The industry still has installed only a few downhole oil-water and gas-water separators, and the technology has languished compared with activity and expectations made during the late 1990s, according to a recent study from Argonne National Laboratory.
Downhole oil-water separators (DOWS) and downhole gas-water separators (DGWS) eliminate the need and associated costs of bringing all produced water to surface and instead allow for direct downhole water disposal.
John A. Veil and John J. Quinn prepared the November 2004 study "Downhole Separation Technology Performance: Relationship to Geologic Conditions" for the US Department of Energy's National Energy Technology Laboratory under Contract W-31-109-Eng-38.
Downhole separators
The study evaluated the two basic DOWS: hydrocyclones that mechanically separate oil and water and the technology that uses gravity for separating the fluids in the wellbore. It also evaluated four types of DGWS: bypass tools, modified plunger rod pumps, electric submersible pumps (ESPs), and progressive cavity pumps (PCPs).
In total, the study provides data on 59 DOWS and 62 DGWS installations and qualitatively discusses another 35.
Table 1 compares qualitative performance with geologic types for 59 DOWS installations. The study rated about 59% of the trials as good with 31% rated poor. For the three categories with production and disposal formations identified, it found that 28 of the trials in sandstone production and injection were poor compared with 50% poor trials for carbonate production and sandstone injection.
Table 2 compares qualitative performance date from the Gas Research Institute (GRI; since merged with the Institute of Gas Technology to form GTI) with geologic formations for 48 DGWS installations. The study rated 54% of trials as successes and noted that all the carbonate-carbonate, carbonate-sandstone, and sandstone-sandstone categories had a high success rate. The study rated 42% of the trials as failures or economic failures.
Almost half of the trials had formations that were not identified. In this subset, the study rated only 30% as successful with 61% being failures.
The study concludes that the analysis of about 120 DOWS and DGWS installations from numerous different countries, states, and provinces was insufficient for determining the likelihood of DOWS or DGWS successes solely based on geology of the production or disposal formation.
Other factors that the study says influence success are:
- Having a formation with good injectivity.
- Eliminating materials that could clog the disposal formation.
- Maintaining good vertical and mechanical separation between the production and disposal formations.
- Installing the equipment in wells with enough hydrocarbon reserves to payback the investment.
- For DGWS, installing the equipment in wells with disposal zones favorable for DGWS should have high permeability, high porosity, and be underpressured.
Recent DOWS activity
The study says that the lack of DOWS sales in recent years has eliminated any active marketing by the three companies that actively sold DOWS in the US in 1999: Centrilift (a division of Baker Hughes Inc), REDA Pumps, and Dresser-Axelson Inc.
The study says that Schlumberger Inc. after acquiring REDA no longer sells the Aqwanot DOW because of reliability concerns. It further adds that Schlumberger continues proprietary development of downhole separation tools and expects to have a field prototype late in 2005 followed by commercialization in 2006.
In Canada, the study found that Quinn Pumps Ltd. has not installed any DOWS recently and had changed its focus to gas wells.
One recent DOW installation was in a Petroleos de Venezuela (PDVSA) well in Venezuela. This DOW was a two-stage hydrocyclone system developed by READ Well Services in conjunction with Wood Group ESP. The study says the system handled 8,000 b/d for 5 months before the unit failed.
In the system, the hydrocyclone separates out the water, which than can be split into two streams: one for disposal downhole and another sent up the annulus to the surface.
The study says C-FER Technologies Ltd., which played an active role in developing the original hydrocyclone DOWS, continues to develop new DOWS technologies, such as for gas-lift.
Another company still developing DOWS tools is Gnesys Inc., which may start a pilot test in late 2004, according to the study.
Recent DGWS activity
The study explains that because water readily separates from gas by gravity, DGWS technology is more about water disposal rather than about the separation process.
A number of vendors market DGWS tools. The study says Harbison Fischer Inc. and Chriscor, a division of Kudu Industries Inc., sell bypass DGWS that are installed at the bottom of a rod pump. These tools work as follows:
- On the pump upstroke, water from the casing-tubing annulus enters the pump chamber through a set of valves.
- On the next downward stroke, these valves close and another set of valves opens, allowing the water to flow into the tubing.
- Water accumulates in the tubing until it reaches a sufficient hydrostatic head that allows it to enter the disposal formation solely by gravity.
The study says that bypass tools are appropriate for 25-250 bw/d production at a maximum depth of 6,000-8,000 ft.
Another DGWS is the modified plunger rod pump. The study says Downhole Injection Inc. (DHI) is the leading vendor of these systems with also Burleson Pump Co. selling some.
These systems incorporate a rod pump with a modified plunger that acts as a solid assembly and an extra pipe section with several sets of valves located below the pump. The tools work as follows:
On the pump upstroke, the plunger creates a vacuum and draws water into the pump barrel.
On the downward stroke, the plunger forces water out of the pump barrel to a disposal zone generating higher pressures than the bypass tool.
The study says modified plunger rod-pump systems are suited for 250-800 bw/d water production from depths of 2,000-8,000 ft.
ESPs also serve in DGWS installations for discharging water downward into a lower injection zone, although the study notes that industry has only installed a few ESP-type DGWS. Both Centrilift and REDA have DGWS systems that include ESPs and a packer that isolates the producing and disposal formations.
The study notes that ESPs can handle much higher flow rates, greater than 800 b/d at deeper depths, more than 6,000 ft.
Petrospec Engineering Ltd. is another company that has introduced an ESP deployed on coiled tubing for shallow and low-power-demand applications, according to the study.
A fourth version of a DGWS includes a progressing cavity pump (PCP). The study lists Weatherford Artificial Lift Systems as selling this type of system that is configured to discharge downward or configured with a bypass tool. Centrilift also provides PCP DGWSs.
The study notes that recent DGWS activity includes the Kudu Industries Inc. development of DGWS that relies on a PCP and a Chriscor downhole injection tool. The Chriscor tool, installed with a beam pump or PCP, has a bypass area that allows water in the tubing string to move downward.
Quinn Pumps (a division of Quinn's Oilfield Supply Ltd.) has two DGWS technologies: T and Q-Sep Gas R. The Q-Sep Gas T pumps water off a gas well and directly injects the water into a disposal zone in the same wellbore, while the Q-Sep Gas R, coupled with a Chriscor injection tool, pumps water upward until it enters the injection zone by gravity.
The study notes that DHI continues to develop and test new DGWS equipment such as a rod pump with a reverse-flow injection (RFI) system and a progressive cavity RFI system for handling high solids content. DHI has pilot-tested also a downhole three-phase separation system that produces separate oil, gas, and water streams, according to the study.
The study also mentions an Austrian company, Rohoel-Aufsuchungs AG, that has developed a subsurface side door (SSD) that allows opening or closing a portion of the tubing to connect the producing and disposal formations, which are separated by packers.
GRI in 1999 found that in 53 DGWS field tests involving 34 operators in the US and Canada, the type of DGWS technology used broke down as follows:
- 60% modified plunger rod pumps.
- 32% bypass tools.
- 8% ESP.
It said gas production rates increased in 57% of the tests, but only 47% of the field tests were successful. GRI attributed about half of the failures to water cycling or poor injectivity.
