Gilsonite as a Borehole Stabilizer: Evaluate Its Effectiveness and Benefits

by N. Davis II and C.E. Tooman, Chevron Services Co.

SPE Members

Copyright 1988, IADC/SPE Drilling Conference

This paper was prepared for presentation at the 1988 IADC/SPE Drilling Conference held in Dallas, Texas, February 28-March 2, 1988

This paper was selected for presentation by an IADC/SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers or International Association of Drilling Contractors and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the IADC or SPE, its officers, or members. Papers presented at IADC/

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Abstract

Gilsonite resin is a naturally occurring mined carbonaceous material classified as an asphaltite. For many years, Gilsonite has been used in drilling fluids as an additive to assist in borehole stabilization. It has been well documented that this material works efficiently to minimize hole collapse in unstable shale sections. However, because Gilsonite is an asphaltite with a high temperature softening point, it has been difficult to duplicate its mending action in laboratory tests conducted at ambient temperatures and low pressures.

Only 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.

Another novel test procedure was developed to discriminate between various types of Gilsonite products to determine the most effective product under differing temperature and pressure environments. This procedure made use of a high temperature, high pressure fluid loss cell using Berea cores as a filtering media. Scanning electron microscopic examination of the test cores from both testing procedures provided insight into the mechanisms of how Gilsonite provides stability under downhole conditions.

These results are discussed in this paper. Field data from wells drilled in widely differing geological environments support the conclusions reached from the laboratory tests.

Introduction

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 documented that these additives can minimize hole collapse in formations containing water-sensitive, sloughing shales.¹²³

The causes of borehole instability are numerous. The reasons for the instability can be mechanical, chemical, or physical in nature. The 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 movement. 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 overpressure 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 shales. 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 names. There are many solutions to this problem. The use of additives to inhibit or partially inhibit the swelling of clay has been well documented. The adjustment of hydraulic

conditions is another solution to reduce mechanical alteration. Knowing and controlling the pore pressure of the problem formations is used often. In this paper, the use of Gilsonite and asphaltic-type additives to minimize physical alteration, and to some extent the chemical alteration, will be discussed.

Evaluating the effectiveness of Gilsonite and other asphaltic-type products in the laboratory has been difficult since most test procedures are performed at ambient temperatures and low pressures.(4) Recently, equipment has been designed to study interaction of drilling fluids with formation rock in the laboratory under simulated downhole conditions.(5) One procedure using this downhole simulation cell has been used to evaluate various water, base and oil-base mud systems on shale stability

under downhole conditions. This joint industry project was sponsored through the Drilling Engineering Association, titled DEA Project 22. A similar project soon to be conducted as DEA Project 38 will study performance of a group of asphaltic-type products and Gilsonite in various muds and on different types of shale. Both projects contain proprietary information and will not be discussed in this paper. Chevron Services has conducted an independent series of tests focusing on Gilsonite. The results of these tests will be discussed.

Gilsonite is a naturally-occurring, solid carbonaceous material which is classified as an asphaltite. It is a relatively pure hydrocarbon without significant amounts of mineral impurities. Gilsonite has a softening point around 370°F.(6) Other asphaltic-type additives including air-blown asphalts and sulfonated air-blown asphalts have a softening point in excess of 240°F. Some of these additives have been treated with a surfactant to provide better water-dispersibility or sulfonated to provide varying degrees of solubility. According to the suppliers of Gilsonite and other asphaltic-type products, these additives are used to help control sloughing shale problems by minimizing shale slippage along microfractures or bedding planes by physically sealing and plugging.(7)

Several manufacturers of asphaltic-type products and several oil companies have attempted to compare the effectiveness of these products with other products which minimize shale sloughing problems by chemical alterations.(8,9) These tests are conducted in a laboratory under ambient temperatures and low pressures. Because Gilsonite and some asphaltic-type products require temperature and pressure to be effective, the results of these tests are skewed toward those additives which control shale problems by chemical reaction. These tests do not compare “apples with apples”, but are really comparing “apples with oranges”.

