DRILLING FLUID ADDITIVES

Drilling Fluid additives come in a huge variety and cover a vast range of chemical types. It is not possible to effectively cover the full range of materials in this context.

“World Oil” publish an annual drilling fluid guide. This is essential for Wellsite Geologist, who will come across many unknown products during their careers and it is highly recommended that copies be kept in all oil company libraries. The use of back copies is a major help when researching old wells that contain additives with only their trade names listed.

Instead of detailing a large range of individual products it will be of greater use to briefly cover the types of materials and their general applications. Individual chemical information should be sought from the “World Oil Guide” with questions addressed to the relevant drilling fluid company when possible.

5.5.1 Fluid Loss Control Additives

Many types of materials are available to control fluid loss in water-based muds. Their application will vary according to the type of mud being used and the chemical environment of the mud. The most common fluid loss control agents are the clays.

5.5.1.1 Clays

The primary fluid loss control agent for most water-based muds are the clay solids present in the system. The starting point for good filtration control is a fluid having the correct particle size distribution of solids. This would be interpreted as a mud having a wide range of particle size, a large percentage of these particles being one micron or less.

In effect, this particle size distribution produces a filter cake having both low porosity and low permeability. To achieve this, an attempt is made to obtain a maximum density filter cake. This simply means a gradation of particle sizes such that each successively smaller particle size plugs openings existing between the larger size particles.

Commercial clays not only develop viscosity but also have the ability to lower fluid loss. Bentonite is particularly suited for the purpose. The ability of bentonite to reduce filtration can be attributed to: (1) particle size, (2) the flat plate-like shape of the particles, and (3) the capacity to hydrate and compress under pressure. Other factors being equal, the finer the particle size, the lower the fluid loss. Bentonite has a larger percentage of fine particles than any other clay.

A deficiency of good bentonite solids in the mud causes increased fluid loss, particularly at increased temperature and pressure.

5.5.1.2 Starch

If the mineral colloids are not enough to lower the fluid loss, organic colloids such as starch may be used. In some muds, the addition of starch not only lowers fluid loss, but stabilises the mud as well. The majority of starch used in the oil industry is made from corn. The starch grains are separated from the corn and specially processed so that they will rapidly and efficiently swell and gelatinise to furnish maximum reduction in fluid loss. This should occur regardless of pH, hot or cold water. Such starches are known as pregelatinised starches. The outside amylopectin shell is ruptured with heat, liberating the amylose inside which absorbs water and swells to form sponge-like bags. The loss of free water from the system aids in the reduction of the fluid loss.

The sponge-like bags will fit into the tiny openings left in the filter cake and lower the fluid loss by a plugging action. At 250°F, polysaccharides are temperature stable. However, when the temperature is raised to 275°F, the chemical will decompose rapidly.

Starches are effective as a drilling fluid stabiliser when evaporate or hydratable shale sections are penetrated and for stabilisation, filtration and rheological control of high electrolyte content workover fluids.

5.5.2 Polymers

With the advances in drilling fluid technology, the usage of water soluble polymers has developed several mud systems that have reduced fluid losses. There are several type polymers that are capable of reducing fluid loss characteristics of a mud system. The type polymers are classified by their action within a mud system. This classification is based upon the polymer’s performance by either adsorption onto the solids or by viscosifying the fluid phase. The most common fluid loss polymers viscosify the fluid phase to reduce fluid loss in a given mud system. These polymers are normally anionic and are less sensitive to their environment. They will develop viscosity in the presence of hardness and salt. However, high concentrations may reduce their effectiveness. Extreme care should be taken when adding a polymer, due to the possible interactions of several chemicals present in a mud system. In field operations, thorough pilot testing is essential before additions are made to the active system.

5.5.2.1 Sodium Carboxyl-Methyl Cellulose (CMC)

CMC is an organic colloid used for filtration control. The structure of CMC is that of a long chain molecule that can be polymerised into different lengths or grades. The material is commonly made in three grades, each varying in viscosity, suspension, and fluid loss reduction qualities.

