American Oil and Gas Reporter - May 2015 - 96

SpecialReport: Gas Compression Technology
directly to the ECDs will keep the oil seal
in place if the VRU overloads or shuts down.
Flash gases are very heavy and subject
to condensing at moderate to high pressures. We recommend VRUs be fitted with
temperature controls on the discharge
coolers that can maintain cooler outlet temperatures up to 150-200 degrees to minimize liquid dropout and potential high gas
recirculation from a liquid cascade.
Many VRUs are designed to shut off on
low suction pressure, then automatically
start as suction pressure increases. On a
VRU handling VRT flash gas, this sudden
start can momentarily reduce the inlet pressure to vacuum levels, allowing oxygen
ingress. Normal control schemes designed
to maintain suction pressure by recycling
compressor discharge gas are problematic in low-pressure VRUs since they could
lead to the same potential high gas recirculation from a liquid cascade.
A better control scheme is to use a slipstream of leaner, high-pressure gas from
the production separator to provide gas to

maintain VRU suction pressure at one-two
psig. This keeps the VRU running constantly to minimize oxygen ingress, and
also helps clear out very rich gas that may
condense when a VRU cycles off.
The D-J Basin facility described in this
article captures 90-95 percent of the flash
gas from the oil, with only a small amount
of gas flowing to the ECDs. The capture
rate depends on the VRUs functioning with
no interruption in service. In practice,
VRUs are not as reliable as most gas compressors and often have run-times of 9095 percent. Most multiple-well facilities
cannot have this much rich gas venting to
atmosphere without limited oil throughput
because of VOC emission limits. Therefore, enough ECD capacity should be provided to handle the full amount of oil flash
vapors (low-pressure separator, VRT and
ECD flash gases).
Operational Flexibility

Any facility should be designed for operational flexibility. One of the character-

istics of shale oil wells is a rapid production decline within the first 12 months. The
typical decline is in the 65-75 percent
range, with long-term oil production at less
than 25 percent of the 30-day initial production rate. Flash gas volumes are a direct function of the oil production rate, so
gas systems on multiwell pads can quickly become oversized.
This can be offset in two ways. First,
a multiwell pad can be fed with additional oil from new wells to keep the gas handling systems at a sufficient operating level. Second, systems can be designed with
multiple operating units, such as two parallel VRUs sized for 50 percent of the gas
flow, or multiple ECDs. This allows surplus equipment to be moved to new pads
as needed.
Well facilities normally are designed for
peak IP rates. This usually is not a problem for pads with one or two wells, but can
lead to high costs and unused equipment
for pads with more wells. One strategy to
maximize the use of these facilities and

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The EPA's "Quad O" regulation mandates that uncontrolled gas releases from
storage tanks be reduced by 95%. To maximize vapor recovery, demand
Blackmer® Oil-Free Reciprocating Gas Compressors.
* Distance-piece design protects crankcase from condensate contamination
and controls vapor leakage and emissions
* Ductile-iron construction reduces thermal and mechanical shock
* PEEK valves deliver superior sealing, eﬃciency and durability
* Precision-ground crankshaft ensures smooth, quiet operation
For more information, please go to: psgpumps.com/aogr515b

96 THE AMERICAN OIL & GAS REPORTER

Process | Energy | Military & Marine

PSG
1809 Century Avenue SW
Grand Rapids, MI 49503-1530
USA
+1 (616) 241-1611
blackmer.com

American Oil and Gas Reporter - May 2015

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