Using mechanical models to investigate the controls on fracture geometry and distribution in chalk

Abstract

Chalk is an important reservoir rock. However, owing to its low permeability, fractures are key to producing hydrocarbons from chalk reservoirs. Fractures in chalk usually form one of three geometric patterns: localized fractures (commonly concentric rings) developed around tips, bends and splays in larger faults; regularly spaced regional fracture sets; and fracture corridors comprising narrow zones of closely spaced parallel fractures. Localized fracture patterns are likely to give only local permeability enhancement; regional fracture sets and, especially, fracture corridors may provide long, high-permeability flow pathways through the chalk. Field mapping shows that both localized fracture patterns and fracture corridors often nucleate around larger faults; however, the fracture corridors rapidly propagate away from the faults following the regional stress orientation. It is therefore not necessary to know the detailed fault geometry to predict the geometry of the fracture corridors, although the fault density can help to predict the spacing of the fracture corridors. Mechanical modelling shows that while localized fracture patterns can form under normal fluid pressure conditions as a result of local stress anomalies around fault bends, tips and splays, fracture corridors can only form under conditions of fluid overpressure. Once they nucleate, they will continue to propagate until they either intersect another fault or the fluid pressure in them is dissipated.Gold Open Access: This article is published under the terms of the CC-BY 3.0 license.Cretaceous chalk is an important reservoir unit in the North Sea, and hosts a number of large fields in the Norwegian, Danish and UK sectors. However, the matrix permeability of chalk is generally very low (typically ,1 mD), and, hence, fractures are often important in controlling fluid flow in the subsurface and are key to producing these fields economically ( Aims and objectivesIn this study we use mechanical modelling techniques to examine the controls on fracture development and resulting geometry in chalk, and compare these results with fracture patterns observed in two chalk outcrops from southern and NE England. In particular, we will examine the relationship between fractures, fracture corridors and larger scale faults, and the impact of fluid pressure on fracture development. We will use elastic dislocation and finite-element models to investigate the in situ stress patterns developed around larger faults under different conditions of deformation and compare these with the observed fracture patterns. The elastic dislocation models simply assume the chalk to be a continuum elastic material, and take no account of fluid pressure. However, the finite-element models allow us to incorporate the effects of fluid overpressure, leading to dilation of the faults and fractures. We model two end-member cases of overpressured chalk: a case with a permeable host rock, in which the fluid overpressure results in a decrease in the effective horizontal stress; and a...

Fundamental controls on fluid flow in carbonates: current workflows to emerging technologies

“…Agosta et al 2010;Cilona et al 2012;Petrachinni et al 2013;Antonellini et al 2014). In this volume, Welch et al (2014) compare fault and fracture systems in two distinct chalk outcrops. Their approach offers a refreshing change in the pursuit of a more broadly applicable framework for fracture prediction.…”

“…These approaches can accelerate the exploration of key uncertainties. Welch et al (2014) explore the mechanical modelling of chalk. Recognizing the common patterns of faults and fractures that occur in different rock types and geological settings, this work explores the impacts of pre-existing structures and pore-fluid pressure on fracture development.…”

The Role of Chemistry in Fracture Pattern Development and Opportunities to Advance Interpretations of Geological Materials

“…Although absolute and even relative timing is notoriously hard to pin down (e.g., Peacock et al, ), fracture sets are commonly interpreted to have formed more‐or‐less contemporaneously over a short period of time (e.g., Caputo, ; Hancock, ). Yet some natural examples (Dunne & North, ) and model results (Bai et al, ; Nick et al, ; Olson & Pollard, ; Welch et al, ) imply that sets can form contemporaneously and/or over a long period of time (millions of years). Model results (e.g., Olson, ) show that complex patterns can arise from simple but protracted (millions of years) loading histories.…”

“…examples (Dunne & North, 1990) and model results (Bai et al, 2002;Nick et al, 2011;Olson & Pollard, 1989;Welch et al, 2015) imply that sets can form contemporaneously and/or over a long period of time (millions of years). Model results (e.g., Olson, 2007) show that complex patterns can arise from simple but protracted (millions of years) loading histories.…”

Numerical modelling to predict fracturing rock (Thanet chalk) due to naturally occurring faults and fluid pressure

“…In this paper, we investigate the impact of fluid pressure on fracture development. Previous work has suggested that the fracturing process in chalk is often governed by changes in fluid pressure, which also affect stress distributions (Welch et al, 2015). Before fracture initiation, the build-up of pore fluid pressure alters the local stress field by reducing effective stresses, thereby leaving the matrix more prone to tensile failure.…”