Geophysical Inversion of Far-field Deformation for Hydraulic Fracture and Reservoir Information

by
Jing Du, Ph.D.

University of Texas at Austin, 2000
Supervisor: Jon E. Olson

The advent of high-resolution surface and downhole tiltmeters has resulted in the unprecedented collection of high quality solid earth deformation data. These data promise more detailed mapping of hydraulic fracture propagation and reservoir compaction, but current inversion models are too simplistic for hydraulic fracture geometry and non-existent for reservoir compaction. This dissertation documents the development of improved modeling tools that greatly enhance hydraulic fracture imaging and reservoir monitoring capability.

Two models have been developed to improve tiltmeter fracture mapping. One is a multi-layered, elastic half-space model that accounts for elastic property variation in layered formations using the moduli perturbation method. As most realistic cases involve layered sedimentary rocks of varying mechanical properties, homogeneous half-space models have limited applicability. Numerical results from the multi-layered model show that the mechanical layering has a noticeable effect on the magnitude of surface tilts that can cause incorrect interpretation of hydraulic fracture depth and width using conventional homogeneous inversion models. Another tiltmeter fracture mapping model has been developed to solve for a non-uniform opening distribution along the fracture plane (fracture opening can vary with height and length). The inversion is done with a penalty function method. A systematic and practical method is proposed to determine the best solution for a non-uniqueness inversion problem. The model can be used to study fracture growth processes such as asymmetric wing development and multi-zone fracture propagation. The impact of data noise on the inversion result is also examined.

Another newly developed application utilizing elastic deformation theory and geophysical inversion methods is a model to map reservoir compaction from surface subsidence data. A semi-analytical model is developed based on a center of dilatation source solution in a homogeneous half-space. Coupled with the same penalty function inversion method used for hydraulic fracture mapping, this model can be used to track reservoir pressure fronts due to primary production or secondary recovery processes. In addition to subsidence, the forward reservoir compaction model can calculate stresses, strains and tilts at the surface and in the subsurface.

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