Lee Conn, Sanjit Roy, M-I SWACO

A SPECIALLY ENGINEERED oil-base drilling fluid system applied in con-junction with a unique real-timehydraulic monitoring service has estab-lished a new benchmark in the drillingof high-temperature and high-pressure(HTHP) wells.

The combination was employed in anexploratory HTHP well in the Norwe-gian North Sea where it reduced drillingdays more than three fold, saving a con-servative $3 million in total projectcosts.

Generally, HTHP wells are defined asthose with an expected static bottomhole temperature (BHT) anywhere from300º to 500ºF and an expected shut-inpressure from 10,000 to 25,000 psi. Suchabnormally harsh environments place

distinct technical demands on a drillingfluid as they present the potential toradically alter its behavior compared tomore conventional applications.

Under conditions of very high tempera-ture and pressure the rheologicalparameters of drilling fluids are difficultto predict without considerable fieldexperience supported by laboratorydata from a Fann 75 HTHP rheometer.The rheology is particularly importantin the deeper sections of HTHP wellswhere the annulus is smaller in diame-ter, thereby creating erratic fluid behav-ior that can have a profoundly negative

impact on equivalent circulating density(ECD) management. While the abnormalpressures experienced in these wellsrequired higher mud weight to controlformation fluids, the fracture gradient ofthe pressured formations does notincrease at the same rate as the pres-sure. Consequently, the so-called drillingwindow between the pore pressure andthe fracture gradient can be very smallon HTHP wells. Hence, if the rheology ofthe drilling fluid is too high, the circulat-ing pressure could break down the for-mation, resulting in lost circulation andincreasing the risks of well controlissues.

Furthermore, the risk of becoming dif-ferentially stuck is magnified on anHTHP well. The weight of the drillingfluid required to control the abnormallypressured zone is much higher than thepressure requirement in nearby forma-

tions. This condition results indifferential pressures betweenthe drilling fluid and pres-sured formations in excess of3,000 psi. If the zones containpermeable formations, the riskof becoming differentiallystuck is elevated. Filtrationmust be tightly controlled,whereby a lubricious filtercake is deposited, which is acritically important factor inavoiding stuck pipe, especiallyin extended reach and high-angle wells. Since invert emul-sion drilling fluids exhibitexcellent filtration character-istics and a low coefficient offriction they provide a practi-cal solution for wells with aBHT of 400ºF or more.

E N G I N E E R I N G A N H T H P F L U I D

A number of factors must be taken intoconsideration when designing or select-ing a drilling fluid for an HTHP well. Therheological profile and filtration stabili-ty of the fluid at elevated BHT is critical,along with its drilling performance atvery high densities. The fluid also mustpossess good hole cleaning, lubricityand corrosion characteristics and beresistant to contamination, includingH2S and CO2 contamination.

For kick detection, the compressibility

and solubility of gas in the mud must beconsidered and understood. The select-ed fluid must pass environmental andtoxicity evaluations where applicable,while also being cost-effective. Thesefactors were taken into considerationwhen designing a new fluid system forHTHP applications.

The new HTHP fluid system can be for-mulated using either linear paraffin orlow-toxicity mineral oil as the base fluid.The lower kinematic viscosity of boththese base oils delivers low to flat rheo-logical profiles that make them effectiveunder a wide range of downhole temper-atures and pressures. Drilling fluidsdesigned with flat to low rheologicalparameters are expected to drill a wellat lower ECD, encounter fewer problemsand successfully avoid instances of lostcirculation, all critical issues in thedrilling of an HTHP well.

An essential component of the newlydeveloped system is a new thermallystable organophilic clay, which is usedas the primary viscosifier. While similarto that used in conventional invert emul-sion systems, the new clay incorporatesa new type of quaternary amine groupthat is thermally stable, thereby makingit oil soluble and increases its tempera-ture stability. The clay used in earliersystems began to thermally degrade at347ºF, while the new product is stable totemperatures in excess of 400ºF.

The exclusive package also contains pri-mary and secondary emulsifiers thatdeliver a temperature-stable emulsion,high-temperature, high-pressure fluid-loss control and preferential oil-wettingof solids.

A specially developed HTHP fluid-losscontrol agent is employed when temper-atures are expected to exceed 300ºF.Owing to a melting point that exceeds400ºF, the new fluid-loss control agentcan be used effectively in high-tempera-ture applications with no adverse effectson rheology.

The end result is a system that remainsstable at high temperatures, is suitablefor narrow hydraulic window applica-tions and reduces ECD while maintain-ing stable low shear rate viscosity. Thereduction in high shear rate viscosity, inturn, reduces total system pressure

52 D R I L L I N G C O N T R A C T O R May/June 2004

Fluid monitoring service raises bar in HTHP wells

RTHS Wellsite Configuration

•• WITS or proprietary protocolsWITS or proprietary protocols•• Serial or Ethernet connectivitySerial or Ethernet connectivity

Driller

PWD Unit

Sensors

Sensors

Sensors

Mud Logger

CompanyManDriller

PWD Unit

Sensors

Sensors

Sensors

Mud Logger

CompanyMan

PPRT

ECD, ESDECD, ESD

Well DataWell Data

PPRTPPRTPPRT

ECD, ESDECD, ESD

Well DataWell Data

ECD, ESDECD, ESD

An advanced real-time hydraulics system (RTHS) at the well-site monitors and measures data wherever available, makingit a perfect addition to real-time onshore drilling and opera-tions centers.

loss. Further, the new system is not onlyeasy to engineer, but displays the behav-ior and accepts chemical treatment sim-ilar to conventional invert emulsion flu-ids. The system also has proven effec-tive in reducing barite sag.

