Orville Welch, Li-Jein Lee
M-1 Drilling Fluids Co.
Houston
A recently developed cationic polymer mud, compatible with conventional polymer additives and designed to meet environmental regulations, significantly minimized the gumbo problems common to the water-sensitive shales in the North Sea.
The cationic polymer mud was used to drill highly reactive Tertiary shale formations which have caused severe gumbo problems on nearby wells drilled with other inhibitive water-based muds.
Although many cationic polymers are toxic, aquatic toxicity tests performed by the Norwegian Statens Forurensningstilsyn (SFT) at the end of the test wells showed results far exceeding the SFT limits on the three species tested.
The mud system on these wells was a seawater-based 15.0 ppg mud enhanced with 3% NaC1. A low molecular weight quaternary polyamine and a high molecular weight cationic polyacrylamide were used to suppress the swelling and dispersion of shales, respectively. Starch and polyanionic cellulose (PAC) polymers maintained fluid-loss control, and a lubricant reduced the torque and drag.
In the southern part of the Norwegian sector of the North Sea, highly reactive, undercompacted claystones are widely encountered at about 4,000-10,000 ft below the sea floor. These formations range from Miocene to Paleocene in age and are part volcanic in origin. X-ray diffraction analyses indicate that smectite, kaolinite, and illite-followed by small amounts of quartz, feldspar, calcite, and chlorite-are the predominant constituents of the claystones from this area. With such a high clay content, these claystones often have cation exchange capacities (CEC) greater than 30.0 meq/100 g. Table 1 shows the mineralogy and CEC of a typical Miocene claystone from the North Sea.
Because of the highly plastic nature and high swelling and dispersive tendencies of the claystones in water-based muds, they can cause severe problems such as bit balling, mud rings on bottom hole assemblies and drillstrings, high torque and drag, and extensive dispersion of cuttings. Specially designed solids-removal equipment is typically needed to remove drilled cuttings.1
Shale stabilization has often been the subject of much research, and numerous inhibitive mud systems have been developed. Most of these inhibitive mud systems rely on common chemicals and anionic polymers for shale stabilization.
Mineral oil-based muds have often been used to prevent these gumbo problems, especially for directional wells. Certain inhibitive water-based polymer mud systems combined with a "dumping and diluting" practice have also been used to minimize the problems.
Because of increasing restrictions on the use of oil-based muds and concerns about heavy metal pollution from barite discharged into the sea, water-based muds with good shale inhibition quality, low dilution rate, and minimal dumping are needed for environmentally sensitive areas. The recent development of a cationic polymer mud system for shale inhibition has received much attention from the drilling fluids industry.
Laboratory studies have shown that water-based muds containing quaternary ammonium salts or other cationic polymers are more inhibitive than conventional water-based muds.2 3
In spite of the inhibitive nature of cationic polymers, earlier field applications had been troublesome because of product incompatibility. This problem now has been overcome because of a better understanding of the behavior of cationic polymers and because of an increase in the available selection of cationic polymers.
INHIBITION BY CATIONIC POLYMERS
Conventional water-based muds use anionic polymers such as partially hydrolyzed polyacrylamide (PHPA) and polyanionic cellulose (PAC) to provide shale inhibition. The inhibitive mechanism is commonly attributed to polymer adsorption on clay particles.4-6
Shale inhibition with anionic polymers is limited because a strong repulsion exists between the negatively charged clay surfaces and anionic polymers. This repulsion makes most of the anionic polymers unlikely to adsorb on clay surfaces. Consequently, anionic polymers can attach only to the positively charged clay edges.
Although this attachment is considered desirable for preventing cuttings from dispersing, the swelling of clays can still occur as a result of hydration of exchangeable cations. Other chemicals, such as potassium compounds, are often used in conjunction with anionic polymers to provide extra shale inhibition.
