POOOS-97

By

D. B. Bennion, F. B. Thomas, R. F. Bietz, A.K.M. JamaluddinHycal Energy Research Laboratories Ltd.

Paper to be presented at the 5th International Conference onCoiled Tubing Technologies, Drilling-Completions-Field Operations-Services

To be held in Dallas, Texas January 8-10, 1997

INTRODUCTIONABSTRACT

What is Underbalanced Drilling? Underbalanced drilling istechnically classified as any situation in which the effectivecirculating pressure of the downhole drilling fluid is less thanthe pore pressure of the adjacent fonnation (preferably over theentire length of the exposed open hole section of the pay). Insome situations, where the formation has high natural pressure,an underbalanced condition can be generated quite easily usingconventional unweighted type drilling fluids (clear brines oroils). This type of drilling is common in such places as theAustin Chalk in Texas and is often referred to as "flowdrilling". In situations where the reservoir pressure is depleted,some type of non-condensible gas (either as a gasified fluid,mist or pure gas injection operation) is generally required toengender a sufficiently low equivalent circulating density togenerate an underbalanced condition. The underbalanceddrilling process is illustrated schematically as Figure I and hasbeen discussed in detail in the literature'.

Coiled tubing has been used with increasing frequency insome underbalanced drilling (UBD) situations because ofperceived superiority to conventional jointed pipe drillingtechnology. Much of the success of an underbalanced drillingoperation centres on the ability to maintain an underbalancedcondition on a continuous basis, transmit effective survey andpressure data back to the surface in a unremitting fashion duringthe underbalanced drilling operation, clean the hole effectivelyand operate in a safe fashion with potentially high flowpressures at surface. Coiled tubing has distinct advantages inthese areas over conventional jointed pipe due to the lack ofrequirements for connections, use of an internal wireline andcapillaries for survey and geosteering purposes, and muchhigher operating surface pressure limitations than conventional

jointed pipe.

Jointed pipe, however, is much less expensive to utilize andis readily available. Also, increasing experience andmodifications in technology (such as EMT surveying tools)have reduced or eliminated many of the problems initiallyassociated with its use for UBD applications. The commonapplication of jointed pipe technology for drilling reducesproblems associated with inexperienced crews implementing therelatively new technology of coiled tubing drilling. In addition,coiled tubing drilling technology is currently limited at 500-700m of horizontal outreach and relatively small hole sizes in orderto maintain sufficiently high annular velocity for adequate holecleaning purposes. High frictional losses, associated with thenecessity of injecting fluid through the entire length of thecoiled tubing string at all times, may also be problematic.

Why Drill Underbalanced? A number of reasons arecommonly provided as motivations for underbalanced drilling.These would include:. reduced invasive formation damage resulting in higher

production rates. increased rate of penetration. reduction in drilling problems such as lost circulation and

differential sticking. elimination of costly and exotic mud systems (andsubsequent disposal of such). ability to flow test while dtilling. flush production during the drilling operation. less environmental impact (generally no mud pits)

This paper presents some of the advantages anddisadvantages of coiled tubing and jointed pipe methods forunderbalanced drilling, primarily from a reservoir andproduction optimization perspective, rather than from a simplemechanical or operational approach.

Not all reservoirs, however, are ideal candidates forunderbalanced drilling and, if the technology is improperlyapplied, more formation damage may result than if a welldesigned and executed overbalanced' operation bad been

utilized2.J.

D.B. BENNION. F.B. THOMAS, R.F. BIETZ, A.K.M. JAMALUDDIN POOO5-972

Jointed Pipe vs Coiled Tubing For Underbalanced DrillingNomenclature and Types of Underbalanced Drilling

The vast majority of wells drilled using underbalancedtechnology have utilized conventional jointed pipe technologyand rotary rigs. More than 85% of the wells drilled using the"artificial" underbalanced type technology in Canada have beendrilled using conventional jointed pipe. The fraction is evengreater if low head and flow drill applications are considered ona worldwide basis. Major advantages and disadvantages withrespect to coiled tubing vs jointed pipe applications can besummarized into the following categories:

There is often confusion as to different tenDs used when onerefers to underbalanceddrilling. To attempt to clarify these,theauthors present the following definitions:

I. Overbalanced Dri//ing. A situation where, at all places inthe exposed target fontlation, the circulating pressure of thedrilling fluid is greater than the effective pore pressure of thefontlation, resulting in total containment of reservoir fluids,other than those produced by milling of the actual bit pathitself.

