Page 400 - Compression Machinery for Oil and Gas

378 SECTION III Applications

passed through a field separator to remove condensate and water. The natural
gas liquids separated at this stage must be transported to a processing plant for
recovery.
A wide variety of gas field configurations exist that produce at different
pressures and flow rates, depending on the size and connectivity of gas pockets
in the field and age of the reservoir (pressure decreases over time as the reser-
voir is depleted). The purpose of gas gathering compression is to raise the gas
pressure from wellhead pressure to approximately 7–10MPa for transport to a
gas processing plant or sales pipeline. This includes compression to compensate
for pressure reductions in field separators or high pressure losses in the well
bore due to flow restrictions from liquid condensate formation. Over the age
of a field the suction pressure of gas gathering compressors change drastically,
with a very low initial pressure ratio of 1.25–1.5 to final suction pressures near
vacuum at well closure [4].
A large number of low-pressure, low-flow wells will require a large number
of small (hundreds of kilowatt) compressors connecting to centralized compres-
sor stations that combine streams. Larger fields with better connectivity and
volume may have field compressor stations with up to 10–20MW of compres-
sion [1]. In addition, over the life a field it may be necessary to restage com-
pressors to higher-head, lower-flow wheels to match reservoir production or
even the purchase of additional compressors as supplemental wells are drilled
to maintain production. In addition, there is significant interest in wet gas com-
pressors that are robust to operation with condensates in order to minimize field
separation and liquid transport requirements.
Typical compression system characteristics for gas gathering applications
are summarized in Table 8.1.

TABLE 8.1 Typical Characteristics for Gas Gathering Applications

Pressure Suction pressures range from near vacuum (0.3–0.7MPa) to
6.5MPa depending on reservoir. Discharge pressures typically
7–10MPa
Temperature 30–35°C suction to 176–190°C discharge
Fluids Natural gas with or without liquids, sometimes corrosive
components also if associated gas
Compression 100kW to 20MW; Highly variable depending on number and size
power of reservoirs
Typical Small reciprocating compressors to larger turbine-driven centrifugal
machinery compressors depending on number of wells, flow rate, and reservoir
size