Linked Diagenesis and Fracture Patterns and Their Effect on Fluid Flow in Fractured Carbonate Rocks

Laubach, S. E., J. E. Gale, R. Marrett, J. E. Olson, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Dolostone reservoirs are important sources of hydrocarbons, and fluid flow within them is commonly influenced by fractures. We use fracture and diagenesis observations and modeling, quantitative fracture population description, geomechanical rock testing and modeling, and a new approach to generating fracture patterns for use in fluid-flow simula­tion to explore how an appreciation of the links between diagenesis and fracturing in car­bonate rocks can enhance hydrocarbon exploration and development success. Mechanical rock properties, which affect fracture architecture, change with diagenesis. Opening-mode fractures can form throughout burial and exhumation under a wide spectrum of loading con­ditions. Fracture architecture depends on rock properties at the time of fracturing (particu­larly subcritical crack index), and hence on pre-fracturing diagenetic processes. Preservation of fracture void space depends in part on cement precipitation synchronous with fracture opening. Typically these cements obey systematic patterns throughout a frac­ture set, and dominantly seal microfractures with apertures smaller than 0.1 mm, reflecting rock-dominated geochemical processes. Later cements have the greatest effect on sealing large fractures and commonly have highly heterogeneous distributions.

We utilize a fracture mechanics based crack growth simulator (rather than purely sto­chastic methods) to generate fracture networks with realistic fracture spacing, aperture and length distributions that depend on measurable rock parameters such as Young’s modulus, subcritical crack index, mechanical layer thickness and tectonic strain. These rock parame­ters depend on the progress of diagenetic reactions and thus diagenetic models can be used to infer suitable fracture attributes in unsampled areas. Using this technique, effects of frac­ture pattern geometry on flow properties such as efficiency of imbibition in water-flooding can be examined and interwell permeability scale-up can be estimated. Model predictions can be checked through comparison with fracture attributes inferred from rock samples that record site-specific aspects of the rock’s diagenesis.