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					                                                                  Oil and Gas Exploration


Assoc. Prof. Vassilios C. Kelessidis1
 Mineral Resources Engineering          Department   Technical   University   of   Crete,
kelesidi@mred.tuc.gr – Greece

CONFERENCE & EXPO, Modern Management of Mine Producing, Geology and
Environmental Protection SGEM 2009, Albena, Bulgaria, June 14-19.

Continuously increasing demand for energy resources drives petroleum industry to
search in very deep waters, drilling very deep wells, encountering adverse conditions of
pressures and temperatures and thus having to resolve a multitude of problems. In many
cases, the problems can be attributed to the performance of drilling fluids which then
presents challenges to drilling fluid industry. Such problems include poor hole cleaning,
high pressure losses, loss circulation zones, fluid gelation, reservoir fluid invasions.
Thus, drilling fluid industry must work hard to overcome these challenges figuring out
solutions which include fluid formulations capable of good continuous performance
under all adverse conditions. In this paper we address these issues, we present the
adverse conditions in the deep drilling environment of pressures and temperatures, and
the implications they have on flow pressures, formation damage and well control. We
analyze the approaches taken by different operators in terms of drilling fluid design and
implementation in such wells from a series of reported case studies. These include
rheological measurements at high temperature and pressures and attempts to model-fit
the data to predictive rheological models which could then be used in appropriate
hydraulic models to estimate pressure drop profiles along the wellbore. High
temperatures present additional stability challenges to drilling fluids which could be
overcome with the addition of appropriate additives. The search for such additives is
continuous and in this paper we discuss various alternatives, the operational procedures
and address also potential problems. Finally, we present possible optimum solutions to
overcome such problems and increase the success rate of drilling fluid performance in
these difficult conditions.
Keywords: drilling fluids, deep drilling, challenges, rheology, mpd

Oil and gas still remain the inexpensive fuels which turn the world economy. Thus,
hydrocarbon exploration has been intense during the past years and industry is now
searching in total depths and water depths never thought before. The environment is
very hostile and challenging for the drilling industry, characterized with high pressures
and high temperatures, hard rock, and presents severe wellbore stability problems [1-4].
Drilling industry has developed drilling technology adequately so that deep wells can be
drilled safely, but in order to produce them these wells need to be completed and
industry has identified technology development gaps which still require design and

International Multidisciplinary Scientific GeoConference SGEM 2009

development [5]. The industry continuously formulates new drilling fluids to deal with
such hostile environments and recent reviews have addressed these issues [6].

The drilling industry is changing and avoids previous practices of heavy overbalanced
drilling, trying to achieve a balanced drilling scenario, and applying Managed Pressure
Drilling (MPD) techniques, monitoring all available parameters, in order to achieve
faster drilling rates. For optimum application of managed pressure drilling, availability
of drilling fluids capable of maintaining their stability under down hole conditions is
essential. Drilling hydraulic simulators are essential and there is a need for fairly
accurate simulators and good and appropriate rheological model such as Herschel-
Bulkley fluid which describe well the behaviour of these complex fluids [7]. In addition,
good hydraulic models are needed to predict pressure drop in annuli for these fluids
covering the whole range of laminar, transitional and laminar flow, especially in new
approaches like casing drilling [8], and recent advances make it possible using simple
approaches [9].

Deep drilling presents great challenges to the industry, requires large and expensive
rigs, encounters hard rock thus producing very low penetration rates, making the whole
effort even more expensive. One of the most recognized drilling challenges for deep
drilling and in particular drilling in deep waters is the smaller tolerance between pore
pressure and fracture pressure gradient which results in narrow pressure margins (Fig.
1), where it is shown the wider pressure margin for shallow water and the much
narrower pressure margin for deep water.


