On-site generated nitrogen cuts cost of underbalanced drilling

This self-contained, skid-mounted nitrogen production unit filters air to produce 92-99% pure gaseous nitrogen for use in underbalanced drilling (Fig. 1).

Underbalanced drilling can loosely be defined as drilling with a fluid medium that exerts a lower hydrostatic pressure than the pore pressure of the formation drilled into. Drilling wells underbalanced allows formation fluids to flow into the well bore and be carried to the surface with the drilling fluid and formation cuttings solids. This process minimizes invasion of the drilling fluids and cuttings into the formation.

During the past 5 years in Canada, some 500 wells have been drilled using underbalanced techniques. The majority of these wells were drilled with a gas such as compressed air, natural gas, or nitrogen. The gas reduces the density of the drilling fluid.

The gas is injected into the drilling fluid and then recovered from the well in a four-phase separator. The solids, hydrocarbon liquids, drilling fluid, and gases are separated and metered.

Most of the underbalanced wells drilled with gas have used nitrogen, and only a relative few have used natural gas or compressed air.

Compressed air is not normally used for underbalanced drilling because the oxygen content in compressed air provides a highly flammable or explosive mixture in combination with hydrocarbon liquids and gases, both downhole and in the surface equipment. The oxygen content of compressed air, approximately 20.9%, also can cause high corrosion rates, particularly at the elevated pressures and temperature encountered during drilling.

There have been many recorded incidents of downhole fires and explosions caused by the use of compressed air for drilling, and these concerns have to some extent limited its use.

Natural gas can be used to gasify drilling fluid for underbalanced drilling. When used in a closed system with precaution taken to eliminate the introduction of air or oxygen, natural gas can lighten drilling fluid quite well. The potential drawbacks to using natural gas, however, are availability of supply at the desired flow rates and pressures, corrosive contaminants (such as carbon dioxide and hydrogen sulfide) in the natural gas, and the cost of the natural gas itself. Furthermore, natural gas has a lower molecular weight than nitrogen, so a larger volume of natural gas is required to achieve a given underbalanced pressure and flow rate condition.

In most cases where natural gas has been used for underbalanced drilling, the well has been in or near previously developed fields or adjacent to operating high-pressure pipelines. If possible, the natural gas is recovered from the separation equipment on site and is returned, with any produced gas, to the pipeline.

Metering and compressing the gas under variable flow rates during drilling and returning the gas to the pipeline can be complicated, and there may be concerns about the safety of natural gas compression at the drill site.

Because nitrogen is inert and inflammable, it is the preferred gas for underbalanced drilling. Nitrogen can be supplied for oil field use by three different methods: cryogenic liquid separation, pressure swing adsorption, and hollow fiber membranes. The selection of nitrogen supply from one of these methods depends on the cost of delivered nitrogen, the required flow rates and pressure, the required nitrogen purity, and the availability and reliability of the equipment for nitrogen generation.

Underbalanced drilling

Underbalanced drilling of horizontal and vertical wells has become an important technology in a growing number of regions, particularly in Canada. Until mid-1994, cryogenic nitrogen was used to drill the majority of underbalanced wells in which a gas phase was required to reduce bottom hole drilling fluid circulation pressures below the formation pressures.

In Canada, the majority of the wells that have been drilled underbalanced using a gas have had total depths of less than 6,000 ft, and gas injection pressures and flow rates have been less than 2,000 psig and 1,500 scfm. Including some deeper wells and horizontal wells with very long lateral sections, nitrogen pump rates have ranged 500-3,000 scfm at pressures up to 2,500 psig for periods exceeding 20 days.

In typical underbalanced drilling operations, gas is injected into the drilling fluid to reduce the hydrostatic pressure of the drilling fluid. Gas injection is done either down the drill pipe or via a parasite string.

For drill pipe injection, the gas is injected upstream of the standpipe into the drilling fluid, so that the combined gas and liquid flows down the drill pipe and bit and then back up the annulus. Parasite injection uses a secondary string, which may be tubing, coiled tubing, or casing, to deliver the gas to a downhole injection point, where the gas enters the annulus to lighten the annular fluid and cuttings returns.

