Gilsonite cement additive was introduced to the oil industry in August 1957, and within a few years over 200 jobs had been performed with it. Designed primarily as a combination low-density, lost-circulation slurry, it has yielded excellent results in areas of incompetent formations as well as in other types of lost-circulation zones. Field results generally show fill-up of 80 to 90 percent in areas where only 50 to 60 percent was possible with other types of slurries.
Gilsonite cement additive — a unique material for oil well drilling
Abstract
Since gilsonite, a solid hydrocarbon, was introduced to the oil industry in August 1957 as a cement additive, over 200 jobs have been performed using the material. These operations have included primary cementing through lost-circulation zones of surface, intermediate and production pipe in both single and multiple stages, as well as various remedial jobs such as squeezing, re-cementing above inadequate fill-up, and plugging back to re-establish drilling-fluid circulation.
Designed primarily as a combination low-density, lost-circulation slurry, gilsonite cement additive has yielded excellent results in areas of incompetent formations as well as in other types of lost-circulation zones. Field results generally show that fill-up of 80 to 90 percent can be obtained in areas where only 50 to 60 percent fill-up was possible with other types of slurries.
The unique properties of gilsonite — low specific gravity, particle-size distribution, impermeability, resistance to corrosive fluids, chemical inertness and low water requirements — result in a slurry having exceptional bridging properties, low slurry weight, compatibility with other slurry additives, and relatively high compressive strength when compared to other slurries of the same weight.
A cement produced with gilsonite is suitable for blocking or plugging an abandoned pipeline or back filling a mine shaft, tunnel or excavation. It contains Portland cement or a mixture of at least two components selected from Portland cement. A cementitious slurry formulated from the cement mix may have a density less than or equal to 1,500 kg/m³ and exhibits good compressive strength.
In the formulation of the cementing composition, it is preferable to employ gilsonite in an amount ranging from approximately one-half to approximately ten times by volume the amount of the cement utilised, depending upon the particular result desired. The lower range is employed where maximum strength is important; the higher range where the various qualities imparted by the gilsonite are most important.
Particle size and mesh selection
Particle size and particle size distribution of the gilsonite determine the strength and porosity-permeability characteristics of the set cement for any given mix ratio.
| Objective | Particle size | Result | Trade-off |
|---|---|---|---|
| Maximum strength | Coarse gilsonite | Highest compressive strength in the set cement | Higher slurry weight; pumpability limits the amount that can be carried |
| Lightest weight, lowest permeability | Minus 50 mesh or finer | Lowest density and porosity-permeability | Strength is sacrificed |
| Bridging across a loss zone | Graded 4 to 100 mesh | Coarse and medium particles bridge; fines are retained in the network | Requires the full distribution rather than a single cut |
| Field conditions in between | Custom blend | Combination of the above characteristics | Must be specified per well |
Conditions are often encountered in the field requiring various combinations of particle size and particle size distribution. The examples above represent the extremes.
The mix must always be pumpable through the system from the mixing point to the final point of placement of the cement slurry. The coarser the aggregate, the less that may be present in any given slurry without impeding pumpability. For example, a cement-to-gilsonite ratio of 1:4 using coarse aggregate is difficult to pump and is likely to plug restricted passages in the system, whereas the same mix using fine aggregate will never plug if the water-cement ratio is high enough.
Petroleum solvent addition
An amount of petroleum solvent, depending upon the amount of gilsonite present, may be added to the wet or dry mix for wetting the surface of the gilsonite particles and causing them to form an intimate bond with casing and earth formations of the bore hole. This prevents corrosion and minimises pulling away of the cement from the casing or bore hole wall by reason of the shrinkage normal to setting of the cement.
Instead of adding the solvent directly to the mix, it may be pumped through the casing and into the cementing zone in advance of the gilsonite-cement slurry.
Introduction: why low-density cementing slurries matter
As the oil-producing industry has continued to grow, the need for a low-density cementing slurry possessing lost-circulation control characteristics has become more and more evident. This is especially so in primary cementing because of the different types of formations being encountered and the need to reduce remedial cementing operations.
These problem formations may range from either porous or cavernous formations to very weak formations that are unable to support the hydrostatic head necessary for drilling and well completion. This latter type of formation will often break down or fracture under hydrostatic loading, resulting in the partial or complete loss of circulation.
Lost-circulation zones encountered during drilling operations may produce many problems in the normal course of completing a well. Increased expenditure can result from reduced drilling rates, fishing jobs and other mechanical difficulties, as well as from loss of large volumes of drilling fluid. Sometimes severe lost-circulation problems may even cause abandonment of a well.
