Polymer Drilling Fluids in the 1990's: Will They Replace Oil-Based Muds?
Authors: Strickland S. Dwight 
Published: December 2007

Technology Today Series

Introduction

The 1990's have seen a surge in new water-soluble polymers available to the drilling industry. Many new polymers offer enhancements or significant improvements. It has often been said that "This polymer system is just as good as oil mud," or "This product will replace oil mud." These claims have negatively affected acceptance of new polymers, retarding their field applications regardless of technical and/or economic merit.

Many new polymers are gaining acceptance because of unique performance characteristics and are proving to be cost-effective. Other polymers have been slow to gain acceptance, not because of improper claims, but because of less-than-satisfactory technical and/or economic performance. Some polymers will never achieve stated claims, but others will be optimized or modified and find an application niche in the industry. No one polymer chemistry or system will replace all the performance characteristics of oil-based fluids; however, some polymers are being used successfully in some oil-mud applications.

Polymer Classification Overview

Polymers are used in virtually all water-based fluids, so the term"polymer" should not have a negative connotation. However, many in the industry view polymers negatively because of a lack of understanding. Also, many drillers, drilling superintendents, and drilling engineers have seen on location that polymers fail to achieve claims.

Polymers are classified as natural, modified natural, and synthetic. Natural polymers originate in nature (i.e., starch). Modified natural polymers are the result of a chemical reaction or modification to a natural polymer (i.e., carboxymethyl starch). Synthetic polymers are chemically reacted monomers [i.e., partially hydrolyzed polyacrylamides (PHPA's)]. The primary benefits of modified and synthetic polymers over natural polymers are increased temperature stability and contamination resistance.

A general understanding of polymer classification and knowledge of the strengths, weaknesses, molecular weight, and functionality can facilitate selection and treatment strategy for polymers.

Performance Characteristics

The following performance characteristics desirable in a drilling fluid are selected from a list of advantages of oil-based drilling fluids and will be used to compare the performances of polymers and oil-based drilling fluids.Thermal stability.Formation stability.Protection of production zone.Lubricity and torque/drag reduction."Drillability."Environmental compatibility.Stuck-pipe prevention.Corrosion protection.Resistance to contamination.

Thermal Stability. Synthetic polymer chemistry has probably made its most significant contribution to the drilling industry in high-temperature/high-pressure applications. During the deep-gas/high-bottomhole-temperature (BHT) drilling of the 1970's and 1980's, oil muds prevailed. During the late 1980's and early 1990's, numerous synthetic polymers proved technically capable and cost-effective at high BHT's and pressures. Synthetic polymers specifically formulated for deflocculating, fluid-loss control, and gel inhibition at high temperature proved to have wide utility. In 1989, Elsen et al. reported the use of a lime-based fluid at densities exceeding 18.0 lbm/gal with calculated BHT's above 350#F. Using limemuds at these hostile conditions would have been impossible without natural, modified, and synthetic polymers. Polymers used in this challenging environment included lignite, starch, polyanionic cellulose (PAC), polyanionic lignon, polymer-grafted lignosulfonate, modified polyacrylate terpolymer, and vinylamide/vinyl sulfonate copolymer. Numerous water-based systems incorporating various polymers have been used successfully and economically above 400 F with densities ranging from 15.0 to 18.0 lbm/gal.

Successful completion of these wells with water-based polymers requires prespud planning and laboratory optimization specific to the drilling objective. These prerequisites must not be compromised. Special programs must be in place with wellsite laboratory equipment and laboratory confirmation for this technology to be applied successfully.

Formation Stability. Various developments in the late 1980's and early1990's focused on improving formation stability by increasing the concentration of shale-inhibiting additives or by combining polymer additives that inhibit reactive shales. Preventing the hydration of swelling and/or reactive shales with water-based systems became the technical challenge of the drilling-fluid industry. A long list of polymers and polymer enhancers were thrust on the industry. Examples are saturated salt/PHPA, polyalcohol, polyglycol, cloud-point polyols, polyglycerine, cationic polymers, cationic starch, carboxymethyl starch, cationic PHPA, black liquids, black powders, potassiumpolyacrylates, potassium cellulosic polymers, polyamino acids, MMH, MMS, and methyl glucoside.

Cationic polymer systems received increased attention in the early 1990's.The industry saw a major operator and a major service company introduce and promote cationic chemistry as a replacement for oil-based muds. The reviews of these systems were mixed. While some reports indicate the success of these systems, more field experience is needed to determine long-termapplicability.

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