CATIONIC DRILLING FLUID IMPROVES ROP IN REACTIVE FORMATIONS

Terry Hemphill
Baroid Drilling Fluids Inc.
Houston

Rob Valenziano, Peter Bale
Baroid Drilling Fluids Inc.
New Orleans

Byron Sketchler
Chevron U.S.A. Inc.
New Orleans

A cationic water-based mud reduced bit balling and increased rates of penetration (ROP) in several Gulf of Mexico wells where highly permeable sands and dispersive formation clays and shales typically cause drilling problems.

The cationic polymers adsorb on the negatively charged sites on shales and clays and immediately inhibit reactive formation solids.

The filtration and mud cake qualities illustrate that effective filtration control is obtainable with the cationic drilling fluid.

Field results indicate that the cationic polymer drilling fluid system:

• Improves shale stabilization in soft-to-medium hard shales

• Minimizes or eliminates bit balling

• Increases penetration rates in softer clays and shales, particularly when drilled with polycrystalline diamond compact (PDC) bits

• Exhibits stable mud properties, including filtration control

• Is tolerant of solids contamination

• Exhibits good permeability plugging/filter cake characteristics

• Is nontoxic.

BACKGROUND

Several classes of cationic polymers have been used for many years in the petroleum industry, especially in the areas of corrosion chemist and enhanced oil recovery.1 Many cationic polymers are used as additives in completion, acidizing, and fracturing fluids, which are commonly clear water systems (i.e., freshwater and brines). The ability of certain cationics to stabilize swelling clays, reduce migration of fines, and wet metal surfaces is well known.

For the drilling industry, field use of cationic drilling fluids has been hampered until recently by the lack of suitable Theological and filtration control agents. The production of stable cationic systems was much more difficult to achieve for solids-laden muds than for clear water fluids.

Recently, the drilling literature has cited several applications of cationic waterbased mud systems used as laboratory and slim-hole coring fluids.2 3 Within the last 2 years, cationic polymer systems have been used in a wide variety of drilling applications in South Texas, offshore Louisiana, and the North Sea.

CATIONIC POLYMERS

Organic polymers can be classified by their electrostatic charges into four basic groups:

• Anionic

• Cationic

• Nonionic

• Amphoteric.

More detailed information on basic oil field polymer chemistry is available.

Nearly all water-based drilling fluid polymers in use today are anionic in nature and thus have a large negatively charged hydrophilic member opposite a small positively charged ion. With cations, the charges are reversed; the large hydrophilic member is positively charged and is opposite a negatively charged ion.

When clays and shales are drilled, their negatively and positively charged surface sites are exposed to the drilling fluid.

Anionic materials will adsorb opposite the clay's positively charged sites (Fig. 1). Usually the ability of anions to stabilize reactive formation particles depends on their ability to encapsulate or cover the exposed surface area, thereby retarding water invasion.

Hydration of clays and shales occurs most readily on the particle edges where both positively charged and negatively charged sites are found.5 In alkaline environments, the predominance of positive charges on the edges is reduced. It is against the negatively charged sites on both the clay faces and edges that cations adsorb, neutralizing the charge imbalance (Fig. 1). Non-adsorbed positive charges along the polymer's backbone induce a "charge-patching" effect in which an envelope or region of localized positive charge is maintained about the clay edges. Flocculation of formation solids readily proceeds under such conditions, reducing the rates of hydration and particle dispersion.

Flocculation and inhibition of clays and shales can be further encouraged by the presence of dissolved salts in the water-based mud.

Prior to this cationic system's first application in Bay Marchand, compatibility checks were performed with several materials used on wells in the area. The addition of anionic materials such as lignosulfonate, partially hydrolyzed polyacrylamide, carboxymethyl cellulose, and polyanionic cellulose to a cationic system will destroy its cationic character and produce unstable fluid properties. Thus, a spotting fluid, a common lost circulation additive, and a borehole stabilization material were tested in the laboratory for their compatibility with the cationic drilling fluid. Results showed that these additives had negligible effects on the system's Theological and filtration properties and were thus approved for field use.

PERMEABILITY
PLUGGING TESTS

Several wells in Bay Marchand have had problems with severe drag on trips, caused by bottom hole assembly balling and the buildup of filter cake across highly permeable sands. It is not uncommon in areas of Bay Marchand to find sand permeabilities of 6-10 darcies as deep as 10,000 ft true vertical depth.