Lost circulation during cementing operations will often be reflected by inadequate fill-up in the annulus and the consequent displacement of slurry into formations away from the well bore. Satisfactory isolation of the different formations may then require re-cementing work above the point of loss and subsequent squeeze-cementing jobs.
Extensive information has been published on lost circulation while drilling. These investigations have been directed primarily to the second phase — lost circulation while cementing — although there is some overlap in areas where cementing slurries with excellent bridging properties may be used advantageously to restore mud circulation.
Attempts have been made to restore circulation while cementing using many types of material. Sometimes sufficiently good results have been obtained simply by reducing the slurry density, whereas other instances have occurred where bridging materials were found to be most helpful but there still remained zones of loss which would not react satisfactorily to either method. Combinations of the two approaches have also been utilised.
Gilsonite as a lost-circulation additive
Gilsonite cement additive is an asphaltene hydrocarbon in granular form. Its particle size varies between 4 and 100 mesh. It is commonly used to control lost circulation and is effective at bottomhole temperatures between 60 °F and 230 °F (16 °C and 110 °C). Typical additive concentrations range from 5 to 50 lb per sack of cement.
The low specific gravity of gilsonite additive helps improve its ability to control lost circulation. However, this feature can also cause the additive to separate to the top of thin slurries and slurries containing dispersants. Adding 2% or more bentonite to the slurry will help prevent separation.
How the bridge forms. The primary use for gilsonite is in restoring lost circulation through the bridging action of the angular gilsonite solids at the point of lost returns. Its effectiveness is due to the particle-size distribution: the larger and medium-size particles bridge, forming a network which retains the finer particles. A dense deposit is formed which is then sealed by the cement. Decreasing slurry weight with an extender assists further by reducing hydrostatic pressure.
Gilsonite as a slurry extender
Gilsonite can be used to lighten the slurry and increase the slurry yield while still providing a relatively high-strength set cement. Large amounts of water are not required. The reduction of slurry density is primarily the result of the low specific gravity of the gilsonite itself.
Mix water requirements
One extra gallon of water is normally used for each 25 lb of gilsonite. Normally a dispersant-type additive is required to prevent gravitational separation of a material having such a wide variance in density from the slurry. Because such a small amount of gilsonite is required, it can usually be blended into the slurry without one.
Thickening time and compressive strength
Gilsonite is an inert solid and, owing to the small amount of additional water required, does not appreciably change the thickening time of the slurry.
Higher compressive strength is generally attainable when solid particles are added to a slurry without adding excessive quantities of water. Laboratory tests indicate that cement containing either gilsonite or ground coal extender has higher strength at all ages than most other available lightweight or lost-circulation slurries at the same slurry weight — although the strength is less than that of the same neat cement system without the gilsonite.
Properties, features and benefits
| Property | Effect in the slurry |
|---|---|
| Low specific gravity | Reduces slurry density and hydrostatic pressure on weak formations. Also the cause of separation in thin slurries |
| Angular particle shape | Bridges across the loss zone rather than flowing through it |
| Graded particle size distribution | Coarse particles bridge, fines are retained, forming a dense deposit |
| Impermeability | The deposit seals rather than filters |
| Chemical inertness | Does not react with cement chemistry or appreciably alter thickening time |
| Resistance to corrosive fluids | Durable in aggressive downhole environments |
| Low water requirement | One extra gallon per 25 lb, so strength is preserved |
| Compatibility with other additives | Blends with standard slurry systems |
Benefits of gilsonite cement additive
Shatter-resistant when perforated
The set cement does not shatter around perforations, which protects zonal isolation during completion.
Does not delay setting
Being chemically inert and requiring little extra water, it leaves thickening time essentially unchanged.
Higher strength than heavy alternatives
Provides greater compressive strength than heavier additives that carry high water requirements.
Improved fill-up
Field results show 80 to 90 percent fill-up where other slurries achieved only 50 to 60 percent.
Wide temperature window
Effective from 60 to 230 °F, covering most primary and remedial cementing conditions.
Dual function
Acts as both an extender and a lost-circulation material in a single addition.
