Integration of Time-Lapse Seismic and Production Logging Data: Jotun Field, Norway
Authors: Johnston David H. , Gouveia Wences P. , Solberg Arne , Lauritzen Morten 
Published: October 2008

Time-lapse (4D) seismic data acquired over the Norwegian North Sea Jotun Field is used to monitor water sweep in the Paleocene-age Heimdal reservoir. Inversion of the seismic differences provides a direct measure of the impedance increase associated with water displacing oil. This highresolution image of water sweep is consistent with production logging tool (PLT) data acquired in seven wells in the field. Combined with integrated reservoir facies interpretations and with other production data, the 4D data successfully derisked several proposed infill well locations and has identified new workover candidates and drilling opportunities in the field. The results of this work demonstrate the strength of applying multidisciplinary team efforts and partner cooperation to optimize asset value through tailored data acquisition programs and drilling.

Introduction

The Jotun field is located on the western flank of the Utsira High, close to the eastern pinch-out of the Tertiary submarine fan system. The reservoir at Jotun is comprised of Paleocene sands shed from the East Shetland Platform and transported across the Viking Graben area onto the Utsira High by gravity flow processes dominated by sandy turbidites. These distal gravity flow deposits display both thin-bedded sands alternating with shales and thicker, more massive sandstones (tens of meters thick).

Jotun Field production is from three structures (Figure 1), the high net-to-gross Elli structure and the lower net-to-gross Elli South and Tau structures. Production from 12 horizontal wells started in late 1999 and reached a peak in 2000. After having exceeded expectations initially, declining production and rising water cut initiated an accelerated infill well program in 2002. Two wells were drilled in the summer of 2002, one marginal having encountered a higher OWC than predicted from reservoir simulation, and the other not completed. The drilling rig was then hot-stacked in order to evaluate further opportunities based on the time-lapse seismic, integrated geological facies interpretation, and production logging studies.

Figure 1. Jotun Field Map(Available in full paper)

Seismic Data Acquisition and Processing

The first 3D seismic streamer survey acquired over Jotun was shot in 1994. A time-lapse feasibility study completed in 1998 suggested that impedance increases in water-swept reservoir would be sufficiently large (~10%) to image with a repeat eismic survey in 2002. That survey was acquired in May and June 2002 using a mixed streamer/OBC system in order to avoid conventional undershooting of the field's well-head platform and production ship. Table 1 compares the acquisition parameters of the 1994 and 2002 surveys. They are nearly identical except for the OBC area.

The streamer components of both surveys were coprocessed in order to maximize repeatability. However, the OBC component of the 2002 survey required a merge and match to surrounding streamer data. The match was complicated by a higher noise level in the OBC data -- impacting repeatability with the 1994 data.