The Drilling Mud Services Section at Chevron’s Drilling Technology Center initiated a study of Gilsonite and asphaltic-type products on an “apples with apples” basis. Products were evaluated using existing testing procedures under ambient temperatures: such as, triaxial testing for shale stability, lubricity evaluation, and effects on filtration control. The results of the initial test series indicated that a more in-depth evaluation was needed under conditions in which the additives begin to function properly. Several new test procedures were designed which included the use of an altere

d high-temperature, high-pressure fluid loss cell, as well as downhole simulation cell. The results of these tests indicate that these procedures can be used effectively to evaluate and discriminate between Gilsonite and asphaltic-type products. In addition, the use of the downhole simulation cell allows the user to discriminate or rank the effectiveness of Gilsonite and asphaltic-type additives with additives that to minimize borehole instability through chemical inhibition problems. These procedures allow the user to compare not only “apples with apples” but “apples with oranges”.

Theory

As 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)

Gilsonite and asphaltic-type materials have been used for many years to stabilize sloughing shales to reduce borehole erosion. It is proposed that the material, added to a mud system prior to encountering a problem shale, would penetrate the shale pore spaces, microfractures, and bedding planes as the bit is penetrating the formation. By a plastic-flow mechanism, Gilsonite would extrude into the pores, fractures, and bedding planes to reduce or minimize filtrate and whole mud invasion, and thus bond the matrix to prevent sloughing. Cagle and Schwertner reported that Gilsonite was superior to the blown asphaltic-type additives because of its higher softening point and fewer impurities. Their paper contained photomicrographs of drilled cuttings and cores which showed Gilsonite extruding into pore spaces and fractures. In addition, it has been proposed that Gilsonite plates out on the borehole wall, creating a thin film. Sulfonated, blown asphalts, however, penetrate deeper into the fractures, due to their high solubility, and do not “plate” as well as the insoluble products.

Testing Procedures

It has been difficult to duplicate the effectiveness of Gilsonite as a borehole stabilizer in the laboratory, as seen in the field. Several companies have published data conducted at ambient temperatures and pressures.(8,9) Many of the tests referenced were or are designed for additives which stabilize shale or clay by partial inhibition or encapsulation. Since Gilsonite and the insoluble blown asphalts require both temperature and pressure for extrusion to occur, these additives do not compare favorably in ambient, low pressure tests. Therefore, to effectively evaluate Gilsonite and asphaltic materials, tests must be designed using higher temperatures and pressures.

The first test, designed and run by Chevron, used existing laboratory equipment. A NL Baroid High-Temperature/High-Pressure fluid loss cell was modified to use a thin 0.5 inch Berea sandstone core as the filter media.(appendix 1) Gilsonite or asphaltic-type materials to be tested were dispersed into the base mud slurry. The mud was then placed into the cell, the temperature and pressure adjusted to specific parameters, the test run, and filtrate collected and measured. Upon completion of the test, the core was removed. After cooling, the core was sliced and examined under a high-powered microscope. The depth of invasion was measured. Using the same slurry composition, additional tests were performed in which the temperature and pressure were increased to determine effective temperatures at which Gilsonite or the asphaltic materials would extrude into the void spaces of the core and whether a thin, inter-matrix filter cake would develop, or continue to further invade the core. This test is useful to discriminate between insoluble manufactured products such as Gilsonites with different softening points, blown asphalts, and soluble sulfonated blown asphalts. The test can be used to select additives that seal off shale micro-fractures and form a plating film at the face of the borehole.

Following the series of tests which used the HTHP filter press/Berea core, Chevron initiated a study of Gilsonite using the Downhole Simulation Cell (DSC).(5) The apparatus was designed and built by TerraTek Inc., Salt Lake City, under the direction of the O’Brien-Goins-Simpson and Associates, Houston. The DSC is a triaxial testing apparatus which can drill through a shale core as large as 6.5 inches in diameter while circulating fluids at temperatures up to 350°F. A schematic of the cell is shown in Figure 1. The complete description of the apparatus can be found in the Simpson, Dearing, Salisbury paper.

For our purposes, the Pierre shale was used. Table 1 shows a complete mineral analysis of the Pierre Shale core. Table 2 shows the composition of the circulating fluid and the Gilsonite additive, and Table 3, the testing parameters. The first test run confirmed that the DSC could evaluate the effectiveness of Gilsonite under simulated drilling conditions. After this initial success, a series of tests were designed to focus on Gilsonite’s effectiveness under varying temperature and pressure conditions. The investigation also included an evaluation of pretreatment versus treatment after instability occurred and an evaluation of low, normal, and high softening point Gilsonites. The results of this testing program, which will be discussed later in the paper, showed the DCS to be a very effective apparatus for evaluating borehole stability.

