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Material Balance in a Pseudo-steady State of Natural Gas Flow |
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Material Balance in a Pseudo-steady State of Natural Gas Flow Authors: Z. Gacek, A. Olajossy
Determining gas resources is a basic calculation problem for reservoir engineering. The knowledge of
resources is crucial for managing the reservoir, creating production strategies, planning equipment, contracts
and evaluating resources. The paper presents a material balance method for determining the amount of
resources with the use of dynamic pressure, without closing the wells and interrupting exploitation of the
reservoir. This circumstance may be of great practical signifi cance. The only assumption of this method
is acquiring stabilized gas fl ow in the reservoir, i.e. pseudo-steady state of fl ow (Mattar & McNeil, 1988).
Therefore, this method can be used for evaluating resources of medium and highly permeable natural gas
fi elds and underground gas storages, which achieves the pseudo-steady state very quickly. The model
relies on the analysis of stabilization of derivative dp/dt in a pseudo-steady state of reservoir. In this case
the gas material balance is calculated in a very similar was as in the classic material balance method. The
only difference is the way in which the pressure data have been used. The classic method makes use of
average reservoir pressure obtained after closing the wells and waiting till the pressure stabilizes. In the
proposed method the dynamic pressure is used without closing the wells and so without interrupting the
exploitation of the reservoir. It follows from the assumption of the pseudo-steady state that the material
balance plot employing the dynamic downhole pressure (pdd /Z vs. Gp) will be parallel to the material
balance plot employing the average static reservoir pressure (p–/Z vs. Gp). By drawing a straight line
linking points pdd /Z for the cumulated gas yield and then extrapolating a parallel line from the initial
pressure, it will be possible to assess the initial resources (OGIP), and determining present magnitude of
resources. It was indicated that resources can be assessed for a steady gas fl ow on a basis of the available
data, available during exploitation of the reservoir, i.e. gas composition, temperature, wellhead pressure,
reservoir temperature, parameters of the wells and production installment. This method can be used for
reservoirs without active water, and for reservoirs and gas storages with active water. The balance verifi
ed on an example of the reservoir „Brzostowo” confi rms the practicability of the proposed method of
a pseudo-steady material balance method. |
2011