|
Technical Advantages
The HSP is designed for low- to mid-volume applications at flow rates of 1 cubic meter to 30 centimeters per day. The benefits are in the details.
The pump's positive displacement design is ideally suited for horizontal and deviated wells. The HSP is free of moving parts or rotating rods.
"The new pumps have been significantly better than the PC pumps due to reduction in tubing wear." Kurt Bair, Senior Petroleum Engineer, Storm Cat Energy.
Ease of Operation
The HSP's clamshell-hooded surface unit is easy to use and easy to maintain. The entire enclosure opens so that all mechanical components are within reach. There is no access panel. The enclosed surface unit is the only one of its kind in the industry. It is extremely compact and quiet, and easily serviced from above ground.
The pump rate can be manipulated using a flow divider (Fig. 1) to produce daily volumes of .1 to 25 cubic meters. This change occurs almost instantly. The flow control valve is located in the surface equipment where it can be easily adjusted.
|
Fig. 1 Flow Divider
The flow of hydraulic oil to the bottom-hole pump dictates the cycles of the pump while the speed with which the oil is sent dictates the frequency of each cycle. A single joystick in the surface unit determines the amount of oil sent to the bottom-hole pump.
It takes one gallon of oil to extend the cylinder and 3/4 gallons to extract the cylinder. For every 1.7 gallons of hydraulic oil pumped from the surface using the joystick, the bottom hole pump produces one gallon of water. Changing rates are easy to accommodate. The range of an HSP is absolutely infinite, whereas other pumps in the industry are limited in their range. For example, an ESP requires a service rig and a complete re-installation to accommodate volume fluctuations.
In contrast, the well deployed with an HSP for two weeks could produce 10 cubic meters of water daily. Inflow levels will not overflow the pump. The joystick can manage changing rates and the rate needed to cycle the bottom-hole pump can be determined quickly. This makes it easy to keep the well unloaded.
Ease of Installation
The pump can be installed and running within a half-day thanks in part to its coiled tubing structure. Coiled tubing enables quick installation of an HSP because it eliminates the need to thread piping together. From service to bottom hole, the pump takes little over an hour whereas jointed pipe takes more than a half-day to install. The HSP capitalizes on all the benefits of coiled tubing, which is also less expensive than other tubing choices.
Average HSP installation schedule:
- Rig up 8 a.m. to 10:30 a.m.
- Pump is run into the well at 11 a.m.
- Pump is on bottom at 1 p.m.
- Surface unit and pipeline are tied in at 3 p.m.
- The surface unit is started and the system starts pumping water.
Ease of Removal and Redeployment
The system is easily removed and re-installed. The HSP runs on three coil tube lines, two hydraulic entry lines and one production exit line. The coiled tubing unit is self contained and equipped with a dual spool trailer, exclusive to the HSP. The entire system can be removed, installed in an alternate well and functional within a day, taking into account all service utilites.
To remove the HSP the hydraulic lines are detached and the surface unit is loaded into a trailer. The tubing is collected and the assembly is removed from the well. Then the pump is disconnected and the well is capped.
The surface equipment is entirely self-contained, there are no building pads to lay and no pump assembly is required. There are simply four fittings and two fluid connections to make to set up the HSP.
In one case, an HSP was pulled out at 7 a.m. and running in a new well by 4:30 p.m.
Success with Frac and Formation Sand
The pump is fitted with a 10-slot self-flushing sand screen, which filters larges particles of frac sand, coal and cement.
The sand screen is a 1.3-metre tube of 1.5-inch outer diameter with 0.01-inch spiral slots. The intake ports of the pump are located within the screen, filtering particles larger than 0.01 inches. The fluid exhausted from the cooling chamber back-flushes the screen clear with each stroke as the HSP pumps particles out of the well.
Unlike other pumps, the HSP has no issues associated with rods spinning inside the tubing or sand being pumped in. It does not have the problems associated with wear on moving components such as rods and tubing caused by corrosion.
Success in Harsh and Deep Environments
A November 2006 study by the Pembina Institute, a leading independent, not-for-profit environmental policy research, consulting and education organization, revealed that the HSP pump efficiency, the ratio of output to input power, was equal or superior to that of the conventional pump jack, progressive cavity pump and hydraulic pump jack.