The three grades are commonly called high viscosity CMC, medium or regular viscosity CMC, and low viscosity CMC. They will also vary in their purity.

Four theories have been advanced to explain the reduction of fluid loss by CMC.

1. A wadding or wedging action of the long chains into narrow openings.

2. A curling up of these long chains into a ball that forms a plug.

3. A filming of the clay particles.

4. Viscosification of the fluid phase.

CMC is an effective fluid loss additive in most water-based muds, working particularly well in the calcium treated systems. It will also stabilise calcium and sodium systems. CMC is not subject to bacterial degradation and performs well in all alkaline pH ranges. CMC becomes less effective

as a water loss control agent as salt concentrations exceed 50,000 ppm. CMC is subject to complete degradation when temperatures exceed 250°F.

The choice of CMC to use depends upon what properties are desired. When suspending properties as well as low filtration rates are desired, the choice will generally be either high or medium viscosity CMC. When less viscosity and a decrease in fluid loss is desired, the low viscosity CMC is adequate.

5.5.2.2 Polyanionic Cellulose

Polypac is an organic fluid loss agent designed particularly for application in muds containing high salt concentration for low solids drilling fluids. The material is a polyanionic long chain polymer of high molecular weight. Pac will give both viscosity and fluid loss control in either fresh or salt water.

Pac is effective in the makeup and maintenance of low solids muds.

5.5.2.3 Chemical Thinners/Dispersants

The use of chemical thinners is not generally required in modern muds. High rates of polymer additions are required to maintain viscosity in the low solids environments and thinning is infrequently required. Environmental restrictions on the use of Chrome (an essential ingredient in most thinners) has also resulted in a general loss of effectiveness and hence a reduction in their use. The change in basic drilling fluid philosophy from one of causing the solids to disperse into the mud, and hence keeping viscosity down, to one of encapsulating solids and preventing their hydration is the single biggest factor in the demise of the thinners/dispersants use.

Improved dispersion generally results in decreased filtration and higher solids. This is a result of an homogeneous distribution of the various particle size solids present in the fluid. Some dispersants are better than others, however, when used in sufficient quantities most are effective.

Lignite and Lignosulphonate (Tannathin, XP-20, and Spersene) all have exceptional ability to lower fluid loss.

The proper use of these materials greatly extends the ability to control the drilling fluid at elevated temperatures and pressures far beyond that which can be obtained with starch or CMC. The materials are not easily affected by bacterial degradation and can be used effectively in high salt or calcium concentrations. It is significant that these chemicals will reduce both the API fluid loss and the HTHP fluid loss as well. At temperatures above 315°F increased amounts of XP-20 should be used and decreased amounts of Lignosulphonate used. The modified lignite is extremely temperature stable above 450°F. Therefore, with increased temperatures, lignite should be utilised to provide proper fluid loss control in dispersed systems.

5.5.2.4 Viscosifiers

Bentonite, Gel or Sodium Montmorillonite have traditionally been the viscosifying base for almost all drilling fluid systems. Bentonite is still the most widely used viscosifier and has particular application in top hole drilling. The material will provide exceptional fluid loss control and provides a base on which many polymers act. Bentonite will provide Yield Point which is essential to hole cleaning and will not suffer the temperature thinning effects that are the trade mark of many polymer systems.

Biopolymer and PHPA polmyers now also dominate the viscosfying functions of drilling fluids.

Biopolymers find application in almost all drilling environments and are untroubled by the bulk of

drilling contaminants. This branch of chemicals is of increasing importance as more is understood about the hydraulics of cleaning horizontal and extended reach wells.

The shear thinning nature of the group enables low solid fluids to be built and maintained at a cheap cost. The systems are relatively stable chemically. The ability of these materials to revert, and their biodegradability, allow for cleaner fluids to be built that clean up at the well’s completion without major stimulation.