A third test was designed to evaluate how Gilsonite works. An attempt was made to shed some light on the mechanisms by which Gilsonite decreases fluid loss to the formation by looking at the drilled shale samples with a scanning electron microscope (SEM). Small pieces of shale were taken from the wellbore edge of the shale which had been drilled with the drilling fluid containing added Gilsonite as previously described. None of the resulting photomicrographs show any detectable Gilsonite, even though the samples were taken from very close to the wellbore edge. Unfortunately, Gilsonite is not easily distinguished in drilling fluids by SEM methods. This particular Gilsonite contains a small amount of silicon which can be used as an energy dispersive x-ray (EDX) tracer. But when the Gilsonite is added to a clay base drilling fluid and used to drill a clayey shale, the silicon from the Gilsonite gets lost in the background.

Thus a third laboratory experiment was designed to determine the mechanism by which Gilsonite decreases fluid loss to the formation. In this case, artificial all-alumina core plugs were injected with Gilsonite-containing fluid at 800 psi across the plug until flow was essentially stopped. Then, small samples of the mudcake and underlying plug were carefully removed and prepared for SEM and EDX study.

The two fluids used in this test were (1) Gilsonite suspended in water and (2) the same Gilsonite-containing drilling fluid used in the previously described tests. The Gilsonite suspension was prepared by adding 2 grams of Gilsonite to 100 milliliters of water then shearing the mixture in a blender for 5 minutes. Prior to the test, the drilling fluid sample was conditioned in a blender for 10 seconds.

The core plugs consisted of fritted aluminum oxide (Alundum) cut 1 inch in diameter and 2 inches long. Alundum plugs were used in order that silicon contained in the solid phase of the two test fluids could be used as a tracer in the SEM and EDX study. Each plug was placed in a high temperature permeameter having modified Hassler type cells and a 1200 psi overburden was applied. Test fluids were pumped through the plugs and when the differential pressure across the plugs reached 600 psi, flow was slowed to maintain that pressure for one hour.

Each of the test flu

ids was tested on plugs at both room temperature and at 250°F. At the higher temperature, a back pressure of 175 psi was used to maintain an all liquid system. Fluid loss to the core during the test was only 1 to 2 milliliters for the Gilsonite suspension.

Samples for SEM and EDX were carefully removed from the influent end of the plugs, dried, cemented on copper plugs, and coated with gold and palladium. The microscopy work was carried out using a JEOL model JSM 840A scanning electron microscope with an EDX attachment. In most cases, a sequence of photos were taken from the filtercake into the core and presented in this paper as a montage.

Test Results

The HTHP fluid loss cell/Berea core test provided data that can be used to discriminate differences between various Gilsonites and asphaltic type additives. Varying temperatures at which the tests were conducted while maintaining the same pressure can distinguish between Gilsonite with varying softening points. Measurement of penetration depth, no penetration, or total invasion or flow through can be used to select a suitable Gilsonite. Table 4 provides the basic drilling mud formulation used in the tests, while Table 5 identifies some of the additives tested along with a generic description of each additive. Many more tests were conducted by our company. The results of some of these tests are shown in Table 6. No penetration or slight penetration by an additive indicates plating at the borehole wall. This indicates that the product is not malleable enough to penetrate into the matrix or there is not enough temperature and pressure to allow the additive to extrude. Deeper penetration shows that the temperature and pressure are enough for extrusion to occur. Complete invasion or flow through the core indicates that additive has been treated for dispersibility and solubility and does not plug or form an inter-matrix filter cake.

An inspection of the core cross-section showed the formation of a thin inter-matrix filter cake with the Gilsonite products. Penetration was limited to less than 2mm. A thin film was formed on the surface of the cores, and definite plugging action was observed.

Inspections of cross-section of a core using a sulfonated blown asphalt manufactured to be approximately 70% soluble shows no plugging action. Additional tests were performed at 250°F and 350°F temperatures while constant pressure was maintained on the cells. In this test, Gilsonite, although processed differently by manufacturers, but with the same softening point, could be discriminated. Several formed an inter-matrix filter cake while others only plated out at the surface. Gilsonite with a lower softening point showed deeper penetration than the higher softening point Gilsonites. The results of this test indicate that the procedure can be used to discriminate between Gilsonites, blown asphalts, and sulfonated blown asphalts as to the degree of plugging action under varying temperature and pressure conditions.

