Introduction to Drilling Fluids

Drilling fluids is an important part in drilling process. The cost of this part can get to the level of 15% of total cost of operations. The cost can be higher if the drilling fluids are not well controlled and the properties are not maintained at their accurate range. Many problems during drilling operations can be caused directly or indirectly by mud. The consequence of bad treatment of mud can lead to drilling problems which take a great deal of time and by consequence cost to remedy them. 

The mud or drilling fluid has many functions. While selecting mud physical and chemical properties to fulfill these tasks, consideration must be given to:

- Environmental issues
- The total cost 
- The impact of drilling fluids on the production


Fig 01- Testing Drilling Fluid in The Field 

I-Drilling Fluids Functions

Removing cuttings generated while drilling is the primary function of the drilling fluid. The process of removing has to be continuously fulfilled. If this process is not well performed, cuttings can affect drilling efficiency. Also, if the cuttings are not transported out of the hole through the annulus, the drill string can become stuck, and the unstuck of it can take time and money or lead to sidetrack the well.

The drilling fluid has to carry cuttings to surface while circulating and suspend them while ceasing circulation. The mud properties have to be carefully designed to carry these requirements. 

Hole cleaning efficiency depends on density, viscosity and annular velocity. Suspending efficiency depends on gelling property; the gel is formed when stopping the circulation. The solids carried to the surface are removed from the mud before pump it again into the well by mechanical devices like shale shakers, desilters and desanders. If the drilling fluid is not treated effectively at the surface, this can lead to dilution and chemical treatment to control the rheological properties of the drilling fluid. 

Preventing formation fluids flow to the well bore

The drilling fluid column exerts a hydrostatic pressure; this pressure has to be high enough to stop any fluid to flow from the formation into the well which can disturb drilling fluid properties and cause uncontrolled situations.

The pressure in the well bore has to guarantee two issues:

- It has to be higher than the pore pressure to do not allow the fluids to flow into the well
- It has to be less than fracture pressure to avoid causing lost circulation

The pressure is related to the density and can be determined by:
P=0.052 x MW x TVD 

- MW: density or mud weight
- TVD: true vertical depth

There are many weighting products used to increase mud weight such as barite, Caco3 and haemitite. They are different in the level of increase that they can provide, for example the high level of barite density (4.2 sg) can help to use lower volume of this mineral to increase the density of the drilling fluids.

The filter cake is also an issue which has to be well studied when drilling through permeable formations. Filter cake can deposit on the wall of the bore hole while fluids seeps into the formation. The loss of drilling fluid products into the formation can plug the pores and cause near well bore permeably decrease. Also if the filter cake is thicker than normal range can cause stuck pipe.

Maintaining wellbore stability

There are many instability problems that can be faced while drilling (ex: instable shales, lost circulation, highly permeable zones)

Shale instability can be caused by two mechanisms:

- The pore pressure of shale is higher than the borehole pressure exerted by the column of mud
- Clays hydration by water contained in mud filtrate.

The shale instability caused by differential pressure can be overcome by increasing mud weight. The clay hydration can be overcome by drilling with non-water base muds, or treating mud with products which can reduce water ability to hydrate clays.

Cooling and lubricating the bit

The cutting process of the rock generates high level of heat at the bit. If the bit is not cooled effectively, it can overheat and wear quickly. The drilling fluid circulation through the bit will cool and lubricate it.

Transmission of hydraulic horsepower

Hydraulic energy is one of the important issues which has to be well designed to improve drilling performances. It provides energy for MWD, LWD tools and mud motors. Sizing the bit nozzles accurately can help to maximize pressure drop at the bit and consequently the jet impact force. The higher pressure drop at the bit helps to remove cuttings from the bottom and keep clean cutting structure.

The pressure losses in the mud circuit from the mud pump down to the bit up to the surface through the annulus are higher in drilling fluids with higher densities, viscosities and solids content. Also, the use of small ID of drill pipe, mud motors and MWD tools reduce the pressure which can be available at the bit.

Facilitate cementing and completing the well

Designing the mud properties properly helps to drill a well into which the casing will be run and cemented effectively. Cementing is important for zonal isolation and better well completion. Run casing in high yield mud can lead to lost circulation by surge mechanism. Also, running casing is easier in cleaned, smooth, in gauge wellbore, and these issues are got by well-controlled drilling fluids. 

Formation evaluation

Drilling is not just getting to target or reservoirs, it is also to collect information about the formations to avoid problems and optimize operations in next projects. The formations are studied and evaluated by the methods:

- Cuttings evaluation 
- Electric logs 
- Coring

The cuttings are got at the surface and analyzed by the mud loggers and geologist. Mud chemistry can affect the cuttings conditions and lead to miss the information which can be collected by studying these cuttings. The information gained by cores or logs can be affected by the filtration properties of drilling fluids. Thicker filter make logging operations less accurate and difficult. Filtrate invasion can mislead the results by cores. 

II- Drilling Fluids Properties

The viscosity

The viscosity is defined as the resistance of fluid to flow. The viscosity of drilling fluid is a function of:
- Viscosity of the continuous phase or the base liquid
- The size, shape and solids particles in the mud which is represent it by the plastic viscosity
- The inter-particle force represents it by the yield point

The plastic viscosity is the resistance to flow caused by the friction between solid particles. The yield point is the resistance to the initial flow or the stress required to start flowing. The yield point is measured under flowing conditions.

