A scale-independent approach to fracture intensity and average spacing measurement

TLDR

Fracture intensity, the number of fractures per unit length, is a critical but difficult-to-establish attribute in subsurface fracture systems, and inadequate constraints can limit reservoir exploitation; traditional measurement methods are inadequate because they ignore the wide spectrum of fracture sizes and the resulting scale dependence. The study proposes an alternative approach that uses fracture-size distributions to measure fracture intensity more meaningfully. This approach counts fractures above a common size threshold using fracture-size distributions, allowing artifact detection, consistent comparisons across locations, and quantification of uncertainties. When applied to two carbonate beds in the Sierra Madre Oriental, Mexico, the size‑cognizant measurements reveal new insights into geologic controls on fracture intensity.

Abstract

Fracture intensity, the number of fractures per unit length along a sample line, is an important attribute of fracture systems that can be problematic to establish in the subsurface. Lack of adequate constraints on fracture intensity may limit the economic exploitation of fractured reservoirs because intensity describes the abundance of fractures potentially available for fluid flow and the probability of encountering fractures in a borehole. Traditional methods of fracture-intensity measurement are inadequate because they ignore the wide spectrum of fracture sizes found in many fracture systems and the consequent scale dependence of fracture intensity. An alternative approach makes use of fracture-size distributions, which allow more meaningful comparisons between different locations and allow microfractures in subsurface samples to be used for fracture-intensity measurement. Comparisons are more meaningful because sampling artifacts can be recognized and avoided, and because common thresholds of fracture size can be enforced for counting in different locations. Additionally, quantification of the fracture-size distribution provides a mechanism for evaluation of uncertainties. Estimates of fracture intensity using this approach for two carbonate beds in the Sierra Madre Oriental, Mexico, illustrate how size-cognizant measurements cast new light on widely accepted interpretation of geologic controls of fracture intensity.