Inversion of surface subsidence data to quantify reservoir compartmentalization: A field study P.A. Fokker n , K. Visser, E. Peters, G. Kunakbayeva, A.G. Muntendam-Bos TNO, Geological Survey of The Netherlands, P.O. Box 80015, 3508 TA Utrecht, The Netherlands article info Article history: Received 11 July 2011 Accepted 14 June 2012 Available online 25 July 2012 Keywords: subsidence inversion compartmentalization abstract Surface subsidence is in many cases an unwanted effect of subsurface exploitation. It can, however, also be used as a reservoir monitoring tool. The present paper reports a field study on the Roswinkel gas field, in which subsidence data were used to reduce the uncertainty about the reservoir architecture. The Roswinkel gas field in the northeast of the Netherlands was in production from 1980 to 2005. Located at about 2100 m depth, it is a severely faulted anticlinal structure, constituting up to 30 reservoir compartments. As a result of its complexity there are large uncertainties about the fault transmissibilities and the strength of the connected aquifer. Consequently, it is possible there are undepleted compartments in the reservoir. Pressure depletion due to gas production causes the reservoir sandstones to compact, leading to surface subsidence. The gas production in Roswinkel has induced subsidence of approximately 17 cm above the center of the field. The subsidence at any point on the surface is a result of compaction over a large area within the reservoir. We estimated the compaction in the reservoir using subsidence data in order to reduce the uncertainties about fault transmissibility and aquifer connection. The subsidence data had been obtained from leveling campaigns and satellite measurements (Persistent Scatterer InSAR). We used a previously developed Bayesian inversion method in which prior knowledge is combined with observations. The prior knowledge on the compaction and the associated uncertainties were generated by Monte Carlo simulations of the reservoir in which the fault transmissibilities and aquifer connectivity were varied. In the meantime, the geological reality was maintained and the production history was honored. The average prior compaction field was a relatively smooth field extending far into the aquifer and typically having an uncertainty of 40%. Our inversion results show a reservoir in which certain large faults divide the reservoir into compartments with different pressure histories. In addition, the aquifer activity appeared to be much weaker than the average prior knowledge suggested. The posterior uncertainty of the degree of compaction was reduced to about 10%. Our study demonstrates that a carefully executed inversion exercise can considerably reduce uncertainties, thus making it possible to identify possibly undepleted compartments in the reservoir. & 2012 Elsevier B.V. All rights reserved. 1. Introduction In many cases the exploitation of deep subsurface resources results in surface movement. Examples are the production of gas (Doornhof, 1992), the injection of CO 2 (Vasco et al., 2010), the production of salt through solution mining (Samiei-Esfahany et al., 2009), and the production of geothermal energy (Allis et al., 2009). Although surface movement is usually undesirable, subsidence or heave measurements can also be used for monitor- ing. The present paper reports a field study on the Roswinkel gas field in the northern Netherlands, where subsidence measure- ments have been used for reservoir monitoring. The Roswinkel field is a gas field in the northeastern part of the Netherlands which originally held some 24 billion sm 3 of gas. It is an anticlinal structure with many faults that give rise to possible compartmentalization, depending on their sealing capacity. There is, however, considerable uncertainty about the compartmentali- zation and the strength of the connected aquifer. Reservoir engineering studies have indicated the possibility of residual gas in compartments which had not been fully depleted. The field was produced by 8 wells from 1980 to 2005. Depletion of the Roswinkel field has led to compaction of the reservoir. As this is a gas reservoir, the effect of the compaction on the reservoir pressure has been small. However, because of the Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/petrol Journal of Petroleum Science and Engineering 0920-4105/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.petrol.2012.06.032 n Corresponding author. Tel.: þ31 88 866 4644; fax: þ31 88 866 4605. E-mail address: peter.fokker@tno.nl (P.A. Fokker). Journal of Petroleum Science and Engineering 96–97 (2012) 10–21