Research paperCharacterizing carbonate reservoir fracturing from borehole data – A case study of the Viséan in northern Belgium
Introduction
Naturally fractured carbonate rocks are often targeted as potential reservoirs for hydrocarbons or geothermal water. In the context of the global energy transition, the interest in deep geothermal energy systems is rapidly increasing. An appropriate geothermal reservoir that meets the demand is dependent on many different factors. One important factor relates to the permeability, which on itself is dependent on multiple properties. Rock fracturing is of major importance for permeability, especially in carbonate reservoirs with low primary matrix porosity and permeability (Warren and Root, 1963; Nelson, 1985). In order to reduce the risks for developing new geothermal systems in such tight reservoirs, understanding fracture characteristics and underlying processes is needed.
From a reservoir perspective, the most interesting parameter to be determined would be the distribution and connectivity of joints (non-cemented extension fractures), since these have a large effect on permeability (Warren and Root, 1963; National Research Council, 1996). Unfortunately, natural joints cannot be quantified from core material directly, since it is impossible to define the cause of a discontinuity between two core samples. Such discontinuities could also reflect drilling induced fractures (Pendexter and Rohn, 1954; Li and Schmitt, 1998) or relate to the coring process or previous research or transport activities. Some veins (cemented fractures) are partially open and could thus also contribute to permeability. These are quantifiable from cores and are the main focus of interest in this study.
In this study, the Viséan (Lower Carboniferous) limestones of the Heibaart DZH1 borehole in northern Belgium were used as a case study. This well was drilled in the Campine-Brabant Basin, a Variscan foreland basin in northern Belgium and the southern Netherlands. In the context of a geothermal project, reservoir characteristics of the carbonates are re-evaluated. The Heibaart DZH1 borehole was chosen as a case study because of the large amount of available core material, the variety of well described lithologies, the known existence of open karstic features, and the availability of multiple geophysical well logs.
To study the factors controlling the development and preservation of these partially open veins, as many variables as possible were taken into account. Four different datasets were used in the case study, regarding lithotype, geochemistry, geophysical well logs and physical features from cores. From the cores, not only partially open veins were quantified, but also features which do not directly contribute to permeability, such as cemented veins, (partially open) vugs and stylolites.
A classical approach to study fracture distributions is by ‘mechanical stratigraphy’ or ‘fracture stratigraphy’. Many studies on these topics have been carried out, based on outcrop studies (Di Naccio et al., 2005; Cooke et al., 2006; Jacquemyn et al., 2012; Lamarche et al., 2012; Ferril et al., 2017; McGinnis et al., 2017; Faÿ-Gomord et al., 2018) or borehole cores and geophysical well logs (Laubach et al., 2009). According to Laubach et al. (2009), mechanical stratigraphy means the subdivision of a stratified rock sequence into mechanical units based on mechanical properties “such as tensile strength, elastic stiffness, brittleness, and fracture mechanics properties”. The authors state that fracture stratigraphy is different since it subdivides a sequence into fracture units based on “extent, intensity, or some other observed fracture attribute”. The present study deals with these fracture parameters, not with mechanical parameters.
In most of the mentioned studies, sequences were subdivided into mechanical units or fracture units based on quantified parameters such as fracture intensity/density/spacing, fracture length/height, fracture propagation or fracture orientation, often normalized for the thickness of beds or defined units. Laubach et al. (2009) discussed borehole studies in which fracture numbers were derived from core material or image logs. Rock property measurements of Young's modulus and Poisson ratio were added. The present study focuses on differences and interdependencies between fracture characteristics and many different variables based on statistical analyses, instead of subdividing a single sequence into units. Furthermore, it does not only take into account the parameters that contribute directly to the permeability of a system, such as partially open veins, but also studies relationships with other features, such as veins and stylolites, which can provide information on the mechanical behavior and the diagenetic history of the rocks.
The aim of the present study is to investigate which factors control the distribution and characteristics of partially open fractures, and to which extent. This study presents a workflow for the characterization of reservoir fracturing from borehole data. Information from both core material and geophysical well logs was taken into account.
Section snippets
Geological setting
In northern Belgium, the southern part of the Netherlands and the western-most part of Germany, a vast amount of carbonates was deposited during the Lower Carboniferous, in a shallow marine basin that is mostly referred to as the Campine Basin (Ziegler, 1990; McCann, 2008) or Campine-Brabant Basin (Bless et al., 1983; Muchez et al., 1987; Muchez and Viaene, 1990). In this study, we consider the Campine-Brabant Basin sensu lato (Fig. 1), as a part of the Northwest European Carboniferous Basin
Datasets
In the case study of the Heibaart DZH1 borehole, four different datasets were used (Fig. 5): 1) lithology; 2) geochemistry; 3) geophysical well logs; and 4) physical features from cores. The lithotype, geochemistry and geophysical well logs were acquired in previous work (Muchez et al., 1987). The physical features from cores were newly acquired in this study. Partially open veins in the cores are the main target of interest.
In the upper 193 m of the Viséan sequence, coring was performed
Physical features from cores
Fig. 6 shows examples of all physical features which were described and measured from the cores: veins, partially open veins, cemented vugs, partially open vugs and stylolites.
In Fig. 7, the frequencies of veins, partially open veins, stylolites, cemented vugs and partially open vugs are plotted along the depth of the borehole. The veins (which are all opening-mode fractures and completely cemented) are most abundant in the upper 110 m of the sequence and around 1263 m and 1288 m (see grey
Interpretations and discussion
The aim of this study was to assess which factors control the distribution and characteristics of (partially open) fractures, and to which extent. In this context, interpretations and explanations are discussed below.
Conclusions
A general workflow was presented for the characterization of fracturing from borehole data by extensive data analyses of different integrated datasets. In the demonstrated case study, four different datasets of the Heibaart DZH1 borehole were used to analyze the factors controlling the distribution of partially open veins and joints, which enhance the permeability of the reservoir. Although joints are most interesting, since these are by definition not cemented, the datasets acquired in the
Acknowledgements
The research is supported by a VITO (Vlaamse Instelling voor Technologisch Onderzoek) PhD grant nr. 1610424. The cores and geophysical well logs of the Heibaart DZH1 borehole were made available by the Geological Survey of Belgium, from the archive KBIN-BGD. In particular, Marleen De Ceukelaire and Tommy Dheuvaert are thanked for their support regarding the material. We would like to thank Prof. Ondrej Bábek, two anonymous reviewers and editor Prof. Reza Rezaee, for their detailed and
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