underbalanced drilling

Based on a workshop sponsored by the Oklahoma Geological Survey and PTTC's South Midcontinent Region on July 11, 2001 in Norman, Oklahoma

BOTTOM LINE

Underbalanced drilling can reduce lost circulation, minimize differential sticking, increase drilling rates, and, most importantly, create higher productivity completions because formation damage is minimized. To maximize benefits, extreme care must be taken to keep drilling and completion operations underbalanced at all times. The rate of return from wells drilled underbalanced is typically higher. Techniques for drilling underbalanced include: (1) lightweight drilling fluids, (2) gas injection down the drill pipe, (3) gas injection through a parasite string, and (4) foam injection. Nitrogen is typically used because of its relatively low generation cost, scale control and low potential for downhole fires. Nitrogen injection down the drill pipe is the most cost effective when electromagnetic measurement-while-drilling (MWD) is used. Despite added cost and time, parasite injection of nitrogen is the preferred method when electromagnetic MWD is not possible. Foams are more stable than aerated systems, but they are more costly.

PROBLEM ADDRESSED

With conventional (overbalanced) drilling practices, problems include lost circulation, differential sticking, low drilling rates and formation damage. Underbalanced drilling, although initially more costly, can reduce these problems.

KEY WORDS:

Corrosion & Scale Control, Formation Damage, Gas Injection, Nitrogen/Foam System, Parasite String Injection, Underbalanced Drilling

SPEAKERS

Underbalanced Drilling Systems, Air/Gas Drilling Equipment
William Rehm, Maurer Technologies Inc.

Nitrogen Membrane Units
Greg Chitty, International Nitrogen Services

Field Experience with Underbalanced Drilling
Don Purvis, BJ Services

TECHNOLOGY OVERVIEW

Effect of Drilling Underbalanced. The primary value of underbalanced drilling is to minimize formation damage. Negative differential pressure between the formation and the wellbore also stimulates the production of formation fluids and gasses. Increased penetration rates are also often observed in wells drilled underbalanced.

Underbalanced drilling does not build a filter cake in the wellbore. During conventional drilling, this filter cake acts as a protective barrier, reducing damage to formation permeability from drill cuttings. When drilling horizontal wellbores, drill cuttings are ground into fine powder. In conventional drilling, if the wellbore does not have a filter cake and becomes overbalanced, this powder is carried into the formation, greatly reducing near-wellbore permeability.

Unlike typical cased-hole completions which can easily be stimulated, if an underbalanced extended-reach open-hole completion develops a near-wellbore region of reduced permeability, the damaged region can significantly reduce the potential production rate. For this reason, preplanning and on-site engineering are critical to maintaining underbalanced conditions.

Underbalanced drilling is not appropriate for most shales, salt, shattered coal sections, unconsolidated sections and chalk. If these lithologies are present up dip, it may be appropriate to drill them overbalanced and then change to underbalanced conditions for the target formation.

Choice of Gas Phase. Air, nitrogen and natural gas have been used as the gas phase in underbalanced drilling. When natural gas is available and can be recovered and reinjected into the supply/sales line, it can be the most cost-effective method to achieve an underbalanced condition. However, this is not typically an option. Drilling with air injection is common, but there are significant corrosion problems and the potential for downhole fires always exists. For these reasons, nitrogen has become the gas of choice for underbalanced drilling.

Cryogenic nitrogen of high purity can be purchased, but price and logistics are unfavorable. Generally the best source is nitrogen that is generated onsite using a membrane unit. Membrane units typically produce 95% purity nitrogen. Purity is such that corrosion can usually be managed and downhole fires are not a problem. Membrane units are available in a range of capacities. Nitrogen cost is a function of feed-air pressure, desired nitrogen pressure and purity, and capacity. A typical 1.5 Mcf unit on duty 24 hrs/day can provide nitrogen for about $2.20/Mcf.

Techniques To Achieve Underbalanced Conditions

Four techniques are currently available to achieve underbalanced conditions while drilling. These include using lightweight drilling fluids, injecting gas down the drillpipe, injecting gas into a parasite string, and use of foam.

Lightweight Drilling Fluids. The simplest mechanism to reduce hydrostatic pressure in the wellbore is the use of lightweight drilling fluids, such as fresh water, diesel or lease crude. The primary problem with this approach is that hydrostatic pressure can not be reduced enough to remain underbalanced in many reservoirs.

Gas Injection Down Drillpipe. With this technique, air or nitrogen is added to the drilling fluid and it is pumped directly down the drill pipe. Advantages of this technique include:

• Hydrostatic advantage gained over entire vertical depth,
• Wellbore does not have to be specifically designed for underbalanced condition,
• Less gas is required to achieve given pressure compared to parasite injection, and
• Penetration rate may be improved.

Disadvantages of this technique include: (1) an overbalanced condition may occur if the well is shut down and (2) exotic MWD systems are required.

Gas Injection Via Parasite String. With this technique, a second pipe is run outside of the intermediate casing. Advantages of this technique include:

• No operational differences,
• Constant bottom hole pressure is achieved, and
• Standard MWD equipment can be used.

Disadvantages of this technique include: (1) additional costs are incurred, (2) additional time is required, and (3) larger diameter surface casing is required.

