Since Gilsonite, a solid hydrocarbon, was introduced to the oil industry in August 1957 as a cement additive, several thousands of 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 reestablish drilling-fluid circulation. Designed primarily as a combination low-density lost-circulation slurry, Gilsonite 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 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.
Introduction of gilsonite un well cements
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 that is necessary for drilling and well completion. This latter type of formation will often break down or fracture under hydrostatic loading, resulting in 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 expenditures 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. Inasmuch as rather extensive information has been published¹²³ 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 restore 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, but it has been difficult to obtain optimum bridging and density without sacrificing other desirable slurry properties such as strength and resistance to corrosive fluids. Some of the more commonly used additives in this application should be mentioned briefly.
One material which has been used quite extensively is bentonite in concentrations from 0 to 12 percent by weight of the cement. The primary purposes of bentonite in cement are to decrease slurry weight and increase slurry volume. These properties are obtained because of the ability of bentonite to absorb relatively large amounts of water. This water, in excess of that required for the cement reaction, substantially reduces the strength of the cement and contributes to the lower resistance of this type of slurry to detrimental down-hole conditions such as corrosive waters and high temperatures.
Diatomaceous earth is another material that is principally used as a lightweight additive. Because of its extremely high surface area, large quantities of water are required to produce a pumpable slurry , with a resultant low compressive strength.
Actually even these two materials do reduce the strength of cement, they will generally provide adequate strength for normal cementing operations and their prevalent disadvantage for use in lost-circulation zones is the lack of coarse bridging particles. Incorporation of coarse additives into these slurries is possible, but may further reduce the compressive strength beyond that which is desired when sufficient quantities are used to effectively control lost returns in relatively severe areas. Materials such as ground nut shells, granulated plastics, and cellophane flakes have been used as bridging agents with excellent results where lost circulation problems were of a relatively minor nature, but they have not been too successful where major difficulties were encountered.
Moderate slurry weight reduction can be attained by the use of pozzolanic additives with cement. Because a pozzolan reacts with the hydrated lime from tile cement to yield cementitious compounds, relatively good strengths are realized and bridging materials may be used with this type of slurry to provide additional benefits. Expanded perlites have been used in the oil industry since 1951 for both lost circulation control and moderately light slurry weight, although in order to have a pumpable slurry extra mixing water is required. The reason for the additional water is that the expanded pellet contains small air cells and when subjected to pressure in a well the walls of these cells will break down and accept water from the slurry, causing a slight dehydration which might result in reduced pumpability if inadequate mixing water were used.
All these various types of slurries which have been discussed briefly have shown ability to control lost-circulation problems under some limited conditions and certain areas of the country may obtain adequate results from their use. However, none of them have shown the overall effectiveness, particularly in the severe lost circulation zones, that has been exhibited by the use of Gilsonite
Source of Gilsonite
Gilsonite is a unique additive for the oil industry's use to combat lost circulation conditions. This material is a solid hydrocarbon that occurs in the Uintah Basin of Utah and Colorado. It was discovered in 1885 by Captain Samuel Gilson, hence the name of the material, Gilsonite. It has been generally grouped as a form of native asphalt, but its unusual properties make it markedly different from the bitumens or asphalts which may range from liquid to definite solid form whether native or processed. Gilsonite is mineralogically classified as an asphaltite. This material was deposited in roughly parallel lines running from a northwest to a southeast direction and it occurs in vertical fissures rather than in beds, pools, or lakes where the bitumens and asphalts are found. Gilsonite also has many uses other than those being discussed here. Its properties make it valuable for both heat and electrical insulation applications, floor tile, roofing materials, acid-resisting paints, electrical-insulating varnish, battery boxes, and brake and clutch linings. It is used in underbody coatings for cars, printing ink, and other items where its black color can be used to advantage.
Gilsonite was first evaluated as a density-reducing additive for cementing slurries because of its low specific gravity (1.07) . Laboratory testing was set up to establish an optimum particle size for these applications with three primary points being considered:
1. Maximum size to permit pumping through down-hole cementing equipment such as float collars and multiple-stage collars .
2. Minimum size to provide excellent control of lost circulation zones .
3. Proper particle size distribution to obtain the two preceding objectives and to also allow slurry preparation with a minimum amount of mixing water.
Physical properties of the material ultimately selected are shown in Table 1.
Physical Properties of Gilsonite
Specific Gravity. . . . . . . . . . . . . . . . . . . .1.07
Bulk Density, lb per cu ft. . . . . . . . . . . . .50
Water requirement, gal per bulk cu ft. . . .2 (approx.)
