THE FUNDAMENTALS OF FLUID CONTROL ARE KEY TO WELL CONTROL

The general functions of drilling fluids are fairly standardized. Since most drilling operations rely on liquid drilling fluids, we will make them our main concern. The eight basic functions of drilling fluids are listed below.

• Transportation of cuttings to surface.
• Suspension of cuttings when circulation is stopped.
• Control of annular pressure.
•  Lubrication and cooling of the drilling assembly.
•  Provision of wall support
• Suspension of drilling assembly and casing
• Delivery of hydraulic energy
•  Provision of a suitable medium for wireline logging.

  

TRANSPORT CUTTINGS TO SURFACE:

The hole must be properly cleaned to prevent cuttings from accumulating in the annulus, which could cause increased torque, drag, fill or hydrostatic pressure.

This may result in stuck pipe, loss circulation, pipe failure or a decrease in penetration. Since cuttings are heavier than the drilling fluid, they are lifted out of the hole by the fluid flowing in the annulus. Gravity will try to cause the cuttings to fall toward the bottom of the hole. The speed at which the cuttings fall depends on particle size, shape, density and fluid viscosity.

SUSPENSION OF CUTTINGS:

Cuttings will try to fall to bottom when circulation is stopped unless the drilling fluid forms a gel-like structure. This gel-like structure should suspend or hold the cuttings in place until circulation is started again. Excessive surge and swab pressures may be caused if the mud remains in a gel-like structure once circulation has started.

ANNULAR PRESSURE CONTROL:

Since formation fluids (oil, water or gas) are under great pressure, they must be balanced or overbalanced to prevent uncontrolled flow. The hydrostatic pressure of the mud in the annulus accomplishes this.

LUBRICATION AND COOLING:

As the bit drills on bottom and the drillstring turns in the hole extreme heat is developed. This heat must be absorbed by the drilling fluid and carried away from the bottom of the hole. The drilling fluid must also lubricate the casing, drillstring and bit. Lubricating properties can be improved by the addition of special materials (dispersants, friction reducers). This may also increase bit life, decrease torque and drag, reduce pump pressure and reduce frictional wear on the drillstring and casing.

WALL SUPPORT:

The formation could fall into the wellbore before casing is set unless support is replaced by the drilling fluid. The amount of support required to prevent this from occurring depends on the formation. Little support is needed in a very firm formation, whereas consolidated or fairly firm formations may be supported just by the mud density. In weak or unconsolidated formations the drilling fluid must have the ability to form a thin, tough wall cake in the hole.

DRILLING ASSEMBLY/ CASING SUSPENSION:

The drillstring and casing weight can exceed many thousands of pounds and develop extreme stress on the rig’s structure. These extreme weights can be partly supported by the buoyant force of the drilling fluid. This force is dependent on the weight of the fluid and the displacement of the pipe.

DELIVER HYDRAULIC ENERGY:

The drillstring and casing weight can exceed many thousands of pounds and develop extreme stress on the rig’s structure. These extreme weights can be partly supported by the buoyant force of the drilling fluid. This force is dependent on the weight of the fluid and the displacement of the pipe.

A high velocity is developed as drilling fluid passes through bit nozzles during circulation. This velocity, or hydraulic force, will keep the area under the bit clean, so the bit will not have to regrind the old cuttings, causing a reduction in penetration rate. The physical properties and velocity of the drilling fluid help keep the area under the bit clean.

SIDE EFFECTS:

The following side effects should be minimized while drilling.

• Open hole formation damage.
• Casing and drillstring corrosion.
• Penetration rate reduction.
•  Circulation, surge and swab problems.
• Lost circulation.
•  Drill string sticking.
•  Wellbore erosion.
• Settling in the pits.
• Mud pump wear.
• Cement and environmental contamination.

Formation damage can appear in two different forms: a reduction in hydrocarbon production or wellbore stability. Many types of drilling fluids will alter formation characteristics, but some formations are more sensitive than others and some fluids more damaging. Particularly sensitive formations (e.g., hydropressured or bentonitic shale may require special drilling fluids, treating chemicals or other considerations.

 CASING AND DRILLSTRING CORROSION

 The steel tubulars in the hole may be subject to a corrosive environment from the drilling fluid and formation. Chemical treatment of the drilling fluid or adding a protective coating to the surface of the steel can minimize the corrosive effect.

PENETRATION RATE PRODUCTION

Many factors affect the penetration rate, but the difference between formation pressure and hydrostatic pressure is the most significant. If the hydrostatic pressure of the drilling fluid is much higher than the formation pressure, a reduction in penetration rate will occur.

CIRCULATION, SURGE AND SWAB PROBLEMS

A thick filter cake can also contribute to surge and swab pressures that might result in a kick. Excessive viscosity limits the flow rate, puts extra stress on the pump and may also reduce penetration rates if sufficient pressure at the bit cannot be achieved.

LOST CIRCULATION

Lost circulation can be caused when hydrostatic pressure exceeds the strength of the formation. High pressures can also be the result of bad tripping or drilling practices, high mud weight and/or fluid viscosity. High drilling fluid and well cost, along with the chance of taking a kick are the results of lost circulation.

DRILLSTRING STICKING

An excessive amount of cuttings in the hole is one cause of pipe sticking, but the most significant type of sticking is when the pipe is embedded in a thick filter cake. Pipe sticking can lead to expensive fishing jobs and increase the well cost.

WELLBORE EROSION

Problems with wireline logging, cementing and stuck pipe are just a few of the difficulties of wellbore erosion. There are two types of wellbore erosion, physical and chemical. Pumping the drilling fluid up the annulus at a lower velocity will help reduce physical erosion. Chemical erosion depends on the chemical reaction between the drilling fluid and the formation.

SETTLING IN THE PITS 

The same gel strength that prevents the cutting from falling in the well when circulation is stopped can also prevent unwanted solids from falling in the pits. Gravity does cause some of the solids to fall to the pit bottom.

MUD PUMP WEAR

Those same solids can cause excessive pump wear if solids are not removed. The most abrasive solid is probably sand incorporated into the fluid while drilling. This sand should be removed by solids control equipment.

CEMENT/ ENVIRONMENTAL CONTAMINATION

Some drilling fluids that are good for drilling operations are incompatible with slurries of cement. A flush, wash or spacer fluid should be used to separate the cement and the drilling fluid.