There are advantages and disadvantages of each method of gas injection, and these are covered in the drilling literature.^1-4 The required nitrogen injection rate and pressure for an equivalent bottom hole pressure are affected by the gas injection method used.

Well geometry, bottom hole pressure, and gas flow requirements determine the injection rates and pressures. Parasite injection system injection rates and pressures are higher and increase more rapidly than those for drill pipe injection, as bottom hole pressures are reduced.

The gas injection rate and pressure vary as a function of well bore parameters, bottom hole pressure, and drilling configuration. Injection gas quality requirements are primarily based on concerns of combustion as the drilling fluids are exposed to and mixed with formation hydrocarbons.

Total underbalanced drilling system costs are also a primary consideration. Until mid-1994, nitrogen for underbalanced drilling in western Canada was supplied from cryogenic plants not central to the major drilling areas and was subject to seasonal demand pressures. Thus, the cost of cryogenic nitrogen was high and a major component of total well costs.

In 1994, the first hollow fiber membrane units were shipped to Canada and placed in service to supply gaseous nitrogen for underbalanced drilling. More than 120 wells have been drilled in Canada using nitrogen supplied from these hollow fiber membrane units.

The projected amount of required nitrogen may be estimated through the use of advanced computer simulation models that analyze multiphase flow of the combined drilling fluid, injection gas, produced formation fluids, and hole cuttings through the drillstring, bottom hole assembly, and surface production equipment.

Although this type of simulation modeling is extremely complex and subject to many uncertainties, it has proven effective on numerous jobs when applied by experienced drilling engineers. Critical data for this type of modeling include bottom hole pressure, flow rates, and temperatures.

Nitrogen supply

Nitrogen for oil field operations can be supplied on site in liquid form or can be produced on site by extracting nitrogen from compressed air.

Cryogenic nitrogen is obtained when air is supercooled to a point where the density differences between nitrogen and oxygen allow for high-purity separation. Generally, liquid nitrogen is obtained from this process, and the liquid nitrogen is stored and transported to the well site for gasification and injection.

Pressure swing adsorption (PSA) equipment can generate gaseous nitrogen from compressed air. PSA units usually have two or more tanks containing a loose or granulated carbon material that adsorbs oxygen. Compressed air is pumped into the tank, the oxygen adsorbs onto the carbon material, and nitrogen flows through to the outlet. When the carbon material in the tank approaches an oxygen saturation point, oxygen sensors on the outlet detect the increasing oxygen content in the nitrogen, and then valves open and close to redirect the compressed air flow to a second tank.

The tank containing the oxygen-saturated carbon material is then depressurized to release the oxygen, and the cyclic process continues. PSA units can provide nitrogen at purities up to 99%. Because of the cost, bulkiness, weight, and mechanical complexity of these units, however, they are not widely used in the oil field, particularly at the high nitrogen flow rates required for drilling.

Hollow fiber membrane nitrogen-generation systems also produce gaseous nitrogen from a supply of compressed air. They use special polymers configured into very small diameter fibers that are bundled together in tubes called modules.

Compressed air flows down the inside of each hollow fiber. Oxygen and water vapor molecules diffuse through the walls of the fibers at a much faster rate than the nitrogen, thus providing separation to produce nitrogen at the outlet. The oxygen and water vapor are vented to the atmosphere.

Multiple modules can be manifolded together to achieve the desired nitrogen flow rates. These units have few moving parts, can be configured onto small skids, and have a large range of nitrogen production rates (Fig. 1).

The selection of nitrogen supply for underbalanced drilling is usually based on both cost and technical specifications (amount of nitrogen needed, required flow rates and pressures, and nitrogen purity) of the drilling operation.

Nitrogen supply costs can vary substantially and depend on factors such as distance from supply points, trucking costs, job duration, total quantity required, and purity. Oxygen corrosion can be a concern if membrane-generated nitrogen is used to drill wells in conjunction with water containing chlorides or if significant amounts of water containing chlorides are produced during the operation.

