Mud engineer

From Wikipedia, the free encyclopedia

A mud engineer (correctly called a Drilling Fluids Engineer, but sometimes referred to as the "Mud Man", though women also do this job today) works on an oil well drilling rig, and is responsible for the drilling fluid, also known as drilling mud which lubricates the drill bit and clears cuttings from the borehole.

1 Use of mud
2 The Job
3 Lost circulation
4 Further Reading
5 See also
6 Notes

[edit] Use of mud

Main article: drilling mud

Mud is a vital part of drilling operations. It provides hydrostatic pressure on the borehole wall to prevent uncontrolled production of reservoir fluids, lubricates and cools the drill bit, carries the drill cuttings up to the surface and forms a "filter-cake" on the borehole wall to prevent drilling fluid invasion. To fulfill these tasks effectively, the mud contains carefully chosen additives to control properties such as viscosity and density.

Drilling mud is usually a shear-thinning non-Newtonian fluid of variable viscosity. When it is under more shear, such as in the pipe to the bit and through the bit nozzles, viscosity is lower which reduces pumping-power requirements. When returning to the surface through the much roomier annulus it is under less shear stress and becomes more viscous, and hence better able to carry the rock cuttings. Bentonite is commonly used as an additive to control viscosity, and also has the additional benefit of forming a mud-cake (also known as a filter cake) on the bore-hole wall, preventing fluid invasion.^[1]

Barite is commonly used to "weight" the mud to maintain adequate hydrostatic pressure down-hole. This is critical in a drilling operation to avoid a kick and ultimately a blowout from uncontrolled production of formation fluids. The "mud-pits" at the surface have their levels carefully monitored since an increase in the mud level indicates a kick has taken place and may require shutting in the well and circulating through heavier drilling mud to prevent further formation fluid production.

Drilling fluid must be chemically compatible with the formation being drilled, salinity must be chosen so as not to cause clay swelling or other problems. Mud can be "oil-based" or "water-based", in many areas oil-based muds are being phased out as they are seen as being less environmentally friendly, although in some formations they are necessary because of chemical compatibility issues.

[edit] The Job

The mud engineer (or drilling fluids engineer) is usually a college or technical graduate, who has been on a special training course, known as "mud school". Prior to drilling, a program will be worked out according to the expected geology. As the hole gets deeper, more mud is required, and the mud engineer is responsible for making sure that it is made up to specification. The chemical composition of the mud will be designed so as not to destabilize the hole. It is sometimes necessary to completely change the mud to drill through a particular subsurface layer.

As drilling proceeds, the mud engineer will get information from the mud logger about progress through the geology and will make regular physical and chemical checks on the drilling mud. In particular the Marsh funnel viscosity and the density are frequently checked. As drilling proceeds, the mud tends to accumulate small particles of the rocks which are being drilled through, and its properties change. It is the job of the mud engineer to specify additives to correct these changes, or to partially or wholly replace the mud when necessary. He or she must also keep an eye on the equipment which is used to pump the mud and to remove particles, and be prepared if the geologists' predictions are not entirely correct, or if other problems arise.

It is sometimes necessary to stabilize the wall of a borehole at a particular depth by pumping cement down through the mud system, and the mud engineer is in charge of this process.

The mud engineer is well supported by the mud supply company with computer aids and manuals dealing with all known problems and their solution, but it is his or her responsibility to get it right in a situation where mistakes can be very costly indeed.

[edit] Lost circulation

One of the most important mud properties is the mud weight (density). If the mud weight exceeds the fracture pressure of the formation, the formation may fracture and large quantities of mud are lost inside it, in a situation referred to as lost circulation. These cracks can also cause water to seep into the well bore or into a hydrocarbon bearing zone, which would likely impede the ability of the formation to produce oil (or require the separation of large quantities of water).

Conversely, if the mud weight is too low it will have a hydrostatic pressure that is less than the formation pressure. This will cause pressurized fluid in the formation to flow into the wellbore and make its way to the surface. This is referred to as a formation "kick" and can lead to a potentially deadly blowout if the invading fluid reaches the surface uncontrolled.

Other important mud properties to be maintained are the YP (Yield Point) which determines the carrying capacity of the mud to suspend and carry the drill cuttings and the ability of the mud to form "mud cake" which forms a lining of the borehole walls.

[edit] Further Reading

ASME Shale Shaker Committee (2005) The Drilling Fluids Processing Handbook ISBN 0-7506-7775-9
Kate Van Dyke (1998) Drilling Fluids, Mud Pumps, and Conditioning Equipment
G. V. Chilingarian & P. Vorabutr (1983) Drilling and Drilling Fluids
G. R. Gray, H. C. H. Darley, & W. F. Rogers (1980) The Composition and Properties of Oil Well Drilling Fluids