Changes

The following procedures have proven useful in [[Fracture analysis|fracture analyses]] of core (after <ref name=pt06r95 />):

The following procedures have proven useful in [[Fracture analysis|fracture analyses]] of core (after <ref name=pt06r95 />):

* Lay out all of the core from a given well for observation. Fit the core together and note continuous segments (see [[Core handling]]).

# Lay out all of the core from a given well for observation. Fit the core together and note continuous segments (see [[Core handling]]).

* Do initial observations while the core is clean and in its whole-core state (unslabbed) (see [[Core description]]). In this state, all [[fracture]]s are present and their angular relationships retained. The core can be slabbed later and a finer internal description done at that time. Large diameter cores are generally preferred to small diameter cores. The larger diameter core depicts more regularly spaced high angle fractures and thus gives a better representation of fracture spacing and a better measurement of reservoir properties from core analyses.

# Do initial observations while the core is clean and in its whole-core state (unslabbed) (see [[Core description]]). In this state, all [[fracture]]s are present and their angular relationships retained. The core can be slabbed later and a finer internal description done at that time. Large diameter cores are generally preferred to small diameter cores. The larger diameter core depicts more regularly spaced high-angle fractures and thus gives a better representation of fracture spacing and a better measurement of reservoir properties from core analyses.

* Have the core analyses (porosity and permeability) data in hand while observing the core to determine the relative effect of the features observed (see [[An overview of routine core analysis]]). Obtain three-dimensional whole-core analyses, if possible, when the fractures or other anisotropic features are predicted to be important.

# Have the core analyses ([[porosity]] and [[permeability]]) data in hand while observing the core to determine the relative effect of the features observed (see [[An overview of routine core analysis]]). Obtain three-dimensional whole-core analyses, if possible, when the fractures or other anisotropic features are predicted to be important.

* Create the fracture stratigraphy (involves steps 5 through 16).

# Create the fracture stratigraphy (involves steps 5 through 16).

* Construct a core deformation strip log at a relevant scale that includes the positioning of major lithology and formation breaks, lost core and noncored intervals, [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=oil-water+contact oil-water]  or [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=gas-water+contact gas-water contacts]] (if available), fractures, and stylolites. Hairline fractures in the core are often difficult to see. These can be enhanced by painting the core with a volatile liquid and observing the drying pattern. Overlooked fractures may “jump out” at the observer by persistence of wetting along hairline fractures reaching the core surface. Additional characterization of hairline or larger fractures internal to the core can be accomplished by core X-radiographs or core computer tomography scans.

# Construct a core deformation strip log at a relevant scale that includes the positioning of major lithology and formation breaks, lost core and noncored intervals, [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=oil-water+contact oil-water]  or [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=gas-water+contact gas-water contacts]] (if available), fractures, and [http://www.merriam-webster.com/dictionary/stylolite stylolites]. Hairline fractures in the core are often difficult to see. These can be enhanced by painting the core with a volatile liquid and observing the drying pattern. Overlooked fractures may “jump out” at the observer by persistence of wetting along hairline fractures reaching the core surface. Additional characterization of hairline or larger fractures internal to the core can be accomplished by core X-radiographs or core computer tomography scans.

* Record the fracture distribution with depth ([[:file:evaluating-fractured-reservoirs_fig2.png|Figure 2]]). (Recording of this and all following data could be done in a tabular format compatible with computer entry to facilitate output compatible with other strip and well logs.)

# Record the fracture distribution with depth ([[:file:evaluating-fractured-reservoirs_fig2.png|Figure 2]]). (Recording of this and all following data could be done in a tabular format compatible with computer entry to facilitate output compatible with other strip and well logs.)

* 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.<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 [[Core orientation]], [[Borehole imaging devices]], and [[Dipmeters]].)

# 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 [[Core orientation]], [[Borehole imaging devices]], and [[Dipmeters]].)

* 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 ([[:file:evaluating-fractured-reservoirs_fig2.png|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 ([[:file:evaluating-fractured-reservoirs_fig2.png|Figure 2]]).

* 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>

# 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 σ₁.<ref name=pt06r95 /><ref name=pt06r145 />

# Describe stylolite distribution (position, rock type, and postulated σ₁.<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.<ref name=pt06r95 /> If present, determine its mineralogy and predicted relative compressive strength.

# 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.<ref name=pt06r95 />

# 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.<ref name=pt06r94>Narr, W., Lerche, I. 1984, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0068/0005/0600/0637.htm 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, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0008/0950/0958.htm Estimating drilling direction for optimum production in a fractured reservoir]: AAPG Bulletin, v. 71, p. 958–966.</ref>

# Estimate or measure fracture spacing and its variability with depth.<ref name=pt06r94>Narr, W., Lerche, I. 1984, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0068/0005/0600/0637.htm 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, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0008/0950/0958.htm 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).<ref name=pt06r135 /><ref name=pt06r95 />

# 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.<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>

# 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.<ref name=pt06r95 />

# 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<ref name=pt06r95 />) (see [[Porosity]]).

# 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.<ref name=pt06r95 />

# 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.

* If needed, set up a checklist of important parameters and document as many parameters as needed on a foot-by-foot basis (on each whole core piece is ideal). Such a checklist should be compatible with direct computer input of the tabular data (see [[:file:evaluating-fractured-reservoirs_fig2.png|Figure 2]]).

# If needed, set up a checklist of important parameters and document as many parameters as needed on a foot-by-foot basis (on each whole core piece is ideal). Such a checklist should be compatible with direct computer input of the tabular data (see [[:file:evaluating-fractured-reservoirs_fig2.png|Figure 2]]).

* Write down impressions and conclusions arrived at before leaving the core. Include thoughts on the relative importance of fractures to production and flow, the permeability of the fractures present, fracture porosity (qualitative), fracture fill and relative compressibility of the fractures, fracture origin and continuity in the reservoir, the percentage of real versus induced fractures, and the fractured reservoir type.

# Write down impressions and conclusions arrived at before leaving the core. Include thoughts on the relative importance of fractures to production and flow, the permeability of the fractures present, fracture porosity (qualitative), fracture fill and relative compressibility of the fractures, fracture origin and continuity in the reservoir, the percentage of real versus induced fractures, and the fractured reservoir type.

==Outcrop analysis==

==Outcrop analysis==