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Abstract

The Krechba field is part of a multi-field gas development in central Algeria where CO₂ is being injected rather than vented to develop carbon storage techniques within an active gas producing reservoir. Surface deformation monitoring has thus far proven to be one of the more useful of a wide array monitoring methods deployed at the site. Using differential interferometric synthetic aperture radar (InSAR), Pinnacle and MDA Geospatial services produced a high-resolution deformation time series of the Krechba field area over a 5 year period between 2004 and 2009.  While the deformation measurements provide a qualitative visual assessment of the injection performance, the real goal of surface deformation monitoring is to determine the location of subsurface strain changes induced by the injected fluid.  A geomechanical model is needed to determine the surface response to a given subsurface strain.  An inversion routine modifies the strain source or sources to find a system that produces theoretical surface motion that best matches the measured deformation.  This paper uses the example at injection well KB-501 to explore how the solution and associated uncertainty is affected by the choice of geomechanical model at this location. The best fit solution and sensitivity of that solution is derived from homogeneous and layered models of varying complexity using both linear and non-linear inversion techniques. The results place boundaries on the possible subsurface pressure distribution associated with CO₂ injection and also quantify the possible benefit obtained from more complex geomechanical modelling for inversion of surface deformation data.  The solutions suggest that the injected fluid at KB-501 remains near the intended depth and is spreading primarily along a northwest-southeast trajectory as anticipated.

Introduction

In August 2004, CO₂ injectors KB-501and KB-503 began placing CO₂ in a 15-20 meter thick target zone roughly 1850 meters below the earth’s surface at the Krechba field. A third injector, KB-502, came online in April, 2005.  Generalized stratigraphic sequences for this target zone illustrate a lithology made up of broad paleovalley deposits consisting of fine grained sandstones and mudstones of variable permeability.  Approximately 1000 meters of very low permeability Carboniferous mudstones lies above the target zone. This cap rock is expected to provide long-term sequestration of the injected CO₂. To monitor the surface deformation resulting from the injection, Pinnacle and MDA made use of both 35-day Envisat and 24-day RADARSAT-2 Fine and Ultra-Fine beam SAR data, processed by MDA's proprietary SBAS-SVD Network Inversion routine.  In 2010, an array of 71 near-surface tiltmeter sites was deployed over KB-501, and an array of high precision differential GPS stations is planned.  This instrumentation will provide further detail of the surface movement over this site.

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 that more input data be acquired to generate the model, and generally require significantly more run time to produce solutions.  The simplest model, from Okada 1992, uses a rectangular dislocation in a homogeneous half-space.  The calculated deformation is only dependent on the dislocation properties of dimension, orientation, position and slip plus the Poisson’s ratio of the formation.  Two general formulation layered models are from Du et al. 1994 and Wang et al. 2003 and 2006.  The former uses a perturbation approach, while the latter employs Hankel transforms to develop a set of Green’s functions that integrate the wave-number spectra functions.  This study uses the method developed by Wang, since it allows relatively quick inversion for dislocation properties in a fixed medium.