Wave-equation migration velocity analysis of the successive vintages of the Sleipner Field - A detailed prediction of the mechanical effects caused by CO2 injection

Abstract

Since September 1996, Statoil and partners have through their carbon capture and storage project at the Sleipner field, located in the central North Sea, injected approximately one million tons carbon dioxide per year into the sand prone reservoir of the Utsira Formation. From previous studies, it is established a significant reduction in both bulk and shear moduli when substituting brine with gas in a reservoir. In addition, multipathing and poor illumination occur as a result of gas saturation indicating that conventional velocity analysis techniques will be insufficient. This contribution examines and determines velocity changes in intra-reservoir layers by the use of target image fitting based on a wave-equation migration velocity analysis (WEMVA). Image-domain tomographic methods, such as the WEMVA method, exploits the divergence between the inadequate depth-migrated image and a perfectly focused version of the data. Reducing the uncertainty of the velocity model will be advantageous in signal processing of the data and estimations of gas layer thicknesses. As a result, better estimation of the amount of carbon dioxide stored in the reservoir can be made. In addition, it will be possible to make a more reliable prediction of migration pathways in the reservoir. It is shown that applying a WEMVA based velocity model constraining velocity changes to known accumulations of carbon dioxide has a satisfying effect on the migrated image. In addition, enhanced focusing of the migrated images for additional reflectors indicates a sufficient and adequate velocity model.