Troubleshooting
| Symptom | Cause | Remedy |
|---|---|---|
| Additive floats to the top of the slurry | Low specific gravity in a thin slurry or one containing dispersants | Add 2% or more bentonite |
| Slurry plugs restricted passages | Coarse aggregate at high concentration — a 1:4 cement-to-gilsonite ratio is difficult to pump | Use a finer cut, reduce concentration, or raise the water-cement ratio |
| Set cement weaker than expected | Fine aggregate selected, or excessive water added | Move to a coarser cut where strength governs |
| Cement pulls away from casing | Normal setting shrinkage without surface wetting of the particles | Add petroleum solvent to the mix, or pump it ahead of the slurry |
| Loss zone not bridged | Single-size material rather than a graded distribution | Specify the full 4 to 100 mesh range so coarse particles bridge and fines are retained |
Frequently asked questions
What is gilsonite cement additive?
An asphaltene hydrocarbon in granular form, with particle size between 4 and 100 mesh, added to oil well cement slurries. It serves two functions at once: it reduces slurry density as an extender, and it controls lost circulation through the bridging action of its angular particles. It has been in use in the oil industry since 1957.
What concentration of gilsonite is used in cement?
Typical additive concentrations range from 5 to 50 lb per sack of cement. Where the formulation is expressed by volume rather than weight, gilsonite is employed at approximately one-half to ten times the volume of cement, depending on the objective — the lower range where maximum strength matters, the higher range where the qualities imparted by the gilsonite matter more.
What temperature range does gilsonite cement additive work in?
It is effective at bottomhole temperatures between 60 °F and 230 °F, which is 16 °C to 110 °C. That range covers most primary cementing and remedial work.
How much extra mix water does gilsonite require?
One extra gallon of water for each 25 lb of gilsonite. This low water requirement is one reason compressive strength is preserved: adding solid particles without adding excessive water generally yields higher strength than lightweight systems that depend on high water content.
Why does gilsonite separate to the top of the slurry?
Because of its low specific gravity, which is the same property that makes it effective at reducing slurry density. Separation occurs mainly in thin slurries and those containing dispersants. Adding 2% or more bentonite to the slurry prevents it.
Does gilsonite affect cement thickening time?
Not appreciably. Gilsonite is an inert solid and requires only a small amount of additional water, so the thickening time of the slurry remains essentially unchanged. This is a practical advantage over additives that interfere with cement hydration.
What particle size should be specified?
It depends on the objective. Coarse gilsonite where maximum strength is required. Minus 50 mesh or finer where lightest weight and lowest porosity-permeability matter and strength can be sacrificed. For bridging across a loss zone, the graded 4 to 100 mesh distribution is needed, because the larger particles bridge and the fines are retained in the network they form.
Why can a 1:4 cement to gilsonite ratio plug the system?
Because the mix must remain pumpable from the mixing point to the point of placement, and the coarser the aggregate, the less can be carried without impeding pumpability. At a 1:4 ratio with coarse aggregate, the slurry is difficult to pump and likely to plug restricted passages. The same ratio with fine aggregate will not plug provided the water-cement ratio is high enough.
What is the petroleum solvent for?
It wets the surface of the gilsonite particles so they form an intimate bond with the casing and the earth formations of the bore hole. That prevents corrosion and minimises the cement pulling away from the casing or bore wall through the shrinkage normal to setting. It can be added to the wet or dry mix, or pumped through the casing ahead of the slurry.
What slurry density can be achieved?
A cementitious slurry formulated with gilsonite may have a density less than or equal to 1,500 kg/m³ while still exhibiting good compressive strength. That combination is what makes it suitable for weak formations that cannot support a conventional hydrostatic head.
Can gilsonite cement be used outside oil wells?
Yes. A cement produced with gilsonite is suitable for blocking or plugging an abandoned pipeline, or for back filling a mine shaft, tunnel or excavation. The same properties that make it useful downhole — low density, bridging, chemical inertness — apply in those applications.
What mesh sizes do you supply?
We supply gilsonite in lump, granular and micronized forms, with mesh sizes of 30–40, 100, 200, 300 and custom cuts to specification. Ash content, softening point and particle size distribution can be adjusted to match your cementing programme. Send us the specification and we will confirm what meets it.
Related pages
Drilling and cementing applications: gilsonite in cementing, gilsonite in drilling fluids, oil and gas applications and bentonite.
Product information: gilsonite overview, specification, properties, micronized powder, lump gilsonite, price, MSDS and HS code.
Other industrial uses: asphalt modification, printing inks, paints and coatings and foundry.
Specify gilsonite for your cementing programme
Send us the bottomhole temperature, the slurry density target, the mesh requirement and the quantity. We will confirm the grade, supply a batch certificate of analysis before loading, and accept third-party inspection by SGS, Bureau Veritas or the surveyor named in your contract.
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