The HSP is an ideal shallow gas exploration device but it was also built for harsh and deep environments. It is designed to fit four-to seven-inch casing and it has been deployed in depths of up to 1,400 meters.
Industry has questioned the HSP's ability to operate at depths greater than 2,000 feet or about 607 meters. However, the HSP has operated at full performance in well depths of up to 4,600 feet or 1,400 meters.
The speed and function of the HSP's hydraulic fluid and piston enable the pump to work in very deep conditions. Performance of the pump in field applications in 2006 and 2007 demonstrates the merits of a hydraulic power approach to downhole pump applications.
Heat Trace
The HSP has an inherent heat trace system that prevents the wellhead from freezing. The system recycles excess hydraulic oil, which manages the temperature of the wellhead. Hot oil routed along flow lines is circulated through the hydraulic hose. It inherently heats when it is pressurized. This system is not dependent on electricity. This becomes a necessity in remote areas where electricity might not be accessible.
A hose is wrapped around the water piping and insulation is wrapped around the hose, which prevents freezing. The pipeline acts as a heat exchanger. It is not necessary to cool oil and generate heat by other means since heat trace occurs naturally in the HSP.
This system is beneficial because it is cost-effective and energy efficient as no extra energy is used to generate heat. The HSP does not require a radiator or fan.
Gas Lock Prevention
The HSP is not reliant on the presence of water in its piston chamber to function, unlike other types of downhole pumps. Positive displacement design of the cylinder and a 100 percent seal enables the pump to move gas just as efficiently as water. Pump volume is based on well depth. This ensures that the cylinder will compress gas at minimum bottom-hole pressure to exceed maximum tubing hydrostatic pressure.
The HSP pump is designed to prevent gas locking. Gas locking will occur when clearance volume, defined as the geometric volume contained in the piston chamber at the end of the stroke, is less than the critical volume relevant to gas locking pressure. These volumes are related to varying pressures by the thermodynamic law of compression under adiabatic and isentropic conditions:
Here, V1 is the volume in the piston chamber at the end of the intake stroke of the pump; V2 is the volume occupied by the compressed gas at the end of the production stroke; P1 is the pressure of the gas when admitted into the chamber; and P2 is the pressure of compressed gas at the end of the production stroke. The exponent r is the ratio of the specific heats of the gas cp/cv. For natural gas mixed with air, this ratio is about 1.29.
When P2 equals the hydrostatic pressure in the coil one obtains values for V2 that establish the critical clearance volume for gas locking. Table 2 below uses V2 as a function of pump depth, which corresponds to the hydrostatic head of the water column in the coil.
Gas locking occurs when gas becomes trapped in the piston chamber. In the HSP pump, this scenario is impossible because of its geometry. Water is moved up to the surface by a piston which pushes the water out of the piston chamber, past a check valve and out to the coil connecting the pump to the surface. The water column contained in the coil rests against that check valve, with a pressure equal to the hydrostatic head of the column. Hence, water can only flow past the check valve when the pressure imparted by the piston on the water in the chamber exceeds that of the hydrostatic pressure.
There are times when gas enters the chamber along with water flows. This gas is compressed during the piston stroke. Gas locking would normally occur when gas entered the chamber in the absence of water. If at the end of the piston stroke the gas pressure is less than the hydrostatic pressure in the water coil the check valve will not open. In this case, the piston becomes gas locked and unable to move any gas to the surface.
Table 2. Critical Clearance Volume for Gas Locking
|
Down-hole
depth (m)
|
Stroke
(inch)
|
P1
(psi)
|
P2
(psi)
|
V1
(cubic inch)
|
V2
(cubic inch)
|
| 1,500 |
73.28 |
35 |
2403 |
359 an> |
13.5 |
| 2,500 |
72.55 |
50 |
4978 |
356 |
10 |
The HSP pump exhibits a clearance volume of about two cubic inches, which is less than the calculated values for V2.
Operation in a Pumped-off State
A pumped-off state occurs when there is no water left downhole. The ability to continue pumping under this condition is inherent in the design of the HSP. In this scenario, the water piston does not produce a flow through the check valve, although it may contain residual water inside the chamber. Gas will be pushed out through the water coil since gas locking cannot occur.