The results of the DSC tests indicate that the apparatus can be valuable in evaluating the effectiveness of additives such as Gilsonite or asphaltic type materials. The Pierre shale core was exposed to a circulating drilling fluid for 45 hours. After 45 hours of exposure, the borehole size of the core sample

increased from 1.25 inches to 2.25 inches, an increase of 80%. Table 7 lists a generic description of various Gilsonite additives used in the tests.

In addition, a core was exposed to the same drilling fluid with an addition of six pounds per barrel of a blended Gilsonite. The borehole did not show any

evidence of hole enlargement after an identical 45 hours of exposure and teeth marks from the bit can be seen. On the initial examination of the borehole wall, a thin, soft, tacky film of malleable Gilsonite could be observed. Another core was also exposed to the same fluid composition containing the blended Gilsonite; however, the test temperature was lowered from 180°F to 125°F. Hole enlargement measured an average of 1.56 inches, a 25% increase and the film was not observed on the core. The lower temperature tests were used to define temperature limitations of this additive.

To confirm that softening point of Gilsonite’s has some effect on its effectiveness. Gilsonite with a low softening point was selected. Figure 6 shows a core exposed to a low softening point 330°F Gilsonite, at 125°F circulating temperature. The average hole size is nearly gauge, with no enlargement.

Another test was designed to determine the effectiveness of Gilsonite treatment after hole instability signs were observable. The core was exposed to the base drilling fluid for 15 hours to initiate hole enlargement, then a treatment with blended Gilsonite was initiated. Enlargement was measured at 2.0 inches as compared to 2.25 inches in the control sample. A film of Gilsonite was observed; however, the film was gummy and not as tough as the sample at 125°F. This test indicates that pretreatment is more effective; however, treatment after observable signs of borehole instability can still mitigate the problem. We also examined the effectiveness of a Gilsonite suspension in a liquid. The core was exposed to such a fluid at a concentration of 4% Gilsonite in suspension.

Enlargement was 0.25 inches over the initial size, an increase of 20%. No borehole plating was observed. This observation was similar to the results of the high-temperature, high-pressure filter cell/Berea core test.

To help verify our proposed mechanism explaining the effectiveness of the Gilsonite for preventing shale instability, the alumina core experiments were set up as described in the procedures section. The resulting photomicrograph for the plug injected with Gilsonite-containing drilling fluid at room temperature shows a very thin filtercake with the drilling fluid solids penetrating not more than about 200 um (0.2 mm) into the core. This penetration depth is supported by the corresponding EDX pattern, ie. the silicon peak reflecting the siliceous drilling fluid solids and the silicon contaminant in the Gilsonite disappears below the midpoint of the montage.

The montage of photomicrographs for the core injected with the same Gilsonite drilling fluid at 250°F shows an equally shallow depth of penetration for the drilling fluid solids despite the fact that there was more fluid lost to the plug during this test. Apparently the Gilsonite used in this test is not very fluid even at 250°F.

To get an even better feel for the effect of temperature on the flow properties of Gilsonite, similar tests using only Gilsonite dispersed in water were carried out. The results from the test done at room temperature show an extremely thin filtercake and the EDX patterns and the photomicrographs show no indication of Gilsonite penetrating more than perhaps 50 um into the alumina plug. Also note the granular shape of the Gilsonite particles making up the filtercake. By itself the Gilsonite does not form a very tight filtercake but in combination with drilling fluid clays it seems to be very effective. The results from plugs injected with the Gilsonite suspension at 250°F show a filtercake about 50 um thick and it is essentially a solid cake of Gilsonite which has been fused. There appears to be only a trace of any remaining granular structure in the filtercake. Farther into the plug at points 3, 4, and 5 in the photomicrographs, it appears that

Gilsonite has flowed through some of the larger pores. This is supported by the EDX patterns for those specific point. Beyond point 5 there is no evidence of Gilsonite penetration even at this relatively high temperature.

Test Interpretation

The results of the two series of tests indicate that these novel procedures can be used to determine the effectiveness of Gilsonite and asphaltic type additives. Test procedures conducted at ambient temperatures should not be used. The Berea core test can be used to provide some measurement of the temperature and pressure conditions in which these additives can be used. Selection can depend u

pon melting point and dispersibility of the drilling fluid. The tests indicate that additives treated for a high degree of solubility do not effectively plug the borehole surface or even form a slight inter-matrix filter cake. With these additives deep invasion does result.

The DCS tests indicate that evaluating these additives under simulated downhole conditions are effective. The tests show that when Gilsonite used in sufficient concentration in a KOH/lignosulfonate can minimize hole enlargement in a Pierre shale. However the tests clearly show that there are temperature limitations and a low softening point Gilsonite should be used in wells with low circulating temperatures.

