ESTER-BASED MUDS SHOW PROMISE FOR REPLACING SOME OIL-BASED MUDS

Patrick Kenny
Statoil
Stavanger, Norway

The use of ester-based mud systems, instead of oil-based mud, helped drill difficult shales and high-angle well sections, including one of the world's longest extended reach wells. Statoil has successfully used ester-based muds on ten wells during a 3-yr period.

Although the results from the ester-based muds were satisfactory, Statoil continues to study other mud systems as alternatives to oil-based muds.

In light of pending legislation regarding the use of oil-based mud in the North Sea, in 1989-90 Statoil began investigating various drilling fluids to replace oil-based systems and found ester-based muds very attractive.

LEGAL AUTHORITIES

In the Norwegian sector of the North Sea, the Olje Direktoratet (OD) gives consent to drilling programs, including mud selection, prior to the drilling of a well. During the drilling of a well the operator must also receive consent from the OD for any major changes to a drilling program, such as changing the casing or mud program.

Oil-based muds or pseudo oil-based muds may only be used if the muds are necessary based on technical or safety reasons. For example, in certain highly deviated sections through troublesome shales, oil-based mud is required for the section to be drilled successfully-an acceptable use of oil-based mud.

The Statens Forurensning-stilsyn (SFT) regulates all discharges offshore Norway. The SFT has set a limit of 1% by weight for oil on cuttings discharged to the sea. All discharge permits, whether for a field development or for a single well, must be approved by the SFT.

The SFT will approve the discharge of ester-retaining cuttings only if the OD has given prior approval and if ester-based mud is being used as an alternative to oil-based mud and not as an alternative to water-based mud.

Originally (that is, until sufficient data were gathered) discharge permits were only granted on a single-well basis. Currently, discharge permits may be applied to longer-term projects. The permits have various requirements, including toxicity testing of whole mud and cuttings, sea bed sampling, and a mass balance calculation. Currently, the SFT has a positive attitude towards ester-based muds, considering them to do little environmental damage and to biodegrade on the sea bed.

ECONOMICS

The ester-based muds were originally planned to be marginally cheaper than the cost of oil-based muds combined with the cost of destroying the cuttings on land. It is debatable whether the costs were actually less. However, on older installations that were not designed for handling oil-contaminated cuttings, the ester-based muds are less expensive to use.

Well CO2 in the Statfjord field illustrates the economics of using ester-based mud. The minimum cost for the handling, transportation, and destruction of oil-contaminated cuttings from the 12 1/4-in. and 8 1/2-in. sections would have been 6 million Kroner ($900,000). This well was to be drilled in the North Sea during the stormiest part of the winter, creating logistical problems with the boats, with the use of the cranes on the rig, and with the storage of the containers.

A conservative estimate of lost time was 6 rig days at a cost of 1 million Kroner/day. The total mud cost with oil-based mud used in the 12 1/4-in. and 8 1/2-in. sections was conservatively estimated at 1.1 million Kroner. At least another 1 million Kroner should be added to allow for equipment problems (conveyor breakdowns, plugging of chutes, etc.).

Thus, the total costs associated with the use of oil-based mud could have been approximately 14.1 million Kroner ($2.1 million). The cost for the ester-based mud was 11.5 million Kroner ($1.7 million).

When ester-based muds first appeared on the market, the cost of a mud formulated to Statfjord specifications was approximately 15,000 Kroner/cu m ($385/bbl). Because of competition and the inevitable decrease in price after the initial wells, an ester-based mud with the same specification now costs less than 10,000 Kroner/cu m. The price could reasonably drop to as low as 8,000 Kroner/cu m in the foreseeable future.

WORKING ENVIRONMENT

The drilling fluids working environment has been greatly neglected by many operators in the past. Every employer is now required to provide a safe and healthy environment for its employees and the contractors' personnel.

This requirement has led to the development of automatic, dust-free systems for mixing mud additives, covered mud tank systems with built-in extractors to eliminate fumes, and adequate ventilation in the vicinity of solids control equipment.