Fig. 1 – Pore and Fracture pressure window in shallow and deep water (adapted from
Rocha et al. [2]

The close pressure tolerance demanded new approaches to pressure control and the
process has been developed, Managed Pressure Drilling, which utilizes friction pressure
and annular back-pressure to control difficult drilling environments. MPD procedures
do not have open circulation system, rather a close loop circulation system is used (Fig.
2), which, depending on the environment may be designed for shore and offshore
applications [10]. These systems allow for the application of back-pressure thereby
enabling custom-designed control of pressure along the wellbore. There are several

                                                                 Oil and Gas Exploration

MPD applications, starting from underbalanced drilling to constant bottom hole
pressure drilling [8]. Some concepts call for dual gradient drilling which is considered
as an option, not only for oil and gas production but also for scientific drilling [11].
There are many parameters that play a part in the pressure profile in the wellbore, like
density, rheology, flow geometry, injection rates, back-pressure (or choke pressure).
The effects of these parameters are different, but interact with one another and full
comprehensive models are needed to take them into account [12]. One should also take
a closer look at the number of fluids present in the wellbore during MPD drilling
because up to four fluids may be encountered simultaneously along the wellbore, gas,
liquid, droplets and solids, thus necessitating the correct modelling approach, not only
in terms of rheology bur also in terms of hydraulics.


Figure 2. Variations of Managed Pressure Drilling for offshore application (adapted
from Hannegan, [10]).

MPD allows the operator to dynamically manage any influxes enabling improved well
control because it provides flexibility to manipulate annular pressure. Applications of
MPD are gaining in momentum and several cases have been noted indicating the
benefits of such techniques. For e.g. Perez-Telles et al. [13] demonstrated the benefits
during drilling in Mexico at depths of 6,424 m with a 1.50 SG drilling mud and with
MPD, only one trip was performed before reaching total depth, thus saving 30 hours in
Bit vibrations also have been identified as deep hard rock drilling problems, while more
efficient hydraulics and fluid delivery systems, like, bigger surface pumps and larger-
diameter drill pipe, have helped increase drilling efficiency and reduce the costs of
drilling during the past couple of decades, and all these require good formulation of
drilling fluids and an understanding of the functions of all additives [3].

Drilling fluids perform a multitude of tasks while drilling, offering hydrostatic
pressures, cooling the bit, transporting cuttings to surface, maintaining wellbore

International Multidisciplinary Scientific GeoConference SGEM 2009

stability. The drilling fluids must have good stability in terms of density and rheology.
In addition, good hydraulics and temperature simulators and phenomenological models
are needed to allow predictions of circulating pressures and temperatures, as the
operating pressure margins between formation fluid pressures and formation fracture
pressures are fairly narrow. However, up until now rheological and density
measurements were performed at atmospheric conditions for most of the wells while for
more demanding wells, measurements were performed for up to 177 °C and 1400 bar.
There are, however, needs, for e.g. in deep gas drilling in the offshore continental shelf
of Gulf of Mexico, which may demand measurements at pressures up to 2100 bar and
temperatures up to 316 °C [11], thus new viscometers have been sought to perform such
measurements [14]. In Fig. 3, rheograms of drilling fluids measured at different
temperatures and pressures, showing the effects of higher as well as lower conditions in
the field are shown [15]. Various polymers are added to WBM, OBM and SBM fluids
to enhance thermal stability and recent work [16,17,18] has demonstrated the ability of
lignite addition to thermal stability of water-bentonite dispersions.

Fig. 3. Rheograms of drilling fluids at high temperature and pressures. The effect is
significant (from Davison et al. [15].

When applying MPD techniques, a good understanding of the effects of the parameters
affecting pressure profile along the wellbore is essential and because the rheology of
MPD fluids plays an extremely important role, appropriate rheological models should
be used and derived from appropriate viscometer studies, using the readings at all six
speeds, and maybe even more, on the viscometer [4,12]. In Fig. 4 an example case for
application of MPD with a back pressure is shown.

Deep water drilling often uses synthetic-base drilling fluids (SBM) with properties that
can address these challenges, because they offer faster penetration rates and enhanced
wellbore stability. In cases, though, where loss circulation may result, like the ones
encountered in Gulf of Mexico, new innovations are needed and have been developed
by the industry like the “Flat Rheology” drilling fluid systems [20,21] which attempt to
keep a constant yield point despite changes in temperature and pressure along the

                                                                   Oil and Gas Exploration

wellbore, thus effectively cleaning the wellbore with the originally designed yield point.
Rheograms for such fluids are shown, for example in Fig. 5, where for the very different
temperatures and pressures, yield stress is not varying much, while rheology changes

Fig. 4. Cases for MPD application: (a) Static pressures are OK, but HH+AFP exceeds
formation strength and losses occur. (b) Possible solution, use lower density and impose
backpressure when static (adapted from Smith, [19]. (Notes: HH=hydrostatic pressure;
AFP=annular flow pressure; BHP=bottom hole pressure; BP=back pressure).

