Volumetric and dynamic uncertainty modelling in Block 22, offshore Trinidad and Tobago

Block 22, located offshore in the North Coast Marine Area (NCMA) of Trinidad and Tobago, contains the Cassra gas accumulation discovered by the Cassra-1 and Cassra-2 wells. Both wells encountered dry, biogenic gas in early Pliocene, shoreface–shelfal sandstone reservoirs.

A best-practice modelling methodology is documented to capture a range of subsurface uncertainties for use in reservoir simulation to generate production profiles in support of pre-development project planning. Calibration of a sequence stratigraphic interpretation from seismic with core data was instrumental in generating a depositional model, within which sedimentary facies could be stochastically distributed and used to constrain the population of petrophysical properties.

Static geological uncertainties were modelled using an uncertainty workflow methodology in commercial three-dimensional (3D) geomodelling software, resulting in multiple, static model realizations and probabilistic GIIP (Gas Initially In Place) distributions for Cassra and some nearby prospects. The multi-regional model required careful selection of realizations for reservoir simulation, based on a ranking scheme that combined GIIP, GRV (gross rock volume) and net:gross (net to gross ratio), and not simply P90–P50–P10 percentiles from the global distribution.

The reservoir simulation phase included a dynamic uncertainty workflow using commercial experimental design modelling software. By including dynamic uncertainties such as horizontal/vertical permeability and well productivity in the workflow, a more objectively defined suite of production profile predictions was achieved.