In contrast to the adsorption on clay edges by anionic polymers, cationic polymers can adsorb on negatively charged clay surfaces as well as the edges. The adsorption of cationic polymers is similar to the common cation exchange reaction.7 8
When cationic polymers adsorb on clay surfaces, exchangeable cations must be desorbed to maintain charge balance.
Fig. 1 shows the desorption of exchangeable cations from smectite as a function of adsorption of quaternary polyamine. The bonding between the polymer and clay is often stronger (because of multisite attachment of the polymer) than the cation/clay bonding. Thus, the adsorption of cationic polymer is usually irreversible and can be used to modify the hydration potential of clays when the site of adsorption is on the basal planes.
The adsorption of cationic polymers on clay surfaces is greatly affected by the molecular size of the cationic polymer. This effect should be considered for encounters with reactive shales with low water contents. The spacing between clay layers in the shales may not be large enough for an oversized cationic polymer; hence, a certain degree of hydration must occur before the cationic polymer has a chance to enter the clay layers.
LABORATORY TESTING
Two low-toxic cationic polymers, a high molecular weight cationic polyacrylamide and a low molecular weight quaternary polyamine, were selected for dispersion and swelling inhibition control, respectively.
The high molecular weight cationic polyacrylamide is a copolymer. It has an average molecular weight of 10 million and a low charge density. Because of its large molecular size, the cationic polyacrylamide adsorbs mainly on the exterior surfaces of clay particles.
Conversely, the low molecular weight quaternary polyamine has a high charge density and a much smaller molecular size. The small molecular size allows it to penetrate clay layers easily to displace exchangeable cations. Because of its high charge density, the polymer will not exchange with other cations once it adsorbs on clay surfaces.
Adsorption of the cationic polyacrylamide and quaternary polyamine were determined to be approximately 0.1 g and 0.3 g per gram of bentonite, respectively.
Hot-rolling dispersion tests with gumbo shales from offshore Louisiana were performed to compare the effects of quaternary polyamine and cationic polyacrylamide. The base mud was a freshwater gel slurry treated with polyvinyl alcohol to reduce flocculation caused by the cationic polymers. About 20.0 g of sized gumbo particles (5-10 mesh) were added and hot rolled 15 hr at 150 F. A 30-mesh screen recovered the gumbo after hot rolling.
Test results showed that cationic polyacrylamide can effectively prevent the gumbo from dispersing into fines by encapsulating the large particles, but it has little control on swelling (Fig. 2). The quaternary polyamine, because of its small molecular size, is not as effective as cationic polyacrylamide in preventing the dispersion into fines (Fig. 3). However, the recovered gumbo particles were firm and showed little signs of swelling.
With both polymers present, the integrity of the gumbo particles is significantly improved, resulting in satisfactory swelling and dispersion inhibition controls (Fig. 4).
Although cationic polymers provide good shale inhibition, their early use in water-based muds was problematic. One of the problems with cationic polymer muds was product incompatibility. For example, cationic polymers react with anionic polymers and cause severe flocculation of clays and barite. Thus, early cationic polymer muds contained no bentonite, barite, or anionic polymers. This incompatibility problem had to be solved for the formulation of a practical 15.0-ppg cationic polymer mud for gumbo shale drilling.
Recent studies have shown that most anionic polymers are incompatible with cationic polymers in freshwater, but both types of polymers may become compatible in the presence of certain electrolytes.6 The concentration of electrolyte required for this compatibility is affected by the charge density of the polymers. Laboratory tests have shown that xanthan gum and PAC polymers are compatible with the cationic polyacrylamide and quaternary polyamine when the chloride level is greater than 30,000 mg/l., indicating that these anionic polymers can be used to control fluid loss and viscosity of the cationic polymer mud.
Another problem associated with cationic polymers is their powerful flocculating capability, which is likely to cause some severe problems at high solids content. An effective deflocculant is needed for this situation; unfortunately, there is no cationic deflocculant available. Nonetheless, laboratory tests showed that certain nonionic polymers and anionic polymers do exert a moderate deflocculating effect on clays flocculated by cationic polymers.
