2. Identifying natural reservoir depletion
Depletion is typically associated with production. However, from our global experience, we also see evidence of “natural” depletion, i.e. where a connected reservoir loses pressure towards an exit point or leak point, perhaps as a result of glacial unroofing, or seal failure.
The effects are that these reservoirs have substantially lower pore pressure than their associated shales. This becomes a real drilling risk in terms of taking losses if the pre-drill model and mud-weight used assumes that the reservoir will have the same pressure as the shales.
These draining reservoirs can be most effectively modeled/predicted by use of regional geopressure studies, as these sands often exit beyond the acreage boundary of a licence block. Once identified, the mud-weights and casing string setting depths can be adjusted accordingly with the minimum of NPT.
As there is a pressure difference between the shales and reservoir, real time monitoring can be used to “see these coming” as rock properties gradually change as they are approached. This also impacts on rock physics models by changing the impedance contrast between sand and shale such that hydrocarbon/water discrimination may not be possible.