Some older installations cannot meet these requirements without considerable expensive upgrading. For example, at Statfjord A, the oldest of the Statfjord platforms, the present equipment and working environment preclude the use of oil-based mud.

In May 1991, Statoil conducted a sampling program while well A27 was drilled with ester-based mud and compared the results to a 1985 study conducted on a well drilled with a low-toxicity oil-based mud. On both occasions, the flow line temperature was 70 C.

The 1991 report concluded "there is doubt that this ester-based mud gave considerably better air quality than when low toxicity oil-based mud was in use. The present concentration of hydrocarbon fumes is less than 10% of that found during the previous study." This report confirmed the claims that the working environment with ester-based muds was better than that for oil-based muds.

One slight problem in the field, however, is to teach the workers that ester-based muds should be treated the same as oil-based muds, especially with regard to protecting clothing.

MARINE TOXICITY

All drilling muds used in Norway have been classified as moderately toxic or highly toxic depending on the results of toxicity tests performed on Skeletona costatum, Balanus improvisus, and Mylitus edulus. Some ester-based muds have been classified as highly toxic because they did not meet the required toxicity levels, but the majority of fluids tested have been classified as moderately toxic.

The overall performance of ester-based muds in marine toxicity tests is considered only marginally superior to oil-based muds. The source of the toxins has not yet been traced, but it is believed to be from the mud additives. The toxicity of the additives must be reduced if the use of these systems is to be continued.

Test results from ether-based muds and polyalpha-olefin-based muds compare favorably with those from ester-based muds.

SEA BED SAMPLING

Cuttings contaminated with ester-based mud are currently allowed to be discharged to the sea once the cuttings have been processed by the solids control equipment. No weight percent limit has been set for the amount of ester retained on the discharged cuttings.

The actual amount of ester retained on cuttings is no less than that for oil-based mud, and the amount usually ranges about 9-20%, depending on the surface area of the cuttings.

Sea bed sampling has been conducted in the area surrounding the Statfjord platforms on a yearly basis. Since 1990, the sea bed samples have been analyzed with a method designed to detect the particular ester used in the mud formulation.

The 1992 survey detected the base ester in significant quantities in only one sample; the concentration was 1.2 mg/kg dry weight. The sample was taken 500 m at a direction of 310 from platform A.

The report concluded that the ester degrades more quickly than hydrocarbons, and the biological analysis showed no additional influences resulting from the use of ester-based mud. Based on these and other surveys, the SFT said: "The required environmental surveys have shown that the base fluid in ester-based muds is almost entirely degraded around the platforms."

APPLICATIONS

Statoil uses minimal environmental impact practices in its drilling operations, and this policy has led to the eventual use of ester-based muds.

Statoil has used ester-based mud in 10 wells, 9 in the Statfjord field and 1 from the Loke subsea template (Table 1).

Well C11 was selected as the first trial of ester-based mud because the well was relatively undemanding and several cores were to be cut. The fluid performed satisfactorily, but there were some problems controlling the theology.

The next well, C03 (a record extended reach well prior to Well C02), was planned to use oil-based mud in the 17 1/2-in., 12 1/4-in., and 8 1/2-in. sections. The 17 1/2-in. section, later under-reamed (UR) to 22 in., was considered impractical for oil-based mud. Water-based mud was also discounted because of fears the under-reamer would ball with cuttings, preventing retraction of the arms and subsequently causing extreme difficulty in pulling it out of the hole.

An ester-based mud was therefore used, and the results were satisfactory. Hole cleaning problems, not surprisingly, were experienced and counteracted by back-reaming.

The next four wells, all on the A platform, used ester-based mud for these same reasons. The success of these wells gave Statoil the confidence to use ester-based mud on Well C02.

Well 15/09-19S, drilled from the Loke template, had extreme difficulty with water-based mud prior to the change to the ester-based mud.

The last two wells used a different base ester and mud system than the previous wells.

WELL C02

Well C02, on the Statfjord C platform, was drilled to 8,761 m (28,743 ft) total measured depth and is currently the world's longest extended reach well. The well was planned with the following mud program: polyanionic cellulose regular/sea water in the 26-in. hole, KCI/polyanionic cellulose/partially hydrolyzed polyacrylamide in the 17 1/2-in. hole, oil-based mud in the 12 1/4-in. hole, and oil-based mud in the 8 1/2-in. hole.