R E A L - T I M E M O N I T O R I N G

In HTHP applications, proper manage-ment of ECD is vital in avoiding well con-trol problems, poor hole cleaning, lostcirculation, ineffective drilling perform-ance and cost over-runs. For HTHP andother narrow-pressure margin wellsreaching their objectives safely and eco-nomically requires accurate knowledgeof downhole hydraulics in real-time.Accordingly, pressure-while-drilling(PWD) tools satisfy many of the needs ofECD management in critical wells, how-ever, the very nature of this technologylimits its use in certain critical opera-tions and wells. Ironically, the tempera-ture limitations of these tools oftenmake them unavailable when pore andfracture pressures converge, resultingin narrow operating windows, which is acritical element of an HTHP well. Seri-ous real-time information gaps alsoexist while running pipe or casing, ormaking connections where downholepressure data is not available in real-time at the surface.

An advanced real-time hydraulics sys-tem (RTHS) has been developed to com-plement, and, in special cases, to substi-tute for PWD tools when they are notavailable. The RTHS models complexdownhole hydraulics in real-time basedon conventional surface-measured inputdata. The software provides ECD pre-dictions if a downhole tool is notinstalled or has failed, or when meas-ured data is not available at the surfacein real-time. If both data sets are avail-able in real-time, the “what is” providedby the downhole tool and the “whatshould be” provided by the RTHS can becompared and successfully used toavoid drilling problems. “True” real-time is achieved by providing resultsfast enough to affect changes in adynamic process before their comple-tion.

The RTHS relies on a continuous streamof surface measured data whereveravailable, making it a perfect addition toreal-time onshore drilling and opera-tions centers increasingly being used tomonitor drilling operations. The system

is monitored onshore by connecting tothe real-time data stream from the rigand transmitting calculated data backto the data provider on the rig and dis-played in front of the driller as virtual orsimulated sensors.

The RTHSemploys sophisti-cated engineeringmodels and a spe-cially trainedhydraulics engi-neer to providecomplete surface-to-TD downholepressure profiles.The system uti-lizes two funda-mental conceptsverified in a widerange of drillingapplications. Thefirst concept sub-divides the entirewell into shortsegments, eachwith their own setof properties andparameters that is continually updatedin real-time. The second concept usesthe best available rheological data forthe drilling fluid by combining data fromvarious measurements. The systemrelies heavily on a sophisticated tran-sient temperature simulator to predictdownhole temperature profiles andadjusts drilling fluid density and rheolo-gy based on temperature and pressure.

The system has substituted for down-hole tools in HTHP applications wheredownhole temperatures exceeded toollimitations and helped reduce wellobservation time while avoiding well-control and lost-circulation incidents.

The RTHS is uniquely available for ECDmanagement while drilling, tripping andrunning casing in narrow operating win-dows where operations would normallybe suspended in the absence of down-hole pressure tools.

The RTHS has been successfully usedon HTHP and other exploratory wells toprevent hydraulics related drilling prob-lems. The system has complementedand substituted for downhole pressuretools while drilling, tripping and otheroperations in the Gulf of Mexico, NorthSea, West Africa and South America,among others.

F I E L D R E S U L T S

The new HTHP fluid system, formulatedwith low-toxicity mineral oil as the basefluid, was used in tandem with the well-site hydraulics monitoring service to

successfully drill an exploratory well inthe Norwegian North Sea with bottom-hole temperature and pressure exceed-ing 350ºF and 14,000 psi, respectively.The combination was used in the critical12 ¼-in. and 8 ½-in. sections, which thebest available offset required 156 daysto drill. The maximum required equiva-lent static density (ESD) was 18 lb/gal,among the heaviest ever recorded in theNorwegian North Sea, with a fracturegradient of 18.5 lb/gal.

Since the well exceeded the tempera-ture limitations of the PWD tool, theRTHS was used and consistently calcu-lated ECDs within 0.1 lb/gal of measuredvalues. Correcting downhole mud den-sity (ESD) for fluctuations in tempera-ture and pressure was crucial in main-taining well stability and integrity. Theaccurate prediction of pressures whiledrilling and tripping dramaticallyreduced well observation time whileavoiding well control and lost-circula-tion incidents. Consequently, the sec-tions were drilled in 46 days, saving aconservative $3 million when comparedto offset wells.

More recently, the combination wasused successfully in a deep shelf well inthe Gulf of Mexico where the tempera-tures were beyond the limits of the PWDtool. n

May/June 2004 D R I L L I N G C O N T R A C T O R 53

9.39.3

9.49.4

9.29.2

100100

200200

00

10001000

20002000

001100 22 4433 55

EC

D(l

b/g

al)

EC

D(l

b/g

al)

RO

P(f

t/h

r)R

OP

(ft/

hr)

Flo

wR

ate

(ga

l/min

)F

low

Ra

te(g

al/m

in)Calculated ECD @Shoe (10,203 ft)

Measured Flow Rate

Measured ROP*10ECDRT

ROP

Flow

Time (hr)Time (hr)

9.39.3

9.49.4

9.29.2

100100

200200

00

10001000

20002000

001100 22 4433 55

EC

D(l

b/g

al)

EC

D(l

b/g

al)

RO

P(f

t/h

r)R

OP

(ft/

hr)

Flo

wR

ate

(ga

l/min

)F

low

Ra

te(g

al/m

in)Calculated ECD @Shoe (10,203 ft)

Measured Flow Rate

Measured ROP*10ECDRT

ROP

Flow

Time (hr)Time (hr)

The RTHS can substitute for downhole tools if they become inoperable dueto tool mechanical failure. The RTHS has been used to guide drilling opera-tion and prevent an extra trip and wasted rig time to replace non-function-al tool.