Abstract:

We examined branch-like fracture sets (joints, shear fractures, and faults) that occur in brittle rock layers across a range of subsurface datasets and scales. Whereas fracture sets are commonly grouped by orientation in order to infer something about their genetic origin, we investigate sets that are believed to be temporally and mechanically related but vary in orientation, specifically, cases where three-dimensional projection of the planes implies hierarchical branching. Examples of these features are: antithetic fault connections; splays and branches commonly found in fault zones, dynamic fracturing, and under injection conditions such as dike swarms. A new analysis method is tested to discern these features from limited sampling data using straightforward geometric relations. A computer script was written to determine all of the potential branch lines, or the intersection lines between two planes in space, along a one-dimensional scanline (e.g. fractures measured along a wellbore). The goal is to identify the spatially dependent types of intersections (a) vertical “Y” shaped branches; (b) horizontal “Y” shaped branches; or (c) 3D arbitrary branches. We compare a few synthetic data sets and a field data set to demonstrate the key patterns.

Introduction

Structural characterization in many subsurface datasets is limited by lack of three-dimensional data at the relevant scale. Wellbore image logs and core provide data on fracture and fault sets at the bedding scale. Seismic data can image fracture corridors and fault zones and discrete faults with larger offsets usually on the scale of features 10s of meters or larger. Typically the structural style is determined from the larger scale, and the challenge is to synthesize smaller scale structures (i.e. wellbore fractures) with the patterns of the larger ones. The patterns of interest to this study are genetically related fractures that are non-parallel and intersect or merge in three dimensions to form tree or branch-like hierarchical structures.

Whereas the terms tree-like or branch-like describe the fracture morphology, the term branching implies the fractures are genetically related surfaces that source from, or merge at, a shared intersection. The branch line is where a master fracture or fault surface splits or branches into two or more surfaces of the same type but with different orientations. Consequently, branching fractures cannot always be grouped into separate fracture sets in the usual way using orientation alone.

The study was motivated by a recent acquisition of a series of subsurface cores and image logs that penetrated several thousand individual fractures that we interpreted as branching fractures linked to a series of fracture swarms (e.g., Fig. 1). We present here a proposed method to help differentiate the three-dimensional nature of the branching using only limited wellbore intersection data. The method is demonstrated on synthetic and field data sets to illustrate key patterns detected from the branch line analysis technique.

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