Examining cores using high-powered microscopy provides some insight into the way Gilsonite works. With sufficient temperature and pressure, the additive becomes malleable and intrudes into microfractures, pore spaces, and bedding planes of the shales. This bonding action “mends” or holds clay platelets together, thus preventing further shale disintegration. In addition, the “tail” of the Gilsonite particle plates or attaches to the borehole wall, forming a tough film which appears to strengthen the wall against the erosional effects of the drilling fluid. Performance of the Gilsonite additive in a drastically different drilling fluid, such as a salt/polymer system, or with a drastically different shale, such as one containing no smectite, might differ considerably from these results and could best be determined by additional DSC testing.

Field Data

The results of these tests are supported by field data in which Gilsonite was used to provide borehole stability. Chevron is using the product in many environments, but only three wells are cited.

In the South Pass area, offshore Louisiana, a blended Gilsonite-lignite was used to reduce torque and drag. Normally wells in this area are deviated approximately 30° from vertical and are drilled with a conventional lignosulfonate mud. On well A, conventional and bead-type lubricants have been used for torque and drag reduction. On well B, 3 lbs/bbl Gilsonite was added at approximately 10,600 feet, measured depth. By comparison, the torque on well B was reduced on a average from 1400 foot-pounds to 900 foot-pounds, a 36% reduction, and drag on a average from 60,000 pounds to 30,000 pounds, a 50% reduction. Caliper log comparisons from these two wells indicate substantial improvement with the use of the blended Gilsonite/lignite.

Another well was drilled in the Eugene Island area of offshore Louisiana. While this area is not known for borehole instability, hole enlargement, however, is a problem. Prior to the introduction of a Gilsonite additive, hole enlargement averaged approximately 50% in a 12.25 inch hole. At the casing point, 4% Gilsonite suspended in a water-base solution, was added to a conventional lignosulfonate mud system. In addition to borehole stability the Gilsonite suspension provided both rheological and fluid loss control. Hole enlargement in the 8.5 inch hole was reduced to an average of 15%.

On a third well recently drilled by Chevron in the Evanston, Wyoming area, hole instability was encountered. The area is in the thrust fault zone known as the hinge line. In this part of the field hole instability is frequently encountered. On this well, excessive torque and drag, bridges after trips, and reaming problems continued to increase. An inhibitive, KOH-gypsum mud system was being used and chemical alteration was not indicated by the cuttings. An onsite analysis by Chevron employees suggested that tectonic stresses probably caused borehole instability. A hole cleaning program was initiated first by viscosifying the mud by increasing the YP from 7 to 15 lbs/100 ft². Secondly, 5 pounds per barrel of a blended Gilsonite was added to the system. Within two days, hole stabilization was achieved and drilling continued without further instability problems.

These well histories confirm the results of the testing program conducted by our company. First, Gilsonite additives, used in sufficient concentration, provide borehole stability and reduce hole enlargement significantly. Second, Gilsonite should be used as a pretreatment; however, it can be used successfully even after borehole instability has occurred.

Conclusions

Based on the results of the testing program our company has conducted and comparing these results with some well histories, the following conclusions are offered.

1. To monitor the effectiveness of a Gilsonite or asphaltic-type product, laboratory tests should be conducted under simulated downhole conditions. One of the most effective laboratory apparatus is the Downhole Simulation Cell.

2. There are differences in Gilsonite additives as to their effectiveness at varying temperatures and to some degree pressure. A simplified high temperature high press filtration cell using a Berea core as a filtering medium can be used to discriminate Gilsonite products and asphaltic-type products concerning plugging ability.

3. Gilsonite is an effective additive to use for borehole stability. The additive plugs off microfractures, bedding planes, and pore spaces, and deposits a thin film on the borehole wall which mitigates hole erosion. It is recommended that Gilsonite be added to a system prior to drilling shale sections where borehole

instability is expected. However, laboratory tests do indicate that treatment after destabilization has been initiated to be effective with minimum concentration values of 3-4 pounds per barrel.

Our company concludes that the downhole simulation cell can be used to evaluate additives which stabilize troublesome shales either through a mechanical action such as Gilsonite or a chemical action such as inhibition. Under these test conditions “apples can be compared with oranges”.

