Gilsonite Cement Additive
Gilsonite Cement Additive-A Unique 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 surf ace, intermediate, and production pipe in both single and multiple stages as well as various remedial jobs such as squeezing, le-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 whre only 50 to 60 percent fill-up was possible with other types of slurries.
The unique properties of Gilsonite such as 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 by Gilsonite is suitable for blocking or plugging an abandoned pipeline or back filling a mine shaft, tunnel or excavations 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 1500 kg/m3, and exhibits good compressive strength.
In the formulation of the cementing composition of the product, 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 utilized, 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 particle size distribution of the Gilsonite determine the strength and porosity-permeability characteristics of the set cement for any given mix ratio.
Where maximum strength is desirable, a coarse Gilsonite Where lightest weight and lowest porosity-permeability are important and strength is to be sacrificed or is of little importance, an aggregate of minus 50 mesh or finer may be used.
Conditions are often encountered in the field requiring various combinations of particle size and particle size distribution. The above examples represent extremes. The mix must, however, 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 impending pump ability.
For example, a cement-gilsonite ration of 1:4, using the coarse aggregate specified above, is difficult to pump and is likely to plug restricted passages in the system, whereas the same mix, using the fine aggregate specified above, will never plug if the water-cement ratio is high enough.
An amount of a petroleum solvent which depends 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, thus preventing corrosion and minimizing pulling away of the cement from the casing and/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.
Gilsonite Cement Additive
INTRODUCTION
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 operation These problem formations may range from either porous or cavernous formation to very weak formations that are unable to support the hydrostatic head that is 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 operation may produce many problems In the normal course of completing a well. Increased expenditures can result from reduced drilling finishing 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. Inasmuch as rather extensive information has been Published 1,2,3 on lost circulation while drilling, these Investigations have been directed primarily to the second phase, lost circulation while cementing, although there is some overlapping In areas where cementing slurries with excellent bridging properties may be used advantageously to rest. ore mud circulation Attempts have been made to restore circulation while cementing using many types of materials 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 of these methods. Combinations of these two approaches have also been utilized.
Gilsonite Lost-Circulation Additive
Gilsonite cement additive is an asphaltene hydrocarbon in granular form. Its particle size varies between 4– and 100-mesh. Gilsonite cement additive is commonly used to control lost circulation. It is effective at bottomhole temperatures (BHTs) between 60° and 230°F (16° and 110°C). Typical additive concentrations range from 5– to 50-lb/sk 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.
The primary use for Gilsonite is in restoring lost circulation due to 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. Thus, a dense deposit is formed which is sealed by the cement. Decreasing the slurry weight by using an extender helps in controlling lost circulation by reducing the hydrostatic pressure.
Gilsonite use as an extender
Gilsonite can be used to lighten the slurry and increase the slurry yield but will still provide a relatively high-strength set cement. Large amounts of water are not required for Gilsonite. The reduction of slurry density is primarily the result of the low specific gravity of the Gilsonite.
Gilsonite mix water requirements
One extra gallon of water is normally used for each 25 lb of Gilsonite. Normally p-eba 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 be blended into the slurry without the use of p-eba.
Gilsonite 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 indicated the cement containing either Gilsonite or ground coal extender have higher strengths 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 systems without the Gilsonite.
Features of Gilsonite cement additive
Gilsonite additive’s low specific gravity 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
Benefits of Gilsonite cement additive
Gilsonite additive can provide the following benefits:
• When perforated, it is shatter-resistant.
• It does not significantly affect the setting time of cement.
• Gilsonite additive can provide higher strength than heavier additives with high water requirements.