Drilling fluid additives which give flexibility when designing and implementing a
drilling campaign, are continuously sought. A fairly interesting twist is with respect to
density control, which has been achieved till today inexpensively with barite. However,
it presents severe technical limitations in deep wells because of barite settling often
called barite sag [22,23]. Recent research [24] has shown that reducing barite particle
diameter by 100 reduces settling, or sag, by a factor of 10,000. Thus, the new approach
is to grind barite from about 75 μm to about 1-3 μm, and this has delivered field benefits
never achieved before by minimizing barite sag but also reducing equivalent circulating
density (ECD). At the same time it enhances bit hydraulics and offers superior hole
cleaning allowing, thus, to operate in very narrow pressure margins, especially for
drilling many mature reservoirs in the North Sea with complex well trajectories and
narrow drilling tolerances [25]. In Figure 6, the rheograms for 13.2 ppg (1.59 SG) fluids
with regular barite and with micron-sized barite are compared with observation of good
rheology. Herschel-Bulkley is the rheological model of choice and the parameters are
indicated in the Figure. The analysis shows, though, that the yield stress in the case of
micron sized barite is fairly lower than the regular one, and this has to be taken into
account. Furthermore, the issue of potential formation damage with such small barite
particles must also be addressed. In addition, phenomenological modeling taking into
account Herschel-Bulkley rheology should also be sought and not only using Bingham
plastic rheological model [23].

HPHT also requires appropriate cement systems and tools which conform to specific
parameters, require special design attention, modified testing procedures and new
products [26] in order to withstand the well high temperatures for the life of the well
without jeopardizing cement properties.

International Multidisciplinary Scientific GeoConference SGEM 2009

Fig. 5. Flat rheology fluid rheograms from two different formulations at different
tempearures and pressures (adapted from Mullen et al. [21]).

               API barite          micron sized barite
              τ y = 4 . 85 Pa      τ y = 0.92 Pa
              K = 0 . 116 Pa * s   K = 0.076 Pa * s
              n = 0 . 841          n = 0.854

Figure 6. Rheograms for API barite loaded and micron-sized barite loaded, 13.2 ppg
drilling fluids (adapted from Oakley [24].

Of course many important issues are not addressed because of the inability to have
appropriate models and techniques for dealing with them. For e.g., while fairly
expensive instruments have been developed to measure rheology at high pressures and
temperatures, what is not really addressed is the calibration of the equipment. All oil
field viscometers are normally calibrated with Newtonian fluids, besides the fact that
they are used to measure rheological parameters of drilling fluids which are highly non-
Newtonian, hence, there is a very strong need for having standards for non-Newtonian
calibration, if there can ever be developed. Furthermore, the simple but also more

                                                                  Oil and Gas Exploration

complex rheological models used to model non-Newtonian fluid rheological behaviour
are normally derived, for oil-drilling industry, using the narrow gap approximation and
Newtonian shear rates, while errors arise for all types of models and in particular for
Herschel-Bulkley rheological models, as it has recently been demonstrated [27].
Another important property of drilling fluids not taken into account is thixotropy,
although, measurements of gel strength at 10s / 10min give a qualitative indication
about the thixotropic state of the fluid, while recent advances have addressed this issue
for drilling fluids [28,29,30].

The continuous search for more hydrocarbons pushes drilling industry to technical
limits and more hardware is needed to enable drilling in high depths where high
temperatures and pressures and harder rock are encountered. These challenges also
demand better formulation of smart drilling fluids.

Drilling techniques involve older and newer procedures, all nowadays referred to as
managed pressure drilling techniques which, using different approaches, depart from
open systems and present a closed loop drilling fluid circulation system. Such
techniques, needed to manage very narrow pressure margins between pore and fracture
pressures, rely on good hydraulic simulators capable of predicting with fair accuracy
pressure profiles along the wellbore.

