Underbalanced operations offer pluses and minuses

Jan. 1, 1996
D. Brant Bennion Hycal Energy Research Laboratories Ltd. Calgary Figure 2 (51679 bytes) Figure 3 (52676 bytes) Figure 6 (26965 bytes) Underbalanced drilling, when properly designed and executed, minimizes or eliminates problems associated with the invasion of particulate matter into the formation. Invasion damage often greatly reduces the productivity of oil and gas reservoirs, particularly in open hole horizontal well applications.
D. Brant Bennion
Hycal Energy Research Laboratories Ltd.
Calgary

Figure 2 (51679 bytes)
Figure 3 (52676 bytes)
Figure 6 (26965 bytes)

Underbalanced drilling, when properly designed and executed, minimizes or eliminates problems associated with the invasion of particulate matter into the formation.

Invasion damage often greatly reduces the productivity of oil and gas reservoirs, particularly in open hole horizontal well applications.

Underbalanced drilling can also minimize numerous other problems, such as adverse clay reactions, phase trapping, precipitation, and emulsification. These problems can be caused by the invasion of incompatible mud filtrates in an overbalanced condition.

The conclusion of this two-part series will cover screening criteria for selecting formations suitable for underbalanced drilling.

Additional benefits of underbalanced drilling include a reduction in drilling time, greater rates of penetration (ROPs), increased bit life, a rapid indication of productive reservoir zones, and the potential for dynamic flow testing while drilling.

Underbalanced drilling is a technique in which the hydrostatic pressure in the circulating downhole fluid system is maintained at some pressure less than the pressure of the target formation. This condition can be generated naturally with low-density fluids (clear freshwater fluids or light hydrocarbon systems) where high natural pressure exists in the formation.

In most situations, underbalance is generated artificially by the concurrent injection of some type of noncondensable gas with the circulating fluid system. The gas most commonly used is N2 because of its availability and transportation concerns with other gases. Underbalanced operations have also used air, natural gas, flue gas, or reduced-oxygen-content air (processed via a semipermeable membrane unit), depending on the specific reservoir situation.

Underbalanced drilling techniques have often been applied to horizontal wells where formation damage concerns have been of particular importance. Compared to vertical wells, horizontal wells have longer fluid contact times, and many of them are completed open hole. Even relatively shallow invasive damage can significantly reduce the productivity of a horizontal well.

Underbalanced technology also has application in vertical wells.

Generating underbalance artificially is most often done mechanically by drillstring injection. A noncondensable gas is injected directly into the drillstring at surface, reducing the density of the entire circulating fluid system in both the injection path (inside the string) and in the returning fluid flowing back to surface in the annulus.

Special surface equipment for pressurized flow, solids separation, cuttings sampling, and well control are required (Fig. 1 [59734 bytes]).1 One drawback of the through-pipe injection method is that conventional mud-pulsed logging techniques cannot be used because of the presence of a compressible gas in the fluid system. In addition, the underbalanced condition is lost or compromised regularly if a rotary rig is used because of the periodic pipe connections during drilling.

Other mechanical configurations, such as a parasite tubing string or concentric drillstring, eliminate this concern and allow continuous underbalanced operation and conventional MWD operations by injection of the noncondensable gas directly into the returning fluid stream at some intermediate location in the annular well bore (Figs. 2 and 3).2

The downsides to this application include added cost and complexity. Another problem is a greater propensity of flushing of the formation because full hydrostatic pressure is applied directly at the drill bit jet ports.

Reservoir characterization and the proper placement of the well in viable producing reservoir pay obviously play a crucial role in determining the final performance of any well drilled either overbalanced or underbalanced.

Advantages

There are many reasons why underbalanced drilling may be considered for a given reservoir application.

Reduced formation damage

Many formations are susceptible to a variety of different types of formation damage during overbalanced drilling operations. Some of the problems include the following:

  • Physical migration of in situ fines and clays caused by high fluid leak-off velocities at highly overbalanced conditions3

  • The invasion of artificial or naturally generated solids present in the mud system into the formation matrix (particularly an issue in open hole completions where penetration of shallow but potentially very severe damage of this type by perforating/fracturing is not normally considered)4

  • A poor knowledge of the formation pore size distribution or a significant bimodal size distribution, which creates difficulty in designing a low-permeability sealing filter cake to inhibit deep invasive damage in an overbalanced mode

  • High permeability zones presenting the potential for severe invasive fluid loss (large macrofractures, highly interconnected large vugs, extremely high permeability sands, or intercrystalline carbonates)