Acknowledgment

The authors wish to express appreciation to Marion Reed, Chevron Oil Field Research Company, Bob Haffner, Clyde Parrish and Clark Christensen, American Gilsonite Company for the assistance in writing this paper. We acknowledge the support of J. D. Combes and T. B. Sumner, Chevron Services for the initiation and completion of the project, and to Chevron Services for permission to publish this paper.

References

1. W. S. Cagle and L. F. Schwertner, “Gilsonite Stabilizes Sloughing Shales”, Oil and Gas Journal, March 27, 1972.

2. J. L. Lummas, “Op Drilling and Holemaker Mud Cut Williston Basin Costs” , Oil and Gas Journal, April 17, 1972.

3. D. La Hue and G. Schooler, “Polymer Retards Water Loss In Shale Formation During Rig’s Lengthy Shutdown”, Drilling, August, 1983.

4. D. B. Anderson and C. D. Edwards, “Fluid Development For Drilling Sloughing and Heaving Shales”, Petroleum Engineer International, September, 1977.

5. J. P. Simpson, H. L. Dearing, and D. P. Salisbury, “Downhole Simulation Cell Shows Unexpected Effects of Shale Hydration on Borehole Wall”, IADC/SPE Conference, SPE17202, February 28, 1988.

6. H. Abraham; “Asphalts and Allied Substances”, Volume I Raw Materials and Manufactured Products, D. Van Nostian Co., Inc. NY City, 1945.

7. Field, L. J. “How Solids Non-Dispersed Mud Usage in Western Canada “, presented at API meeting, Calgary, Alta, May, 1968.

8. Drilling Specialties Company, “Soltex Shale Inhibitor”, Technical Sales Memorandum, 1986.

9. J. A. Wingrave, E. Kubers, Jr., C. F. Dority, D. L. Whitfill, and D. P. Cords, “A New Chemical Package Produces An Improved Shale-Inhibitive Water-based Drilling Fluid System”, SPE 16687, Presentation at SPE Fall Technical Conference, September, 1987.

10. F. F. Sullivan, “Drilling Fluid Treatment” -application No.109, 136, Canadian Patent, January 20, 1971.

APPENDIX 1

GILSONITE AND BLOWN ASPHALT TEST

I. Method Summary

A prehydrated bentonite-lignosulfonate base slurry is prepared. Using a high-temperature, high pressure fluid loss cell, a Berea sandstone core is sealed in the cell using silicone or a special core holder equipped with O-Rings. The test additi

ves, Gilsonite products, asphalts and sulfonated blown asphalt products are added to the base slurry samples at concentrations of 6 ppb. The cells are loaded with the test slurry and the mud is filtered through the Berea core under elevated temperature and pressure conditions. This test determines the capability of a product to build an intermatrix filter cake and limit the depth of invasion of the mud filtrate.

II. Equipment

HTHP Fluid Loss Cell

Balance (± 0.01 g)

Dispersator or equivalent high shear mixer

Graduated Cylinder, 500 cm³

Hamilton Beach Mixer

Beaker, 150 cm³, 1000 cm³

Spatula

API Test Calibration Bentonite

Sodium Hydroxide: 5N NaOH Solution

Lignosulfonate

Gilsonite or Asphaltic Additive

Silicone or Berea Core Holder with O-Rings

Berea Core

III. Procedure

A. Base Slurry Penetration

1. Measure 350 cm³ of deionized water into a 1000 cm³ beaker.

2. While stirring with Hamilton Beach mixer, slowly dust in 18 grams of API Test Calibration Bentonite. Stir for 30 minutes.

3. Age a minimum of 16 hours in a sealed container at room temperature.

4. After aging, restir base slurry for 5 minutes in HB mixer. While stirring, dust in 2 grams of lignosulfonate and add 2.5 cm³ 5N NaOH. Stir for an additional 30 minutes.

B. HTHP Cell Preparation

1. Cut a Berea core to fit snug inside the cell or cut the core to fit into a core holder that fits in the cell. Core thickness should be approximately ½”.

2. If the core is cut to fit the cell, coat the outer edge with silicone to effect a seal.

C. Test Procedure

1. Add 6 ppb of test product to a portion of the base sample and stir for 30 minutes.

2. Place mud in the HTHP cell and slide the Berea core into place in the cell in a snug position. Close cell and hookup apparatus.

3. Obtain HTHP filtrate measurements under 500 psi differential pressure using the desired temperature (150-300°F). Run test for 30 minutes duration.

IV. Report

Obtain filtrate volume and measure invasion depth on Berea core.