Good and representative rheological models are also needed for the prediction of the
performance of the new type of drilling fluids which can withstand the extreme
environment. The model of choice should be the Herschel-Bulkley rheological model
which describes well complex fluid rheology and appropriate modeling is necessary for
the hydraulics of such fluids.

We address these issues and present the challenges ahead. Furthermore, we point out
areas that have not been dealt with so far, because in the very more complex and
difficult environment, such issues have to be properly addressed and resolved which
will enable the industry to develop practices for safer and more economical drilling.


1. R. Bland, G. Mullen, Y. Gonzalez, F. Harvey, M. Pless, Drilling fluid meets deep
   gas drilling challenges, Drilling Contractor, May/June, 50-52, 2005.
2. L.A.S. Rocha, P. Junqueira and J.L. Roque, Overcoming deep and ultra deepwater
   drilling challenges, paper OTC 15233 presented at the 2003 Offshore Technology
   Conference held in Houston, Texas, U.S.A., 5–8 May 2003.
3. JD. Rogers, SW Lambert, S Wolhart, Benchmarking deep drilling and completion
   technologies, GasTIPS, Spring, 5-8, 2004.
4. V.C. Kelessidis, Sustainable drilling for oil and gas: Challenging drilling
   environments demand new formulations of bentonite based drilling fluids, 3rd
   International Conference on Sustainable Development Indicators in the Minerals
   Industry, June 17-20, Milos, Greece, 2007.

International Multidisciplinary Scientific GeoConference SGEM 2009

5. D. Hahn, M. Atkins, J. Russell, B. Pearson, Gulf of Mexico shelf deep ultra-hp/ht
    completions - current technology gaps, paper SPE 97300 presented at the 2005 SPE
    Annual Technical Conference, Dallas, 9–12 October, 2005.
6. C. Cameron, Deepwater drilling fluids – what’s new? Paper AADE-05-NTCE-79
    presented at the AADE 2005 National Technical Conference and Exhibition,
    Wyndam Greenspoint, Houston, Texas, April 5-7, 2005.
7. V.C. Kelessidis, R. Maglione, C. Tsamantaki and Y. Aspirtakis, Optimal
    determination of rheological parameters for Herschel-Bulkley drilling fluids and
    impact on pressure drop, velocity profiles and penetration rates during drilling, J.
    Petrol. Sci. Eng., 53, 203-224, 2006.
8. M. B.Oyeneyin,, V.C. Kelessidis, G. Bandelis and P. Dalamaritis, Developing a
    managed pressure drilling strategy for casing drilling operations, Advanced
    Materials Research 62-64, 456-465, 2009.
9. K. Founargiotakis, V.C. Kelessidis, R. Maglione, Laminar, transitional and turbulent
    flow of Herschel-Bulkley fluids in concentric annulus, J. Can. Chem. Engr., 86,
    676-683, 2008.
10. D M. Hannegan, Managed Pressure Drilling, A “new” way of looking at drilling
    hydraulics, SPE 2006 -2007 Distinguished Lecturer Series, 2007.
11. G. Meyers, Ultra-deepwater riserless mud circulation with dual gradient drilling,
    Scientific Drilling, No. 6, 48-51, 2008.
12. S Tian, G Medley, CR Stone, Parametric analysis of MPD hydraulics, paper
    IADC/SPE 108354-PP presented at the 2007 IADC/SPE Conference and Exhibition
    held in Galveston, Texas, 28–29, March 2007.
13. C. Perez-Tellez, H. Duno, O. Casanova, W. Colombine, C.P. Lupo, J.R. Palacios,
    L.D. Medina, Successful application of MPD technique in a HP/HT well focused on
    performance drilling in southern Mexico deep fractured carbonates reservoirs:,
    paper IADC/SPE 122200 presented at the IADC/SPE Conference & Exhibition, 12-
    13 February, San Antonio, Texas, 2009.
14. WJ Gusler, ML Pless, JE Maxey, PE Grover, JJ Perez, J Moon, TR Boaz, A new
    extreme HP/HT viscometer for new drilling fluid challenges, paper IADC/SPE
    99009 presented at the IADC/SPE Drilling Conference, 21-23 February, Miami,
    Florida, USA, 2006
15. J.M. Davison, S. Clary, A. Saasen, M. Allouche, D. Bodin, V-A. Nguyen, Rheology
    of various drilling fluid systems under deepwater drilling conditions and the
    importance of accurate predictions of downhole fluid hydraulics, paper SPE 56632
    presented at the SPE Annual Technical Conference, Houston, Texas, 3–6 October
16. V.C. Kelessidis, C. Tsamantaki, A. Michalakis, G.E. Christidis, P. Makri, C.
    Papanicolaou, A. Foscolos, 2007. Greek lignites as additives for controlling
    filtration properties of water–bentonite suspensions at high temperatures, Fuel, 86,
17. V.C. Kelessidis, G.E. Christidis, P. Makri, V. Hadjistamou, C. Tsamantaki, A.
    Michalakis, C. Papanicolaou, A. Foscolos, 2007. Gelation of water–bentonite
    suspensions at high temperatures and rheological control with lignite addition,
    Applied Clay Sci., 36, 221-231.
18. V.C. Kelessidis, D. Marinakis, C. Tsamantaki, Laboratory assessment of drilling
    fluid formation damage in sandstone cores and mitigation with lignite additives for