  • Susceptibility to aqueous or hydrocarbon-phase traps which may result in the retention of water-based or hydrocarbon-based fluid filtrates, possibly leading to a permanent reduction in the productive capacity of the near well bore region from adverse relative permeability effects5 6

  • Potential adverse reaction between invaded filtrate and the formation (swelling clays, deflocculatable clays, formation dissolution, chemical adsorption, wettability alterations, etc.)4

  • Potential adverse reaction between invaded filtrates and in situ fluids (emulsions, precipitates, scales).4

Increased ROP

Many underbalanced drilling operations have significantly greater ROPs than conventional overbalanced applications. The underbalance improves overall drilling time significantly in extended reach horizontal sections, improves bit life, and reduces drilling costs.

Indication of productive zones

Because the hydrostatic pressure of the circulating fluid system in a truly underbalanced operation is less than the formation pressure, a condition of net outflow of formation fluids should occur.

Proper flow monitoring of the produced fluids at the surface can provide a good indication of productive zones and act as a valuable aid in the geosteering of the well (in a horizontal application). Transient holdup in the horizontal portion and slug flow in vertical sections of the pipe must be taken into account when correlating the location of the surges of produced fluids to productive zones.

Significant production of liquid hydrocarbons (gas is usually flared) during the drilling operation may provide some early cash flow to offset some of the additional costs of an underbalanced drilling operation. Up to 2,000 cu m (11,000 bbl) of oil or hydrocarbon liquids have been produced during some underbalanced drilling operations in Canada.

MWD

A major drawback in past underbalanced drilling operations was the inability to use measurement while drilling (MWD) to geosteer while using gas-charged fluid systems. The exception is with a parasite or concentric drillstring configuration, which allows pulsed logging up an entirely liquid-filled drillstring.

Electronic telemetry tools, however, can directly transmit downhole information back to surface while drilling, even in an underbalanced well. Depth and temperature limitations on these tools still currently limit their applicability in some wells, but technology advances will make these tools applicable to even deeper wells.

Test while drilling

Some operators take advantage of the flowing well to conduct either single or multirate drawdown tests during drilling. These tests help evaluate the productive capacity of the formation and formation properties in a static mode or while drilling ahead.

The analysis of these tests is complicated. Especially in horizontal wells, a number of productive zones may be simultaneously flowing, particularly later on in the drilling operation. Valuable insight into formation properties can still be obtained, however.

Disadvantages

The primary reason for drilling underbalanced must be related to economics. The increased cost and other potential downsides of underbalanced drilling should be offset by a potential significant increase in well productivity or other technical concerns.

Underbalanced drilling is not a solution to all formation damage problems. Damage from poorly designed and executed underbalanced drilling programs can rival or even greatly exceed the damage which may occur in a well-designed conventional overbalanced drilling program.

A proper understanding of some of the potential problems is essential prior to implementing any underbalanced drilling program.

Expense

Underbalanced drilling is usually more expensive than a conventional drilling program, particularly in a sour environment or in the presence of adverse operational or surface conditions.

There is little advantage to drilling a well underbalanced if the well is not completed underbalanced. Underbalanced completions often result in additional costs for snubbing equipment required to strip the drillstring from the hole. A portion of this expense may be offset by increased ROP, reducing drilling and rig time.

If the well can be drilled in a truly underbalanced fashion, limited or no completion work will be required. This will reduce the cost of extensive, expensive completion and stimulation treatments which may often be required in severely damaged horizontal and vertical wells.

Safety concerns

The technology for underbalanced drilling and completing wells continues to improve. Recent developments in surface control equipment and rotating blowout prevention equipment and the increased use of coiled tubing have increased the reliability of many underbalanced drilling operations.

Drilling and completing the wells in a flowing mode, however, always adds safety and technical concerns.

The use of air as the injected gas, although economical, can cause concerns with respect to flammability and corrosion problems.

Considerable work has been conducted recently in high pressure testing to ascertain safe combustible limits of produced mixtures of natural gas, oil, and drilling mud with both conventional and oxygen-reduced-content air.2 7

Flash envelope testing is usually required for each particular reservoir fluid system/gas composition under consideration.

Formation damage

Many underbalanced drilling operations conducted in the past had disappointing results because the underbalanced condition was not maintained 100% of the time during drilling and completion operations.8 9

Because the formation pressure is greater than the circulating fluid pressure in a truly underbalanced operation, there is no impetus for the formation of any type of sealing filter cake on the surface of the rock.

