ABSTRACT
Cuttings transportation during in non-vertical boreholes is necessary
for oil and gas wells. Adequate cuttings removal from a well in
drilling is critical for cost-effective drilling as high annular
cuttings buildup often leads to high risk of stuck pipe, reduced rate of
penetration and other impediments to standard drilling and completion
procedures.
This study investigates how rheological parameters influence the
removal of cuttings in non-vertical boreholes. It contributes to work
already done to ensure efficient hole cleaning process. In this study,
the rheological parameters examined were the flow index (n), consistency
index (K), plastic viscosity (PV), mud yield point (YP), YP/PV ratio,
apparent viscosity and effective viscosity. Fifteen mud samples, three
annular velocities (3.82, 2.86 and 1.91 ft/sec) and three hole angles
(30o, 45o and 70o) were considered. An Excel Spreadsheets program was
used to determine the parameters. The results of this study show that,
higher annular mud velocities are required for efficient hole cleaning
in directional wells than in vertical wells. Increasing values of YP,
YP/PV ratio and K promote effective cuttings transport while the value
of n should be low. Effective and apparent viscosities also should be
high.
CHAPTER 1
INTRODUCTION
1.1 Problem Definition
Many materials of engineering interest must be handled and
transported as slurries or suspensions of insoluble particulate matter.
Transportation of cuttings in non-vertical boreholes is of no exception.
Almost the same thing occurs whereby the cuttings act as the solids in
the drilling fluid. In spite of the many technological advances that
have accompanied the drilling of non-vertical boreholes, one significant
remaining challenge is effective cuttings transport, particularly in
deviated wells.
The transportation of cuttings during drilling has a major influence
on the economics of the drilling process. Problems that can occur as a
result of inefficient hole cleaning from cuttings include reduced weight
on bit, increase risk of pipe stuck and inability to attain the desired
reach, reduced rate of penetration (ROP), extra cost because of the
need of special additives in the drilling fluid, extra pipe wear,
transient hole blockage which can lead to lost circulation and wasted
time for wiper tripping. These problems have prompted significant
research into cuttings transport during the past 50 years. (Kelessidis,
2004).
Hole cleaning relying on viscous fluids in laminar flow for drilling
has proved to be inefficient because of the inability to rotate the
string to agitate bedded cuttings. Alternatively, a high fluid flow to
induce turbulent flow regime is more effective for hole cleaning, but
difficult to achieve because of high friction pressures in the
drillpipe. Therefore a bed of cuttings is almost always present in
non-vertical boreholes. For laminar flow, the distance that a particle
will travel (downstream) before it falls across the annulus clearance
can be calculated using Stokes’ law and the local viscosity while
flowing can also be calculated. This analysis may be easily applied to
optimize mud selection and wiper trips. Applying this model to high
low-shear rate-viscosity (LSRV) gels shows that they may perform well
inside casing but are expected to do a poor job of hole cleaning in a
narrow openhole horizontal annulus without rotation.
For turbulent flow in horizontal wells, the concept of using annular
velocity (AV) as a measure of hole cleaning is insufficient. A more
complete term called AVRD is introduced, which is the product of the AV
and the square root of the hydraulic diameter. This term can be used to
compare cuttings transport in turbulent flow in horizontal wells of
different cross sectional areas. (Leising et al., 1998).
Rheology which is the study of the flow and deformation of fluids is
an important contributing factor to these problems. Rheology describes
the relationships between shear rate and shear stress. Pilehvari, Azar,
and Sanchez2,16 state that fluid velocities should be
maximized to achieve turbulent flow, and mud rheology should be
optimized to enhance turbulence in inclined/horizontal sections of the
well bore.
The purpose of this study is to investigate how rheological
parameters influence the removal of cuttings in non-vertical boreholes.
1.2 Objectives
The objectives of this research are:
· To present a review of cuttings transport in vertical, directional/horizontal well bores.
· To provide a critical review of how rheology affects cuttings transportation in non-vertical boreholes.
· To identify the critical parameters that affect effective
removal of cuttings in the drilling of non-vertical boreholes.
· To propose a methodology for analysing the rheological
parameters that affect cuttings transportation in non-vertical
boreholes.