DOWS reduce produced water disposal costs

John A. Veil
Argonne National Laboratory
Washington, D.C.
Bruce G. Langhus
CH2M Hill Inc.
Tulsa
Stan Belieu
Nebraska Oil & Gas Conservation Commission
Lincoln, Neb.

A relatively new technology, downhole oil/water separators (DOWS), can reduce the cost of handling produced water.

DOWS are devices that at the bottom of the well separate out water from the oil and gas stream. Some of this water is reinjected into another formation or horizon, while the oil, gas, and remaining water are produced to surface.

Water is the largest-volume waste stream associated with oil and gas production.

Treatment and disposal of water represent significant costs for operators. But costs are greatly reduced if the water does not have to be produced to surface and treated before being reinjected into another horizon.

DOWS may also aid in recovering additional oil, as well.

Other terms for DOWS are Dhows or Dials (dual injection and lifting systems).

This article summarizes a report on the technical, economic, and regulatory feasibility of DOWS technology that was funded by the U.S. Department of Energy and prepared by Argonne National Laboratory, CH2M Hill, and the Nebraska Oil & Gas Conservation Commission.¹

Benefits

Besides the fact that DOWS can save operators money by reducing produced water disposal costs, DOWS have other economic and environmental benefits.

In North America, more than half of the wells with DOWS increased their oil production after installation of these bottom hole devices.

Another benefit is in cases where surface processing or disposal capacity is a limiting factor. Here, the use of DOWS to dispose of some produced water can allow more production from that field.

Simultaneous production and injection using DOWS has the added benefit of minimizing the opportunity for contamination of underground sources of drinking water through leaks in tubing and casing during the injection process.

DOWS technology

A DOWS system includes many components, but the two primary components are an oil/water separation system and at least one downhole pump.

The individual components have been well tested and proven in the oil field. The challenge for DOWS designers is to build separation systems and pumps that work together in the confined space of a 7 in. or smaller casing in a bottom hole environment.

DOWS technology holds tremendous promise but is still in its infancy.

Similar devices have been used to a much greater extent for downhole gas/water separation. However, this article covers only oil/water separators.

Types available

Two basic types of DOWS have been developed. One uses hydrocyclones ( Fig. 1 [46,795 bytes]) and the other relies on gravity separation.

Hydrocyclone DOWS can handle flow up to 10,000 b/d,² while gravity separator DOWS can handle up to 1,000 b/d.³ Hydrocyclone DOWS, however, are significantly more expensive.

Several alternative designs are available from different vendors.

Hydrocyclone DOWS have been paired with electric submersible pumps (Fig. 2 [61,161 bytes] and Fig. 3 [97,137 bytes]), rod pumps (Fig. 4 [112,157 bytes]), and progressing cavity pumps. Gravity separator DOWS have used only rod pumps (Fig. 5 [101,453 bytes] and Fig 6 [104,103 bytes]).

To fit into 51/2-in., or 7-in. casing, DOWS are designed as long, slender devices. The full DOWS assembly may be 100 ft in length.

Most DOWS installations have the producing zone above the injection zone.

DOWS can potentially be used for waterflooding. Also, these devices could be used for reverse coning to reduce water influx into oil-producing zones.

Economics

Conversion of a well from a regular pump to a DOWS is relatively expensive. Total costs include the device itself and well workover expenses.

Hydrocyclone DOWS are fairly expensive. For example, the cost of an electric submersible pump DOWS system is about two to three times the cost of replacing a conventional electrical submersible pump. These DOWS can cost in the range of $90,000 to $250,000, excluding the well workover costs, which can exceed $100,000.⁴

Costs are somewhat lower for the gravity separation DOWS, ranging from $15,000 to $25,000.^5-8 One complete gravity separator DOWS installation, for example, cost $140,000 (Canadian).⁹

DOWS installations may not be cost-effective for all wells. Knowledge of the reservoir and historical production is important before deciding to install these devices.

The characteristics of wells that are likely to work well with DOWS include:

• High water/oil ratio
• Presence of a suitable injection zone that is isolated from the producing zone
• Compatible water chemistry between the producing and injection zones
• Properly constructed well with good mechanical integrity.