Oxygen corrosion is the only significant corrosion concern with membrane-generated nitrogen. Oxygen corrosion can occur in the presence of water containing significant amounts of chlorides and pH levels less than 7. In these conditions, only a few parts per million oxygen can cause corrosion.

Oxygen corrosion can be effectively controlled in the drilling environment, however, by use of a drilling fluid that does not contain significant concentrations of chlorides, such as crude oil, diesel, or an oil-based mud, or for a water-based mud containing chlorides by raising the pH level to 9 or higher and using a corrosion inhibition program.

There have been few instances of oxygen corrosion with membrane-generated nitrogen, and these have occurred when good practices were not followed.

In general, for drilling operations in which nitrogen flow rates are less than 500 scfm or more than 3,000 scfm, required pressures exceed 3,000 psig, or nitrogen injection is needed for under 2 days, then liquid nitrogen may be less expensive than membrane-generated nitrogen.

The costs for liquid nitrogen usually include the lease of the pumper or tanker, mileage, operator charge, and the nitrogen itself. For membrane systems, costs include the lease of the membrane unit, the rental of air compression equipment, diesel fuel for the air compressors, trucking charges, and air compression and membrane unit operators.

Liquid nitrogen is sold on a per Mcf-delivered basis, and NPU equipment is leased on a per day basis.

An operator should obtain nitrogen bids from both liquid suppliers and membrane units suppliers to ensure the lowest costs for the specific needs of each drilling job.

Nitrogen production units

Membranes used in nitrogen production units (NPUs) are made of a special polymer extruded into very small diameter hollow fibers. Each hollow fiber has a circular cross section and uniform wall thickness and is so small that several may fit through the eye of a needle. The small fiber size provides a huge surface area to separate large quantities of nitrogen.

Compressed air is directed to the NPU equipment, where it is first filtered to remove particulates and entrained water and oil droplets down to about 0.1 m diameter. The clean compressed air, at 120-350 psig, enters the inlet of the membrane module and travels down the fiber tubes.

Oxygen and water vapor molecules permeate the wall of the fiber membrane and are vented to atmosphere. The product nitrogen remains inside the fiber and is collected at the outlet connection, where it is metered. Permeation of the oxygen and water vapor through the membrane wall is governed by Fick's and Henry's laws and is not a matter of molecule size, per se.

The hollow fiber membranes can produce nitrogen purities up to 99.9%. The total flow rate of nitrogen product from the membrane is inversely proportional to the purity. For most drilling applications, in which 92-95% nitrogen purity is sufficient, the membrane efficiency is generally 50-67%, depending upon the particular NPU equipment used. Membranes have intrinsic performance characteristics that are a function of temperature and pressure, and thus different membranes may have different optimum operating pressures and temperatures at selected purities.

Equipment configuration

The NPU receives compressed air from one or more primary compressors at pressures ranging from 100 to 350 psig. The product nitrogen is pumped, with about a 20-40 psig pressure drop, to the suction of a booster compressor where its pressure is increased to that required for injection into the drillstring.

NPUs have three major components: an air filtration system, an array of air separation modules, and a control panel.

The air filtration system usually consists of a scrubber, a coalescing filter, and a particulate filter. Some NPUs also include an activated carbon bed filter and possibly a refrigerated air dryer. The activated carbon bed filter removes aerosol-sized and smaller oil droplets down to a concentration of a few parts per billion. The refrigerated air dryer reduces the relative humidity into the carbon bed to improve oil droplet filtration.

The array of hollow fiber modules are manifolded together to accept the clean compressed air feed and to collect and deliver the nitrogen product. The oxygen and water vapor permeate stream is also collected from each membrane module and piped at near atmospheric pressure to the outside of the NPU skid, where it can quickly and harmlessly dissipate into the atmosphere.

The control panel on the NPU allows monitoring and control of the operation. Control panel design and function vary greatly depending on the manufacturer. Some panels measure flow rates, temperatures, purity, and pressure drops across the NPU precisely, yet others only provide simple output of flow rate and nitrogen purity.

