Wells – Processes – Separating material entering well
Reexamination Certificate
2000-10-25
2003-04-22
Neuder, William (Department: 3672)
Wells
Processes
Separating material entering well
C166S105500
Reexamination Certificate
active
06550535
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
Downhole oil water separators, Submersible pumps and submersible pump control systems.
2. Description of the Background Art
In the production of oil and gas from geologic formations, water is often produced along with the hydrocarbons. To make the hydrocarbons useful, they must be separated from the water, typically near the point of production. In the case of crude oil, this is typically accomplished by the use of a “heater treater” or “gunbarrel” separator located at the surface. These separators work on the principle of gravity separation; that is the oil is separated from the water by placing the mixture in a container, and allowing the lighter oil to float to the top of the container, the water to the bottom, and withdrawing the oil from the top, and the water from the bottom. Other methods are used to separate the oil from the water. Centrifugal separators, such as hydrocyclone separators, separate the oil from the water by causing the mixture to rotate and then withdraw the oil from the center of rotation, and the water from the outer diameter. All of these separators rely on the difference in densities between oil and water, and tend to separate via gravity or centrifugal force.
Over the past couple of decades, interest has increased in downhole separation of oil and water in oil wells because of the numerous advantages of this technique. The most important advantage is that the water produced can be immediately re-injected back into the formation without removing it from the well bore. This eliminates the need for above ground tanks, separators, and water disposal systems, reducing costs and the possibility of environmental damage. Downhole separation also is also simpler and cheaper than above ground separation. In addition, higher temperatures downhole often result in more complete separation.
The prior art suggests several combinations of downhole separator, pump and control system. In general, gravity type separators have been used exclusively in low flow rate wells, and dynamic type separators have been used with higher flow rate pumps. Gravity type separators are simpler but are limited in maximum flow rate because the mixed fluid must have sufficient dwell time in the separator to work. The dwell time needed depends on a number of factors such as the type of oil being produced, the size of the separator, the temperature, and oil droplet size going into the separator, but it is typically tens of minutes for oil wells. Because well bores are typically about 4 inches in diameter, and pipe units greater than 30 feet are difficult to handle using conventional workover rigs, the maximum downhole separator volume is about 0.5 barrels resulting in an average dwell time of 15 minutes at a production rate of 50 barrels per day. This would be about the largest flow rate achievable using a 30 foot long downhole separator. Gravity separators are limited in flow rate, but are much simpler than any other alternative.
Conversely, dynamic type separators such as hydrocyclone separators are seldom used in low flow rate wells because the stream of pumped fluid has insufficient kinetic energy to operate the separator. Dynamic separators are used at flow rates greater than 200 barrels of fluid per day. Dynamic separators can be used at lower flow rates, but additional kinetic energy must be added by accelerating the fluid using an impeller. This type of separator is more complex and expensive than gravity separation in low flow rate situations, and is normally used only in conjunction with submersible centrifugal pumps that offer a ready source of mechanical energy to operate the separator.
In practice, submersible centrifugal pumps are used at higher flow rates because they are inefficient and expensive to use at lower flow rates. Mechanically actuated pumps such as sucker rod pumps or progressing cavity pumps most often used at the lower flow rates. As a result, the use of gravity separation and mechanically actuated pumps is a compatible combination. U.S. Pat. No. 3,915,229 is an example of such an approach. At higher flow rates, gravity separation is impractical because of limitations of the volume of the separator downhole, therefore dynamic separators are typically used in conjunction with submersible centrifugal pumps.
The use of mechanically actuated pumps with gravity separators poses several significant problems. First of all, three connections to the surface are required. The first is the conduit for oil, the second a conduit for water, and the third is the mechanical power transmission mechanism. Mechanical power transmission mechanisms are typically bulky, and produce a reaction force that must be borne by some means. For example, a sucker rod pump uses a 1 inch diameter rod reacted by heavy walled tubing that is 2.5 inches in diameter that transports one fluid (typically oil) to the surface and power from the surface to the pump. This leaves a very small space, less than 0.75 inches to conduct the other fluid (typically water) to the surface using an additional pipe in parallel with the tubing/rod string. Typically there is very little room for this additional conduit let alone a sufficiently sized separator volume. A more serious problem arises as a result of the fact that the pump is typically located below the separator if one pump is used. This requires that the power transmission mechanism be routed through or around the separator to activate the pump. This further reduces the volume of the separator and because these mechanisms are large, this creates a serious design problem for the separator resulting in complexity, expense, and loss of efficiency.
U.S. Pat. No. 3,915,225, addresses this problem by locating the separator below the mechanical pump, eliminating the need for the power transmission mechanism to pass through the separator. Under this method the unseparated fluid enters the separator from the well bore to replace the oil is withdrawn by the pump, and the water exits the well bore by gravity into a disposal zone below the separator. The pump is activated when the oil level in the separator reaches a predetermined level. This approach will work only if the water will flow away from the well bore without being pumped into the formation under pressure. This is almost never the case in most oil wells. Both the oil and the water need to be pressurized for reliable operation.
U.S. Pat. No. 4,766,957 improves upon the methods of the '225 patent by providing two pumps, one to pump the separated oil to the surface, and another (if required) to pressurize the water being injected back into the disposal zone. The use of two pumps, while feasible, creates additional complexity and expense over any system that uses a single pump.
U.S. Pat. No. 5,697,448 further improves upon the methods of the '225 and the '957 patents by providing two mechanically actuated rod pumps powered by a single rod, where the oil is pumped on the up stroke and water is pumped on the downstroke. This method has two drawbacks that are addressed by the present invention. First, the method of the '448 patent is only useful with a sucker rod pump, and not useful with a submersible pump, leading to the problems already discussed. Second, because the oil is pumped on the up stroke and water is pumped on the downstroke the ratio of water to oil must be known and incorporated into the structure of the pump. If the ratio of water to oil is not known, changes with time, or is calculated incorrectly, either oil will be injected into the water zone or water will be produced with the oil. The use of two pumps, although powered by a single power source, further complicates the system and adds an additional expense.
A much more practical approach would be to use a single electrically powered submersible pump in conjunction with a downhole gravity separator. This would allow the power to be transmitted on a relatively small, flexible electrical cable and provide for automatic adjustment of the relative amounts of oil and
Bauman, Dow, McIntosh & León, P.C.
León, Esq. Alberto A.
Neuder William
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