ABSTRACT
Appropriate selection of rheological models is important for
hydraulic calculations of pressure loss prediction and hole cleaning
efficiency of drilling fluids. Power law, Bingham-Plastic
Herschel-Bulkley models are the conventional fluid models used in the
oilfield. However, there are other models that have been proposed in
literature which are under / or not utilized in the petroleum industry.
The primary objective of this study is to recommend a rheological model
that best-fits the rheological behaviour of xanthan gum based biopolymer
drill-in fluids for hydraulic evaluations. Ten rheological models were
evaluated in this study. These rheological models have been posed
deterministically. Obviously this is unrealistic so these deterministic
models are replaced by statistical models by adding an error
(disturbance) term and making suitable assumptions about them.
Rheological model parameters were estimated by least-square regression
method. Models like Sisko and modified Sisko which are not conventional
models in oil industry gave a good fit. Modified Sisko model which is a
four parameter rheological model was selected as the best-fit model
since it produced the least residual mean square. There is 95% certainty
that the true best-fit curve lies within the confidence band of this
function of interest.
CHAPTER ONE
INTRODUCTION
1.1 INTRODUCTION
The use of rheological models to approximate the behaviour of
non-Newtonian fluids is very paramount in the oil and gas industry
especially during drilling, well completion, workover and acidizing. In
drilling operations, mathematically designed rheological models are used
to describe the viscous forces to develop frictional pressure loss
equations. Accurate prediction of pressure losses help in the
determination of bit optimization hydraulics, estimation of equivalent
circulating density (ECD) and drilling fluid compressibility. The
benefits of a more accurate estimation of ECD is adequate hole cleaning
efficiency to enhance total drilling rate which in turn reduces total
drilling cost. Prevention of circulation loss, maintenance of
under-balanced drilling conditions and detection of potential kick are
achieved if ECD is rightfully predicted (Bailey and Peden, 2000).
Estimated model parameters help to perform other hydraulics
calculations.
Power Law and Bingham Plastic models are widely used for hydraulics
evaluation. They are assumed for standard API hydraulics calculations.
Herschel-Bulkley, Roberston-Stiff and Casson models have been accepted
to some extent in the petroleum industry. These models and the
corresponding hydraulic calculations do provide a way for fair estimates
of hydraulics for conventional wells using simple drilling fluids as
asserted by Guo and Hong in 2010. Power Law model predicts shear stress
well at low shear rate (in the annulus) and Bingham Plastic model
describes the characteristics of drilling fluid at high shear rate (in
the drill pipe).
Biopolymer drill-in fluid is a complex fluid formulated with several
compositions to desired properties for optimum performance particularly
in unconventional wells. It is a water soluble 'rheology engineered'
drilling fluid designed to optimize the performance of rotary drilling.
It is a complex high molecular weight (MW) polymer with a strong bond
between the chains of its molecules which is efficiently used in
unconventional wells like onshore and offshore horizontal wells, coiled
tubing drilling and slim holes. The elastic structures of biopolymers
make them have a higher carrying capacity than the other polymers
applied in the petroleum industry during drilling. Due to the complex
nature of this type of fluid and its unusual behaviour, it is very
prudent to use a more precise rheological model to characterize its
behaviour over a full range of shear rate to achieve a proper hydraulics
evaluation.
Drill-in fluids are specially designed fluid system for drilling
through the reservoir interval of a wellbore. They are basically
formulated to drill the reservoir zone successfully, often a long,
horizontal drainhole, to minimize damage and optimize the production of
the exposed zones and to enhance the well completion needed. It contains
additives that can principally control filtration loss and facilitate
optimum carrying capacity. Its composition may be brine with right
aggregate size (salt crystals or calcium carbonates) and polymers
(www.oilfield.slb.com). Brian et. al (1997) asserted that polymers
typically used as drill-in fluids are xanthan gum, starch, cellulose and
scleroglucans. Hemphill et al. in 1993 proposed that Herschel-Bulkley
model which is a three-parameter model is more likely to approximate the
non-Newtonian behaviour of polymeric fluids.
This study focuses on ten rheological models proposed in various
literatures and come out with a statistical criterion to select the most
likely model to predict the rheological characteristics of xanthan gum
base biopolymer drill-in fluids.
1.2 PROBLEM DEFINITION
The use of rheological models in the characterization of the
behaviour of non-Newtonian fluid aids in the evaluation of drilling
fluid hydraulics. The Power Law and / or Bingham Plastic models are more
often used in evaluating hydraulics of drilling fluid in the oilfield
during drilling operations. These are used because their resultant flow
equations are simple and it is also easier to estimate parameters of the
models by explicit solutions. However, none of these is able to predict
the behaviour of the fluids over the wide ranges of deformation rate
during the circulating of drilling fluid system throughout the wellbore.
The advent of computers makes it realistic to estimate parameters of
more complicated models and thereby deriving expression of pressure drop
as function of flow rate. Rig site computers are now readily available
making the requirement for simple parameter estimation and easily
manipulated flow functions redundant and provide a platform conducive to
more rigorous analysis.