Melting point, deg F. . . . . . . . . . . . . . . . .385 to 525
Following selection of a Gilsonite for use in cement slurries and further study of the material itself, it was anticipated that the following advantages could be attained:
1. Low slurry densities because of the specific gravity of the solid Gilsonite rather than because of addition of large volumes of water .
2. Higher compressive strengths at a specific slurry weight by virtue of adding solid particles instead of adding water.
3. Compatibility with existing retarders, accelerators, low-water-loss materials and other slurry additives inasmuch as the solid hydrocarbon would be chemically inert in a cement system.
4. Protection against premature dehydration of the slurry because the Gilsonite is impermeable and non-porous.
5. Equal or superior resistance to corrosive waters because the material itself is chemically resistant to both acidic and alkaline solutions.
6. Excellent control of lost-circulation zones with a granular bridging material strong enough to withstand moderate squeeze pressures and yet soft enough to permit pressure deformation providing an effective seal against loss.
Further laboratory tests were then instigated to determine the validity of these concepts , particularly compressive strength, compatibility with other additives, lack of porosity of the Gilsonite, and the effectiveness in sealing porous or fractured simulated formations. Test methods in general were those described in API RP 10B for compressive strength and thickening time and API RP 29 for lost circulation, although slight modifications were necessary because of the granular particles.
Compressive-strength tests were conducted using various ratios of Gilsonite with different basic cementing compositions under varying curing temperatures. The results of these tests show that at equal slurry weight, higher strengths are generally attained with Gilsonite, which permits the use of lower water ratios than do other low-density or lost-circulation-control additives .
Thickening-time tests were made on the Pan American High-pressure Thickening-time Tester at simulated well depths of 8, 000 and 10,000 ft to determine the effect of Gilsonite on the pumpability of neat cement, and cement containing bentonite, a calcium lignosulfonate retarder or a pozzolanic additive. There was virtually no difference in thickening times of these slurries containing 0 or 50 LB of Gilsonite per sack, as shown in Table 2, indicating the chemical inertness of Gilsonite and the practicality of using other additives in conjunction with it. Another series of compressive-strength tests was made using Gilsonite with calcium chloride as an accelerator. It was found that acceleration of compressive-strength buildup was obtained in about the same order of magnitude, percentage-wise, as occurs with other types of cementing slurries.
Density measurements were made on set specimens cured under 0 and 3,000 psi pressure to measure any difference which would result from compression of air in void spaces and indicate porosity within the particles of Gilsonite There were no measurable differences discovered in this group of tests thereby leading to the obvious conclusion that no voids exist within the Gilsonite particles.
The ability of Gilsonite to control lost-circulation problems was tested by the two general methods outlined in API RP 29. Effectiveness in porous formations was measured on a graded-limestone gravel bed having a permeability of approximately 450 darcys while fractures were simulated by use of slots of various widths machined in metal slugs. Complete duplication of field conditions existing in lost circulation zones is very difficult and the results of these tests are not to be construed as absolutely valid data with respect to what volume of loss might be expected in a well. However, as a comparative method for evaluating lost-circulation materials the gravel bed has been found to be useful from the standpoint that a material which performs best during the tests will generally provide superior results In the field. Table 3 shows a portion of the data obtained during these tests comparing Gilsonite to expanded perlite, an additive which has probably been more widely used as a bridging agent to reduce slurry loss than any other prior to the introduction of Gilsonite. As concentration of the additives was increased, the Gilsonite definitely permitted less loss of slurry and in actual field usage Gilsonite has also been superior in providing maximum fill-up through lost-circulation zones.
Laboratory tests have also been performed to determine the effect of various oils on Gilsonite cement since the Gilsonite itself, being a hydrocarbon, is soluble to a certain extent. It has been found that Gilsonite is more soluble in solvents such as kerosene and naphtha than it is in various crude oils which were available for conducting tests. Tests were then made wherein a Gilsonite cement column was placed in the annular space between 1-in. and 3-in. pipe and cured for 3 days; a hole was drilled through both pipes and the cement, and kerosene was pumped through the simulated perforation for a measured period of time at 4 gal per min. The samples, after testing, were sawed in two. During the limited time of the test there was no detrimental effect noted on the cement, and there was only a slight dissolving of exposed Gilsonite particles. It is anticipated that there will be very little effect beyond tile exposed particles of Gilsonite because of the cement boundary between particles, but further testing is being considered for longer periods of time and at higher temperatures than the room temperature used during this first series of tests.
