document preview

Abstract
The RSE-1 well operated by Total is located within the Rousse field in the southwest of France. The 43 year old, depleted gas well has undergone extensive research in order to determine its suitability for CO2 storage. The main objective, in terms of containment, is to avoid the injected CO2 migrating back to the atmosphere or reaching potable aquifers, either through the well or the caprock itself. Openhole data from 43 years ago consisted only of 1-arm caliper and compressional sonic data. In order to predict geomechanical effects on the formation from production or CO2 injection, sonic shear data was acquired to compute elastic properties and the direction of maximum horizontal stresses in anisotropic zones. Breakout orientations from multi-arm/finger-calipers, corresponding to the minimum horizontal stress direction are used to improve the geomechanical analysis, especially in areas of strong stress anisotropy. This paper focuses on the acquisition of good quality stress data through casing essential for a geomechanical analysis of the caprock using sonic and cement/borehole imaging tools.

Since azimuthal sonic data is very sensitive to acoustic anisotropy, the available log was processed together with directional survey data to reveal a 75m-thick stress-induced anisotropic zone in the lower part of the caprock. The fast shear azimuth, which is associated with maximum horizontal stress direction as dispersion curves show the sonic anisotropy being the result of a horizontal stress imbalance, suggested a ENE-WSW compression. Evaluation of 3-D cement imaging data showed borehole breakouts along most of the annulus. The cement thickness map is equivalent to a 36-arm caliper through casing and the computed maximum hole diameter correlates well with the 1-arm openhole caliper data acquired 43 years ago. The associated breakout direction within the zone of anisotropy was determined to vary between NNW-SSE and NNE-SSW.

The fast shear azimuth computed during sonic processing is approximately perpendicular to the breakout direction acquired from the borehole images within the 75m-thick anisotropic zone. The overall NNE-SSW compression was confirmed by other well geometry data from the region. These results suggest that both measurements complement each other very well in a cased-hole environment and could be further used for geomechanical modeling.

Introduction
Geological storage of CO2 is recognized worldwide as a possible solution to mitigate climate change. Along with capacity and injectivity, containment is believed to be the main factor in geological storage performance. In particular seal integrity must not be jeopardized by the mechanical effects of storage operations. Even if fractures do not start or propagate, permeability of rocks and faults may drastically increase as they undergo stress changes and deformation [Sibson 2003]. As a result, the mechanical response, in particular of the seal, to the loads induced by well drilling and completion as well as CO2 injection must be assessed when evaluating the suitability of possible sites, designing safe injection operations as well as planning reliable monitoring techniques.

The main objective of this study was to characterize the stress field in the caprock of the dolomite reservoir into which Total is currently injecting CO2, and in particular whether stress is anisotropic. Using the acquired data, anisotropy due to stress within the caprock formation was detected and the associated minimum and maximum horizontal stress directions determined, which is a necessary basis to provide information on stress and permeability orientations.