Page 228 - Hydrocarbon

Reservoir Dynamic Behaviour 215

9.3.3.2. Movement of gas–water contact during production
As the gas is produced, the pressure in the reservoir drops, and the aquifer responds
to this by expanding and moving into the gas column. As the gas–water contact
moves up, the risk of coning water into the well increases, hence the need to
initially place the perforations as high as possible in the reservoir.
The above descriptions may suggest that rather few wells, placed in the crest of
the field are required to develop a gas field. There are various reasons why gas field
development requires additional wells

the need to provide excess deliverability to meet swing requirements as agreed in
the sales contract
the reservoir will not be homogeneous and certain areas will require closer well
spacing to drain tighter parts of the reservoir in the same time frame as the more
permeable areas are drained
the reservoir may not be continuous and dedicated producers will be required to
drain isolated fault blocks
the reservoir may have a flat structure and therefore it may be impossible to place
perforations at sufficient height above the water contact to avoid water coning. In
this case, a lower production rate is necessary, implying more wells to meet the
required production rate.

9.3.3.3. Pressure response to production
The primary drive mechanism for gas field production is the expansion of the gas
contained in the reservoir. Relative to oil reservoirs, the material balance calculation
for gas reservoirs is rather simple; the RF is linked to the drop in reservoir pressure
in an almost linear manner. The non-linearity is due to the changing z-factor
(introduced in Section 6.2.4, Chapter 6) as the pressure drops. A plot of (P/z)
against the RF is linear if aquifer influx and pore compaction are negligible. The
material balance may therefore be represented by the following plot (often called
the ‘P over z’ plot) (Figure 9.11).
The subscript ‘i’ refers to the initial pressure, and the subscript ‘ab’ refers to the
abandonment pressure; the pressure at which the reservoir can no longer produce
gas to the surface. If the abandonment conditions can be predicted, then an estimate
of the RF can be made from the plot. G p is the cumulative gas produced, and G the
GIIP. This is an example of the use of PVT properties and reservoir pressure data
being used in a material balance calculation as a predictive tool.
From the above plot, it can be seen that the RF for gas reservoirs depends on
how low an abandonment pressure can be achieved. To produce at a specified
delivery pressure, the reservoir pressure has to overcome a series of pressure drops;
the drawdown pressure (refer to Figure 10.2), and the pressure drops in the tubing,
processing facility and export pipeline (refer to Figure 10.13). To improve recovery of
gas, compression facilities are often provided on surface to boost the pressure to
overcome the pressure drops in the export line and meet the delivery pressure specified.
Typical RFs for gas field development are in the range 50–80%, depending on
the continuity and quality of the reservoir, and the amount of compression installed
(i.e. how low an abandonment pressure can be achieved).