Olav Hansen

Consolidation patterns during initiation and evolution of a plate-boundary decollement zone: Northern Barbados accretionary prism

Borehole logs from the northern Barbados accretionary prism show that the plate-boundary decollement initiates in a low-density radiolarian claystone. With continued thrusting, the decollement zone consolidates, but in a patchy manner. The logs calibrate a three-dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid, and which portions have consolidated. The seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid-rich conduit extending down the decollement surface. Fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch.

Monitoring CO2 Injection Into a Fluvial Brine-filled Sandstone Formation At the Snøhvit Field, Barents Sea.

500 ktons of CO2 injected during 16 months has been monitored by well pressure and 4D seismic data. Clear 4D responses can be observed as amplitude increases and time delays at the lower part of the reservoir, up to at least a km away from the injector. CO2 saturation, increased water pressure and cooling of the rock all contribute to the change in seismic response, and it is challenging to separate the effects. Pressure sensitivity on seismic velocity is interpreted to be higher than expected. The observed pressure buildup and falloffs during shut-ins constrain reservoir volume in good communication with the well and the permeability distribution. Together these data provide a picture of a more heterogeneous reservoir than originally expected. By constraining models by both seismic and pressure data, more confidence is obtained.

Sedimentology and unexpected pressure decline: the HP/HT Kristin Field

Abstract Since production began in the HP/HT Kristin Field off mid-Norway, reservoir pressure in each of the three mid to late Jurassic reservoir units (the Garn, Ile and Tofte formations) has declined significantly more rapidly than was initially predicted. In the Garn Formation, the Tofte Formation and to some extent also the Ile Formation, this has occurred at least partly because an unusual distribution of reservoir properties led to bias in the four-well appraisal dataset and this in turn resulted in an overestimation of reservoir properties. Of particular importance to this bias was the fact that very good but unrepresentative reservoir properties were encountered in all three reservoir zones in the discovery well located in the centre of the field. These, it is now realized, are not even typical of most of the central part of the field but are, instead, restricted within one, small, anomalous area. Study of cores and thin sections indicates that in each reservoir unit this directly reflects a concentration of more energetic depositional facies in the area while less energetic facies are present on three sides. This pattern was not predictable from the original dataset and seems to have arisen because there was structural control upon facies positioning during accumulation of the reservoir section. This influenced the distribution of cleaner, coarser grained, more proximal depositional facies and, ultimately, reservoir quality distribution and pressure development. What is interesting about Kristin Field is that the structural influence upon sedimentation is observed within the footwall stratigraphy of a major relay structure where the primary provenance direction was on the hanging-wall side. This pattern is the reverse of what is normally reported in tectono-stratigraphic studies.

Assessment of Time-lapse Seismic Repeatability Using Repeated Test Lines At the Kristin Field

We present a work flow for assessing the feasibility of time-lapse (4D) seismic for a reservoir where the expected time-lapse signal is small and mainly consists of time shifts. The feasibility assessment is achieved by acquisition of repeated 2D test lines and full 3D processing. Noise extracted from the measured difference data is used in a seismic modelling study to demonstrate that time-lapse seismic may be feasible for expected production effects at the Kristin field.