Investigate causes of anomalous pressure behavior
A major reservoir found in multiple fields across southern Kuwait and
the neutral zone separating that country from Saudi Arabia had been producing
oil intermittently under natural depletion since its discovery in the 1960s.
In 2010, after production had been shut down for several years, the
operator examined static reservoir pressure data for historical trends and
noticed two things. First, even during prolonged periods with little-to-no
production over the past decade, pressures still showed a significant decline.
Second, reservoir pressure was rapidly approaching the bubble point. The next
step appeared to be implementing waterflood operations to maintain pressure and
improve ultimate recovery.
However, before trying to determine the requirements for water
injection, the operator formed an internal team to investigate two possible
causes of the anomalous pressure decline. One scenario was crossflow among
reservoirs via the wells due to mechanical problems. The other was
communication with nearby producing fields through a common regional aquifer.
Build dynamic model of reservoir and regional aquifer
The team built a simple material balance model, estimated crossflow
volumes required to match historical pressures, and analyzed well data for any
indications of crossflow. Not only was simulated crossflow production
insufficient to account for observed pressure trends, but temperature surveys
and geochemical analyses of oil from various zones yielded no evidence of
To investigate the second scenario, the operator reviewed pressure and
production data for the same reservoir across multiple, closely spaced fields.
These data strongly suggested that communication through the aquifer could have
caused local pressure to decline when production in a nearby field
To conduct a more comprehensive and realistic study of the regional
aquifer, the operator worked with a team of Schlumberger petrotechnical
experts. From multiple data sources, Schlumberger created a static
100-by-100-km 3D structural model of the aquifer across four fields, including
15 horizons and five facies. Using computer-assisted history matching (CAHM)
technology, they calibrated the regional model with historical static
In petrophysical logs, tar mats had been observed near the oil-water
contact in two fields. These could act as significant barriers to fluid flow,
and were added to the model. Petrophysical property distributions were adjusted
using historical production to further constrain the model.
With limited control points, however, tar mat transmissibility and many
aquifer properties remained uncertain. Thus, multiple versions of the model
could match the same pressure and production data. To minimize uncertainties,
15 alternative history-matched realizations were achieved using the CAHM