The HPHT Kristin field (Haltenbanken, Norwegian Sea) — its geology and surprises during early production

The Kristin field is a high-pressure, high-temperature (approx. 900 bar, 165°C) gas-condensate field located towards the western margin of Haltenbanken (65° N, 6.4° E) at approx. 4600 to 4850 m bsl. The Jurassic Garn, Ile and Tofte formations constitute the reservoir with estimated total in-place volumes of approx. 100 MSm3 of condensate and 100 GSm3 of gas. Structurally, the field is a north-south elongated, east-dipping horst block, almost completely surrounded by normal faults. Faulting took place during shallow burial of the reservoir formations from the late Middle Jurassic to the Early Cretaceous. In the west, the Base Cretaceous Unconformity erodes the fault scarp and cuts across all three reservoir formations. All 11 production wells (as of February 2008) and 3 of 5 exploration and delineation wells are located in the crestal area in the west.
The sand-dominated Garn Formation is approx. 100 m thick and of rather poor quality (log- and core-based permeability on average ca. 5 mD). Deposition of the Garn Fm took probably place in a shoreface environment. The heterolithic Ile Formation is ca. 85 m thick with log - and core-based permeabilities (subzone averages) ranging from 40 to 2000 mD. This interval is interpreted to have been deposited in a tidal delta setting with river flow from west to east. The sand-dominated Tofte Formation is approx. 125 m thick with average log- and core-based permeability of approx. 200 mD.
Since production began in November 2005, reservoir pressure has declined more rapidly than had been predicted in all three reservoir levels (for some wells up to 5 times). At each reservoir level there are probably different reasons why this has occurred. Within the Garn Formation, permeability seems to decrease even more rapidly towards the east of the field than had been realised based on well log data and depositional models. It is unclear whether this is primarily a facies trend or a combination of facies and diagenesis associated with the erosional scarp which cuts the west of the field. Within the Ile Formation, the rapid pressure decrease partly reflects the unpredicted presence of stratigraphic barriers to vertical flow which have become obvious only following post production MDT data (both flooding surfaces and poor communication between sandy tidal channels which are the main reservoir). A further possible component for pressure decline is fault seal. In the Ile Formation, faults seem to have very low permeability with one of them — a major N-S striking, field-internal fault of up to 40 m throw — being indicated by history matching to be completely tight. Faults in Garn and Tofte seem to be more open but still much less permeable than clay-smear based models would predict; this points to cementation as a major fault seal mechanism, which is in line with observations from cored small faults. Because of limited well penetration and short production history, the Tofte interval is still poorly understood.