2. Near Balanced Drilling. A situation where, at all places inthe exposed target fonnation, the circulating pressure of thedrilling fluid is greater than the net effective pore pressure.This results in a containment of reservoir fluids, as describedin point I above, but attempts are made to reduce themagnitude of the circulating bottomhole pressure (generallyby reducing the density of the circulating drilling fluid) toreduce the degree of overbalance pressure applied to thefonnation to some nominal value to increase ROP and reducemud losses and invasive fonnation damage. In a horizontalwell (or vertical well with significant open hole pay), thedegree of overbalance pressure may vary along the length ofthe exposed fonnation face due to frictional pressure dropeffects.

3. Flow Drilling. A situation which occurs when aconventional water or oil based mud system is used in asituation where high, naturally occurring, reservoir pressureis present. This results in an inherent underbalance pressurecondition occurring in all, or a portion of, the exposedformation and results in flow of reservoir fluids to surface.

I. Safety and surface pressure control issues2. Continuous BHP maintenance issues3. Rate of penetration issues4. Hole cleaning issues5. Total drilling time issues6. Continuous circulation issues7. Mud spillage and environmental issues8. MWD capabilities9. Rig/site considerations, footprint10. Surface hole/casing considerationsII. Hole size limitations12. Depth limitations13. Rotation issues14. Torque, drag and weight on bit issues15. Downhole motor issues16. Tubing life limitations17. Torque limitations18. Flow hydraulics limitations19. Orientation and steering difficulties20. BHA considerations21. Availability issues22. Experience issues23. Economics24. Hybrid rig applications4. "Artificial" Underba/anced Drilling. This UBD technique

has received much attention recently. In this situation,reservoir pressure is too low to allow an underbalancedbottomhole pressure condition to be established withconventional water or oil based drilling fluids. To reduce thedensity of the circulating mud column (and correspondinglythe hydrostatic component of the circulating bottomholepressure during the drilling process), a low density drillingfluid must be used. This is commonly achieved byconcurrently injecting some type of non-condensible gassimultaneously with the drilling fluid (usually nitrogen,natural gas, air or oxygen content reduced air or flue gas),using pure gas or mist, foam or suspending solid hollQwspheroids in the circulating fluid system. Typically, thereturn of excess formation fluids to surface during such adrilling operation is taken as evidence that an underbalancedcondition has been obtained, but this does not necessaryimply that an underbalanced pressure condition existsthroughout the entire wellbore (which is the optimum desiredscenario), only that an underbalanced pressure conditionexists at some point within the exposed formation.

Some of the more significant of these areas will now bediscussed in more detail.

Safety and Surface Pressure Control Concerns

Safety and well control concerns are always a major issue inany underbalanced drilling operation, particularly in situationswhere high reservoir pressures or sour fluids exist. Due to thesmooth and continuous surface of a coiled tubing string and thebuilt-in stripping mechanism in the CT injection unit, thisprovides perceived superior safety when operating at ~ighannular surface return pressures in an underbalanced drillingoperation.

Considerable development work has occurred in the last fewyears in the development of a variety of different types ofrotating control heads for similar control flexibility foroperations using jointed pipe4).6.1. The new generation of controlheads allows rotating operation at pressures of up to 2500 psiwith a 5000 psi static pressure rating. The rotating working

COILED ruBING-THE FUruRE OF UNDER BALANCED DRILLING?P0005-97

pressure is still less than for conventional CT, which mayresult in CT being a potential consideration for situations wheresour fluids or high surface pressures are expected. Sourcirculating fluids in the drill string, when connections arerequired for jointed pipe operations, may also aggravate safetyconsiderations. CT drilling operations are also quieter thanconventional jointed pipe operatio.ns, thus reducing noiseconcerns with drilling personnel and local residents.

prevent rapid fluid fallback in the vertical annular section of thewellbore. Bottomhole pressure fluctuations, however, insituations where significant volumes of liquid are beingproduced from the fonnation, can be minimized throughapplication of these techniques but not totally avoided.

