Diagenetic controls on fracture permeability and sealing

To cite this paper: Int. J. Rock Mech. & Min. Sci. 34:3-4, paper No. 204.

Copyright © 1997 Elsevier Science Ltd

Copyright © 1997 Elsevier Science Ltd Int. J. Rock Mech. & Min. Sci. Vol. 34, No. 3-4, 1997 ISSN 0148-9062 To cite this paper: Int. J. RockMech. &Min. Sci. 34:3-4, Paper No. 204

D I A G E N E T I C C O N T R O L S ON F R A C T U R E P E R M E A B I L I T Y AND S E A L I N G Randall Marrettl;

S. E . L a u b a c h 2

1 Department of Geological Sciences, The University of Texas at Austin, Austin, Texas 78712-1101 USA z Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713-8924 USA ABSTRACT

Although the degree to which fractures are open and interconnected in the subsurface governs their ability to transmit fluid, information on in situ fracture apertures and connectivity is usually incomplete. Fracture apertures remaining open and effective for fluid flow (hydraulic apertures) depend on a number of factors that reflect fracture growth, diagenesis, and the modern state of stress. Of these, diagenesig mineral precipitation and dissolution within fractures---has received little systematic study. Yet observations in petroleum reservoirs show that diagenetic minerals prop open some fractures but close others. Study of coupled diagenesis and fracture processes yield insight into fracture permeability and sealing. For example, using microstructural evidence for the timing of fracture formation relative to cementation episodes, fracture openness can be estimated from cement volumes in rock matrix, even where large fractures are not sampled. Interaction of diagenesis and fracture processes might also play an active role in governing fracture growth. This could moderate the partitioning of permeability and porosity among fractures of different size. Copyright © 1997 Elsevier Science Ltd

KEYWORDS F r a c t u r e • d i a g e n e s i s • fluid f l o w • effective stress • scaling • c a t h o d o l u m i n e s c e n c e FRACTURE

DIAGENESIS

Current-day effective stress is widely viewed as the prime control on variation in fracture aperture (and fracture closure). Yet mineral deposits in natural fractures are widespread, ranging from isolated crystals lining open fractures to massive cements that completely fill fractures. Such mineral fills can preserve or destroy fracture-system permeability (Dyke 1995), and they can have a strong influence on the sensitivity of fractures to changes in effective stress. Systematic studies of cement patterns within hydrocarbon reservoir fractures are rare. This partly reflects inadequate sampling of subsurface fracture populations, making trends in cement patterns challenging to recognize. In addition, many fractures in outcrop are poor analogs to those in reservoirs because they have undergone different fracture-fill histories and near-surface alteration of fracture-filling mineral suites. Nevertheless, core studies show that diagenetic modification of fracture pore space augments rock and fracture strength such that open, conductive fractures can be sustained for long periods in some

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geochemical and tectonic settings, whereas fractures in other rocks are partly or completely occluded. In some sedimentary rocks, shifts in degree of fracture fill from open to filled occur locally over vertical distances of only a few centimeters. Direct measurement of the attributes of large fractures is extremely challenging because such fractures rarely intersect wellbores where they can be observed. However, in some cases the properties of fractures can be inferred from observation of diagenetic minerals in the rock matrix and associated microstructures (Laubach, Milliken 1996). In this paper we present qualitative predictions of the degree of mineral fill in fractures as an example of this approach (Figures 1-4). Microfractures can share not only diagenetic mineralization with macrofractures, but also orientations (Laubach 1997) and size distributions (Marrett 1997). In many ways, microfractures and macrofractures are simply different size fractions of the same fracture sets. This insight offers the potential for using scaling relations to quantitatively link fracture attributes, including diagenetic characteristics, across the gap between micro- and macrofractures. Here we speculate that, where diagenetic effects play an active role in fracture growth, diagenesis may moderate fracture scaling and consequently the partitioning of permeability and porosity among fractures of different size.

