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Controls on Fracture Geometry in Chalk

Michael J. Welch¹, Christine Souque², Viki Wood¹, and Rob J. Knipe¹
¹Rock Deformation Research Ltd., Leeds, UK
²IFP Energies nouvelles, Rueil-Malmaison, France

Cretaceous chalk is an important reservoir unit in the North Sea, with a number of producing fields. Fractures are key to producing from chalk reservoirs due to the low permeability of the chalk matrix. Most chalks are heavily fractured, but the geometry and distribution of the fractures can be highly variable. Understanding the controls on fracture geometry and distribution is therefore vital to predict fluid flow through the chalk reservoir. The aim of this paper is to use field data from chalk outcrops in southeast and northeast England to identify the main fracture geometries, and then to use various numerical and analytical modelling techniques to understand the controls on those geometries.

Two outcrops were studied for this project: Pegwell Bay in southeast England and Flamborough Head in northeast England.

Porosity measurements on samples from the two formations suggest that the Burnham Chalk (porosity 12%) is more compacted than the Welton Chalk (porosity 17%). We can use a simple Mohr-Coulomb failure criterion combined with linear elasticity to calculate the strain at which tensile and shear failure will occur for chalk in different states of compaction, by using published data to estimate mechanical properties based on porosity (e.g. Bell et al. 1999). As Figure 2c shows, the failure mode varies for chalk with different porosities: at 1.4x fluid overpressure, more compact chalk (porosity >13%) will fail in tension before it fails in shear, whereas more porous chalk will fail in shear before it fails in tension, which is consistent with the observed deformation style.

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012