The effect of high temperatures with regard to softening and agglomeration of the Gilsonite has also been studied. Up to temperatures of 240 F. , with pure Gilsonite being tested, there was no apparent sticking together of the separate particles, indicating no appreciable softening of the material. Because of the temperature reduction during circulation, there should not be any formation of a plastic mass while circulating Gilsonite at bottom-hole static temperatures of 300 F. or slightly above .
As would be expected inasmuch as the specific gravity of Gilsonite is very nearly the same as that of water, almost identical slurry factors are obtained upon the addition of Gilsonite as would be obtained with the addition of an equal weight of water. Volume-wise 1 bulk cu ft of Gilsonite (50 LB) plus the 2 gal of mixing water occupies 1.02 cu ft so that slurry yield with this additive is relatively high for each sack of cement used.
Gilsonite has been used in nearly all areas of the country for many different cementing operations. Table 4 lists tile types of jobs performed in the various areas. The amount of Gilsonite used on these jobs in respect to the type of cementing composition and other additives is given in Table 5.
A discussion of the different types of jobs in the various areas and a summary of the various well conditions encountered in the different locations where Gilsonite cement and other types of cement compositions have been used should be of benefit in assessing the potential value of this additive.
In the Oklahoma area most of the Gilsonite cement jobs that have been performed were squeeze jobs to correct inadequate fill-up. In one southern Oklahoma field a 1,500 psi dry test was obtained after a squeeze job using 25 LB of Gilsonite per sack of API Class A Cement with a low-water-loss additive and an accelerator. This successful repair job followed six other attempts with various types of cementing compositions during which it was impossible to attain a satisfactory squeeze pressure. Since this initial success, four other wells in the same field which were reported after Table 4 was prepared have been satisfactorily squeezed on the first attempt using the Gilsonite cement with squeeze pressures of 2, 000 to 4, 500 psi. These jobs were in a rotten shale section and consideration is now being given to using Gilsonite cement for primary cementing in an attempt to avoid the remedial jobs. In another field one zone in a well was to be isolated for saltwater disposal. After three unsuccessful attempts, with 100-to 150-sack batches of API Class A Cement with a low-water-loss additive each time, to obtain squeeze pressure through the perforations a blend of 15 LB of Gilsonite per sack of cement plus a low-water-loss additive was tried and a pressure of 4, 000 psi obtained after 15 sacks were displaced into the formation. Excess cement was backwashed and the squeeze tested to 3,000 psi. After cement had set under 1,500 psi pressure, no cement was found inside the pipe and the perforations were tested to 3,000 psi without any leaks. Other squeeze jobs have also been satisfactorily performed with Gilsonite cement after 2 to 5 failures with other types of cements , and as a consequence interest has definitely increased in the potential value of 10 to 25 LB of Gilsonite per sack of cement for this type of application.
Only a few Gilsonite cement slurries have been used in the north central Texas area inasmuch as lost circulation has not been a serious problem, but there was one very interesting operation performed. Insufficient fill-up was obtained on the primary cementing job without Gilsonite so 8 perforations were placed in the casing over a 2-ft interval above the top of the cement at approximately 3,850 it. A pozzolanic-cement slurry containing 100 LB of Gilsonite per sack and mixed at 10.8 LB per gal was circulated through these perforations to approximately 2,700 ft where a split was found in the 5 1/2-in. casing. The packer was then raised above this split and the remaining slurry displaced at this point to bring the cement column to 2,100 ft. The upper stage was overdisplaced with 5 bbl of water and the split casing was later repaired by squeezing . A significant observation from this job was that a relatively high concentration of Gilsonite was successfully pumped through 8 perforations and through a split in the pipe without any premature bridging.
In southern Arkansas there is a field which has from 2 to 5 gas pays and usually requires the use of a two-stage cementing collar plus considerable squeeze work to get proper fill-up and allow separate fracturing of each zone. Three Gilsonite-cement jobs with a pozzolan-cement blend have been done in this area without the benefit of a stage collar and with adequate fill-up being obtained and no squeezing required to segregate the producing intervals. Other jobs in this general area have utilized Gilsonite for its bridging effect in squeeze cementing , with all jobs reported thus far being achieved satisfactorily.
The Texas Gulf Coast area has primarily used this material for plugback operations for lost circulation in fractured and faulted formations where several wells have been lost. Very good results have been obtained in restoring circulation. In another field Gilsonite slurries have been the only type which would permit circulation while setting surface and conductor pipe .
