Page 400 - Hydrocarbon Exploration and Production Second Edition

Managing the Producing Field 387

production forecasts. In addition, the model is used to predict the outcome of
alternative future development plans. The criterion used for selection is typically
profitability, but the operating company may state other specific objectives.
Some specific examples of the use of data gathered while monitoring the
reservoir will now be discussed.
If the original FDP was not based on a 3D seismic survey, which is a commonly
used tool for new fields, then it would now be normal practice to shoot a 3D survey
for development purposes. The survey would help to provide definition of the
reservoir structure and continuity of faulting and extent of reservoir sands, which is
used to better locate the development wells. In some cases time lapse 3D seismic,
‘4D’, surveys carried out a number years apart (see Chapter 3), are used to track the
displacement of fluids in the reservoir.
The data gathered from the logs and cores of the development wells are used to
refine the correlation, and better understand areal and vertical changes in the
reservoir quality. Core material may also be used to support log data in determining
the residual hydrocarbon saturation or the residual oil saturation to water flooding
left behind in a swept zone.
Production and injection rates of the fluids will be monitored on a daily basis. For
example, in an oil field we need to assess not only the oil production from the field
(which represents the gross revenue of the field), but also the GOR and water cut.
In the case of a water injection scheme, a well producing at high water cut would be
considered for a reduction in its production rate or a change of perforation interval
(see well performance below) to minimise the production of water, which not only
causes more pressure depletion of the reservoir but also gives rise to water disposal
costs. The total production and injection volumes are important to the reservoir
engineer to determine whether the depletion policy is being carried out to plan.
Combined with the pressure data gathered, this information is used in material
balance calculations to determine the contribution of the various drive mechanisms
such as oil expansion, gas expansion and aquifer influx.
Fluid samples will be taken in selected development wells using downhole sample
bombs or the MDT tool to confirm the PVT properties assumed in the
development plan, and to check for areal and vertical variations in the reservoir. In
long hydrocarbon columns, of about 1000 ft, it is common to observe vertical
variation of fluid properties due to gravity segregation.
Reservoir pressure is measured in selected wells using either permanent or non-
permanent bottom hole pressure gauges or wireline tools in new wells (RFT, MDT,
see Section 6.3.6, Chapter 6) to determine the profile of the pressure depletion
in the reservoir. The pressures indicate the continuity of the reservoir and the
connectivity of sand layers. They are used in material balance calculations and in the
reservoir simulation model to confirm the volume of the fluids in the reservoir and
the natural influx of water from the aquifer. The following example shows an RFT
pressure plot from a development well in a field which has been producing for some
time (Figure 16.3).
Comparing the RFT pressures to the original pressure regime in the reservoir
yields information on both the reservoir continuity and the depletion. The disconti-
nuities in pressure indicate that there is a shale or a fault between sands A and B which