FIELD TESTS
The cationic polymer mud system was used on two directional wells to drill the 12 1/4-in. intervals in which severe gumbo problems were anticipated. Both wells were drilled from an offshore platform located in the southern Norwegian sector of the North Sea. For Test Wells A and B, the section intervals began at 5,438 ft and 5,290 ft, and hole inclinations were 27 and 0-25, respectively. The bottom sections of the holes were underreamed to 15 in. before the section total depth (TD) was reached.
MUD FORMULATION
The mixing of the 15.0-ppg cationic polymer mud started after the 13 3/8-in. casing was cemented. To ensure polymer compatibility, seawater enhanced with 3% by weight NaC1 was used as the makeup water. Once the chloride level was raised, the polymers were added through the hopper with the mixing pumps. No special shearing device was used for the mixing.
On the first well, a small amount of prehydrated bentonite was added for barite suspension, and on the second well, xanthan gum was used. The addition of bentonite or xanthan gum was terminated because the incorporation of drilled solids in the system provided more than adequate barite suspension once drilling started.
Starch was used for fluid-loss control initially on the first well. The starch was replaced with a low-viscosity PAC polymer because of biodegradation resulting from a lack of biocide in the system. On the second well, the low-viscosity PAC polymer was used for fluid-loss control.
To prevent bit balling and to reduce torque and drag, a lubricant was added to the system at a concentration of 1% by volume. Polyvinyl alcohol, a nonionic polymer which also can provide shale stabilization, was used as a deflocculant to aid in controlling the rheology. Table 2 shows polymer concentrations used for mixing the cationic polymer mud.
The cement plug and casing shoe were drilled out with a weighted seawater/polymer mud to avoid unnecessary cement treatment. After approximately 10 ft of new formation were drilled and the leak-off test was performed, the seawater/polymer mud was displaced with the cationic polymer mud. A high-viscosity gel spacer was used to reduce cement contamination.
Normal drilling operations started after the displacement. Table 3 lists the properties of the cationic polymer mud shortly after drilling commenced.
SOLIDS CONTROL
The only equipment used on both wells for solids control was shale shakers. The Norwegian SFT restricts overboard discharge and onshore disposal of the liquid cationic polymer mud, and this restriction precludes the use of a low-speed centrifuge for barite recovery, although there was one onboard for this purpose.
On Test Well A, three conventional double-deck shale shakers were used to remove shale cuttings. Initially, the shale shakers were dressed with 30 mesh screens on top and bottom to prevent mud loss over the shakers.
Drilling operations for the first 2,000 ft were trouble-free. No signs of torque or bit balling were encountered during drilling. A couple of short trips were made, and the borehole was in a very stable condition.
Although most of the cuttings removed over the shakers were firm and well-defined, a large number of fine cuttings were incorporated into the system because of the coarse screens. With a fast drilling rate, this led to a quick buildup of fine solids. The shaker screens were changed to 60 mesh to improve solids removal while keeping the mud loss minimal at the shakers. In spite of this change, fluid-loss and high-viscosity problems occurred when the content of low-gravity solids reached 8% by volume.
Rheology measurements of the high-viscosity mud showed a rather flat rheological profile characterized by low plastic viscosity, high yield point, and high gel strengths. The viscometer readings at low shear rates were also unusually high, indicating that the fine solids were flocculated.
Drilling operations had to be suspended to recondition the mud. The causes of the problems were attributed to inadequate solids control improper mud additives and inefficient maintenance procedures. The system was partially displaced when the content of low-gravity solids finally reached a maximum of 13% by volume.
After the displacement, a few changes were made to reduce the rapid solids buildup problem. A fine spray of seawater was applied over the shakers to enhance solids removal with finer screens. A larger volume of mud with low gel strengths was also maintained so that entrapped cuttings could settle out more easily. These changes allowed a tolerable control of the low-gravity solids level. The well was drilled to section TD without any more viscosity or fluid-loss control problems.
