DRILLING FLUID SYSTEMS

Drilling fluid systems have continuously evolved to their present state.

Systems that for many years performed extremely well have recently become “extinct” with the advances being made in Polymer Chemistry. Systems that were once thought to be highly effective have been superseded as more sophisticated testing has revealed either shortcomings in their performance or that their effects on the formation are unacceptable. An example of this is the demise of the CL/CLS (Chrome Lignite/Chrome Lignosulphonate) System.

Early drilling fluids were only concerned with the drilling aspects of wildcatting. Mud Systems were designed to get cuttings from the hole and to control subsurface pressures only. The understanding that a mud with density control which balanced formation pressures would reduce hole problems, resulted in further formulation changes. Speciality completion fluids were formulated as production EOR methods were refined.

Solids levels in the fluid were of little concern and many systems including the Gyp and CL/CLS systems actually required high solids levels to be effective.

Emphasis over the years has therefore changed from a mud that will “get you to TD”, to a fluid that will provide maximum formation protection, return the maximum amount of geological information, provide the fastest possible drilling environment and remain cost effective.

Many differing systems have evolved as these goals change from operator to operator, and from well to well to best suit the conditions.

In general, basic description categories are recognised, although almost all have numerous subsets.

Drilling fluids have generally been divided into water-based and oil-based systems. A new breed of synthetic systems is now being pushed to market. These “new” muds will continue to infiltrate the drilling world as costs decrease and as environmental pressures make their upfront cost less important or regulation makes their use mandatory.

5.6.1 Water-Based Systems

Water-based muds are described by their major constituents. Names vary occasionally for the same system, depending on the emphasis required at the time of drilling. Systems are often converted from one type to another adding to the definition confusion.

Water-based names will ideally contain any major salt additions and the principal polymer used in its construction. Examples include KCL/Polyacrylamide, Seawater/Drispac.

Names will also often include a trade name if it is well recognised (or the mud company has done a good marketing job) eg Freshwater Polysal or Spersene/XP-20.

5.6.2 Oil-Based Systems

Oil-based systems commenced in the Gulf of Mexico with the addition of diesel to the drilling fluid to aid lubricity. Increases in the concentration were seen to aid in the inhibition of shale swelling.

The development of the Invert Oil Emulsion was the next logical development. Variations in the oil/water ratio have been made over the years as emulsifying technology improved and the cost of oil escalated. Simply put, the more water in the emulsion the less oil and hence the lower cost.

5.6.3 System Summary See table below.

The aim of effective fluid selection is to match formation chemistries with down-hole expectations and to provide an inhibiting environment to the drilled cuttings and the well bore. All muds have advantages and disadvantages.

5.7 NEW DRILLING FLUID SYSTEMS

Mud System Advantages Disadvantages

Spud Muds Cheap, Simple Chemistry No Inhibition

Gel Polymer Cheap, Low Solids, Versatile Little Inhibition Lime Systems Cheap, Solids Tolerant,

Relatively Inhibitive Salt Systems Relatively, Inhibitive, Low

Solids, Moderate Price Poor Filtrate Control, Poor Lubricity, Weight Limitations

Excellent Inhibition, Excellent Lubricity

A range of new drilling fluid systems (most proprietary) is finding a way to market as environmental pressures continue. The search continues for a fluid that will replace oil-based drilling muds at a reasonable cost. Different companies have had different approaches to the problem and a wide range of “exotic” materials have been tested, or are being tested. These include peanut oil and palm oil. Adverse effects on blowout preventer rubbers can occur and oil fluorescence can be masked. It is up to the wellsite geologist to ensure he is aware of the properties and individual characteristics of the base fluid that is being run.

5.7.1 Synthetic Systems (Novadril)

This is a low toxicity, synthetic-based emulsion system that offers inhibition, wellbore stability, lubricity, and temperature stability that previously could only be achieved with toxic oil-based mud systems.

Novadril is formulated with a base carrier; a synthetic oligomer that is not produced directly or indirectly from crude oil.

The system is formulated as an invert emulsion in which the synthetic oligomer based fluid forms the continuous phase. A brine solution serves as the dispersed phase. As with an oil based system, oil/water ratios can range from 90/10 to 60/40. Oligomers can be contrasted to mineral oils which are manufactured from crude by a refining process. These mineral oils comprise a broad range of hydrocarbons of varying toxicities, including some aromatics.

5.7.2 Oligomers do not contain toxic hydrocarbons.

This makes the system non-toxic and it will not upset wellsite hydrocarbon searching. Physical properties vary from oil-based systems principally by the higher viscosities that are experienced in the oligomers. Other beneficial properties of the replacements are the high flash point and low pour values that the systems exhibit. High flash point provides a safer environment as there are not volatile components which could ignite and the low pour point enables the system to be run in the coldest of environments. All oil-based alternatives are expensive.

5.7.3 Envirotherm

This is a mud system designed for high temperature drilling that is chrome-free and acceptable for drilling in environmentally sensitive areas. Envirotherm is temperature stable in excess of 400°F (240°C). Envirotherm utilises two products; a proprietary chrome-free lignosulphonate and a water-soluable, polymeric resin as the primary temperature stabilisers.