West Texas is a notorious area for lost circulation problems and Gilsonite has contributed materially to successful cementing operations in this area. In one field considerable trouble is encountered when setting production pipe because of two very weak formations. Prior to the introduction of Gilsonite all types of bridging materials and lightweight slurries had been tried with at least two or three stages being required. Most of these jobs showed fluctuations in pressure while pumping which indicated the breaking down of the weak formations while cement was being circulated. Five single-stage jobs have been reported using Gilsonite with a steady buildup of pressure from the time cement reaches the formations until the top plug was landed. Temperature surveys indicated fillup of 90 percent or better on each of these jobs whereas other types of slurries required more slurry volume and staging in order to attain this fillup.
The majority of the jobs in the New Mexico area using Gilsonite cement have been setting production liners in gas wells where extremely weak formations exist. The first two jobs run in this area were on gas-drilled wells at a depth of approximately 6,000 ft. with a 5 1/2 in. production liner being set in a 6-3/4-in. hole. Both jobs were cemented using API Class A Cement containing Gilsonite. After the liner was cemented the drill pipe was raised to reverse out any excess cement slurry above the top of the liner with Gilsonite-cement slurry being returned to the surface on both jobs, indicating that cement had completely circulated the liner. Calculations on both jobs indicated that only 25 to 35 percent excess volume of Gilsonite cement was needed to circulate the liners compared to 200 percent excess which was used with other types of additives without being able to circulate completely because of lost-circulatlon zones .
In another field in New Mexico cement could not previously be circulated on a long string because of a weak coal bed lying near the top of the producing zone. The wells in this area that have been cemented using Gilsonite have been very successful with Gilsonite slurry being circulated out during the jobs indicating no loss of slurry to the thief zones.
A considerable amount of Gilsonite has been used in the Texas Panhandle-Kansas area to combat severe lost circulation problems caused by extremely weak or porous formations and also to provide a better cementing job where lost circulation was not a problem .
Because of the difficulty encountered from lost returns while drilling in one field, a well finally had to be shut down because the hole could not be filled with mud containing other lost-circulation materials. The fluid level was approximately 650 ft. from the surface. Gilsonite cement was used to set a 7-in. casing at the depth of lost circulation and returns were established for the first time. Drilling was continued with returns being lost at four additional formations. In each instance API Class A cement containing 100 LB of Gilsonite per sack of cement was used in a plugback-cementing operation which allowed a standing pressure of 800 to 1,500 psi to be placed on each zone of loss. When drilling was resumed through each plug, complete circulation was regained. Many wells have been completed in this area using Gilsonite cement where prior to its introduction some of the holes had to be abandoned because of the severe lost-circulation formations encountered.
Nine wells have been completed in another field using a pozzolan-cement mixture containing Gilsonite in concentrations of from 25 to 30 LB per sack of the pozzolan-cement blend. Nearly every well in this field will be completed in two producing zones, which are either broken or fractured limestone embedded in fairly thick, rotten-shale sections .Prior to the use of Gilsonite, lost returns had not been as great a problem in setting of the production string as has been communication between zones caused by the channeling of the cement slurry or deterioration of the shale formation's strength by fresh water from the slurry during cementing . Consequently every well had to be block squeezed to isolate the pay zones for fracturing and acidizing . Because of the scouring action of the Gilsonite in the cement slurry, it appears to be helping remove the filter cake in the annulus during cementing. Nine wells have been completed in this field using Gilsonite cement and each has been successful in that no block squeezing was required either before or after fracturing and acidizing. Since this particular application has only recently been brought to our attention, we have been unable to secure laboratory data to support the field performance prior to preparation of this paper. However, testing is currently under way to provide more information along this line .
Fifteen unsuccessful squeeze jobs were performed on another well using many types of cementing slurries in an attempt to seal off the perforations. It was desired that the perforations should hold a standing pressure of 3,000 psi after drilling out. When the well was squeezed using a Gilsonite slurry , the perforations held a standing pressure of 2,500 psi after drilling out; however, it failed to hold a pressure of 3,000 psi. Another squeeze job was performed successfully against the existing Gilsonite cement with a standing pressure of 3,000 psi being attained. Although this Gilsonite-cement squeeze job itself did not hold the desired pressure in this particular well, it dld provide sufficient strength and bridging action for a second squeeze job to be successful where 15 previous squeezes were of no benefit.
