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Abstract
The Krechba field is part of a gas development in Central Algeria where CO2 is being injected to develop carbon-storage techniques within an active gas-producing reservoir. In 2004, three CO2 injectors (KB-501, KB-502, and KB-503) began placing CO2 in a 15- to 20-m thick target zone at a depth of roughly 1850 m. Above the target zone lies approximately 1,000 m of very low permeability carboniferous mudstones. This caprock is expected to provide long-term sequestration of the injected CO2.

Surface-deformation monitoring measured using differential interferometric synthetic aperture radar (InSAR) has proven to be one of the more useful of a wide array of monitoring methods deployed at Krechba. The unique, horseshoe-shaped uplift pattern around KB-502 has been the subject of several technical papers to date. The analysis presented in this work, however, is unique in that it does not place any constraints on the location of strain sources that produce the measured uplift.

The results of this analysis, which indicate strain sources shallower than the injection horizon, coincide well with formation properties determined from logs and 3D seismic surveys. The implications of fluid intrusion into depths shallower than the injection interval, even if that fluid is displaced formation water rather than CO2, are significant for the project operators.

Introduction
In August 2004, CO2 injectors KB-501 and KB-503 began placing CO2 in a 15- to 20-m thick target zone roughly 1850 m below the earth’s surface at the Krechba field (Fig. 1). A third injector, KB-502, came online in April 2005. Stratigraphic sequences for this target zone show broad paleovalley deposits consisting of fine-grained sandstones and mudstones of variable permeability. Approximately 1000 m of low-permeability, carboniferous mudstones lie above the target zone. This caprock is expected to provide long-term sequestration of the injected CO2. To monitor the surface deformation resulting from the injection, the operating company and service company used both 35-day Envisat and 24-day RADARSAT-2 fine and ultrafine beam SAR data processed by a proprietary SBAS-SVD network-inversion routine.

Interpretation of the surface deformation to determine the location of subsurface strains requires a model that propagates subsurface strain to the surface. More complex models require more input data be acquired and generally require significantly more runtime to produce solutions. The simplest model (Okada 1992) uses a rectangular dislocation in a homogeneous halfspace.

The calculated deformation is only dependent on the dislocation properties of dimension, orientation, position, and slip, in addition to the Poisson’s ratio of the half-space. Two general-formulation layered models exist, one from Du et al. (1994) and the other from Wang et al. (2003 and 2006). The former uses a perturbation approach, while the latter develops a set of Green’s functions that integrate the wave-number spectra functions. This study uses the method developed by Wang et al. because it allows relatively quick inversion for dislocation properties in a fixed medium.