New Opportunities from 4D Seismic and Lithology Prediction at Ringhorne Field, Norwegian North Sea
Authors: Johnston David H. , Tiwari Upendra K. , Laugier Bernard 
Published: April 2012

Abstract
Time-lapse (4D) seismic data from the Ringhorne field in the North Sea are usedto monitor water movement in both Paleo-cene and Jurassic age reservoir sands, improve existing geologic and simulation models, and enable more cost-effectivefield operations. The structural complexity of the reservoirs, their proximityto the high-impedance Cretaceous chalk, and a rela-tively small 4D responserequired a significant effort in seismic acquisition and processing whichresulted in a highly repeat-able survey. In addition to the 4D interpretation, Vp/Vs derived from simultaneous elastic inversion is diagnostic of sand andprovides additional constraints on Ringhorne subsurface models. Connectedvolumes based on Vp/Vs correlate to areas of water sweep seen in the 4D dataand reduce uncertainty in 4D interpretation. Relative P-wave impedance changescalculated from inversion are consistent with pre-survey 4D predictions. The 4Dseismic and inversion results help explain water break-through timing andimprove our understanding of field production history. The data have resultedin an increase in the re-serves base and the identification of additionalinfill well opportunities.

Introduction
Time-lapse (4D) seismic data provide an opportunity to identify and quantifychanges in reservoir properties that occur dur-ing hydrocarbon production. 4Dseismic has been extensively used to identify areas of bypassed and undrainedoil, improve the existing geologic model, and enable more cost-effective fieldoperations.
The Ringhorne field is located in block PL027 (100% ExxonMobil* interest), offshore Norway in the North Sea. The Ring-horne Jurassic and West (Ty) fieldswere discovered in 1997 and 2003, respectively, with first Jurassic/Westproduction oc-curring in 2003. There are two commercial hydrocarbon units inthe Ringhorne field. The Lower Jurassic fluivial and shallow marine sandStatfjord reservoirs form in a structural horst-block trap on a basement high. The deep water Paleocene Ty sands (Ringhorne West) stratigraphically pinch outonto the Ringhorne high (Figure 1).
The Ty sand is a single seismic cycle, sub tuning thickness reservoir whichdirectly overlasy the regionally extensive Creta-ceous chalk. The sands areabout 85% net to gross, an average porosity of 30%, with multi-Darcypermeability. They are being depleted by a strong natural water drive. Thelight oil (API 39) has similar properties and is in pressure communication withthe Jurassic oil reservoirs. The Jurassic sands are found in three fault blocks(Figure 1) underlying the chalk and are depleted primarily by waterinjection.
A 4D feasibility study conducted prior to acquiring the monitor surveysuggested that impedance changes resulting from wa-ter displacing oil would be7–8%. While above what is normally considered the detectable limit for 4Dapplication, the prox-imity of the reservoirs to the high-impedance chalk addedcomplications. A relatively low amplitude change of 20% (relative to the chalkreflectivity) and potential side-lobe interference from the chalk reflectionrequired that the 4D data be highly repeatable.