A deterministic methodology for prediction of fracture distribution in basaltic multiflows

Abstract

Abstract. The fracture distribution in basalt flows is a direct result of thermal processes. Thus basalt flows present a unique opportunity to characterize a nearly perfect deterministic system with its fundamental physical parameters. Fracture distribution data collected on cliff exposures of basalt flows near the Idaho National Engineering and Environmental Laboratory (INEEL) are combined with calculations of cooling rate and temperature distribution from a finite element model to construct a predictive methodology for fracture spacing. The methodology is based on an empirical power law relationship between inverse cooling rate and fracture spacing. The methodology may be applied to unexposed basalt flows of approximately elliptical cross section whose thickness and width are constrained only by geophysical or borehole data if sufficient fracture data on nearby exposed flows are available. The methodology aids waste remediation efforts at sites involving contaminant transport through fractured basalt, such as the INEEL and the Hanford site in Washington, as well as involving transport and fluid flow through volcanic or intrusive rocks where thermal processes are responsible for fracturing. IntroductionColumnar tensile fractures in basalt flows are driven by thermal elastic strain due to differential cooling of the basalt, as opposed to regional tectonic extension. Given the low matrix permeability of basalt, characterizing the spatial distribution of thermal contraction fractures is the key to understanding the overall permeability structure of multiple basalt flows. The goal of the geological portion of the integrated study, and the subject of this paper, is the development of a methodology to predict fracture distribution in basalt flows when only limited knowledge of the flow geometry is available. Obtaining fracture information on basalt flows not exposed at the surface is challenging, as geophysical methods have resolution falling below that required to image the fractures directly and core fracture measurements sample a very small volume. The We integrate field data on basalt flow geometry and fracture spacing with numerical constraints on the thermal history during fracturing. The result provides a methodology to estimate fracture spacing as a function of cooling rate, controlled by flow thickness and geometry. Although this work is part of a study for hazardous waste containment, aspects are directly applicable to water, oil, or gas reservoirs in thermally fractured volcanic or intrusive rocks.

Scales of columnar jointing in igneous rocks: field measurements and controlling factors

“…Most studies agree that the length of a polygon-bounding fracture segment is a function of the cooling rate ∂T/∂t (Tomkeieff 1940;DeGraff et al 1989;Budkewitsch and Robin 1994;Lore et al 2001). Slower cooling creates wider (stouter) columns; faster cooling creates narrower (slenderer) columns.…”

“…The currently most widely accepted view for columnar jointing is contractional cooling, first proposed by Raspe (1776), which states that the network of fractures forming the column boundaries develops due to mechanical stress buildup while the lava cools and contracts. This idea stands at the origin of numerous papers with different investigation methods, including in situ observation of columnar jointing formation (Peck and Minakami 1968), numerical models using thermal and/or mechanical equations (Jaeger 1961;Long and Wood 1986;DeGraff et al 1989;Budkewitsch and Robin 1994;Lore et al 2000Lore et al , 2001Kattenhorn and Schaefer 2008) and analogue experiments with starch desiccation (Müller 1998;Toramaru and Matsumoto 2004;Goehring and Morris 2005;Goehring et al 2006Goehring et al , 2009.…”

“…The local cooling rate, commonly inferred to be the sole control on the size of columns (Tomkeieff 1940;DeGraff et al 1989;Budkewitsch and Robin 1994;Lore et al 2001; dashed arrow in Fig. 1), acts through the balance of stresses.…”

Implications of spatial reservoir uncertainty for CO2 sequestration in the east Snake River Plain, Idaho (USA)

“…Fracture densities are greatest in the upper and lateral portions of the basalt flows where convective heat transfer to the atmosphere resulted in elevated thermal gradients during emplacement (Lore et al 2001;Schaefer and Kattenhorn 2004). Column bounding fractures, observed in plan view, define the edges of a polygonal network of columns that, in the upper colonnade, are cross-cut by column normal fractures, see, e.g.…”

“…Fig. 3 in Lore et al (2001). In order to represent this complex flow geometry at reservoir (kilometer) scale and maintain the computational efficiency required for Monte Carlo-type simulation runs, this system is modeled as a bimodal heterogeneous stochastic continuum.…”