5.7.4 A Cationic Water-Based Mud System

The Cationic system is a newly developed water-based mud system, utilising exotic cationic polymers to provide exceptional shale stabilisation. It is designed for drilling water-sensitive shale formations containing highly reactive clay minerals.

Cationic polymers tend to be adsorbed more strongly than anionic polymers by attaching to clay surfaces dominated with negative charges. This strong adsorption can be utilised for better encapsulation and swelling suppression, hence superior shale inhibition. Drilling fluids containing properly selected cationic polymers have been proven to be more inhibitive than conventional water-based muds.

Previous utilisation of cationic polymers to formulate practical drilling fluids was unsuccessful due to their high toxicity and strong interaction with other mud additives. The toxicity made former cationic polymers envrionmentally unacceptable; while the interaction made them less effective as severe flocculation and precipitation often occurs with bentonite, drill solids, anionic polymers, and weight materials.

By applying advanced cationic polymer chemistry, the cationic system prevents the undesired interactions. Cationic polymers with low toxicity are carefully selected to provide shale inhibition without sacrificing environmental accountability.

The cationic systems consist of an encapsulating polymer (MCAT), a swelling suppression polymer (MCAT-A), and chloride-enhanced water phase. The system is formulated to be compatible with conventional anionic polymers and weight materials. A non-ionic polymer also can be used as a secondary shale encapsulator to enhance inhibition.

Because of its highly inhibitive nature, the cationic system can be considered as an alternative to oil-based muds. The low toxicity nature of the cationic system makes it desirable for drilling operations in offshore and other environmentally sensitive areas.

5.7.5 Causes of Shale Inhibition

Many shale-related drilling problems can be attributed to hydration (water adsorption) of clay minerals, which changes the physical strength of shales, leading to disintegration of cuttings, swelling, and sloughing into the wellbore. To stabilise a shale formation, and to prevent shale cuttings from dispersing, hydration of clay minerals must be reduced during drilling.

The most troublesome swelling clays are smectite, illite, and mixed-layer clays. They are commonly referred to as 2:1 layer clays. Each layer is composed of AO-O-OH (octahedral) sheet sandwiched between two Si-O (tetrahedral) sheets. Due to ion substitution in the octahedral and/octahedral sheets, an overall charge is generated on the surfaces of each layer. The negative charges are balanced and the layers weakly held together by interlayer cations. During hydration, water molecules can be attracted to interlayer cations as well as to clay layers by hydrogen bonding.

The type of interlay cation is important in determining the amount of water that can be attracted;

for example, sodium attracts more water molecules than calcium or potassium.

Two mechanisms are often cited to account for the hydration of clay minerals; they are surface (crystalline) hydration and osmotic (intercrystalline) hydration.

During surface hydration, one to two layers of water molecules can be adsorbed by both interlayer cations and clay surfaces. The distance between clay layers increases as the interlayer cations move away from the clay surfaces. As a result, volume of the clay may double upon surface hydration.

Certain types of clays such as smectite with sodium or lithium as the interlay cation, can absorb water molecules continuously through the development of diffused double layers. This phenomenon is similar to osmosis, which is the transfer of water through a semi-permeable membrane. It is often called osmotic hydration. Volume increase associated with osmotic hydration is usually several times greater than that due to surface hydration.

A shale tends to undergo surface hydration immediately upon contact with water and may continue to absorb water osmitically, depending on the type of clay minerals, exchangeable

cations, and its water content. Absorption of cationic polymers on clay surfaces can reduce surface and osmotic hydration.

5.7.6 Inhibition

To maintain shale stability, it is necessary to prevent clays from hydrating and swelling. The stabilising effects of the cationic systems are primarily derived from two cationic polymers, which provide polymer encapsulation and swelling suppression, respectively.

Since over 90% of the clay surface is negatively charged, sites for cationic (+) polymer attachment are readily available. This is in sharp contrast to the attachment of anionic polymers which occurs at the positively-charges edges. Thus, cationic polymers can provide more effective shale inhibition than anionic polymers.

The higher molecular weight cationic polymer is the primary encapsulating agent for the cationic system. Its large molecular size allows it to be attached to the exterior surfaces of clay particles through ionic bonding. A protective layer of polymer is formed to prevent cuttings from dispersing. A secondary encapsulating agent enhances shale stability by attaching to shale particles through hydrogen bonding.

The inhibition provided by MCAT-A is of a different nature. Because of its small molecular size, it can penetrate the clay layers and adsorb on the interior surfaces. The adsorbtion of MCAT-A results in a complete displacement of interlayer cations, consequently retarding surface hydration and impeding osmotic swelling of the clays. The absorption of MCAT-A is rapid and more effective than the common cation exchange reaction.

Cuttings to surface have the appearance of oil-based cuttings due to the very low swelling tendencies of the system. This improvement in inhibition enables the Wellsite Geologist to see considerably more sample than was previously possibly.

Continuing advances in polymer chemistry and the reduced use of hydrocarbon-based products will lead to steady improvements in the quality of returns coming across the shakers.

6 Abbreviations

See separate files (wsgman_OMV_app_6_a & _b).