Obviously, the lack of filter cake is advantageous with respect to formation damage concerns. A filter cake can act as a barrier to fluid and solids invasion, however.

If the formation is abruptly (or gradually) exposed to periodic pulses of overbalance pressure, very rapid and severe invasion of filtrate and associated solids may occur. The damage may be more significant than if a properly designed overbalanced system had been used in the first place. The invasive depth and profile can often be minimized in many overbalanced systems with proper mud and bridging agent design.

Loss of underbalance

There are a large number of reasons why an underbalanced condition may be lost during drilling:

  • If a rotary rig is used, the underbalance is compromised each time gas injection must be terminated to make a pipe connection. Circulating out to pure gas prior to each pipe connection tends to minimize the effect of these overbalanced pulses, but increases in pressure of several hundred to several thousand kPa are still common in some operations (Fig. 4 [38421 bytes]).

  • Periodic kill jobs to conduct bit trips result in balanced pressure or full hydrostatic pressure being required to control the well unless the string is snubbed out. A compression wave occurs in front of the pipe when the string is run back in the hole rapidly, further aggravating the overbalanced condition.

    If the string cannot be removed in an underbalanced mode and run in slowly after bit replacement, a new bit should be run and the well terminated when the bit expires if the depth is close to the desired total length in the target formation, rather than tripping out for another bit run of a few hundred feet.

  • Periodic hydrostatic kill jobs to conduct conventional mud-pulsed logging programs for MWD and geosteering purposes can have adverse effects because of fluid invasion. The use of electronic telemetry MWD tools can eliminate this problem for wells less than approximately 2,500 m true vertical depth.

  • If a concentric or parasite string is used to obtain continuous underbalance, full hydrostatic pressure will be present at the drill bit jets. Orifice effects will drop this pressure somewhat as the fluid moves through the jets, but possible flushing and an overbalance may still exist directly at the rock/bit interface. This overbalance would not be detected by downhole pressure recorders adjacent to the bit, because pressure will drop rapidly as the fluid leaves the bit area. The pressure in the majority of the returning fluid column will be controlled by the parasite/concentric string injection scheme.

  • Local depletion effects may occur in situations where formation permeability is low, underbalance pressure is high, or reservoir volume accessed by the well is limited. As in any well production application, a pseudo-steady-state flow condition will begin to be forced in zones of the reservoir which have been penetrated during an underbalanced drilling operation and are in a condition of dynamic flow.

    Then, the flowing equilibrium sand-face pressure will ultimately approach that of the circulating underbalanced fluid. Even a slight increase in effective downhole pressure, which the operator may consider to be still well within a condition of true underbalance based upon the original reservoir pressure, will result in an overbalanced condition in the near well bore region.

    The degree of severity of this problem will depend on the reservoir parameters under consideration and the speed at which the formation tends to repressure the depleted zone during the overbalanced period. It is likely that unless formation permeability is extremely high, some invasion may occur prior to repressurization.

    Although high-permeability formations will repressure more quickly, it is also likely that these zones will be operated in a less underbalanced condition because of surface flow restrictions. Thus, there will be less margin for error with an overbalanced pulse because the drilling operations will be much closer to the original reservoir pressure than in a lower permeability scenario.

  • A poor knowledge of original reservoir pressure may result in operating in an overbalanced condition. Accurately metered flow of excess oil, water, or gas from the formation is a good indication that a true underbalanced condition has been achieved. Flow may not always be practically measurable in formations which exhibit very low permeability and correspondingly low production rates.

  • The intersection of multiple reservoir zones which may be at significantly different pressures, because of permeability barriers, may result in crossflow between individual zones. There is the possibility that underbalance may be obtained in some higher pressure zones, yet overbalanced conditions and invasive crossflow from higher pressure zones may occur into lower pressure zones penetrated by the well.

  • Slug flow and liquid holdup occur in the vertical section of the well bore in most operations where gas and liquid are concurrently injected. The slug flow results in problems with sizing of surface equipment to handle periodic high-rate surges which occur at surface conditions. Resulting downhole pressure swings and surges may be comparable in magnitude to those induced in making pipe connections in rotary drilling operations. Some invasion in lower pressure or depleted reservoir zones may occur (Fig. 5 [55834 bytes]).

  • The underbalance must be maintained by delicate control and the use of special surface injection and control equipment. Unfortunately, the operation is therefore at the mercy of smooth and trouble free operation of all of the surface equipment and uninterrupted supply of the noncondensable injected gas to ensure a continuous underbalance.