Reducing viscosity in any drilling fluid can be achieved by: 
- Reduction of solids by mechanical treatment or dilution
- Neutralization of attractive forces between particles.

Mud weight or density

The density is the weight per unit of volume. During operations mud weight has to be well controlled and need adjustment. If the mud weight is less than the required level of density to drilling safely can permit the formation fluids to flow into the well and lead to well control situation and if it is higher than the appropriate level it can lead to lost circulation situations. 

The real mud weight under circulation is greater than the density while drilling fluid is in static. This density is called equivalent circulating density (ECD). When designing hydraulics, the ECD has to be less than fracture gradient.

Filter cake

The filter cake is formed when mud solids deposit on the walls of the hole. The filtration is the loss of fluids from mud into the formation. The main objective of controlling fluid loss is forming thin filter cakes while drilling through permeable formations and preventing excessive fluid loss (filtrate).

Loss of drilling fluids in formation can lead to high water consumption and thick filter cake which can cause a tight hole, increased torque and consequently lead to stuck pipe.

Solids content

Solids can be: 

- Added to the drilling fluid in order to increase viscosity or weight
- Accumulated in the fluid like drilled cuttings or disintegrated clay particles.

The treatment of solids has to be effective to remove undesired solids which do not contribute to beneficial properties.

Gel strength

Gel strength represents the attractive forces under static conditions (non-flow conditions). Contrary to gel strength, the yield point represents the attractive forces under flowing conditions. The gel strength gives an indication of the required pressure to initiate flow after ceasing circulation for period of time. Also, it gives an indication about the ability of the drilling fluid to suspend cuttings when mud is stationary. 

Gels are classified into two types: 

- Progressive that starts low but increase with time
- Fragile that starts high and increase slightly with time

Progressive gels are not accepted in drilling fluids because they can create excessive pump rates to break circulation which can lead to lost circulation situation.

III- Drilling fluids composition

Drilling fluid is consisted of liquid phase and solid phase. In some cases where two liquids are used to prepare the drilling fluid, the mud is described as two liquid phase this case one liquid is defined as continuous phase and emulsified in the other one that is called discontinuous phase. For example in the oil based mud, the continuous phase is the oil, and the discontinuous phase is the water. 

There is a verity of liquids which can be used as base fluid: water, crude oil, diesel. The products which are added to mud system have to be able to dissolve in the base fluid as salts for density, and chemicals for alkalinity.

Another factor that can affect mud properties is the reaction between the formation and the drilling fluid. For example salts can come from salts sections and ions can come from clay reactions.

Solids are two types:

- Mud products are classified in two classes: active and inert. Active solids react with the base fluid to give mud a particular property such as the bentonite that is used for adjusting viscosity. Inert solids do not react with the base fluid like the barite that is used for increasing mud wight.
- Drilled solids or cuttings and they also classified to active and inert according to the formation which is drilled.

IV- Drilling fluid classification

The drilling fluids are classified into two classes according to the continues phase or basic liquid: water based mud and oil based mud.

Water based mud

- Non dispersed system: this type of mud is used at top holes as spud mud or lightly treated muds. Chemicals like dispersants and thinners are not used to disperse cuttings and clay particles.
- Dispersed system: in deeper wells where high densities are required to control the well, mud is dispersed. Potassium-containing products are frequently added to the mud to provide shale inhibition. 

- Calcium treated systems: calcium and magnesium are added to fresh water drilling fluids in order to inhibit clays and shale swelling. High concentration of soluble calcium is used to control hole enlargement and sloughing shale to prevent any formation damage. Calcium mud systems resist to salt and anhydrite contamination but are vulnerable to gelation and solidification at high level of temperature.

- Polymer system: polymers of long chains, high molecular weight are used to encapsulate cuttings to prevent dispersion and coat shale for inhibition. They are also used to increase viscosity and reduce fluid loss. A verity of polymers is added for these purposes such as cellulose and natural gum based products. KCL and NaCl are added to provide higher shale stability. The polymer system is temperature limited system; they cannot be used for temperature higher than 300 F°. 

- Salt water systems: this type of drilling fluids is used to drill through salt formation. the system is saturated and have chloride concentration up to 190000 mg/l. 

Oil based mud

In oil based mud the base fluid is the oil. At the early days crude oil was used as continuous phase but it suffered from many disadvantages: low gel strength, limited viscosity and beside the mud properties, the safety hazards were a real issue. 

Oil based mud is classified into three types:

- Full oil system (water content less than 05%)
- Invert oil emulsions (water content from 5 to 50%)
- Synthetic oil based mud

Invert emulsion system

They are water in oil emulsions. The emulsified phase is the calcium chloride brine and the continuous phase is the oil. It is characterized by its low electrical stabilities and high fluid loss values. Oil systems require additional gelling agents for viscosity. These types of systems have many advantages. They are not very reactive. They are also stable at high temperature and pressure, high contamination resistance and not corrosive, but also they have some drawbacks: they are expensive and harmful for environment.

Synthetic muds

synthetic fluids are used to replace oil based muds. They have the same advantages without the environmental hazards. Esters, ethers and isomerized alpha olefins can be used to prepare this type of drilling fluids.

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