RE: TR-117 Nov 2023 10:38
And this from the CPR
Remaining uncertainty about reservoir quality and dynamic performance
1. There may be sub-seismic resolution baffles to production which reduce the pressure
responsiveness of the Main Reservoir and adversely affect the production profile.
2. Condensate banking may increase the skin (excess pressure drop) of the wells, reducing
the gas productivity index and the economic condensate recovery.
3. The tortuosity in the near wellbore region may choke mass flowrates at high superficial
velocities, reducing communication between the bulk reservoir and the wellbore,
reducing the gas productivity index. The extremely rapid recovery of wellhead pressure
on shut-in supports this assessment and (2) above.
6. Water production may increase more quickly than expected, reducing performance and
life expectancy of one or more of the wells; and potentially requiring a well workover to
remedy the problem. There is initial evidence that well A4 (aka SF4) is producing above
the trend water production rate from 2012 to 2017, and this may be because it is
producing at a lower wellhead pressure and higher flowrate.
7. If the water flowrate in well A4 were to increase significantly, a smaller tubing size (2
3/8”) would need to be installed to enable the well to continue to flow 8. The P50 and
P90 schedules for start of well A4 operation after installation of this smaller tubing
(“accelerator string”) are February and May 2024, respectively. P50 and P90 Capex of
£0.5m and £0.7m, respectively, are included in the cash flow calculations.
8. Each of the frequent short-term shut-ins and flowrate changes during the first year of
production has sent a pressure pulse to the reservoir that follows the same evolution
from early time to stabilised time (Figure 2). Hence, the flowing well head pressure on
any day is the super-position of the reservoir’s individual response to each flowrate
change since August 30th, 2022. It is not possible to correctly interpret the reservoir’s
performance in the short run without sophisticated computational analysis9 and a
reliable conversion of well head pressures to bottom hole pressures. Furthermore, there
is evidence of choking in the near wellbore region and so the flowing wellhead pressures
may not correctly be “seeing” the gas further into the reservoir (refer (3) above). This is
evident from the automated history matching of wellhead pressure undertaken by the
operator’s consultant for the period August 2022 to June 2023. (By contrast, the
automated history match of wellhead gas flowrate for the period 2014-2017 against an
almost constant separator pressure (30-32 barg) showed little deviation from actual.)
There is therefore inherent uncertainty in the production forecast from the reservoir
simulation, and this uncertainty increases with increasing wellbore flowrates.
.
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