | Result |
|---|---|---|
| Gilsonite (insoluble asphaltite) | Extrudes under temperature and pressure, penetrates under 2 mm, forms a thin inter-matrix filter cake and plates on the surface | Definite plugging action |
| Air-blown asphalt (insoluble) | Similar mechanism but lower softening point and more impurities | Plugs, but less effectively at elevated temperature |
| Sulfonated blown asphalt, ~70% soluble | Dissolves and invades deep into the matrix rather than bridging it | No plugging action observed |
A product treated for high dispersibility and solubility disperses beautifully into the mud and then passes straight through the formation face. It does not plug and it does not form an inter-matrix cake. The insolubility that makes gilsonite harder to mix is the same property that makes it seal.
Why ambient laboratory tests mislead
Gilsonite and the insoluble blown asphalts require both temperature and pressure before extrusion can occur. Most standard test procedures run at ambient temperature and low pressure.
The consequence is systematic: results are skewed toward additives that control shale by chemical reaction, because those function at room temperature. The comparison is not like for like. An additive whose mechanism needs 180 °F to activate scores poorly against one that works at 70 °F, and the ranking inverts downhole.
Two procedures were developed to correct this.
| Test | Method | What it discriminates |
|---|---|---|
| HTHP fluid-loss cell with Berea core | Standard HTHP cell modified to use a 0.5-inch Berea sandstone core as the filter medium; additive dispersed in base mud, temperature and pressure set, core sliced and examined afterwards | Depth of invasion, plating versus inter-matrix cake versus flow-through. Separates gilsonites of different softening point from blown and sulfonated asphalts |
| Downhole simulation cell (DSC) | Triaxial apparatus that drills through a shale core up to 6.5 inches in diameter while circulating fluid at up to 350 °F | Actual hole enlargement under simulated drilling conditions, including mechanical versus chemical stabilisers side by side |
Published work on this includes Cagle and Schwertner on gilsonite for sloughing shales (Oil and Gas Journal, 1972) and Simpson, Dearing and Salisbury on downhole simulation cell behaviour (IADC/SPE 17202, 1988). Further joint industry work was carried out under DEA Projects 22 and 38.
What the downhole simulation testing showed
Pierre shale cores were exposed to circulating drilling fluid for 45 hours. The results below are the headline figures.
| Condition | Circulating temperature | Hole enlargement after 45 h | Film observed? |
|---|---|---|---|
| Base fluid, no gilsonite (control) | 180 °F | 1.25 in to 2.25 in — 80% increase | No |
| 6 lb/bbl blended gilsonite | 180 °F | No enlargement — bit teeth marks still visible | Yes — thin, soft, tacky malleable film |
| 6 lb/bbl blended gilsonite | 125 °F | 1.56 in average — 25% increase | No |
| Low softening point grade (330 °F) | 125 °F | Near gauge, no enlargement | Yes |
| Treatment after 15 h of instability | 180 °F | 2.0 in against 2.25 in control | Yes, but gummy and less tough |
| 4% gilsonite suspension in liquid | 180 °F | 0.25 in over initial — 20% increase | No plating observed |
The two findings that change how you specify. First, match the softening point to the circulating temperature, not to the maximum bottomhole temperature. The standard grade failed at 125 °F while the 330 °F low softening point grade held the hole near gauge at the same temperature — the material has to soften to work. Second, pretreat. Treating after instability appeared still reduced enlargement, but only from 2.25 to 2.0 inches, against zero enlargement when the additive was in the system from the start.
Core examination
Cross-sections of the test cores showed a thin inter-matrix filter cake forming with the gilsonite products, with penetration limited to under 2 mm and a thin film on the core surface. Definite plugging action was observed.
Cores run with a sulfonated blown asphalt at roughly 70% solubility showed no plugging action at all. In alumina core work with SEM and EDX, drilling fluid solids penetrated no more than about 200 microns — 0.2 mm — and the same shallow depth held even at 250 °F, indicating the material remains far from fluid at that temperature.
Field results
Laboratory findings are supported by well histories across different geological environments.
| Location | Problem | Treatment | Result |
|---|---|---|---|
| South Pass, offshore Louisiana | Torque and drag on wells deviated about 30° from vertical, conventional lignosulfonate mud | 3 lb/bbl blended gilsonite-lignite from about 10,600 ft | Torque 1,400 to 900 ft-lb (36% reduction); drag 60,000 to 30,000 lb (50% reduction). Caliper logs showed substantial improvement |
| Eugene Island, offshore Louisiana | Hole enlargement averaging 50% in a 12.25-inch hole | 4% gilsonite suspension added to conventional lignosulfonate mud at the casing point | Enlargement in the 8.5-inch hole reduced to 15% average, with rheological and fluid loss control as well |
| Evanston, Wyoming | Instability in the thrust fault hinge line — excessive torque and drag, bridges after trips, reaming. Cuttings showed no chemical alteration, pointing to tectonic stress | YP raised from 7 to 15 lb/100 ft² for hole cleaning, then 5 lb/bbl blended gilsonite | Hole stabilised within two days; drilling continued without further instability |
The Wyoming case is the most instructive of the three. The mud was already an inhibitive KOH-gypsum system and the cuttings ruled out chemical alteration, so adding more inhibitor would not have helped. The cause was mechanical, and the fix was a mechanical stabiliser.
