BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
The recognition of the impact of drill string rotation on hole cleaning was one of the factors that
encouraged the use of rotary steerable systems.
Turbulence
Bottom hole assembly design becomes important because the flow restrictions of some of the
components, such as the measurement-while-drilling tools or positive displacement motors, may
not allow sufficient flow to reach turbulence in the annulus. Turbulence can, of course, be induced
at some constant flow rate by decreasing the annular area. This may be more difficult to
accomplish in a horizontal well because of limitations on the design of drill strings and tool
assemblies. An increase in the effective viscosity of the drilling fluid will decrease the Reynolds
Number, and thus increase the flow rate required to obtain turbulent flow. Thus, a lower effective
viscosity fluid may be desirable in the horizontal well, but attention must also be given to cleaning
the larger diameter portion of the upper hole or possible washed-out sections where it is impractical
to maintain turbulent flow.
It is interesting to point out a basic fact between static fluid pressure and dynamic fluid pressure.
Static fluid pressure is a function of true vertical depth, while dynamic fluid pressure, or ECD, is a
function of measured depth. Thus, the ECD will continue to increase, albeit slowly, even after the
hole becomes essentially horizontal. The fracture pressure could be reached if the hole was
extended to sufficient depth and the fluid density is close to the fracture gradient.
ADVANTAGE Hole Cleaning Modeling
Baker Hughes Drilling Fluids’ ADVANTAGE SM hole cleaning simulator is an invaluable
engineering tool. Hole cleaning analysis is a critical technology for annular pressure management.
The hole cleaning of highly deviated wells is so complex that it can only be adequately analyzed
with the use of a simulator. The hole cleaning models within the Advantage System Engineering
software can be used to predict minimum flow rate for hole cleaning (optimization mode) or to
analyze cuttings transport and cuttings bed formation based on a user-defined flow rate (analytical
mode). The hole cleaning model is based on the balance of forces acting on cuttings in the annulus.
Advantage System hole cleaning models provide the drilling engineer with a useful tool for
identifying areas in the wellbore where problems related to hole cleaning may occur. Input
parameters include cutting size and density, mud weight and rheology, ROP, survey, drill string,
wellbore geometry and flow rate. The models uses a force balance analysis within each grid and
identifies the most difficult grid for hole cleaning.
Advantage System hole cleaning modeling is routinely used in well planning and drilling phases to
predict hole cleaning efficiency and analyze cuttings beds. This information is useful in identifying
the location and size of cuttings beds, and their impact on annular pressure (ECD). In many
instances, the root cause of annular pressure spikes and increases is poor hole cleaning (annular
cuttings loading) rather than flow rate or drilling fluid properties
Further information on ADVANTAGE hole cleaning modeling is available in the ADVANTAGE
manual and is provided during ADVANTAGE training. There follows two examples from
ADVANTAGE simulations.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-15