                                                                  Oil and Gas Exploration

    high temperature fields, SPE 107762 presented at the SPE European Formation
    Damage Conference, Scheveningen, The Netherlands, 30 May–1 June, 2007
19. K Smith, MPD helps to make problems disappear, Drilling Contractor, Sept/Oct, 48-
    49, 2006.
20. J. Lee, J. Friedheim, B. Toups, E. van Oort, A new approach to deepwater drilling
    using SBM with flat rheology, paper AADE-04-DF-HO-37 presented at the AADE
    Drilling Fluids Conference, held at the Radisson Astrodome in Houston, Texas,
    April 6-7, 2004.
21. GA. Mullen, P-B Tanche-Larsen, DE. Clark, A Giles, The Pro’s and Con’s of flat
    rheology drilling fluids, paper AADE-05-NTCE-28 presented at the AADE 2005
    Drilling Fluids Conference, held at the Wyndam Greenspoint in Houston, Texas,
    April 5-7, 2005.
22. W Dye, G Mullen, New technology to manage barite sag, paper AADE–02-DFWM-
    HO-12 presented at the AADE 2002 Technology Conference, Houston, Texas, April
    2 - 3, 2002.
23. P.R. Paslay, U.B. Sathuvalli, M.L. Payne, A phenomenological approach to analysis
    of barite sag in drilling muds, paper SPE 110404 presented at the 2007 SPE Annual
    Technical Conference and Exhibition held in Anaheim, California, U.S.A., 11–14
    November 2007.
24. D Oakley, Specially treated drilling fluid weighting agent facilitates development of
    maturing reservoirs, paper AADE-06-DF-HO-27, presented at the AADE 2006
    Fluids Conference held at the Wyndam Greenspoint Hotel in Houston, Texas, April
    11-12, 2006.
25. D Oakley, D Cullum, Advanced technology makes new use of age-old drilling fluid
    agent, Drilling Contractor, May/June, 96-100, 2006.
26. S Haidher, S Kale, S Affes, K Nair, HPHT cement system design, east coast case
    history, paper SPE/IADC 104048 presented at the SPE/IADC Indian Drilling
    Conference, 16-18 Oct. in Munital India, 2008.
27. V.C. Kelessidis, R. Maglione, Shear rate corrections for Herschel-Bulkley fluids in
    Couette geometry, Appl. Rheol. 18:3 (2008) 34482-1 – 34482-11, 2008.
28. R. Jachnik, Drilling fluid thixotropy and relevance, Annual Transactions of the
    Nordic Rheology Society, 13, 121-126, 2005.
29. B Herzhaft, A Ragouilliaux, P Coussot, How to unify low shear rate rheology and
    gel properties of drilling muds: a transient rheological and structural model for
    complex wells applications, paper IADC/SPE 99080 presented at the IADC/SPE
    Drilling Conference, 21-23 February, Miami, Florida, USA, 2006.
30. V.C. Kelessidis. Investigations on the thixotropy of bentonite suspensions, Energy
    Sources Part A, 30, 1729-1746, 2008.