The progressing cavity pump (PCPs) is a standard lift system for oil production as well…
A representative example for PCP installation is to decrease the fluid level in gas wells. Herewe look at the development process in co-operation with the operator Rohölf-Aufsuchungs AG (RAG) in Austriathe pump supplier Netzsch Oilfield Products GmbH and the Institute for Process Technology and Machinery (University Erlangen-Nuremberg).
Reservoir pressure
Reducing the reservoir pressure in the reservoirs featuring intensive water drift causes a high water inflow. Due to the water inflow the bottom flow pressure increases at the perforation. Thus the gas inflow from the producing horizons decreases or ceases. Lifting water decreases the dynamic level and reduces the bottom flow pressure.
The completion for such wells is designed to remove the water through the tubing and free gas through the annulus.
In 2004RAG started a project in cooperation with Netzsch to try PCP for dewatering of gas wells for the first time. The gas well Weizberg 1 (Fig.1) was chosen for the pilot project.
The well Weizberg 1 in the oilfield Weizberg (Upper Austria) is about 1700m deep and is cased with a 7-in production casing.
On the basis of the production rate produced by a sucker rod pump in the mid-90s PCPNTZ 350 *180ST25 was designed for a production rate of about 30m3/day. The pump assembly consists of rotorstatorstop pintorque anchor and gas separator.
The drive-head assembly consists of drive head NDH*045KW*15T with a pressure-capsulated
three-phase motor (motor power 22KW). The pump speed was controlled by a frequency converter.
Optimisation process
The exact analysis of the failed stators showed obviously that free gas diffusion in the bonding layer caused a partial loss of elastomer from the stator tube.
Besides the bonding and the elastomer were much more encroached due to the bad lubrication properties of water and gas mixturein comparison to producing oil and water mixture.
The important development steps were: Improvement of the bonding between the elastomer and stator tube.
In addition to the standard bonding system with the field-tested binders a metal wire mesh had to be welded on the inner side of the stator. This mechanical construction complements the bonding with binders.
The successful operation in the gas well over a longer period proved this construction. Since installation of the special stators the problem of the elastomer losses has not appeared anymore.
The laboratory technical analyses of the elastomer’s behaviour in a gas/water mixture showedthat a limited swelling can not be prevented. The only solution to prevent the volume increase of the elastomer material and to avoid a high pressing between rotor and statoris to intentionally reduce the rotor diameter.
The pump installations in other gas wells showed that the pump design must be very carefully adjusted for each well.
The analysis of pump failure showed that the rods in gas wells elongated considerably stronger than in oil wells. For this reason the usual calculation methods are not suitable for the installation case.
Successively the rotor spacing method for gas well application was corrected using the experience of the real operation. Therefore the length of the rotor had to be changed.
The rotors of PCPswhich are to be installed in gas wellsare ca. 10percent longer than the rotors in the oil production wells.
As already repeatedly mentionedthe pump loading is considerably higher due to the water-gas mixture than at the oil-water production.
For this reason the pumps will be designed for application under a lower stage pressure. It means that at the same differential pressure the pump in gas wells has about 15percent more stages and thus the stator is approximately 15percent longer than in oil wells.
In general there are two options to install the tubing or suction pipe. Their installation can be either below or above the perforated interval.
The preferred option to install the suction pipe is below the perforated intervalwhere the free gas will be separated in the annulus as a natural separation (Fig.2).
Depending on the area in the annulusdifferent volumes of gaseous liquid can be separated. In the separation process the gas percolates upwards through the liquid column with a minimum speed of 550m/h.
It is obvious that higher volumes of gaseous fluid can be handled when the separation area is increased. As long as downward fluid velocity does not exceed the separation velocity of the gas no or only a minimum amount of gas will pass through the pump.
Summary and outlook
Changing from sucker rod pumps to Netzsch PCPs for deliquification of gas wells was a very successful step at RAG (Fig.3).
By the use of PCPs an increasing number of wells can be produced economically now. Beside an optimal design adaption of the pump system for each well separatelythe main issue for optimisation is automation of the whole system.
Neverthelessthe most important aspect in the complete process is the close co-operation between the pump supplier and operator.o
Enter 65 or at www.engineerlive.com/iog
Joerg Eitler is vice-president and sales director with Netzsch Oilfield Products GmbHSelbGermany.
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