Changes

''Type 4''—Impart no positive [[reservoir quality]] but create strong reservoir anisotropy and inhomogeneity<ref name=pt06r95>Nelson, R. A., 1985, Geologic analysis of naturally fractured reservoirs—Contributions in Petroleum Geology and Engineering #1: Houston, TX, Gulf Publishing Company, 320 p.</ref>

''Type 4''—Impart no positive [[reservoir quality]] but create strong reservoir anisotropy and inhomogeneity<ref name=pt06r95>Nelson, R. A., 1985, Geologic analysis of naturally fractured reservoirs—Contributions in Petroleum Geology and Engineering #1: Houston, TX, Gulf Publishing Company, 320 p.</ref>

}}

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Anticipated exploration and development problems associated with these four reservoir types are summarized in Table 1 and Table 2.

Anticipated exploration and development problems associated with these four reservoir types are summarized in Table 1 and Table 2.

! Reservoir Formation Problem || Approach

! Reservoir Formation Problem || Approach

|-

|-

| Reservoir performance is controlled by sedimentary facies, lithology and/or geometry || Determine sedimentary characteristics and origin of facies through [[core description]] and construct predictive models for lateral variability and heterogeneity of reservoir units

| Reservoir performance is controlled by sedimentary facies, lithology and/or geometry || Determine sedimentary characteristics and origin of facies through [[core description]] and construct predictive models for [[lateral]] variability and heterogeneity of reservoir units

|-

|-

| Reservoir performance and formation permeabilities are negatively impacted by presence of [http://dictionary.reference.com/browse/detrital detrital] or [http://dictionary.reference.com/browse/authigenic authigenic] clays (very common) || Identify and map reservoir facies with the least detrital clay content; avoid treatment of formation by [[Stimulation#Acidizing|acidization]] or injection of any fluids; use [[Drilling_fluid#Oil-based_muds|oil-based muds]]; enhance recovery through [[Stimulation#Hydraulic_fracturing|artificial fracturing]] of the formation<ref name=pt06r93>Moslow, T. F., and R. W. Tillman, 1986, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0250/0271.htm Sedimentary facies and reservoir characteristics of Frontier Formation sandstones, southwestern Wyoming], in C. W. Spencer, and R. F. Mast, eds., Geology of Tight Gas Reservoirs: AAPG Studies in Geology Series 24, p. 271–295.</ref>

| Reservoir performance and formation permeabilities are negatively impacted by presence of [http://dictionary.reference.com/browse/detrital detrital] or [http://dictionary.reference.com/browse/authigenic authigenic] clays (very common) || Identify and map reservoir facies with the least detrital clay content; avoid treatment of formation by [[Stimulation#Acidizing|acidization]] or injection of any fluids; use [[Drilling_fluid#Oil-based_muds|oil-based muds]]; enhance recovery through [[Stimulation#Hydraulic_fracturing|artificial fracturing]] of the formation<ref name=pt06r93>Moslow, T. F., and R. W. Tillman, 1986, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0250/0271.htm Sedimentary facies and reservoir characteristics of Frontier Formation sandstones, southwestern Wyoming], in C. W. Spencer, and R. F. Mast, eds., Geology of Tight Gas Reservoirs: AAPG Studies in Geology Series 24, p. 271–295.</ref>

| Best to have fractures of one orientation than multiple intersecting trends

| Best to have fractures of one orientation than multiple intersecting trends

|}

|}

==Evaluation==

==Evaluation==

[[file:evaluating-fractured-reservoirs_fig2.png|300px|thumb|{{figure number|2}}Schematic of typical features to measure and record in core analysis. © van Golf-Racht, 1982; courtesy of Elsevier. Typical recording format from <ref name=pt06r108>Reiss, L. H., 1980, The reservoir engineering aspects of fractured formations: Houston, TX, Gulf Publishing Company, 108 p.</ref>; courtesy of Gulf Publishing Co.]]

[[file:evaluating-fractured-reservoirs_fig2.png|300px|thumb|{{figure number|2}}Schematic of typical features to measure and record in core analysis. © van Golf-Racht, 1982; courtesy of Elsevier. Typical recording format from <ref name=pt06r108>Reiss, L. H., 1980, The reservoir engineering aspects of fractured formations: Houston, TX, Gulf Publishing Company, 108 p.</ref>; courtesy of Gulf Publishing Co.]]

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:<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]]).

# Have the core analyses ([[porosity]] and [[permeability]]) data in hand while observing the core to determine the relative effect of the features observed. 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. 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 [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 [http://www.thefreedictionary.com/radiograph X-radiographs] or core computer [http://www.merriam-webster.com/medical/tomography 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 [http://www.thefreedictionary.com/radiograph X-radiographs] or core computer [http://www.merriam-webster.com/medical/tomography 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 [[Dipmeter]].)

# 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., and S. M. Luthi, 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 [[Dipmeter]].)

# 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., and S. L. Dean, 1985, Hackle plume geometry and joint propagation dynamics, in O. Stephansson, 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 [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=proppant 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 [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=proppant 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 [[Unconformity|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 [[Unconformity|unconformities]].

# Observe the width and width variation of the fractures. Measurements of width could be made with a scale, [http://www.merriam-webster.com/dictionary/micrometer micrometer], or caliper 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, [http://www.merriam-webster.com/dictionary/micrometer micrometer], or caliper 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., and I. Lerche, 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., and J. H. Howard, 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 [[Wikipedia:Stress (mechanics)|stress]] directions and the origin and continuity of the fracture system(s).<ref name=pt06r135 /><ref name=pt06r95 />

# Determine principal [[Wikipedia:Stress (mechanics)|stress]] directions and the origin and continuity of the fracture system(s).<ref name=pt06r135 /><ref name=pt06r95 />

# Determine the relative timing of [[deformation]]al 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 [[deformation]]al 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>

[[Category:Geological methods]]

[[Category:Geological methods]]