Formación del Comité Interno de Protección Civil

Drilling fluids serve many purposes but their primary functions are to

• Lift formation cuttings to surface

• Control subsurface pressures

• Lubricate the drill string

• Clean the hole

• Aid in formation evaluation

• Protect formation productivity 5.2.1. Cuttings Lifting

Clearing the hole of cuttings is an essential primary function of a drilling fluid system and is often the most misinterpreted and abused. Drill solids are heavier than the mud and will tend to slip downward against the flow. This slip velocity when the fluid is in viscous of laminar flow is directly affected by the thickness or shear characteristics of the mud. The relationship between mud velocity and thickness to enable cutting removal is important and if velocity is low due to pump rate or enlarged hole sections, then the mud must be thickened and vice versa.

Water based muds are thickened by adding bentonite, large volumes of solids, flocculation or by the use of special additives. This provides the operator with a choice of options, however the use of bentonite is the most popular as it is relatively cheap. When using bentonite, sometimes a thinner needs to be added to prevent flocculation and water loss control problems.

The use of large quantities of solids is an undesirable solution if it is not required to increase mud weight for subsurface pressure control. Usually a mud selection is a compromise of all the various problem solutions and often the lifting capability is not effective. What may have begun as a simple mud thickening problem is complicated by the resulting effects on the other mud objectives.

5.2.2. Subsurface Well Control

It is always desirable to utilise the lowest possible mud weight to achieve maximum drilling rate and lost circulation problems are minimised. However, the hydrostatic pressure applied by the mud must be greater than the highest formation pressures to effect pressure control.

To determine the mud weight required, it is necessary to obtain predicted formation pore pressures and the fracture gradient. The mud weight selected must exceed the formation pore pressures in each section but to minimise drilling problems and still not exceed the fracture pressure. This is sometimes a fine balancing act between satisfying well control and not exceeding the rock strength in weak zones.

Formation pressure and temperature prediction is usually found be using offset well data but can also be predicted (refer to section 2). Normal formation pressure gradients are 0.465psi/ft but vary from region to region. It is important that overpressure are predicted and monitored for during drilling.

Once the formation pressures for a section are known, a safety margin must be added and then mud weight calculated:

P_F = Formation pressure, psi TVD = True vertical depth, ft

Example: A formation pressure has a pressure of 4,020psi at 8,500ft, a safety margin of 600psi is desired, what is the required mud weight ?.

ppg

Safety margins are usually around 0.2ppg but may vary according to conditions.

Example, a mud with a 700psi safety margin at 10,000ft will only provide a 350psi margin at 5,000ft. It may be decided to use an increased mud weight at the shallower depths if the margin is too small.

To calculate pressure at a given depth and mud weight the calculation is:

TVD

Mud weight is increased by the addition of heavy solids.

5.2.3. Lubrication

Lubrication and cooling are also important functions of the mud. Working life of expensive equipment can be prolonged by adequate cooling and lubrication. Problems such as excessive torque, drag and differential sticking are also reduced.

Lubricants include bentonite, oil, detergents, graphite, asphalts, special surfactants and walnut shells. Bentonite acts as a lubricant by reducing friction between the wall cake and the drill string. Oil is less used today due to the environmental impact and disposal problems and similar to graphite as it also requires oil as a carrier. Asphalt is usually added for its other properties but also acts as a lubricant. Surfactants have been claimed to lubricate but this should be analysed as they are more expensive.

5.2.4. Bottom-Hole Cleaning

Thin fluids with high shear rates through the bit are the most effective at hole cleaning and means that viscous fluids can be used if they have shear-thinning characteristics. In general fluids with low solids contents are more effective in hole cleaning.

5.2.5. Formation Evaluation

Drilling fluids have been effect greatly by the requirement for quality formation evaluation.

Viscosity may be increase to ensure improved cutting lift, filtration may be reduced to reduce fluid invasion or special fluids used instead of the mud system for logging and well testing. The procedures for mud conditioning before logging have become standard today.

The type of mud will also have an effect, e.g. oil based mud make evaluation of potential producing formations difficult and salt water fluids can mask permeable zones.

Thick filter cake can interfere with side wall coring information and water or oil invasion affects resistivity logs.

The formation evaluation programme must take all of these considerations into account to obtain the best results.

5.2.6. Formation Protection

In the past it has been proven that the drilling process and fluids will cause damage to producing formations and the utmost precautions should be taken to minimise this damage.

The ideal protection policy is to keep all foreign fluids away from the formation, however in most cases this is impractical, unless air drilling, and hence the drilling fluid should be selected according to conditions. For instance, oil based mud can be used when it is desirable to keep water off a zone, however oil based mud may be more damaging to gas zones than salt water fluid, etc. Salt water fluid with high calcium content have also been effective.

To help minimise invasion, reduction in the filtration rate may be employed but reliance on static surface testing as assurance may be misleading on actual downhole filtration rates.