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When evaluating a fractured reservoir, the analyst must follow these steps<ref name=pt06r95 />:
 
When evaluating a fractured reservoir, the analyst must follow these steps<ref name=pt06r95 />:
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* Determine the origin of the fracture system found or the type of fracture system that is being explored for based on geometric characteristics of the fractures, their distribution in three dimensions, and empirical models of fracture system genesis (<xref ref-type="bibr" rid="pt06r135">Stearns and Friedman, 1972</xref>).
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* Determine the origin of the fracture system found or the type of fracture system that is being explored for based on geometric characteristics of the fractures, their distribution in three dimensions, and empirical models of fracture system genesis<ref name=pt06r135>Stearns, D. W., Friedman, M. 1972, Reservoirs in fractured rock: AAPG Memoir 16, p. 82–100.</ref>.
* Determine the reservoir properties of both the rock matrix housing the fractures and the fracture system (<xref ref-type="bibr" rid="pt06r145">van Golf-Racht, 1982</xref>).
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* Determine the reservoir properties of both the rock matrix housing the fractures and the fracture system<ref name=pt06r145>van Golf-Racht, T. D., 1982, Fundamentals of fractured reservoir engineering—Developments in Petroleum Science No. 12: Amsterdam, Elsevier Scientific Publishing Co., 710 p.</ref>.
 
* Determine the degree of fluid flow interaction or crossflow in either a qualitative or quantitative manner.
 
* Determine the degree of fluid flow interaction or crossflow in either a qualitative or quantitative manner.
 
* Classify the fractured reservoir on the basis of what the fracture system provides to overall reservoir quality.
 
* Classify the fractured reservoir on the basis of what the fracture system provides to overall reservoir quality.
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* Relate fracture distribution to rock type.
 
* Relate fracture distribution to rock type.
 
* Record the dip of fractures either real or apparent.
 
* Record the dip of fractures either real or apparent.
* Back up core observations with appropriate logs from the same zones as core from the well for effective extrapolation to uncored wells (see “Formation Evaluation of Naturally Fractured Reservoirs”). Record the strike of features if the core is oriented core or if it is locally oriented either mechanically or by oriented logs such as the Borehole Televiewer, Formation MicroScanner, or high resolution dipmeter (<xref ref-type="bibr" rid="pt06r101">Plumb and Luthi, 1986</xref>). (For more on these methods, see the chapters on “[[Core orientation]]” in Part 3 and “[[Borehole imaging devices]]” and “[[Dipmeters]]” in Part 4.)
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* Back up core observations with appropriate logs from the same zones as core from the well for effective extrapolation to uncored wells (see “Formation Evaluation of Naturally Fractured Reservoirs”). Record the strike of features if the core is oriented core or if it is locally oriented either mechanically or by oriented logs such as the Borehole Televiewer, Formation MicroScanner, or high resolution dipmeter<ref name=pt06r101>Plumb, R. A., Luthi, S. M., 1986, Application of borehole images to geologic modeling of an eolian reservoir: 61st Annual Technical Conference of the Society of Petroleum Engineers, New Orleans, LA, Oct. 5–8, SPE 15487, 11. p.</ref>. (For more on these methods, see the chapters on “[[Core orientation]]” in Part 3 and “[[Borehole imaging devices]]” and “[[Dipmeters]]” in Part 4.)
 
* Look for intersection angles of fractures as expressed on the outside surface of the core or on the ends of the samples and record the true or apparent angles (Figure 2).
 
* Look for intersection angles of fractures as expressed on the outside surface of the core or on the ends of the samples and record the true or apparent angles (Figure 2).
* Determine which of the fractures in the core are natural or induced (<xref ref-type="bibr" rid="pt06r69">Kulander and Dean, 1985</xref>).
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* Determine which of the fractures in the core are natural or induced<ref name=pt06r69>Kulander, B. R., Dean, S. L., 1985, Hackle plume geometry and joint propagation dynamics, in Stephansson, O. ed., Fundamentals of Rock Joints: Proceedings of the International Symposium, Bjorkliden, Sept. 15–20, p. 85–94.</ref>.
* Describe stylolite distribution (position, rock type, and postulated σ<sub>1</sub>) (<xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>; <xref ref-type="bibr" rid="pt06r145">van Golf-Racht, 1982</xref>).
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* Describe stylolite distribution (position, rock type, and postulated σ<sub>1</sub><ref name=pt06r95 /><ref name=pt06r145 />.
* Determine fracture plane morphology, paying particular attention to any partial mineralization along the fracture planes that might act as a natural proppant during depletion (<xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>). If present, determine its mineralogy and predicted relative compressive strength.
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* Determine fracture plane morphology, paying particular attention to any partial mineralization along the fracture planes that might act as a natural proppant during depletion<ref name=pt06r95 />. If present, determine its mineralogy and predicted relative compressive strength.
 
