BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Physical and Chemical Properties of Fluids
The physical and chemical properties of a drilling fluid play an important role in the success of a
drilling operation.
The properties of drilling fluid are perhaps the only variables of the entire drilling process that can be
altered rapidly for improved drilling efficiency. These properties usually receive the greatest
attention.
API Recommended Practices
The American Petroleum Institute (API) has set forth numerous recommended practices designed to
standardize various procedures associated with the petroleum industry. The practices are subject to
revision from time-to-time to keep pace with current accepted technology. One such standard is API
Bulletin RP 13B-1, “Recommended Practice Standard Procedure for Field Testing Water-Based
Drilling Fluids”. This Bulletin described the following drilling fluid measurements as necessary to
describe the primary characteristics of a drilling fluid:
• Density – for the control of formation pressures
• Viscosity and Gel Strength – measurements that relate to a mud’s flow properties
• Filtration – a measurement of the mud’s loss of liquid phase to exposed, permeable formations
• Sand – the concentration of sand (solid particles < 74µ) being carried in the mud
• Methylene Blue Capacity – an indication of the amount of reactive clays present in the mud
• pH – a measurement of the alkaline / acid relationship in the mud
• Chemical Analysis – qualitative and quantitative measurement of the reactive chemical
components of the mud
Chemical properties, such as chloride content, total hardness, etc., are important. They are discussed
in Chapter 4 of this manual, “Contamination of Water-Base Fluids”.
Density
The density of any fluid is directly related to the amount and average specific gravity of the solids in
the system. The control of density is critical since the hydrostatic pressure exerted by the column of
fluid is required to contain formation pressures and to aid in keeping the borehole open. Fluid density
in English units is commonly expressed in lb m /gal (lb m /ft 3 in some locations) and in specific gravity or
g/cm 3 in countries utilizing the metric system.
The density of any fluid should be dictated by formation pressures. The density must be sufficient to
promote wellbore stability. The pressure exerted by the fluid column should ideally be only slightly
higher than that of the formation to insure maximum penetration rate with minimal danger from
formation fluids entering the well bore.
The common method for checking the density of any drilling fluid is the mud balance. The mud
balance consists of a supporting base, a cup, a lid, and a graduated beam carrying a sliding weight. A
knife edge on the arm rests on the supporting base. It has become common in many locations to use
pressurized mud balances as these are considered to be more accurate.
Rheology, Viscosity and Gel Strength Relationships
The rheolgical properties, viscosity and gel strength of drilling fluids describe the ability of the fluid
to transport cuttings while drilling and suspend them when circulation is interrupted. Frequently, the
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 1-5