1. Field results on over 100 primary-cementing jobs indicate that in lost-circulation areas where only 50 to 60 percent fill-up can be attained with other types of lightweight or lost-circulatlon additives, 80 to 90 percent fill-up can be realized by the use of Gilsonite in a cementing slurry . Also it has been found that 25 to 50 LB of Gilsonite per sack of cementitious material will generally be an optimum amount for controlling lost-circulatlon zones and that, where weak formations exist, the bridging properties of Gilsonite will allow use of 93.5 to 101.0 LB per cu ft (12.5 to 13.5 LB per gal) slurry weights for excellent cementing results whereas other types of slurries have not been satisfactory even at much lower weights .
2. In most areas where Gilsonite has been used it appears that its scouring action is very good in removing drilling fluid and filter cake from the hole, resulting in a better cementing job without the necessity of block squeezing . This information has been derived from field reports which indicate that squeeze jobs have not been required when Gilsonite cement was used for primary cementing in an area where squeeze cementing had previously been necessary for satisfactory producing-zone isolation. In other areas where this material has been used there have been no reports of squeeze operations on a Gilsonite cement primary-cementing job. Laboratory research has been instigated to evaluate more fully the advantage of Gilsonite in this respect.
3. Both laboratory and field data show that Gilsonite is compatible with existing cementing compositions and additives and the only precautions necessary are to minimize the use of high water-ratio additives which may materially reduce the strength and durability of the Gilsonite blends. Accelerators, retarders, and low-water-loss additives can and should be used when specific well conditions warrant them. Both laboratory investigations and field results on production pipe indicate the absence of serious Gilsonite cement deterioration because of hydrocarbon solubility. Separation of individual Gilsonite particles by relatively impermeable cement boundaries should prevent dissolving of Gilsonite except that which is exposed by perforating .
4. Compressive strengths attained with Gilsonite are higher than for other lightweight additives when comparison is made on the basis of slurry weight. The primary reason for this is because weight reduction is obtained by use of a low specific gravity solid rather than by large volumes of water .
5. Because of its excellent bridging properties, certain precautions should be recommended when using Gilsonite cement. Because it does have a low specific gravity and may float and accumulate in a slurry of low viscosity, water should not be used ahead of the slurry because of potential dilution and viscosity reduction. The preferred fluid ahead of Gilsonite would be a mixture of bentonite and water (300 LB per 15 bbl) . The slurry should also be mixed at the recommended weight throughout the job.
Because of the advantageous scouring action of Gilsonite in mud removal and the possibility of premature bridging, it is also believed that a bottom-cementing plug should not be necessary and that the use of wall-cleaning devices should be held to a minimum to reduce possible restrictions in the annulus. This is not intended to imply that wall scatters are unnecessary nor that they must be removed to allow circulation of Gilsonite. Gilsonite has been pumped without complication into annuli containing scatters, and this is primarily a warning against too-close spacing of these devices over exceptionally long intervals of pipe .
The authors wish to express their appreciation to the management of the Halliburton Oil Well Cementing Company for permission to prepare and publish this paper.
For release: May 1, 1959
NOTICE TO EDITORS: Permission is hereby granted to reprint this paper on or after May 1, 1959, providing that the auspices under which it was presented be conspicuously acknowledged, the authors' names and affiliation be stated, and that the original title, if not used, be stated in the acknowledgment . If reprinted in installments, the foregoing conditions apply to each installment.
GILSONITE--A UNIQUE ADDITIVE FOR OIL-WELL CEMENTS
Knox A. Slagle and L. Gregory Carter
Halliburton Oil Well Cementing Co.
For presentation at the
Spring Meeting of the Pacific Coast District
Division of Production
Baltimore Hotel, Los Angeles, Calif.
April 30-May 1, 1959
(The statements and opinions expressed herein are
those of the authors and should not be construed as
an official action or opinion of the Institute.)
1. Scott, P. P. , Jr. and Lummus , J. L: New Developments In The control of Lost Circulation, Petroleum Branch, AIME , Paper No. 516-G, 1955.
2. Liedholm C. C: When Lost Circulation Besets You, Oil Gas J. , July 2 (1956).
3. Clark, E. H. , Jr: You Can Reduce Lost Circulation Problems , Part 1, Oil Gas J. , July 2 (1956); Part 2, Oil Gas J July 16 (1956)
4. API RP 10B: Recommended Practice For Testing Oil-Well Cements; 7th Edn. , Jan. , 1958.
5. API RP 29: Recommended Practice on Standard Field Procedure for Testing Drilling Fluids, 4th Edn., May, 1957.