On Test Well B, three double-deck, high-speed shale shakers were used for solids control. The new solids-control equipment immediately proved more than adequate; 105-mesh screens were effectively used at the beginning of the drilling operations. As drilling progressed, finer shaker screens (145-165 mesh) were installed to remove the fine cuttings. With this more efficient solids-control equipment, the second well was drilled to section TD without any severe viscosity or fluid-loss control problems.
MUD MAINTENANCE
For optimal shale inhibition, it is important to maintain proper polymer concentrations during drilling because much of the cationic polymers will be removed from the system via solids control or lost to the formations.
On Well A, polymer concentrations as well as mud properties were maintained through use of weighted premixes in combination with direct addition of polymers to the active system. A fine stream of seawater was also necessary to compensate for fast evaporation.
The pH of the system was maintained in a range of 8.0 9.5 with caustic soda. The caustic was added in diluted liquid form to avoid degradation of the polymers. No soda ash or other chemicals were used for hardness treatment.
Instead of weighted premixes, small batches of unweighted premix with elevated polymer and chemical concentrations were used on Test Well B for daily maintenance. This method was more effective than the previous method. The prehydration of polymers in seawater also avoided polymer "fish eye" formation and localized high viscosity mud in the system. Both polymer concentrations and mud volumes could be controlled efficiently by regulating the transfer rate of the unweighted premix.
Two test methods were used to monitor the concentration of cationic polymers. On both wells, an ammonium extraction test was used for quantitative determination of the cationic polyacrylamide. This test was quite accurate and reliable. The ammonium test was conducted on a daily basis to maintain the concentration of cationic polyacrylamide at about 1.0-1.5 lb/bbl during drilling operations.
A clay flocculation test and mass balance were used to monitor the concentration of quaternary polyamine on Test Well A. However, this method was inadequate for quantitative determination because the consumption rate of the polymer was greatly affected by the type of formation drilled.
To solve this problem, a new test method was introduced on Test Well B. This new test method used the strong interaction between the polymer and a strongly anionic chemical, which causes a white precipitate to be formed even at high electrolyte content. The amount of precipitate, which can be determined visually or by turbidimeter, is a function of the concentration of the quaternary polyamine.
This test method allowed close monitoring of the concentration of quaternary polyamine during drilling. The concentration of quaternary polyamine was maintained between 3.0 lb/bbl and 6.0 lb/bbl on Test Well B. Table 4 shows the typical properties of the cationic polymer muds just before section TD for both wells.
Corrosion inhibitors were not used in the cationic polymer mud. The corrosion rate was determined on the second well by running corrosion rings for approximately 100 hr. The results indicated a minor corrosion ( < 3 mpy) even though the system pH was in the low range. The low corrosion rate was attributed to the use of the lubricant, which contains a metal wetting surfactant that minimizes both bit balling and corrosion.
TOXICITY STUDY
Toxicity studies on the cationic polymer mud and the cuttings exposed to the mud were conducted at the section end of Test Well A according to SFT protocol. Organisms used for the toxicity tests were: Skeletonema costatum (algae), Balanus improvisus (barnacles), and Mytilus edulis (mussels). Both the mud and cuttings passed the SFT test by wide margins, indicating only a minimal impact on the environment (Table 5).
CONCLUSIONS
The purpose of cationic polymer mud is to inhibit shales effectively. Field test results indicate that the cationic polymer mud system used was indeed more inhibitive than conventional water-based muds.
During both field tests, no bit balling or gumbo rings around drillstrings were observed. Minimal efforts were required to move the drillstring during bit trips, except for a few tight spots encountered on Well A. These tight spots were probably caused by key seats. The slick hole conditions were attributed partially to the use of a lubricant and partially to shale stabilization from the mud system.
On Test Well B, large caved-in claystones were observed several times while the well was drilled with a 15.0-15.3 ppg mud weight. Caliper logs run after section TD, however, revealed a mostly in-gauge hole, except for a few zones of enlargement immediately below the casing shoe and in the Eocene claystone section where hole angle had been built.