    Equipment or supply problems may result in the physical loss of underbalance for a period of time. This problem may negate much of the effort expended to drill underbalanced in the portion of the well drilled prior to that time, because of invasive damage effects associated with either killing the well or exposing it to a balanced shut in condition with mud in the hole.

Spontaneous imbibition

Because of adverse capillary pressure relations, it is possible for the formation to imbibe water-based (and in some cases hydrocarbon-based) fluids in the near well bore region. These fluids may reduce permeability from rock/fluid or fluid/fluid incompatibility or reduce flow capacity from aqueous or hydrocarbon-phase trapping and relative permeability effects.

The absence of a sealing, very low permeability filter cake can result in potentially more severe problems with imbibition. The filter cake created during a conventional overbalanced operation can act as a barrier to long-term spontaneous imbibition effects (as long as high initial spurt loss is not present).

A detailed discussion of aqueous-phase trapping and countercurrent imbibition effects is in the literature.5 6 Fig. 6 illustrates the basic mechanism of an aqueous phase trap. In an underbalanced drilling operation, imbibition effects can cause phase trapping and damage problems in a number of different reservoir scenarios:

  • Water-wet gas reservoirs in a dehydrated state of subir-reducible saturation.

    These types of formations are common in very tight gas reservoir scenarios or in zones which have undergone significant regional migration of gas over geologic periods of time (Fig. 7 [41582 bytes]). Because of the very low initial water saturation, there is a strong tendency for the formation to imbibe countercurrently the water-based mud filtrate to reach an equilibrium capillary pressure value.

    The greater the difference between the initial and true irreducible water saturation exhibited by the formation, the more severe the problem. Because of the asymptotic nature of most capillary pressure curves near the irreducible water saturation, the practical magnitude of applicable underbalance pressures is insufficient to counteract most countercurrent spontaneous imbibition effects.

  • Water-wet oil or gas bearing formations at relatively low underbalance pressures out of the capillary transition zone.

    In this situation, the water saturation in the formation directly adjacent to the well bore is at some saturation level close to the irreducible saturation. The presence of a water-based fluid in the well bore is equivalent to the artificial placement of a water/oil or water/gas contact directly adjacent to the formation face.

    If the underbalance pressure is relatively low, the formation can still imbibe fluid in a countercurrent fashion, causing damage. This problem can be largely mitigated by operating at a relatively significant underbalance condition to ensure a sufficient drawdown present to counteract this effect.

    A proper understanding of the wettability, initial water saturation, and water/oil or gas/water capillary pressure characteristics of the formation is essential to minimize concerns with capillary imbibition effects.9

  • Oil-wet formations with subirreducible oil saturations.

    Imbibition can occur in oil-wet retrograde condensate formations producing under the dew point pressure, in gas-bearing formations containing naturally oil-wet minerals (pyrobitumen, elemental sulfur, asphalt precipitates, or residual heavy bitumen saturations), or in a formation with subirreducible oil saturation caused by displacement of an original oil column from the zone by gas over geologic time.

    Water-wet formations in general will not spontaneously imbibe oil-based fluids, and conversely oil-wet formations will not imbibe water-based fluids. If high fluid loss conditions exist during overbalanced conditions, these fluids can still be easily displaced via pressure in a damaging fashion into the near well bore matrix.

    Thus, a proper understanding of formation wettability, coupled with the selection of the proper base fluid, can minimize some of the problems associated with countercurrent imbibition effects.

Glazing

The size and quantity of cuttings in the circulating fluid stream depend on the formation type, bit type, ROP, and fluid system. Fluid systems used in underbalanced drilling operations and gas/air drill operations may suffer from several problems.

Glazing or mashing is a polishing of the surface of the well bore caused by direct action of the bit at the formation face (particularly when severely hard formations are drilled at low ROPs or if the bit is dull or damaged). The formation face can be polished also by a poorly centralized drillstring.

The glaze generally consists of formation fines, which are generated and milled by bit action. A thin, pottery-glaze-like paste coats the surface of the formation.

Straight gas drilling operations are particularly sensitive to this problem because of the poor solids transport properties of most pure gas systems, very fine dust cuttings generated when turbine-type mud motors are used, and the poor heat transfer capacity of gas. The resulting very high rock/bit temperatures aggravate the glaze formation process.

Glazing in general tends be a relatively shallow process extending only a few millimeters into the formation. Cased and perforated completions, therefore, rarely encounter significant productivity impairment from this problem. The damage is easily penetrated with a typical perforation charge.

