BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
by utilizing a variety of highly soluble formate salts to achieve the required density. By selecting
the correct salt, any density in the range 1.0 (8.34 ppg) to 2.35 S.G.(19.6 ppg) can be achieved with
a solids-free system.
The advantages of a solids-free system are significant. The plastic viscosity of a fluid is greatly
affected by the quantity of suspended solids. Consequently, clear brine systems will exhibit lower
plastic viscosities than conventional fluid systems. This will result in lower equivalent circulating
densities (ECD) and lower pressure losses downhole, resulting in greater drilling efficiency. These
two effects are particularly important in slim hole drilling where, because of the narrower pipe and
annulus diameters, the reduction of downhole pressure losses and ECDs is essential to ensure good
rates of penetration while preventing fracturing of the formation.
Environmental considerations are playing an increasingly significant part in drilling activities
throughout the world, and drilling fluids have come under particularly close scrutiny, both onshore
and offshore, in terms of their environmental potential. The CLEAR-DRILL system has been
developed partly in response to environmental pressures for cleaner drilling fluid systems. This has
been achieved by utilizing biodegradable salts with very low acute toxicity in conjunction with
benign polymers and by eliminating the routine use of weighting materials, many of which contain
quantities of heavy metals such as lead, cadmium, and mercury.
Clearly, no system can be acceptable solely on environmental grounds; a high level of performance
must also be present. Fluids in the CLEAR-DRILL system provide optimum rheological
properties, excellent stability and a very high degree of inhibition to reactive shales. The absence of
barite helps to ensure that the CLEAR-DRILL fluids are non-damaging to the pay zone, thus
eliminating the need for more complicated “drill-in” fluids. Other salts such as sodium chloride or
potassium chloride can be used as bridging materials, providing the necessary fluid loss control
with materials which are readily removed during the well clean-up operation. The CLEAR-
DRILL system comprises three brines: sodium formate, potassium formate, and cesium formate.
All these salts can be used either to formulate solids-free drilling fluid systems or in conjunction
with conventional weighting materials to build regular drilling fluids.
Function of Formates
The use of clear brines for drilling is not a new idea. Sodium chloride and calcium chloride have
been available for many years and the former, in particular, has been widely used as the basis for
low-weight drilling fluids. These brines however, have significant limitations. Sodium chloride is
limited to a maximum density of 1.2 S.G. (10.0 ppg) while calcium chloride brines suffer from a
high degree of incompatibility with the majority of polymers used in conventional drilling fluids.
The use of highly soluble salts to provide density rather than insoluble weighting materials can
have a major effect on solids loading within the system. The benefit of a clear brine system
increases with increasing density as shown in the figure below.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-29