Nitrogen purity is controlled by varying the back pressure on the membrane modules.

Electricity for the control panel is generally supplied via a small on-board diesel-fueled generator. For extreme cold weather operations, NPUs are configured with insulated housings and either a forced-air diesel-fueled heater or electrical heat tracing.

There are currently four manufacturers of NPU equipment: MG Generon, Air Liquide, Permea, and Praxair. Most of these companies prefer to lease the NPU equipment rather than sell it. Most oil companies prefer to lease the equipment also, because of the equipment's high cost and the unknown lifetime and performance degradation of the membrane modules over time. There are some significant differences in NPU design and membrane performance among the manufacturers.

To date, NPUs have been built and placed into service with capacities varying from 700 scfm to 3,000 scfm, and most of the units have a 1,500-scfm capacity.

Nitrogen purity

To date, for nearly all horizontal underbalanced drilling operations using nitrogen, the nitrogen purity requirement has been based on the concern of hydrocarbon combustion or flame propagation as the hydrocarbons produced from the formation come into contact and mix with the drilling fluids.

The nitrogen purity required to prevent combustion is a function of pressure, temperature, and composition of hydrocarbons encountered. Previous work by the U.S. Bureau of Mines and others has indicated that methane may combust in oxygen concentrations as low as 12-14%, but this lower explosive limit of oxygen content decreases to 8% or less as pressure and temperature increase. Subsequent studies by Canadian operators and one Canadian university have indicated that the minimum amount of oxygen required to initiate and propagate a flame may be as low as 6% in the presence of hydrogen sulfide and under high temperature, high pressure conditions.

Other drilling applications

In addition to its use in horizontal underbalanced drilling, nitrogen is often used to drill oil and gas wells under other conditions. The most common practice is to drill with straight dry nitrogen instead of compressed air or misted compressed air.

Although leasing NPU equipment increases the total daily drilling costs slightly, drilling with straight dry nitrogen can reduce total well costs by eliminating concerns about downhole fires or explosions, increasing penetration rates, improving bit life, reducing the number of drillstring trips, reducing or eliminating corrosion, and improving safety.

The use of NPU-generated nitrogen in some traditional air drilling areas (West Texas, eastern Oklahoma, and the San Juan basin, for example) has become commonplace and often the method of choice both to cut costs and improve drilling operations.

Nitrified mud drilling is also becoming popular, particularly in areas where water-based muds cannot be used, rates of penetration with liquid muds are slow, there are lost circulation zones, aeration of mud may cause excessive corrosion, or drilling mud invasion and permeability damage are concerns. In these applications, nitrogen is injected into the drilling fluid at rates as low as 300 scfm or as high as 3,000 scfm, depending on operating conditions. This technique has been applied successfully in East Texas, eastern Oklahoma, the Texas panhandle, the Permian basin, the Overthrust belt, and the San Juan basin.

Bibliography

1. Downey, R.A., "On-Site Generated Nitrogen for Oil and Gas Well Drilling and Other Applications," paper No. 95-107, Canadian Association of Drilling Engineers/Canadian Association of Oilwell Drilling Contractors Spring Conference, April 1995.

2. Allen, P.D., "Nitrogen Drilling System for Gas Drilling Applications," Society of Petroleum Engineers paper 28320, September 1994.

3. Teichrob, R.R., "Low-pressure reservoir drilled with air/N2 in a closed system," OGJ, Mar. 21, 1994, pp. 80-90.

4. Fried, S., and McDonald, C., "Nitrogen Supply Alternatives for Underbalanced Drilling," First International Underbalanced Drilling Conference & Exhibition, October 1994.

The Author

Robert A. Downey is president and CEO of Energy Ingenuity Co. in Englewood, Colo. Energy Ingenuity Co. was the exclusive marketer for the use of the nitrogen production units in the oil and gas industry for the Generon subsidiary of the Dow Chemical Co. Downey has a BS in petroleum engineering from the Colorado School of Mines.

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