Very heterogeneous formations containing large vugs or natural fractures also tend to be less sensitive to this type of damage because of the inability of the glaze to occlude large porosity features.

Relatively homogeneous sandstone or carbonate formations completed open hole tend to be the most susceptible to this type of damage. In carbonate formations the glaze tends to be dominated by acid-soluble limestone or dolomite constituents and can often be removed by a tubing-conveyed light acid wash.

Silicate-based glaze generated in sandstone formations is more difficult to remove.

Macroporosity invasion

In formations which exhibit macroporosity (very large open fractures, large interconnected vugs), gravity-induced invasion of circulating drilling fluid and solids can occur on the lower side of a horizontal well bore (Fig. 8 [25313 bytes]).

If the fractures or vugs are small and the underbalance pressure sufficient, the natural orifice jetting action of the fluid from these features into the well bore will be sufficient to counteract this phenomenon. If low underbalance pressures or very large porosity features are present, resulting in a low superficial fluid velocity at the well bore/porosity feature intersection, gravity-dominated invasion could occur.

Extremely permeable zones

It is unfortunate that one of the best applications of underbalanced drilling technology, that of extremely high permeability formations (macrofractured chalks, grossly vugular carbonates, highly unconsolidated high permeability sands) also presents one of the major challenges.

Effective control of these formations when they exist at naturally high initial pressure, even at relatively low underbalance pressure conditions, becomes problematic. The risks associated with handling huge volumes of produced fluids and high pressures on surface becomes too costly and risky to consider underbalanced drilling, particularly in extreme or offshore operating conditions.

Improvements in surface handling and control equipment may allow underbalanced drilling technology to be applicable to a wider spectrum of formation applications of this type.

Risk of failure

Underbalanced drilling, like any advanced technology, often needs a company champion to put forth a case for a good application. In many companies, having a successful first application of any new technology is important for that technology to be considered for future applications.

Therefore, proper selection of a good candidate reservoir for underbalanced drilling is doubly important for a first operation. A poorly executed operation resulting in a failure will likely result in the technology being discounted as too risky for future applications where it may be advantageous.

References

1. Lunan, B., Surface Control Systems for Underbalanced Drilling, Journal of Canadian Petroleum Technology, September 1995.

2. Teichrob, R., et al., Use of a concentric drillstring system for underbalanced completion.

3. Eng, J., et al., Velocity Profiles in Perforated Completions, JCPT, October 1993.

4. Bennion, D.B., Thomas, F.B., Bennion, D.W., and Bietz, R.F., Fluid Design to Minimize Invasive Damage in Horizontal Wells, paper presented at the Canadian SPE/CIM/CANMET International Conference on Recent Advances in Horizontal Well Applications, Calgary, Mar. 20-23, 1994.

5. Bennion, D.B., Bietz, R.F., Thomas, F.B., and Cimolai, M.P., Reductions in the Productivity of Oil and Low Permeability Gas Reservoirs Due to Aqueous Phase Trapping, JCPT, November 1994.

6. Bennion, D.B., Thomas, F.B., Bietz, R.F., and Bennion, D.W., Water and Hydrocarbon Phase Trapping in Porous MediaDiagnosis, Prevention and Treatment, paper presented at the 46th Annual Technical Meeting of The Petroleum Society of CIM in Banff, Alta., May 14-17, 1995.

7. Mehta, R., et al., Flash Tests to Determine Combustible Limits for Underbalanced Drilling, paper presented at the Unitar Conference, Houston, February 1995.

8. Bennion, D.B., and Thomas, F.B., Underbalanced Drilling of Horizontal Wells: Does it Really Eliminate Formation Damage? Society of Petroleum Engineers paper 27352 presented at the SPE International Symposium on Formation Damage Control, Lafayette, La., Feb. 7-10, 1994.

9. Bennion, D.B., and Thomas, F.B., Recent Investigations Into Formation Damage in Horizontal Wells During Overbalanced and Underbalanced Drilling and Completion Procedures, paper presented at the 2nd Annual Conference on Emerging TechnologyCoiled TubingHorizontal WellsExtended Reach and Multilaterals, Aberdeen, June 1-3, 1994.

The Author

D. Brant Bennion is president of Hycal Energy Research Laboratories Ltd. in Calgary and is responsible for research and development in multiphase flow in porous media and formation damage. He has worked at Hycal Energy Research Laboratories since 1979.

Bennion graduated with distinction from the University of Calgary in 1984 with a BS in chemical engineering. He has authored more than 30 technical papers and has lectured in North and South America, Asia, Europe, Africa, and Australia.

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