DOWS will not work well in wells that produce a low API gravity oil. ^{3 10}

Installations that meet these requirements must still remain in good operating condition long enough that the accrued monthly savings can offset the initial purchase costs.

Existing installations have a mixed track record. Some DOWS have remained in service for more than 2 years, but others have failed within a few days.

This situation is understandable given that fewer than 50 DOWS have been installed in North America through the end of 1998. The technology is new and is still being refined and improved with each successive installation.

Installations to date

Table 1 [68,572 bytes PDF Format] and Table 2 [57,720 bytes PDF Format] include information on 37 DOWS installations in North America. ¹ Key statistics from these data include:

• Hydrocyclone DOWS comprise more than half of the installations to date (21 compared with 16 gravity separator DOWS)
• Canada has 27 installations compared to 10 in the U.S.
• Four producing areas have 27 installations: Southeast Saskatchewan, East-central Alberta, Central Alberta reef trends, and East Texas
• Seventeen installations are in 51/2-in. casing, 14 are in 7-in. casing, 1 is in 85/8-in. casing, and 5 are unspecified.
• Twenty installations are in wells producing from carbonate formations, while 16 installations are in wells producing from sandstone formations. One trial did not specify the lithology.

DOWS appeared to work better in carbonate formations, showing an average 47% increase in oil production (compared with an average 17% for sandstone formations) and an average 88% decrease in water brought to the surface (compared with 78% for sandstone formations).

• Oil production increased in 19 trials, decreased in 12, stayed the same in 2, and was unspecified in ⁴. The top three performing hydrocyclone DOWS wells showed a 457 to 1,162% oil production increase. One well lost all oil production. The top three gravity separator DOWS wells had a 106 to 233% oil production increase. One well lost all oil production.
• All 29 trials for which both pre-installation and post-installation water production data were provided showed a decrease in water brought to the surface. The decrease ranged from 14% to 97%, with 22 of 29 trials exceeding a 75% reduction.
• The data on injectivity and the separation distance between producing and injection formations do not correlate well with the decrease in water volume brought to the surface.

Problems

Some installations experienced problems that impeded proper DOWS operations.

At least two installations suffered from low injectivity of the receiving zone. In both cases, incompatible fluids contacted sensitive reservoir sands, which plugged part of the permeability.

Several installations noted problems of insufficient isolation between the producing and injection zones. If isolation is not sufficient, the injectant can migrate into the producing zone and then short-circuit into the producing perforations. This results in recycled produced water, with oil production rates dropping to nearly zero.

Other DOWS have been plugged by fines or sand, although recently developed desanding equipment appears to mitigate plugging problems.¹¹

Several trials had to be canceled prematurely because of corrosion and scaling problems. Finally, some of the early installations suffered from poor design features.

Regulatory issues

Because the technology is still new, no regulatory requirements for DOWS exist in many jurisdictions. The U.S. Environmental Protection Agency (EPA) does not yet have a formal position on regulating DOWS.

Four states-Colorado, Oklahoma, Louisiana, and Texas-have developed either regulations or administrative guidelines for DOWS. Those states regulate DOWS with requirements comparable to or less stringent than those for regular Class II injection wells.

Some concern exists that the EPA may decide DOWS are not covered under the definition of a Class II well, thereby potentially leading to stricter requirements that could hinder future use of DOWS.

It is important for EPA and state regulators to develop reasonable regulatory requirements for DOWS that do not necessarily impede their use in the future.

Acknowledgments

This work was supported by the U.S. Department of Energy, Office of Fossil Energy, National Petroleum Technology Office (NPTO) under contract W-31-109-Eng-38. We thank Nancy Holt, project manager at NPTO, for supporting and encouraging this work.

Numerous oil and gas operators and equipment suppliers in the U.S. and Canada provided information on DOWS installations. Finally, we acknowledge Dan Caudle for his technical advice and assistance.

References

1. Veil, J.A., Langhus, B.G., and Belieu, S., "Feasibility Evaluation of Downhole Oil/Water Separation (DOWS) Technology," prepared for U.S. Department of Energy, Office of Fossil Energy, National Petroleum Technology Office, by Argonne National Laboratory, CH2M-Hill, and Nebraska Oil & Gas Conservation Commission, January 1999.
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The Authors