P R E D I C T I O N OF F R A C T U R E O C C L U S I O N Cement can be divided into three categories on the basis of the timing of cement precipitation relative to fracture growth (Figure 1; Laubach 1988; Laubach, Milliken 1996): prekinematic, where cement precipitates before fractures open; synkinematic, where cement is precipitated during fracture opening; andpostkinematic, where cement precipitates in static fractures. Rocks may have several fracture-opening events, as well as repetitive sequences of mineral precipitation, so this classification must be referenced to a specific fracture event. Cement types can be defined based on observations of paragenetic relations within large fractures, or by combining conventional petrographic documentation of mineral precipitation sequences in the rock matrix and observation of mineralized microfractures to identify the time of fracture opening. Because large fractures are rarely encountered in core, the latter approach has the best potential for improving information on subsurface fracture attributes. According to this classification, synkinematic and postkinematic cements are those available to fill fractures. However, analysis of a suite of sandstones of varying depositional environment, depth of burial, diagenetic history, and tectonic setting shows that where fractures are completely filled, the cements responsible are overwhelmingly postkinematic (Laubach 1989, 1997 and unpublished). In these rocks, large fractures (mechanical apertures > 1 mm) tend to preserve open fracture pore space only if little or no postkinematic cement is present. Moreover, in two formations where more than 50 macrofractures were observed, postkinematic cement occludes intergranular porosity to about the same extent that it fills fracture porosity. Fractures are a variety of porosity and thus are susceptible to being filled with cement. This is particularly true for fractures that are not growing. Thus fractures are more likely to be filled with cement and closed to fluid flow in areas where volume of postkinematic cement is high. Synkinematic cements, on the other hand, are associated with crack-seal microstructures that show fractures were opening as cement precipitated (Laubach 1997). Evidently in the geochemical and tectonic environments sampled, the increase in fracture volume due to crack opening was greater than the decrease due to cement precipitation. Thus, volume of postkinematic cement can be used to predict the permeability of a natural fracture

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system. Figures 2 and 3 show comparisons of post-stimulation production outcomes for two sets of well pairs together with measurements of cement type at a number of depths. The first well pair is from Lower Cretaceous sandstone in the East Texas Basin, and the second is from Upper Cretaceous sandstone in southwestern Wyoming. Both formations are natural gas reservoirs, and the depositional environment, depth, and stimulation for each pair of wells are similar. In these examples, matrix contains reservoir storage volume and fractures provide intrareservoir flow pathways. Key differences between wells are postkinematic cement volume and overall rate of fluid production (producer versus non-producer), which we interpret to reflect the permeability of the natural fracture system in the vicinity of the well. In these cases uniformly high volumes of postkinematic cement accurately predict low fluid production and vice versa. Many areas show evidence of extreme heterogeneity in the mix of cement types, and by implication, the degree to which fractures are filled with cement. Figure 4 shows typical patterns of vertical variability in proportions of various cement types in Permian sandstone from west-central Texas. Observation of large fractures in this formation show that open and filled fractures exist in proximity, just as these predictions imply. FRACTURE SCALING AND DIAGENESIS Mechanical apertures of fractures (including both preserved fracture porosity and diagenetic cement) largely reflect processes of fracture growth. Fracture linkage and incremental propagation are generally considered to be the primary mechanisms of fracture growth. Models of fracture growth dominated by incremental propagation (Clark et al. 1995) and by fracture linkage (Cladouhos, Marrett 1996) show that these mechanisms provide viable explanations for the commonly observed power-law scaling of natural fracture populations. Extension fractures having mechanical apertures smaller than 1 mm and larger than 10 cm follow similar power-law scaling (Marrett 1997), although individual data sets typically span only one to two orders of magnitude in this scale range. The physical processes of fracture linkage and incremental propagation may have significant impact on the geometry of individual fractures and the scaling of fracture populations. However, chemical processes of diagenesis that precipitate synkinematic cement in fractures might also govern fracture growth. If the rate of synkinematic cementation keeps pace with the rate of fracture opening, at least locally, then continued growth of a fracture requires breakage not only of material along the fracture tip but also where the fracture has healed. This diagenetic strengthening of fractures could affect the relation between fracture length and mechanical aperture, as well as the length and aperture statistics of fracture populations. Consequently, diagenetic processes might moderate key aspects of both fracture geometry and scaling. Recent attempts to quantify the relation between mechanical aperture and length for extension fractures have resulted in significant disparity (Hatton et al. 1994; Vermilye, Scholz 1995; Johnston, McCaffrey 1996). New data (Marrett et al. 1997) collected from microfractures in low-porosity sandstones using the SEM-CL technique (Laubach, Milliken 1996; Laubach 1997) extend the range of available data by three orders of magnitude (Figure 5). Macrofractures in the sandstones have dimensions comparable to those determined for fractures in the previously studied rocks. Each of the data sets shows considerable scatter, part of which might reflect sampling limitations (e.g., measurement along chords of fracture surfaces) and differences in fracture linkage maturity, host layer thickness and host rock strength. However, variation between data sets as well as additional scatter could result from different modes of synkinematic cementation (e.g., veins of Johnston, McCaffrey 1996, show crack-seal textures, whereas