* Measure the relative size or height of the fractures, paying particular attention to any rock features that tend to control the vertical extent of the fractures, such as lithology breaks, bedding planes, stylolites, or unconformities.
 
* Measure the relative size or height of the fractures, paying particular attention to any rock features that tend to control the vertical extent of the fractures, such as lithology breaks, bedding planes, stylolites, or unconformities.
* Observe the width and width variation of the fractures. Measurements of width could be made with a scale, micrometer, or calipers or by impregnation with epoxy or plastic for either thin section measurement of epoxy width or dissolution of matrix leaving the width at the depth approximated (<xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>).
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* Observe the width and width variation of the fractures. Measurements of width could be made with a scale, micrometer, or calipers or by impregnation with epoxy or plastic for either thin section measurement of epoxy width or dissolution of matrix leaving the width at the depth approximated<ref name=pt06r95 />.
* Estimate or measure fracture spacing and its variability with depth (<xref ref-type="bibr" rid="pt06r94">Narr and Lerche, 1984</xref>; <xref ref-type="bibr" rid="pt06r96">Nolen-Hoeksema and Howard, 1987</xref>).
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* Estimate or measure fracture spacing and its variability with depth<ref name=pt06r94>Narr, W., Lerche, I. 1984, A method for estimating subsurface fracture density in core: AAPG Bulletin, v. 68, p. 637–648.</ref><ref name=pt06r96>Nolen-Hoeksema, R. C., Howard, J. H., 1987, Estimating drilling direction for optimum production in a fractured reservoir: AAPG Bulletin, v. 71, p. 958–966.</ref>.
* Determine principal stress directions and the origin and continuity of the fracture system(s) (<xref ref-type="bibr" rid="pt06r135">Stearns and Friedman, 1972</xref>; <xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>).
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* Determine principal stress directions and the origin and continuity of the fracture system(s<ref name=pt06r135 /><ref name=pt06r95 />.
* Determine the relative timing of deformational events from cross-cutting relationships or paragenetic sequence (<xref ref-type="bibr" rid="pt06r77">Lindquist, 1983</xref>).
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* Determine the relative timing of deformational events from cross-cutting relationships or paragenetic sequence<ref name=pt06r77>Lindquist, S. J., 1983, Nugget Formation reservoir characteristics affecting production the Overthrust Belt of southwestern Wyoming: Journal of Petroleum Technology, July, p. 1355–1365.</ref>.
* Relate fracture distribution to rock properties and rock fabric (such as composition, porosity, preferred grain orientation, bedding, and cross-bedding). Structural and petrological descriptions go hand in hand (<xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>).
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* Relate fracture distribution to rock properties and rock fabric (such as composition, porosity, preferred grain orientation, bedding, and cross-bedding). Structural and petrological descriptions go hand in hand<ref name=pt06r95 />.
 
* Select samples for additional petrophysical or petrological determinations (such as X-ray, thin sections, and permeability) (see “[[Thin section analysis]]” and “[[SEM, XRD, CL, and XF Methods]]”).
 
* Select samples for additional petrophysical or petrological determinations (such as X-ray, thin sections, and permeability) (see “[[Thin section analysis]]” and “[[SEM, XRD, CL, and XF Methods]]”).
 
* Estimate the permeability of properly oriented individual fractures from core analyses (see “Permeability”).
 
* Estimate the permeability of properly oriented individual fractures from core analyses (see “Permeability”).
* Qualitatively estimate fracture and matrix porosity interaction or at least determine if there is any evidence of impedance to cross-flow (<xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>) (see “Porosity”).
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* Qualitatively estimate fracture and matrix porosity interaction or at least determine if there is any evidence of impedance to cross-flow<ref name=pt06r95 />) (see “Porosity”).
* Determine the fractured reservoir type (<xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>).
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* Determine the fractured reservoir type<ref name=pt06r95 />.
 
* Photograph important relationships shown in the core for documentation in reports, if needed.
 
* Photograph important relationships shown in the core for documentation in reports, if needed.
 
* Select samples for mechanical testing, if appropriate.
 
* Select samples for mechanical testing, if appropriate.
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* For a so-called quantitative measurement station (having enough fracture measurements to be statistically meaningful), all fractures in the measurement area should be recorded and should number from about 100 to 150 fractures. The actual number should depend on the complexity of the fracture patterns present.
 
* For a so-called quantitative measurement station (having enough fracture measurements to be statistically meaningful), all fractures in the measurement area should be recorded and should number from about 100 to 150 fractures. The actual number should depend on the complexity of the fracture patterns present.
 