Specialty products, such as sized cellulose fiber material and asphaltite, have been successfully used in this field in anionic KOH/lime systems to maintain a wide particle size distribution (PSD) to promote plugging and the formation of a compressible filter cake.

The permeability plugging apparatus (PPA) is an effective tool for determining and modifying the plugging characteristics of drilling fluids.6 It has been used successfully at the rig site to monitor filtration rates and filter cakes produced by various drilling fluids. The PPA results obtained in the laboratory for various drilling fluids have correlated very well with performance in the field. For the Bay Marchand area, a maximum total PPA value of 12 cc/30 min is recommended for drilling highly permeable or severely depleted sands where minimum filtrate loss and cake buildup is required.

Prior to use of the cationic polymer drilling fluid in the field, several PPA tests were run with bridging and plugging additives and shale stabilization products to determine their effects in the cationic environment. Tests were performed with both a clean cationic fluid and one contaminated with solids. All samples were hot rolled for 16 hr at 150 F.

The base cationic mud, contaminated with 20 lb/bbl Bay Marchand drill solids, provided excellent fluid loss properties: an American Petroleum Institute (API) fluid loss of 3.9 ml/30 min and a PPA value of 10.0 ml/30 min. Additional amounts of the filtration control additive, a modified starch for the cationic system, provided the best filtrate reduction with an API fluid loss of 2.0 ml/30 min and a PPA value of 8.8 ml/30 min. PPA values of this magnitude have been obtained in the field previously only with weighted KOH/lime systems heavily treated with lost circulation products.

Although spurt losses were not noted in these laboratory tests, PPA tests were run on Sidetrack E, the second cationic polymer drilling fluid application in Bay Marchand. Spurt losses measured at the well's total depth were 4.0 ml/30 min.

DYNAMIC FILTRATION TESTS

In the last 10 years, extensive research has been conducted with dynamic filtration devices because the filtration characteristics of a drilling fluid may not always be adequately defined by static measurements. The dynamic properties of a fluid are important considerations in drilling fluid selection. 7

Inadequate control of drilling fluid filtration is associated with the following:

• Excessive clay swelling and borehole instability

• Excessive torque and drag

• Differential pressure sticking

• Increases in trip time because of excessive filter cake buildup

• Formation damage.

The static API high temperature/high pressure (HTHP) fluid loss test does not detect the effect of fluid shear upon the orientation of solid particles that form a filter cake. 8

The dynamic filtration rates can be greater than the API HTHP values if the filter cake is eroded by the applied shear force. If cake forms, the dynamic filtration rates can be equal to or less than the HTHP rates depending upon the thickness of the filter cake that forms during the allotted filtration period.

Selected fluid formulations similar to those described for the PPA tests were used in dynamic HTHP evaluations with a Farm model 90 at 175 F. and 500 psi with a 3 darcy core and a shear rate of 96 sec-1. The cationic system's starch derivative was found to optimize the dynamic filtration rate and maintain an erodible filter cake. Excessive amounts of the cellulose product increased the fluid loss. The fluid loss levels obtained with the Farm model 90 tracked those of the PPA as long as some filter cake was formed during dynamic filtration.

The laboratory PPA and dynamic HTHP results showed the system provided good filtration and filter cake properties, thereby supporting its use in the Bay Marchand drilling environment.

FIELD APPLICATIONS

To date, the cationic drilling fluid system has been used on four wells/sidetracks in the Bay Marchand field, an area known for its reactive clays and shales. Drilling problems such as bit balling and tight hole are common in this area.

Information from the field corroborates earlier laboratory findings and documents the system's increased performance levels over those of KOH/lime anionic water based muds. Table 1 lists seven Bay Marchand wells that were drilled with KOH/lime or cationic drilling fluid systems. Well design data and directional information for each of the referenced wells are provided in Table 2. Data pertaining to penetration rates, drill bit consumption, and drilling costs for the wells are found in Table 3.

CLAY AND SHALE STABILITY

Clays and shales taken from the Bay Marchand area can be described as moderately reactive. Cation exchange capacity (CEC) levels of formation cuttings from this area have been measured as high as 28 meq/100 g, with average CEC levels of 18-19 meq/100 g. Fig. 2 contains a profile of formation CEC levels vs. depth for Bay Marchand cuttings collected during drilling with the cationic polymer systems. X-ray diffraction of collected shales has shown them to be composed of approximately 55% illite, 16% quartz, and 12% smectite, with other minerals making up the balance.

