BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
Figures 11-7 and 11-8 above illustrate very well the usefulness of the ADVANTAGE hole cleaning
modeling. The only difference between the two examples is that the drilling fluid density in Figure
11-7 is 11.0 ppg and in Figure 11-8 is 13.0 ppg. This change in density reduces the flow rate
necessary to clean the hole from 760 to 569 gpm.
Because of the many variables involved in hole cleaning it is absolutely essential to use a simulator
to study the combined effects and be able to examine “what if?” situations.
Hole Cleaning Guidelines
The single most important factor related to hole cleaning in deviated wells is flow rate. During
directional drilling operations drilled cuttings will settle on the low side of the hole and form a
stationary bed if insufficient annular fluid velocity is used. The critical flow rate required to
prevent cuttings bed formation can be determined from ADVANTAGE.
Typically few hole cleaning problems exist in vertical or horizontal sections. Most problems
associated with hole cleaning are seen on deviated wells and occur in the 40 – 60 degree section
where gravity effects can cause cuttings beds to slump down the hole.
Typical flow rates to aim for in ERD wells:
Hole Size
Typical Flow Rates
17 ½” 1100 gpm minimum. Some rigs achieve 1250 – 1400 gpm.
12 ¼” Aim for 1100 gpm (although 800 – 1000 gpm is typically achieved). If
1000 gpm is not achievable, ensure tripping procedures are in place for
poorly cleaned hole.
8 ½” Aim for 500 gpm.
When correctly designed, both laminar and turbulent flow regimes will effectively clean a deviated
well. In general increasing the viscosity of a fluid in laminar flow will improve hole cleaning as
will a reduction of the viscosity of a fluid in turbulent flow. It is important that one or other regime
is selected and that the transition zone between the two is avoided.
Generally viscous fluids in laminar flow are preferred because:
• It is possible to achieve higher cleaning capacity.
• Viscous fluids give better transport in the near-vertical sections.
• Viscous fluids have better suspension characteristics when circulation is stopped.
• It is difficult to achieve “turbulent flow” except in small hole sizes.
Experience has shown that good drilling fluid rheological properties are extremely important to
hole cleaning when drilling a high angle well. Studies show that the effects of drilling fluid
rheological properties and annular flow regime are mutually dependent. In the laminar flow regime,
increasing fluid viscosity will improve hole cleaning and this is particularly effective if the Yield
Stress is high. In the turbulent regime, however, reducing fluid viscosity will help removing
cuttings. Therefore the drilling fluid rheological properties should be designed to avoid the
transitional flow regime. For hole sizes above 8 ½”, the annular flow is laminar under most
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-17