The first two sections were relatively straight forward and problem free. While the 17 1/2-in. hole section was drilled, it was discovered that the oil-based mud planned for the 12 1/4-in. section had too high a friction factor for the section to be drilled to the programmed depth. The friction factor was initially obtained from data from a previous well and was immediately confirmed by tests at an independent laboratory.

Water-based muds were not even considered because no known water-based mud has a sufficiently low friction factor. The four muds in Table 2 are lab formulations of possible mud systems; field muds were not available for testing at the time. Based on these lab tests, it was decided to use Mud B instead of Mud A. Shortly after deciding on Mud B, it was found to be impossible to use any of the well annuluses on Statfjord C platform for cuttings injection, leading to the "nightmare" situation of transporting cuttings to land.

For economic reasons, ester-based Mud C was then chosen. Ester-based Mud D was discounted because it was not yet field proven and because Well C02 was an unsuitable test well.

The mud was displaced into the hole using only viscosified ester-based mud as a spacer. The mud provided excellent hole conditions and hole cleaning through the entire 5,332 m of the 12 1/4-in. interval. The performance of the mud exceeded all expectations, and the theology profile was the most efficient yet obtained with ester-based muds. The theology was critical because of the mud's effects on circulating pressure, equivalent circulating density, barite sag, and cementing.

The only problem was some barite sag, which seems virtually unavoidable in invert emulsion muds and with the demands of drilling extended reach wells. Table 3 lists the mud properties and system formulation for this hole section.

The success in the 12 1/4-in. section indicated the 8 1/2-in. section would also be fairly successful. The following demands on the mud in this section were among the most extreme ever placed on a drilling fluid. It should:

• Help prevent differential sticking with an overbalance of 100 bar (1,500 psi)

• Prevent fracturing of the coal zone in the upper Brent formation with a fracture pressure equivalent to 1.72 sp gr (14.3 ppg)

• Drill with a weight lower than the theoretical collapse pressure

• Clean the hole efficiently

• Enable successful cementing of the 1,500-m, 7-in. liner

• Prevent settling because it could create a well control problem with the reservoir exposed.

Table 4 lists the mud properties and formulation used to drill this interval successfully.

Several problems were experienced at one particular depth while tripping. The shale was particularly troublesome, having a collapse pressure less than the mud weight. Subsequent logs showed this zone to be washed out severely; that is, the hole collapsed at this point because the mud weight was lower than the collapse pressure.

The 7-in. liner could not be run to total depth, but the completion program was not adversely affected. Nonetheless, the cement bond was excellent. Overall, the drilling fluid performed well and played an integral part in this well.

PLANS

Statoil plans to continue using pseudo oil-based muds as long as they meet both its and the legal authorities' requirements. The mud companies should continue research to reduce the toxicity of their fluid systems.

Several recent developments with water-based muds and pseudo oil-based muds show promise as possible replacements for oil-based muds: polyglycol/polyglycerine systems, glycol systems, ether systems, polyalphaolefin systems, biodegradable diesels, and biodegradable base oils.

MYTHS DISPELLED

The successful use of ester-based mud on these wells, especially the difficult extended reach wells, helped dispel several common myths about these muds:

• Ester-based muds attack the various rubber elements in the drilling equipment system.

  This problem was not serious and is rarely mentioned anymore as a concern.

• Ester-based muds cause poor cement jobs.

  This situation is not true if good cementing practices are followed, just as with any other mud system.

• Ester-based muds are nontoxic.

  With regard to the SFT's toxicity testing procedure, the same problems are encountered with the ester-based muds is with standard low-toxicity, oil-based muds.

• There is only one ester-based mud system on the market.

  Ester-based muds are obtainable from all the major mud companies.

ACKNOWLEDGMENT

The author wishes to thank Den norske stats oljeselskap AS (Statoil) and its partners for permission to publish this article.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.