Capillary suction time (CST) testing can be used to measure the relative reactivity between base fluids and clays and shales.9 CST data illustrate the reactivity of Bay Marchand clays in deionized water and three filtrates (Fig. 3). These tests indicate that the seawater/cationic fluid filtrate best stabilizes the two cuttings, composite samples.

Erosion tests have also been run on shales and clays from Bay Marchand welts. In one set of tests, cuttings from various depths (2,674, 3,150, 3,800, and 4,010 ft) were exposed to seawater/cationic polymer and seawater KOH/lime fluids. The fluids were formulated to have nearly identical Theological profiles. The samples were hot rolled at 150 F. for various periods of time and then passed over an 8 mesh screen. Shale pieces remaining on the screen were then dried and weighed. A plot of cuttings erosion over time showed that the cationic polymer fluid provided a more inhibiting environment.

Sizes of the gumbo pieces taken at the shale shakers were remarkably large. During the drilling of Well D, the cuttings integrity was quite good. The interior sections of the shales were dry and the cuttings moved across the shakers as individual pieces. There were no semifluid masses of sticky cuttings slowly moving over the shaker screens in "glop" fashion, something often seen offshore when Gulf of Mexico gumbo is drilled.

Evidence of increased shale stability was provided by particle size distribution analysis. A mud sample from Well F contained 11.19% by volume low gravity solids (LGS), equivalent to 101 lb/bbl. Analysis showed that although the sample contained a large volume of LGS, the

BIT BALLING

In wells where the cationic drilling fluid system has been used, the occurrence of bit balling has been minimized or eliminated. Two minor incidents of bit balling were noted on Well D and were eliminated by pulling the bit off bottom and circulating 5 min. No incidents of bit balling were noted on Sidetrack E.

These field results are consistent with laboratory data generated on a full-scale simulator, in which no signs of visible bit balling were noted in tests involving the cationic drilling fluid . 2 Cationic polymers can preferentially wet metal, as do cationic corrosion inhibitors. It is thought that the cationic polymer forms a film on the surface of the drilling bit, keeping the cut shale pieces from sticking to the bit while under pressure. With the cationic fluid's improved cuttings stability, the cut formation particles have reduced surface area exposed for hydration. This reduces bit balling potential.

The lack of bit balling with the cationic polymer system in the field has allowed sustained use of polycrystalline diamond compact (PDC) bits in recent drilling programs. Previously, the routine occurrence of severe bit balling with PDC bits prohibited their use on development wells. With the cationic drilling fluid, bit usage per well and time spent on bit trips have been greatly reduced (Table 4).

PENETRATION RATES

The improved clay and shale stability and the near-elimination of bit balling have increased ROP in the field. Instantaneous ROPs on Well D, where long intervals of controlled drilling to maintain deviation angle were not required, usually ranged between 200 and 400 ft/hr. Occasionally, instantaneous ROPs up to 1,000 ft/hr were recorded. In the lower interval section drilled with a PDC bit, average ROPs of 90 ft/hr in the firmer clays and shales and 130 ft/hr in the sands were recorded. Penetration rates on Sidetrack E were lower than those on Well D because drilling rates were usually controlled to maintain hole angle. Still, ROPs averaged 46 ft/hr for the section.

The average ROPs on Well D were significantly greater than the 30-40 ft/hr levels recorded in the same formations on previous offset wells that were drilled with the KOH/lime System. Fig. 4 contains ROP curves for Well D and for a composite of Wells A and B (the curve is an average of the normalized ROP levels of each well for the interval for which information is available). Generally, ROP increases of 150-200% were common with use of the cationic polymer system and PDC bit.

The improvement in penetration rates is best shown by a plot of depth vs. days for three Bay Marchand development wells (Fig. 5):

• Well B-The previous well on the platform in which a KOH/lime mud system was used

• Well C-A "bench mark" well that previously held the depth-vs.-days record in the Bay Marchand field

• Well D -The well in which the cationic polymer system was first used.

Well D was drilled to 8,500 ft nearly 2 days faster than Well C and nearly 3 days faster than Well B.

The increased performance in penetration rates with the cationic drilling fluid/PDC bit combination seen in Well D was nearly doubled in Well F, where average ROPs were greater than 135 ft/hr for the 3,318 ft section (Table 4). Such sustained drilling performance had not been seen previously in this field with the use of the water-based KOH/lime alternative mud system.

FLUID PROPERTIES

Cationic drilling fluids used in Bay Marchand were formulated to have mud rheological and filtration properties similar to those of the KOH/lime systems used previously. Table 4 contains typical product concentrations used in the field as well as brief descriptions of individual product functions in the cationic polymer system. The cationic polymer drilling fluid systems used in Bay Marchand have produced fairly stable Theological and filtration properties (Table 5).