Alternate injection configurations, such a parasite tubingstring or microannular injection techniques (see Figures 3 and4) can provide stable and unifonn bottomhole pressure profilesfor jointed pipe operations similar to those seen during CTopenstions. These modified UBD operations increase expense ofthe well, however, both from an equipment perspective and alsooperationally due to the fact that more non-condensi~le gas isrequired to maintain the same bottomhole pressure in aconcentric or parasite string application in comparison to astandpipe injection mode in the same situation.

If high surface injection pressures are required, CT mayhave a nominal safety consideration, due to the fact that theentire length of the high pressure CT spool is at surface in alocation exposed to the rig crew and ongoing operations. Thepossibility of a leak or safety concerns is present, butappropriate monitoring of the fatigue and cycle life of the CTstring should address many of these considerations.

Rate of Penetration (ROP) IssuesTypically, jointed pipe underbalanced drilling operationshave had an exceptional safety record when combined withproperly designed surface control equipment and experienceddrilling personnel. Hence, all factors being considered. forsweet, low surface pressure applications, CT does not exhibitany appreciable advantage with respect to safety considerationsover conventional jointed pipe.

Generally, ROP is significantly increased by UBD operations.All factors being equal, ROP increases in CT vs jointed pipeoperations are comparable, but this assumes that equivalent holesize, weight on bit. torque and drag considerations, etc. arepresent which is often not the case as will be discussed shortly.ROP may be compromised in some CT applications, particularlyin extended reach applications, due to the fact that limitedweight can be applied to the bit by the action of the CT injectorat surface, and much of this force may be expended in drageffects as the CT string is forced around the horizontal bendand down the helical well path that is typically generated whenattempting to drill horizontally with CT.

Continuous Maintenance oran Underbalanced BottomholePressure Condition

One of the main motivations for underbalanced drilling, inmany situations, is the elimination or minimization of invasiveformation damage caused by fluid and solids losses to thefonnation under conventional overbalanced conditions. Much ofthis benefit is negated if the underbalance pressure condition is-periodically compromised2.J. This phenomena is schematicallyillusb"ated as Figure 2. Coiled tubing bas a major advantageover jointed pipe in this case, due to the fact that there is nonecessity to break for coMcctions. If standpipe injection of thenon-condensible gas being used to generate an artificiallygenerated underbalance pressure condition is occurring, thismay result in bottomhole pressure fluctuations which may, ifimproperly handled, result in conditions of periodic hydrostaticoverbalance pressure being encountered downhole. This maynegate much of the benefit of the UBD operation (with respectto fonnation damage minimization). Damage may actually beworse in some cases in comparison to the same situation thanif a well-designed overbalanced drilling fluid {which has thecapability to fonn a stable and bridging and. hopefully, readilyremovable filter cake) bad been utilized.

Hole Cleaning Issues

Hole cleaning presents a major problem in many UBDoperations. Hole cleaning is affected by fluid rheology. cuttingssize and concentration (which is in turn a function of bit typeand ROP) and annular fluid velocity. The majority of CToperations are conducted using 2" or smaller CT strings.whereas hole size can be 6.25" or larger. This may result inhigh pump rates and frictional losses in the CT string beingrequired in order to maintain sufficient aMular velocity tomaintain effective hole cleaning. Inability to rotate coiled tubingmay further exacerbate the problems with hole cleaning. Theuse of larger CT strings (to reduce frictional drop in the stringand increase corresponding annular velocity due to a reductionin annular size) may be useful. Increased use of aggressive bits(such as POC's) to reduce cuttings size may also be useful. butmay be offset by problems with torque generation and highamplitude torque variation.Bottomhole pressure fluctuations using jointed pipe can be

minimized by making fewer connections (drilling with doubleor triple pipe stands - triples are becoming more common withthe increasing use of top drive rigs for UBD jointed pipeapplications), faster connections, circulating to pure gas toattempt to unload as much fluid from the horizontal and annularsection as possible prior to making a connection, and leavingthe annulus open during connections to allow bleedoff to

The ability to continually or periodically rotate conventionaljointed pipe may result in superior hole cleaning ability due tokeeping a competent cuttings bed from forming. Working theCT string may also assist in hole cleaning ability in certainsituations. Many CT applications for UBD are drilled slim holewhich reduces the problems associated with low annular fluid

4 D.B. BENNION. F.B. THOMAS, R.F. BIETZ. A.K.M. JAMALUDDIN P0005-97

velocities in the horizontal section, although problems may stillbe apparent in larger diameter uphole sections.

injection is required in this application and thereforeconventional MWD technology can be used with no problems.

