Geometry of layer-bound fractures based on fracture density and aperture-depth plots from resistivity image logs of deviated wells

https://doi.org/10.1016/j.jsg.2021.104372Get rights and content

Highlights

  • Shape of layer-bound fractures can be inferred from fracture density-depth plots.

  • 3-dimensional aperture geometry can also be inferred from aperture-depth plots.

  • High resolution resistivity or acoustic image logs are used for this purpose.

  • The borehole must be deviated and traverse layers with layer-bound fractures.

  • Density and aperture plots can jointly detect multilayer and unbound fractures.

Abstract

High resolution borehole resistivity image logs provide valuable bedding and fracture data from subsurface rocks including fracture type, orientation and aperture. The present study shows that common average shape and aperture geometry of uniformly located layer-bound fractures can be inferred from scan-line density and aperture-depth plots using image log data from a deviated well.

The proposed method extends the use of borehole image logs and allows 3-dimensional (3D) modeling of fractures and fractured layers from subsurface data. So far, such 3D fracture studies could only be conducted using outcrop data. The approach is quite robust because correspondence between fracture geometry and image log plots is independent of fracture length and orientation, and applies to any fracture shape and aperture geometry.

Joint interpretation of fracture scan-line density and aperture-depth plots, and bedding picks from borehole images also helps differentiate isolated/dispersed, unbound and multilayer fractures. Abundance of fractures that breach bed boundaries gives an idea about the cohesive bonding strength of bedding contacts and pave the way to construct fracture stratigraphy with several fractured layers that are separated by interfaces of varying bonding strength or by ductile beds. 3D fracture geometry may also provide some information on the type and mode of fractures and geo-mechanical conditions under which the fractures were generated.

Introduction

High resolution borehole image logs are mainly circumferential resistivity or acoustic records of borehole wall features, which are either logged during drilling (LWD) or separately as wireline logs (e.g. Prensky, 1999). Borehole images provide fracture scanline density (number of fracture per unit length), orientation and aperture (Luthi and Souiate 1990) as well as bedding and breakout measurements, which are used for a wide range of purposes such as modeling fractures (e.g. Trice 1999; Dashti et al., 2018; Spaid et al., 2016; Aghli et al., 2020), identification of faults, fracture clusters (e.g. Li et al., 2017), unconformities (e.g. Grace et al., 1999) and determining in-situ stress orientation and magnitude (e.g. Barton et al., 1988; Brudy and Zoback 1999; Brudy and Kjorholt 2001; Zoback et al., 2003; Djurhuus and Aadnoy 2003). Borehole image logs along with wireline logs also open the possibility for characterization of diagenesis, facies, and sequence stratigraphy (e.g. Prosser et al., 1999; Rider et al., 1999; Wilson et al., 2013).

Despite the wealth of fracture measurements, borehole image interpretation falls short of providing sufficient information on characterization of layer-bound fractures. A common aspect of layer-bound fractures is that their abundance, height, shape, aperture and other attributes are controlled mainly by mechanical layer properties and cohesive strength of layer boundaries (Helgeson and Aydin 1991, Narr and Suppe 1991, Vermilye and Scholtz 1995, Wu and Pollard 1995, Bai and Pollard 2000a,b, Cooke and Underwood, 2001, Underwood et al., 2003, Cooke et al., 2006, Laubach et al., 2009, Bazalgette et al., 2010, Guo et al., 2017, McGinnis et al., 2017, Dashti et al., 2018, Procter and Anderson 2018, Afshar and Luijendijk 2019).

Critical attributes of layer-bound fractures include fracture outer shape and 3 dimensional (3D) aperture geometry (i.e. Odone et al., 2007; Alzayer et al., 2015; Olson 2016); relative abundance of unbound and multilayer fractures and cohesive strength of layer boundaries. These attributes bear clues as to origin and type of fractures (Stearns and Friedman 1972; Helgeson and Aydin 1991), provide building blocks for mechanical or fracture stratigraphy (Laubach et al., 2009) and also play some role in fracture connectivity and permeability (i.e. Odling et al., 1999; Gudmundsson 2000; Cooke et al., 2006).

Restricted access to data from subsurface rocks has forced researchers to focus on outcrop data to study these critical fracture aspects and arrive at general theoretical models (e.g. Guerriero et al., 2010; Strijker et al., 2012; Ukar et al., 2019). Unfortunately, the immense variation of factors that influence such aspects of fracture precludes inference of conditions in subsurface from outcrop studies (Laubach et al., 2019).

