Why GARP Works (technical)
Reason #1 – Reduction of Back Pressure on the reservoir
- The back pressure exerted at the bottom of a column of liquid increases as the true vertical height distance and density of the liquid increases.
- The differential pressure between the interface of the reservoir and the wellbore is a major contributing factor in determining the production rate from the reservoir. The higher the pressure differential, the higher the production rate and conversely the lower the pressure differential, the lower the production rate. The wellbore pressure is calculated by the sum of the pressures in the surface separator + the weight of the column of gas + the weight of the column of liquid + any frictional forces from flow.
- Since a rod pump requires gravity to prime the pump with liquids, a liquid column must exist above the pump. Therefore rod pumps cannot recover liquids below the pump or lower the liquid level below the pump.
The GARP technology lowers the back pressure on the reservoir by reducing the weight of the column of liquids above the reservoir. If a surface compressor is utilized, additional reduction of back pressure on the reservoir may also be possible by using the compressor suction inlet to lower the separator pressure below the sales line pressure. The compressor can also be utilized to compress the gas to overcome the sales line pressure for gas sales. The production increase realized depends on the amount of increase in differential pressure, the system permeability and porosity of the reservoir (matrix + natural fractures + induced fractures). More permeable and porous reservoirs will respond with higher production rates for comparable pressure differential increases.
Reason #2 – Reduction of Gas Interference in a Rod Pump
- Free gas in a rod pump reduces the efficiency of the pump to pump liquids since the gas takes up space in the pump that could have been occupied by liquids. If free gas is present, the rod pump will compress the gas and will generate heat and vibration that may cause premature mechanical failures in the pump.
- Conventional down-hole gas separators rely on gravity and density differences between the gas and liquids for separation
- Rod pumps are single acting and generally pump liquids only on the up-stroke. During the down-stroke, the pump plunger is filling with liquids and no fluid velocity is induced by the pump. This period of no velocity allows the gas to rise up and out of the gas separator.
- It may be counter intuitive, but adding length to the dip tube over the minimum critical length does not increase the gas separation capacity of the gas separator. The minimum critical length is that length that prevents the gas bubbles from reaching the end of the dip tube for a given production rate and a given SPM of the pump. If the gas bubbles do not exit the gas separator before the beginning of the up-stroke, the gas bubbles will eventually make it into the pump intake. Basically the gas bubbles travel two steps downward (during the up-stroke) and one-step upward (during the down-stroke).
- Cross-sectional increases in the annular region inside the down-hole gas separator increases the capacity of the separator to provide gas free liquids to the rod pump. This occurs because gas bubbles are drawn down faster in the gas separator by the action of the pump than the natural rise of gas bubbles in the liquid. If the cross sectional area is larger, the corresponding velocity of the liquid is lower and the gas bubbles do not travel as far down the gas separator which means the gas bubbles are closer to the top of the gas separator. Therefore the natural rise of the gas bubbles will carry the bubbles up and out of the separator before the bubbles are drawn back down again when the pumps begins the upstroke.
- Conventional gas separators also rely on tubing anchors to prevent movement of the tubing string during the rod pumping cycle. During normal pumping operations, gas and liquids flow around the tubing anchor in both directions. Gas from the reservoir must travel up and around the tubing anchor to reach the surface. Any falling liquids that condense from the gas in the shallower and colder regions of the well-bore must travel down and around the tubing anchor to reach the pump. The cross sectional area around the tubing anchor and the casing is less compared to the cross-sectional area of the casing. This smaller cross-sectional area causes a restriction in the flow path around the tubing anchor. Gas may be attempting to flow up around the tubing anchor while liquids are simultaneously attempting to flow down and around the tubing anchor resulting in a “traffic jam”, which results in a buildup of free gas below the tubing anchor that can eventually reach the intake of the pump. The buildup of liquids above the tubing anchor also increases back pressure on the reservoir.
The GARP technology increases gas separation by eliminating the tubing anchor and also provides for a larger cross sectional area of the downhole gas separator by the utilization of a packer type separation system. The GARP downhole separation system provides lower liquid velocities traveling down to the pump intake and therefore a higher capacity and more efficient separation system. Additionally, a solids separation and containment system is deployed which reduces the risk of solids interfering with the packer and pump.
Why GARP® Works (less technical)
Back Pressure Analogy – Swimming Pool and Inflatable Flotation Devices
Anyone who has been at the bottom of a swimming pool has experienced the increase in pressure exerted on their ears by the water column above them. Additionally anyone who has tried to deflate a pool float has experienced that as the pressure inside the float depletes, the more difficult it is to remove the remaining air in the float. The same is true with a well. The higher a column of liquid is above a reservoir, or the deeper you go into a pool, the more pressure the column of liquid exerts. And, like a pool float, as the pressure in the reservoir or the pool float approaches the pressure in the wellbore or atmospheric pressure, the lower the production rate will be from the reservoir or the slower the pool float will deflate. Liquids and gases travel from regions of high pressure to regions of lower pressure. Therefore, GARP works by raising the production rate from a well by raising the differential pressure between the wellbore and the reservoir.
Gas Separation Analogy – Garden Hose
Partially covering the end of a water hose with your finger causes the water to flow faster than if the end of the hose is not covered. The reason is that the same amount of water is traveling through the hose in both instances; therefore, the speed of the water has to be higher for the partially covered end since it has a smaller cross sectional area. In the same fashion, liquids travel at less speed downward to the pump intake in the GARP gas separation system since the pathway has a larger cross sectional area. Less speed is desirable since gas bubbles in the liquid stream do not travel as far down the separator on each up-stroke of the pump. If the gas bubbles don’t travel down as far, they also don’t have as far to travel upward to exit the top of the separator during the no-flow period (during the down-stroke of the pump). The speed of the rise of the gas bubbles due to density differences with the liquid is much slower than the downward speed induced by the rod pump. To summarize, if the gas bubbles do not exit out of the top of the separator before the beginning of the up-stroke of the pump, the gas bubbles will eventually be drawn down into the intake of the pump regardless of the length of the dip tube of the separator.