Total Drilling Time The advent of CT technology minimized some of theproblems associated with downhole MWD technology since, dueto the continuous nature of the CT string, a continuous trouble-free internal wireline approach could be used to directlytransmit the data from the bottomhole MWD assembly back tosurface allowing for continuous telemetry measUrements whilemaintaining a continuously underbalanced condition.

Drilling times may be reduced using CT due to the fact thattime for connections may account for up to 25% of the totaldrilling time (in soft fonnation applications) when drilling withconventional jointed pipe. Since cOMCctions arc not requiredfor a CT application, this represents direct reduced drilling timeand cost savings for the drilling operation. CT can also betripped much faster than conventional jointed pipe, which mayalso reduce on-site time if multiple trips are required in aparticular job.

The use of newer EMT technology. has allowed thesuccessful application of MWD technology to jointed pipeapplications with many of the benefits of a continuous wirelineused in a CT system. With EM technology, an electromagneticpulse is used to transmit the survey and pressure data directlythrough the ground back to a surface receiver. The technologyhas been used extensively and successfully in many UBDapplications. Depth and temperature limitations are the majorproblems with this technique, as well as the inability of certaintypes of fonnations to effectively conduct the EM signal.

Continuous Circulation Issues

Due to the nature of coiled tubing, continuous circulation ispossible both while drilling ahead. and also while hipping. Theadvantages of continuous circulation in maintaining a uniformBHP while drilling have been discussed previously. The abilityto continuously circulate with CT while tripping, in general,reduces reaming requirements, allows for backreaming, whiletripping out (if required) and allows the hole to be cleaned andmaintained in a better condition.

Rig/Site Considerations - Footprint

CT drilling has some advantages over a conventional jointedpipe rig with respect to site considerations in remote locationsor sites with limited space. A CT unit is a smaller, mo~portable assembly than a conventional drilling rig, and typicallyrequires only about 50% of the footprint space on site foroperation in comparison to an equivalent system using jointedpipe technology. This lowers environmental impact on site(which may be critical in certain sensitive areas) as well asreducing site construction, use and abandonment costs).

Mud Spillage Issues

Generally, UBD operations have less environmental impactthan conventional drilling, due to an absence of mud pits whena closed loop surface conb'ol type system is utilized. CT use inUBD further minimizes environmental impact as, once again,due to a lack of necessity for connections, mud spillage on thesite during connections when jointed pipe is being used and thepotential for contamination rising from such spillage isminimized.

Surface Hole Considerations

If CT is in use in a new drilling application. some adverseconditions may be imposed by the fact that a small conventionalrig will generally be required in order to spud the well. run thesurface and possibly intermediate casing or. in some cases. topull the production equipment from an existing well. This. ofcourse. may obviate some of the potential economic advantagesof CT drilling since two complete sets of mechanical drillingequipment must be moved onto and ofT site with the economicand logistical hassles which this entails.

Measurement While Drilling (MWD) Issues

The ability to MWD during an underbalanced drillingoperation is essential. both from a geosteering and trajectorycontrol perspective for a horizontal well. and also due to thefact that real time measurement of the BlIP is essential toensure the success of any underbalanced drilling operation.

In the early days of artificial UBD drilling. conventional mudpulsed telemetry was used. Since the presence of anincompressible fluid in the drill string is a pre-requisite for mudpulsed telemetty, this necessitates the cessation of non.condensible gas injection in order to allow the propagation ofa successful suite of data to surface. This, of course, resulted inthe appl ication of full hydrostatic pressure to the formation andnegated the underbalance pressure condition and, hence, most,if not all, of the benefits ofunderbalanceddrilling with respectto formation damage mitigation were compromised. If aconventional mud system is used for a flow drilling application,this, of course, is not a problem as no non-condensible gas

Hole Size Limitations

Although there are some reported cases of larger hole sizesbeing drilled with coiled tubing, the current conventional upperlimit for CT drilling is presently about 6.25". Larger hole sizes,if required, typically tend towards using conventional jointeddrillstring. CT drilling in the past has tended to be restricted tomore slim hole applications and reentries, rather than largediameter new drill applications.

As CT drilling is extended to using larger diameter CT