* For a so-called qualitative measurement station (not having enough fracture measurements to be statistically meaningful), only general fracture trends are recorded, along with a judgment of relative abundance of the individual fracture orientations. These are often used when spot checking areas between statistical measurement stations. If no major change in orientation and intensity of fracture patterns is observed away from nearby statistical measurements stations, the use of qualitative stations gives valid intermediate data.
 
* For a so-called qualitative measurement station (not having enough fracture measurements to be statistically meaningful), only general fracture trends are recorded, along with a judgment of relative abundance of the individual fracture orientations. These are often used when spot checking areas between statistical measurement stations. If no major change in orientation and intensity of fracture patterns is observed away from nearby statistical measurements stations, the use of qualitative stations gives valid intermediate data.
* At the individual measurement stations, the analyst should record as much of the following information as possible (<xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>):
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* At the individual measurement stations, the analyst should record as much of the following information as possible<ref name=pt06r95 />:
* Each individual fracture measurement at the station should record as much of the following data as possible (see <xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>):
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* Each individual fracture measurement at the station should record as much of the following data as possible (see <ref name=pt06r95 />):
 
* At convenient times, the fracture data should be plotted in preliminary form on either rose diagrams or pole plots (π diagrams). Such preliminary plotting is necessary in the field to establish trends and application to simple geological fracture models. In this way, working interpretive models can be created and altered or updated while field data are still being gathered. The observer should always examine fracture patterns in light of their relationship to their localities and to local structural configuration.
 
* At convenient times, the fracture data should be plotted in preliminary form on either rose diagrams or pole plots (π diagrams). Such preliminary plotting is necessary in the field to establish trends and application to simple geological fracture models. In this way, working interpretive models can be created and altered or updated while field data are still being gathered. The observer should always examine fracture patterns in light of their relationship to their localities and to local structural configuration.
 
* The number and frequency or spacing of quantitative measurement stations are generally high in the early stages of study in a region and decrease in relationship to qualitative stations throughout the study.
 
* The number and frequency or spacing of quantitative measurement stations are generally high in the early stages of study in a region and decrease in relationship to qualitative stations throughout the study.
* When dealing with outcrops containing a predominance of either contractional fractures or fractures related to unconformity surfaces (<xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>), much of the previous quantitative orientation data will be ill-defined due to their isotropic or irregular distribution in orientation. In these outcrops, matrix block size (fracture spacing in three dimensions) are very important, as are lateral distribution and lithology.
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* When dealing with outcrops containing a predominance of either contractional fractures or fractures related to unconformity surfaces<ref name=pt06r95 />, much of the previous quantitative orientation data will be ill-defined due to their isotropic or irregular distribution in orientation. In these outcrops, matrix block size (fracture spacing in three dimensions) are very important, as are lateral distribution and lithology.
 
* Photograph all outcrops measured and take block samples (about 10” × 6” × 5”) of all major units of interest for petrophysical and possibly mechanical testing.
 
* Photograph all outcrops measured and take block samples (about 10” × 6” × 5”) of all major units of interest for petrophysical and possibly mechanical testing.
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* Document fracture systems geometry
 
* Document fracture systems geometry
 
* Document fracture morphology
 
* Document fracture morphology
* Determine fracture type (origin) Application of observations to empirical models using data from steps 1 through 5 (<xref ref-type="bibr" rid="pt06r135">Stearns and Friedman, 1972</xref>; <xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>)
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* Determine fracture type (origin) Application of observations to empirical models using data from steps 1 through 5<ref name=pt06r135 /><ref name=pt06r95 />
 
* Predict fracture distribution and extent Extrapolation using fracture type and observations
 
* Predict fracture distribution and extent Extrapolation using fracture type and observations
 
* Estimate fracture spacing and spacing variability
 
* Estimate fracture spacing and spacing variability
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* Estimate reservoir properties at depth
 
* Estimate reservoir properties at depth
 
* Estimate fracture and matrix interaction
 
* Estimate fracture and matrix interaction
* Correlate small scale petrophysical properties with large scale reservoir engineering tests (<xref ref-type="bibr" rid="pt06r1">Aguilera, 1980</xref>; <xref ref-type="bibr" rid="pt06r145">van Golf-Racht, 1982</xref>)
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* Correlate small scale petrophysical properties with large scale reservoir engineering tests<ref name=pt06r1>Aguilera, R. 1980, Naturally fractured reservoirs: Tulsa, OK, PennWell Books, 703 p.</ref><ref name=pt06r145 />
* Determine fractured reservoir type Correlate matrix and fracture properties and their communication to determine relative contribution of the fracture system and potential recovery problem (<xref ref-type="bibr" rid="pt06r95">Nelson, 1985</xref>)
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* Determine fractured reservoir type Correlate matrix and fracture properties and their communication to determine relative contribution of the fracture system and potential recovery problem<ref name=pt06r95 />
 
* Make conclusions relevant to the type of evaluation
 
* Make conclusions relevant to the type of evaluation
  

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