The objective of this study is to demonstrate how fracture density and aperture-depth plots from borehole images contribute to modeling of layer-bound fractures in subsurface by extracting information on facture outer shape, aperture geometry, presence and abundance of unbound and multilayer fractures and cohesive strength of bed boundaries.

The first part of the paper shows that shape and aperture profile of layer-bound fractures can be inferred using fracture scan-line density and aperture-depth plots from a deviated well, whatever the shapes and aperture profiles are, provided that (i) fractures share common average shape and aperture profile, (ii) fractures are randomly located and (iii) horizontal lines at any level do not intersect fracture edge more than twice.

The second part focuses on unbound and multilayer fractures. Joint interpretation of fracture scan-line density and aperture depth plots can differentiate layer-bound fractures from dispersed, unbound and multilayer fractures; find their relative abundance and estimate cohesive strength of bedding contacts.

Finally, fracture data from a few example illustrating the way information on fracture geometry can be obtained from actual borehole image data, as an alternative to outcrop studies. The example wells are also intended to expose the potential of borehole image logs in modeling fracture stratigraphy (Laubach et al., 2009) and to provide a glimpse into some other aspects of layer-bound fractures such as the relationship between layer thickness, aperture and fracture density ((Vermilye and Scholtz 1995; Rijken and Pollard 195, Wu and Pollard 1995, Olson 2003; Renshaw and Park 1997; Bai et al., 2000; Schultz et al., 2008; Underwood et al., 2003; Hooker et al., 2009; Corredetti et al., 2017).

Fractures are too complex and difficult to predict and have many complication. Therefore a section is devoted to highlighting the shortcomings, limitations and ambiguities of the proposed method of analysis.

Section snippets

Materials, conventions and definitions

In this section, some definition, conventions, data requirement and limitations are surveyed. This is followed by definition of terms such as “layer bound fractures” and “fracture stratigraphy” as they are used in this work.

Fracture density-plots of layer-bound fractures

This section investigates the relationship between fracture shape and fracture scan-line density-depth plots from a deviated well across a layer with strata-bound conductive fractures. Fractures can assume a wide variety of shapes from ranging from simple elliptical or rectangular shapes (Bai and Pollard 2000a Passchier et al., 2021, Alzayer et al., 2015) to hooked and complex shaped related fracture coalescence (Eichhbul and Aydin, 2003). Therefore different fracture shapes are used as

Fracture aperture profiles of layer-bound fractures

This section aims at demonstrating that fracture aperture-depth plots across a layer with layer-bound fractures contain all necessary information to construct 3D aperture geometry of layer-bound fractures, whatever the aperture profile is, provided fractures share a common average aperture geometry underlying variations and irregularities.

Identification of dispersed, layer-bound and multilayer fractures

The aperture plots can be contoured by color coding apertures between preset size limits as explained above (Fig. 14). Such plots not only help determine aperture variation along horizontal and vertical axis of fractures and construct 3D aperture geometry, but also help differentiate dispersed/unbound fractures and multilayer fractures from layer-bound fractures (Hooker et al., 2013; McGinnis et al., 2017). Unbound/dispersed/isolated fractures have box shaped aperture plots (Fig. 15a), because

Limitations and ambiguities

The limitations and ambiguities are surveyed in this section. It is not possible to recount all possible pitfalls. Only a few possible limitation and ambiguities are covered below. The effects of violating the three conditions for the method to work are discussed first.

Case studies

Three examples from the Middle East oil fields are presented in this section. The examples include aperture and density-depth plots from borehole images, mechanical-stratigraphy, and image snapshot from highly deviated wells through nearly horizontal layers. The interval for fracture density calculation was set to five fractures and step size to one fracture in these wells, instead of fixed interval and step size.

The three wells are from different fractured carbonate oil reservoirs of Cambrian

Summary and conclusion

The method proposed here shows it is possible to obtain information on layer-bound fracture outer shape and aperture profile from deviated borehole image logs. Such 3D observations and measurements have so far been possible only in outcrop studies. Joint interpretation of fracture density and aperture-depth plots not only infers fracture shape and aperture models but also differentiates unbound, layer-bound and multilayer fractures and provide information on bonding strength of bedding contact (

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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