Good filtration properties, usually difficult to maintain in highly flocculated systems, can be maintained with the use of the special additives this system requires. Fig. 6 contains a plot of both API and HTHP filtration rates recorded during the drilling of Well F. Filtration properties remained more stable when low-to-moderate levels of drilled solids were incorporated into the system.

The highly flocculated and inhibitive nature of the cationic polymer drilling fluid allows the system to be very tolerant of solids contamination. On Well D, excessive mud viscosities and gel strengths did not develop during drilling at the sustained elevated penetration rates. Most anionic waterbased polymer mud systems do not perform as well under these conditions.

Of the four applications of the cationic polymer system in Bay Marchand, Sidetrack E had the highest levels of low gravity solids, as this system was used previously on Well D. Fig. 7 shows the LGS level, the yield point, and the 10-min gel strength plotted against depth. With the LGS levels ranging between 21 and 107 lb/bbl (2.3-11.8% by volume), the rheological properties of the system remained fairly stable, which confirms the cationic system's ability to reduce clay and shale hydration and dispersion levels.

One interesting aspect of cationic drilling fluids concerns the amount of active cationic polymer in the system. According to clay/polymer adsorption theory, cationic polymer concentrations should deplete as the clays and shales flocculate. To ensure newly drilled cuttings are quickly stabilized, an excess of the cationic polymer must be maintained in the system at all times. For Well D, the active cationic polymer concentration was maintained at 1.5-2 lb/bbl, a level that provided good cuttings stability. Treatments on subsequent applications of the system in Bay Marchand maintained 22.5 lb/bbl active cationic polymer.

TOXICITY

Toxicity levels of the cationic water-based systems have been evaluated from monthly samples and from samples collected at the wells' total depths. All samples were analyzed by an independent laboratory in accordance with current Environmental Protection Agency (EPA) protocol.

The 96-hr LC50 values for a 9:1 seawater/mud suspended particulate phase (SPP) using Mysidopsis bahia have exceeded 1 million ppm SPP for all mud samples tested. These results far exceed the current minimum 96-hr LC50 lower limit of 30,000 ppm SPP set by the EPA for discharge of drilling fluids in the U.S. Outer Continental Shelf. Thus, this cationic water-based drilling fluid is environmentally acceptable in U.S. waters and elsewhere.

REFERENCES

1. Borchardt, J.K., "Cationic Organic Polymer Formation Damage Control Chemicals," Oilfield Chemistry, American Chemical Society, Chapter 10, 1989.

2. Hemphill, T., and Clark, R. K., "The Effects of PDC Bit Selection and Mud Chemistry on Drilling Rates in Shale," SPE 22579 presented at the 1991 SPE Annual Technical Conference and Exhibition, Dallas, Oct. 6-9, 1991.

3. Beihoffer, T.W., Dorrough, D.S., and Schmidt, D.D., "The Development of an Inhibitive Cationic Drilling Fluid for Slim-Hole Coring Applications," IADC/SPE 19953 presented at the IADC/SPE Drilling Conference, Houston, Feb. 27-Mar. 2, 1990.

4. Lauzon, R.V., "Water Soluble Polymers for Drilling Fluids," OGJ, pp. 93-98, Apr. 19, 1982.

5. Sawhney, B.L., "Selective Sorption and Fixation of Cations by Clay Minerals: A Review," Clay and Minerals, Pergamon Press, Vol. 20, pp. 93-100, 1972.

6. Newhouse, C.C., "Successfully Drilling Severely Depleted Sands," IADC/SPE 21913 presented at the 1991 IADC/SPE Drilling Conference, Amsterdam, Mar. 11-14, 1991.

7. Chesser, B.G., Clark, D.E., and Wise, W.V., "Dynamic and Static Filtrate Loss for Monitoring Filter Cake Quality Improves Drilling Performance," SPE 20439 presented at the SPE Technical Conference, New Orleans, Sept. 23-26, 1990.

8. Fisk, J.V., Shaffer, S.S., and Helmy, S., "The Filterability of Drilling Fluids," SPE 20438 presented at the SPE Technical Conference, New Orleans, Sept. 2326, 1990.

9. Wilcox, R.D., Fisk, J.V. Jr., and Corbett, G.E., "Filtration Method Characterizes Dispersive Properties of Shales," SPE Drilling Engineering, pp. 149-158, June 1987.

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