Foam Versus Two Phase Flow. A nitrogen foam system is less damaging to water sensitive formations and has been used on a limited basis. However, the additional nitrogen requirements to generate stable foam have made this cost prohibitive in most cases. Aerated systems with gas/liquid ratios varying from 10-to-1 to 50-to-1 are simple and flexible, but pressure control/gas surging can be a problem. Because of this, and wanting to remain continuously underbalanced, the margin of safety for aerated systems is typically larger than for more stable systems, such as foams. Foams also exhibit some sensitivity to hydrocarbons, so large inflows of hydrocarbons can destabilize them. Temperature limits of current foams, about 180° F, restrict the use of foam to depths less than 12,000 ft.

CANADIAN WILLISTON BASIN EXPERIENCE

Pan Canadian Petroleum Limited undertook an underbalanced drilling program using nitrogen injection down the drillpipe in the early 1990s. Conclusions from analyzing more than 120 underbalanced horizontal wells include:

• If properly designed, underbalanced drilling can be used effectively to reduce formation damage and increase productivity in horizontal wells drilled in underpressured or pressure-depleted reservoirs.
• Accumulation of liquids and solids in the annulus was the primary cause of pressure surges that resulted in periodic episodes of overbalanced conditions. Acid was required to remove drilling fines imbedded in the formation during overbalanced episodes.
• By utilizing electromagnetic MWD and by changing operating parameters, the magnitude of pressure surging was reduced, resulting in significant increases in well productivity.

More than 550 horizontal and more than 300 vertical wells have now been completed underbalanced in Canada. This technology has been applied to more than 25 wells in the Williston Basin. In the U.S. portion of the Williston Basin, both parasite and drillpipe injection methods have been utilized, but due to problems with corrosion, scaling and MWD tool limitations, parasite injection has been the preferred method.

U.S. WILLISTON BASIN CASE HISTORIES

Bottineau Co., North Dakota. Two nitrogen assist wells were drilled in an underpressured, 150° F, high permeability carbonate reservoir. The first well utilized a 1" parasite string, nitrogen and mud pulse MWD. Design in this well called for drilling approximately 100 psi overbalanced to control formation fluid influx. Fluid loss in the formation was reduced by a factor of 10 from previous wells that had drilled 600 psi overbalanced. The second well was drilled using drillpipe injection and nitrogen generation equipment using wet-connect procedures. H₂S was not present. Corrosion and scaling were not a problem in either well.

Ward Co., North Dakota. A horizontal well was drilled underbalanced in a 160° F underpressured reservoir. H₂S ranged from 2-4% and average water cut was 70% with a 9.8 ppg formation brine. The well was drilled using drillpipe injection, nitrogen generation and wet-connect system. Even though oxygen concentration in the nitrogen was less than 2% and an aggressive chemical program was used, corrosion and scaling could not be effectively controlled.

Fallon Co., MT and Bowman Co., North Dakota. More than 25 wells have been drilled in this area using gas assist. A high-pressure cryogenic pump was used to inject 99.8% pure nitrogen through a 1.5" parasite string to establish nitrogen/fresh water circulation. Air compressors were used to re-inject nitrogen during operation and a high pressure pump and a nitrogen storage vessel were on standby to supply additional product when necessary. This method reduced nitrogen requirements by 5-10 fold. A reduction of the oxygen content to less than 2%, reverting to a parasite string, and a continued chemical treatment program allowed the use of a nitrogen membrane unit. Natural gas was used in place of the nitrogen in areas where it was economic to supply via a temporary pipeline.

LESSONS LEARNED FROM THE CASE HISTORIES

Underbalanced drilling by nitrogen injection can be a cost effective method to maximize production in low permeability horizontal wells if all precautions are taken to remain underbalanced. If the system becomes overbalanced at any time, the advantages of underbalanced drilling are negated.

• Oxygen concentrations greater than 2% cause excessive corrosion in the Williston Basin.
• Nitrogen injection down the drillpipe is the most cost effective when electromagnetic MWD is possible.
• Despite added cost and time (compared to injection into the drillpipe), parasite injection of nitrogen is the preferred method when electromagnetic MWD is not possible.
• On-site modeling to optimize nitrogen and fluid injection rates should be done to reduce cost and prevent an overbalanced condition.
• Accurate modeling of bottomhole pressure is possible.
  

CONNECTIONS:

William Rehm
Maurer Engineering Inc.
2916 West T.C. Jester
Houston, TX 77018
Phone: (281) 955-1307 Fax: (281) 970-1755
Email: billrehm@worldnet.att.net

Greg H. Chitty
International Nitrogen Services
5123 Hiltonview Rd
Houston, TX 77086
Phone: (281) 885-4723 ext. 305 Fax: (281) 885-4732
Email: chittyg@n2services.com

Don Purvis
BJ Services Co.
4005 NW Expressway, #410E
Oklahoma City, OK 73116
Phone: 405-767-9207 Fax: 405-810-9174
Email: dpurvis@bjservices.com


For information on PTTC’s South Midcontinent Region and its activities contact:
Charles Mankin, Director, Oklahoma Geological Survey
100 E. Boyd St., Room N131, Norman, OK 73019-0628
Phone 405-325-3031, Fax 405-325-7069, Email cjmankin@ou.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

The not-for-profit Petroleum Technology Transfer Council is funded primarily by the US Department of Energy’s Office of Fossil Energy, with additional funding from universities, state geological surveys, several state governments, and industry donations.

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