Selecting a grade
| Well condition | Grade to specify | Reasoning |
|---|---|---|
| Low circulating temperature, under about 150 °F | Low softening point, around 330 °F | The material must soften to extrude. A high softening point grade stays rigid and only plates, if that |
| Normal circulating temperature | Standard, around 370–400 °F | The general-purpose range; extrudes readily at typical downhole conditions |
| HPHT wells | High softening point, 400–430 °F+ | Holds structure at elevated bottomhole temperature rather than flowing away |
| Known problem shale ahead | Any matched grade, added as pretreatment | Pretreatment gave zero enlargement against 2.0 inches for remedial treatment |
| Instability already developing | Matched grade at 3–4 lb/bbl minimum | Still mitigates, though the film forms gummy rather than tough |
| Torque and drag on a deviated hole | Blended grade | Field-proven at 3 lb/bbl for 36% torque and 50% drag reduction |
Concentration below the effective threshold does not give partial performance — it gives no film. The published minimum is 3 to 4 lb/bbl, with 5 to 6 lb/bbl used where instability is expected or already present.
Properties
| Property | Value |
|---|---|
| Classification | Asphaltite — naturally occurring solid carbonaceous material |
| Purity | Relatively pure hydrocarbon, without significant mineral impurities |
| Specific gravity | 1.05 |
| Softening point | 330 °F to 430 °F+ depending on grade |
| Acid value | Low |
| Iodine number | Zero |
| Solubility | Soluble in aromatic and aliphatic hydrocarbons; insoluble in water |
| Water dispersibility | Requires a sulfonated form for water based systems |
Frequently asked questions
How does gilsonite stabilise a borehole?
Mechanically rather than chemically. Under sufficient temperature and pressure it becomes malleable and extrudes by plastic flow into pore spaces, micro-fractures and bedding planes, bonding the shale matrix so clay platelets are held together. At the same time it plates onto the borehole wall, forming a tough film that resists erosion from circulating fluid. It does not inhibit clay swelling — that is a different mechanism requiring a different additive.
Why do ambient laboratory tests undervalue gilsonite?
Because the mechanism requires both temperature and pressure to activate. At ambient conditions the material stays rigid and does not extrude, so it scores poorly against chemical inhibitors that work at room temperature. The ranking inverts downhole. Evaluation needs a downhole simulation cell or an HTHP fluid-loss cell with a core as the filter medium.
What concentration of gilsonite is needed?
3 to 4 lb/bbl is the published minimum effective concentration. Field cases used 3 lb/bbl for torque and drag reduction and 5 lb/bbl where instability was already developing; downhole simulation work used 6 lb/bbl for pretreatment. Below the threshold the film does not form at all, so underdosing gives no partial benefit.
Which softening point should be specified?
Match it to the circulating temperature, not the maximum bottomhole temperature. In testing, a standard grade allowed 25% hole enlargement at 125 °F circulating, while a low softening point grade around 330 °F held the hole near gauge at the same temperature. For HPHT wells the 400 to 430 °F+ range is appropriate. The material has to soften to work.
Is it better to pretreat or to treat after instability appears?
Pretreat. In controlled testing, gilsonite in the system from the start gave no measurable hole enlargement over 45 hours, while treatment begun after 15 hours of instability reduced enlargement only from 2.25 to 2.0 inches. Remedial treatment still works and is worth doing, but the film that forms is gummy rather than tough.
Why does a soluble sulfonated asphalt not stabilise as well?
Because solubility and plugging are opposed. A product treated for high dispersibility and solubility disperses easily into the mud and then invades deep into the formation rather than bridging the face. Core cross-sections of a roughly 70% soluble sulfonated blown asphalt showed no plugging action. The insolubility that makes gilsonite harder to mix is the property that makes it seal.
How deep does gilsonite penetrate the formation?
Under 2 mm in Berea core testing, with a thin inter-matrix filter cake and a film on the surface. In alumina core work examined by SEM and EDX, drilling fluid solids penetrated no more than about 200 microns, and the same shallow depth held at 250 °F. Shallow penetration is the objective — deep invasion means the material is not sealing.
Can gilsonite fix chemically induced shale instability?
Only partly, and indirectly. By sealing the formation face it limits the filtrate reaching the clay, which reduces hydration. But it does not inhibit swelling chemically. If the cuttings show chemical alteration, an inhibitor or encapsulator is the primary answer. Where cuttings rule out chemical alteration and the cause is tectonic stress, bedding plane slippage or erosion, gilsonite is the correct choice.
Does gilsonite also reduce torque and drag?
Yes. In a South Pass field comparison offshore Louisiana, adding 3 lb/bbl of blended gilsonite-lignite from about 10,600 ft reduced torque from 1,400 to 900 ft-lb, a 36% reduction, and drag from 60,000 to 30,000 lb, a 50% reduction. The comparison well was already running conventional and bead-type lubricants for the same purpose.
Does it work in water based mud?
Yes, but the sulfonated form is required, because untreated gilsonite is a hydrocarbon and will not disperse in water. See water based muds for the explanation. In oil based and synthetic systems the untreated material is used directly.
What forms and particle sizes are supplied?
Granular and micronized powder, with mesh sizes of 30–40, 100, 200, 300 and custom cuts. Softening point, ash content and particle size distribution are adjusted to the well conditions, and the measured softening point is stated on the batch certificate of analysis rather than quoted as a range.
What should be confirmed before ordering?
The measured softening point of the batch, not the nominal grade range; the circulating temperature the material will see; whether the problem is mechanical or chemical in origin; and whether the material is going into a water based system, in which case the sulfonated form is needed. Send us those four and we will confirm the grade.
Related pages
Drilling applications: gilsonite in drilling fluids, water based muds, oil based muds, deepwater drilling, gilsonite in cementing, oil and gas overview and bentonite.
Product information: what is gilsonite, specification, properties, powder and granular, price, MSDS and HS code.
Specify a borehole stabilising grade
Send us the circulating temperature, the mud system, the shale conditions and whether you are pretreating or responding to instability already underway. We will confirm the softening point grade, state the measured value on the batch certificate of analysis, and accept third-party inspection by SGS, Bureau Veritas or the surveyor named in your contract.
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