The positive displacement design of the pump allows it to continuously pump up to 100 percent gas for prolonged periods of time. In practice, the pump should not be operated for more than eight hours in the pumped-off state, to prevent the possibility of dry-running the middle gland seals.
Determining a Pumped-off State
Operators can readily determine if the well has been pumped-off while the system is in operation by monitoring the gauge pressure of the oil flow as it leaves the surface pump. A gauge located above ground at the wellhead easily determines the amount of gas in the pump. When water is present in the coil the surface unit generates a pressure surplus in the oil coils to raise the water column. The surplus shows up as a large pressure maximum on the gauge. When water runs out downhole, the pump is no longer pushing against the water column. Consequently, the pressure maximum read from the surface gauge will be noticeably lower, while the pump driver's rpm will momentarily speed up to account for reduced resistance.
This ability to rapidly detect the sudden water cutoff downhole, without the need for reading the flow conditions downhole, is unique to the HSP design. A pumped-off state is easily determined without fluid shots or down-hole gauges. The hydraulic pressure gauge detects gas interference or compression. When the pump is lifting fluid at full stroke the gauge will show the pressure required to lift it. If there is gas in the pump, it must be compressed to equal the pressure of the fluid inside the tubing. When a normal working pressure level is reached, contents are sent to the surface.
Furthermore, the operator can gauge the downhole pressure in a similar fashion while the surface unit increases the initial pressure in the oil coil until water flow is produced. Given the depth, coil sizes and surface pressure in the oil coil, the downhole pressure can be determined either from the pump's performance curves, or by a quick calculation based on the friction losses in the coil.
Cooling and Lubricating
The pump is self-lubricating and self-cooling as a result of its design configuration.
The principal source of heat generated inside the pump during operation comes from the motion of the piston rod against the middle gland seals and the pistons against their piston seals. The contributions of the water and oil temperatures are minimal; they tend to equalize the temperature of the entire pump.
The general differential equation for the passage of heat through a continuous medium:
Where (x, y, z) are Cartesian coordinates, T is the temperature, A is the thermal diffusivity and T is time. Apply to this equation a transformation of the time variable to account for the speed of the rod V:
To obtain the final form of the governing equation:
This last equation is applicable to the cooling and lubricating mechanism of the HSP. With the boundary conditions that T ---> 0 as (x, y, z) are distant from the interface (that is, r=sqr(x2+y2+z2) ---> 00), the heat flux Q at the interface is known, and the region outside the source area is adiabatic:
Where T0 is the temperature away from the interface and K is thermal conductivity of the rod material.
The value of Q is undetermined. It is expressed as the ration of the friction force F and the rod speed V, where F is given by:
F = u-P-Ac
Where u is the fluid (water or oil) viscosity, P is the fluid pressure, and Ac is the are of the interface.
Therefore, Q's final expression takes the form:
Q = u-P-Ac-V
Upon closer analysis, the sheer force of the fluid at the interface is not included. The omission is justified on the basis of the low speed of the rod, which yields a surface sheer stress = 1/u - d/dn V that is at least two orders of magnitude smaller than F above.
This heat dissipation model was applied to the selection of the various HSP seals. Seals and rider bands on the pistons yielded insignificant heat values, although seals in the middle gland were exposed to significant temperature gradients depending on the seal materials considered. For example, H poly seals were experiencing the same heat rates as the piston power inputs. Teflon-based seals were shown to be one to two orders magnitude less in heat generation.
Interface clearances will play a significant part in the friction generated. If the clearance between the outer diameter of the rod and the inner diameter of a seal is smaller than 0.001 inch, then the sheer T will become significant. If the lubricant fluid disappears, the sliding contact is running dry, in which case the friction force F now requires the dry static coefficient of friction for the two materials. In this case, the heat generated can have harmful consequences.
Within the design parameters of the pump, these factors do not contribute significant heat sources at the seal interfaces. Some of the heat generated flows to the surrounding metal lattice of the pump, (barrel, middle gland, etc,) which acts as a massive heat sink, and some moves through the fluids (water and oil), and is quickly removed from the pump.