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Vermilye, Scholz 1995, avoided such veins). Fracture population scaling might also reflect the influence of synkinematic cementation. Mechanical apertures sampled along scanlines commonly follow power-law scaling with scaling exponents between 0.75 and 0.85 (Marrett 1997). However, some data sets describing fractures in which synkinematic cementation repeatedly bridged or sealed the fractures during growth have produced exponents greater than one (e.g., Clark et al. 1995). Macrofractures in low-porosity sandstones exhibiting crack-seal textures also produce exponents in excess of one (Figure 6), as do microfractures in such sandstones (Marrett et al. 1997). Although reliable conclusions cannot yet be drawn from existing data, they suggest that synkinematic cementation does affect fracture scaling. Fracture scaling, in turn, may play a fundamental role in the permeability of a fracture system (Marrett 1996). Postkinematic cementation of extension fractures occludes fracture porosity after cessation of fracture growth. Microfractures tend to be more thoroughly sealed than associated macrofractures, suggesting that postkinematic cementation is scale dependent like other aggregate properties (Marrett 1996). One simple hypothesis stems from the fact that smaller fractures have greater surface area-to-volume ratios than do larger fractures. Because fracture-sealing cement must grow from fracture surfaces, there may be a natural tendency for smaller fractures to be sealed more efficiently than larger fractures. Such postkinematic diagenesis would enhance the dominance of larger fractures in terms of fracture system permeability (Marrett 1996). In the future, fracture scaling relations may provide an empirical basis for predicting the attributes and spatial frequencies of macrofractures from observations of microfractures. For such predictions to be most useful in quantifying reservoir properties, it will be necessary to understand fracture scaling in the context of diagenesis. Diagenetic effects on a fracture system include not only reduction of stress sensitivity, occlusion of fracture porosity and reduction of fracture permeability, but possibly moderation of fracture growth as well. To the extent that fracture growth governs both fracture geometry and scaling, diagenesis could moderate the partitioning of permeability and porosity among fractures of different size. ACKNOWLEDGMENTS We thank S. Doenges for comments on the manuscript. Our research on use of scaling concepts and microstructural observations in reservoir characterization and simulation is supported by DOE contract no. G4S51732 and by the following companies in the UT Applied Structural Petrology consortium: Chevron, Conoco, Exxon, Union Pacific Resources, and Oxford Instruments.

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References References Cladouhos T.T, Marrett R. 1996. Are fault growth and linkage models consistent with power-law distributions of fault lengths? dournal of Structural Geology, 18:2-3, 281-293. Clark M.B., Brantley S.L., Fisher D.M. 1995. Power-law vein-thickness distributions and positive feedback in vein growth. Geology, 23:11, 975-978. Dyke C.C. 1995. How sensitive is natural fracture permeability at depth to variation in effective stress? In Myer, L.R, Tsang, C.-F., Cook, N.G.W, and Goodman, R.E. (Editors), Proceedings Fractured and Jointed Rock Masses Conference, Balkema, Rotterdam, 81-88. Hatton C.G., Main I.G, Meredith P.G. 1994. Non-universal scaling of fracture length and opening displacement. Nature, 367:2, 160-162. Johnston J.D., McCaffrey K.J.W. 1996. Fractal geometries of vein systems and the variation of scaling relationships with mechanism, dournal of Structural Geology, 18:2-3, 349-358.

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Laubach S. E. 1988. Subsurface fractures and their relationship to stress history in East Texas Basin sandstone. Tectonophysics, 156:4, 495-503. Laubach S. E. 1989. Paleostress directions from the preferred orientation of closed microfractures (fluid-inclusion planes) in sandstone, East Texas basin, U.S.A. Journal of Structural Geology, 11:5, 603-611. Laubach S. E. 1997. Amethod to detect natural fracture strike in sandstones. American Association of Petroleum Geologists Bulletin, 81:4. Laubach S. E., Milliken K.L. 1996. New fracture characterization methods for siliciclastic rocks. In Aubertin, M., Hassani, F., and Mitri, H. (Editors), Proceedings 2nd North American Rock Mechanics Symposium. Balkema: Rotterdam, 1209-1213. Marrett R. 1996. Aggregate properties of fracture populations. Journal of Structural Geology, 18:2-3, 169-178. Marrett R. 1997. Permeability, porosity, and shear wave anisotropy from scaling of open fracture populations. In Hoak, T.E., Blomquist, RK., and Klawitter, A. (Editors), Fractured Reservoirs: Descriptions, Predictions and Applications. Rocky Mountain Association of Geologists Guidebook. Marrett R., Ortega O., Reed R., Laubach S. 1997. Predicting macrofracture permeability from microfractures. American Association of Petroleum Geologists Annual Convention Official Program, 6. Vermilye J.M., Scholz C.H. 1995. Relation between vein length and aperture. Journal of Structural Geology, 17:3, 423-434.

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