Page 408 - Hydrocarbon Exploration and Production Second Edition

Managing the Producing Field 395

The design parameters will typically be based on assessments of
fluid flowrates (oil, water, gas) and their variations with time
fluid pressures and temperatures and their variations with time
fluid properties (density, viscosity)
the required product quality.
During the production period of the field, managing the surface facilities involves
optimising the performance of existing production systems. The operating range of
any one item of equipment will depend on the item type, for example liquid–gas
separator, and its selection at the design stage, but there will be maximum and mini-
mum operating conditions, such as throughput. The minimum throughput may be
described by the turndown ratio
Minimum throughput
Turndown ratio ¼ 100%
Design throughput
Below the minimum throughput an equipment item such as a gas compressor
will not function. The process must therefore be managed in a way which keeps
production above that of the minimum throughput.
Often a more common concern is the maximum capacity of the item of
equipment, since optimising performance usually means maximising possible produc-
tion. For an individual equipment item such as a separator, increases in the maximum
capacity may be achieved by monitoring the operating conditions, such as temperature,
pressure, weir height, and fine-tuning these conditions to optimise the throughput.
This fine-tuning of specific items of equipment is ongoing, since the properties of the
feed change over time, and is performed by the process engineer and the operator.
Records of the operating conditions of the equipment items are kept to help to
determine optimum conditions, and to indicate when the equipment is performing
abnormally.
The surface production system consists of a series of equipment items, such as
that illustrated below, which shows the maximum oil handling capacity of the items.
The maximum capacity of the system is determined by the component of the
system with the smallest throughput capacity.
This very simplified example indicates that the export pump is limiting the
system throughput to 45 Mb/d, although the production potential of the wells is
50 Mb/d. If the pump was upgraded or a duplicate pump was installed in parallel to a
new capacity of, say 80 Mb/d, then the system capacity would become limited by the
separator. Identifying and then uprating the item which is limiting the capacity is
called de-bottlenecking. It is common to find that solving one restriction in the
capacity leads on to the identification of the next restriction, as in the above example.
Whether or not de-bottlenecking is economically worthwhile can be determined
by treating it as an incremental project and calculating its NPV. The operators and
engineers should constantly try to identify opportunities to de-bottleneck the
production system. A de-bottlenecking activity may be as simple as changing a valve
size, or adjusting the weir height in a separator.
The above example is a simple one, and it can be seen that the individual items
form part of the chain in the production system, in which the items are dependent