Insufficient mud weight, abnormal tectonic stresses' and fractures induced by a subsidence of the field were attributed to the main causes of this well bore instability. The mud weight was therefore increased to 15.5 ppg to stabilize the well bore. Similar problems had also been encountered previously with oil-based and water-based muds.
Because of improper solids control, fine solids buildup had caused some viscosity problems on the first well, resulting in excessive dilution and treatment. The problem was greatly minimized on the second well with the use of better shale shakers. The effectiveness of the solids-control equipment demonstrates the advantage of cationic polymers which encapsulate the cuttings, thereby promoting the removal of the cuttings from the mud system.
With effective solids-control equipment, the amount of mud built and lost over the shakers was significantly reduced on the second test well. Table 6 lists the total mud volume and barite discharged from the two test wells and other offset wells in the area.
Effective solids control also can help reduce the consumption of cationic polymers, which was dependent on the type of formation penetrated, the content and particle size of reactive solids, and the relative concentrations of the polymers in the mud system.
The polymer consumption rate could be established with quantitative determination methods. In general, a faster depletion rate of cationic polymers was observed when smectite-rich claystones were drilled, while a slower depletion rate was observed with less reactive formations.
The depletion rate of quaternary polyamine was observed as greater than that of cationic polyacrylamide, especially when cuttings were mechanically disintegrated into clay-size ( < 2m) particles. The quaternary polyamine easily adsorbs on the basal planes of clays, whereas the cationic polyacrylamide adsorbs on the exterior surfaces of clays.
The correlation between polymer consumption rate and content of reactive clays generally agreed with laboratory results.
Changes in the concentrations of the cationic polymers were often reflected in the physical conditions of the cuttings. During both field tests, it was observed that when the concentration of the encapsulating polymer was low, cuttings became smaller but remained moderately firm. Conversely, when the concentration of the swelling-suppressing polymer was low, cuttings became softer and mushy.
These changes in cuttings quality as a function of polymer concentration also agreed with dispersion test results obtained in the laboratory.
ACKNOWLEDGMENT
The authors would like to thank M-1 Drilling Fluids Co. for permission to publish this article and the field personnel for their cooperation.
REFERENCES
- Kendall, H.A., and Norton, P., "Clay Mineralogy and Solutions to the Clay Problems in Norway," Journal of Petroleum Technology, January 1974, p. 25.
- Beihoffer, T.W., Dorrough, D.S., and Schmidt, D.D., "The Development of an Inhibitive Cationic Drilling Fluid for Slim-Hole Coring Applications," SPE paper 19953, presented at the 1990 SPE/IADC Drilling Conference, Houston, Feb. 27-Mar. 2, 1990.
- Smith, J.E., "Taking Care of Your Redbed Problems," presented at the 30th Annual Southwestern Petroleum Short Course, Lubbock, Tex., Apr. 27-28, 1983.
- Fleming, C.N., "Moderate pH, Potassium, Polymer-Treated Mud Reduces Washout," SPE paper 14758, presented at the 1986 SPE/IADC Drilling Conference, Dallas, Feb. 10-12, 1986.
- Liao, W.A., and Siems, D.R., "Adsorption Characteristics of PHPA on Formation Solids," SPE paper 19945, presented at the 1990 SPE/IADC Drilling Conference, Houston, Feb. 27-Mar. 2, 1990.
- Retz, R.H., Friedheim, J., Lee, L.J., and Welch, O., "An Environmentally Acceptable and Field-Practical, Cationic Polymer Mud System," SPE paper 23064, presented at the Offshore Europe Conference, Aberdeen, Sept. 3-6, 1991.
- Theng, B.K.G., Formation and Properties of Clay-Polymer Complexes, Elsevier, 1979.
- Brindley, G.W., and Brown, C., "Crystal Structures of Clay Minerals and Their X-Ray Identification," Mineralogical Society, London, 1980.
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