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A ''fractured reservoir'' is one in which naturally occurring fractures either have or are predicted to have a significant effect on reservoir fluid flow in the form of (1) increased reservoir [[permeability]], (2) increased [[porosity]], and/or (3) increased permeability [http://wiki.seg.org/wiki/Dictionary:Anisotropy anisotropy]. Four basic types of reservoir fractures can be defined ([[:file:evaluating-fractured-reservoirs_fig1.png|Figure 1]]):

A ''fractured reservoir'' is one in which naturally occurring [[fracture]]s either have or are predicted to have a significant effect on reservoir fluid flow in the form of (1) increased reservoir [[permeability]], (2) increased [[porosity]], and/or (3) increased permeability [http://wiki.seg.org/wiki/Dictionary:Anisotropy anisotropy]. Four basic types of reservoir fractures can be defined ([[:file:evaluating-fractured-reservoirs_fig1.png|Figure 1]]):

{{quotation|''Type 1''—Provide the essential porosity and permeability to the reservoir

{{quotation|''Type 1''—Provide the essential porosity and permeability to the reservoir

| Reservoir performance and formation permeabilities are negatively impacted by presence of detrital or authigenic clays (very common) || Identify and map reservoir facies with the least detrital clay content; avoid treatment of formation by acidization or injection of any fluids; use oil-based muds; enhance recovery through artificial fracturing of the formation<ref name=pt06r93>Moslow, T. F., Tillman, R. W., 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 Spencer, C. W., Mast, R. F., 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 detrital or authigenic clays (very common) || Identify and map reservoir facies with the least detrital clay content; avoid treatment of formation by acidization or injection of any fluids; use oil-based muds; enhance recovery through artificial fracturing of the formation<ref name=pt06r93>Moslow, T. F., Tillman, R. W., 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 Spencer, C. W., Mast, R. F., eds., Geology of Tight Gas Reservoirs: AAPG Studies in Geology Series 24, p. 271–295.</ref>

|-

|-

| Reservoir performance is dictated by origin and distribution of natural fractures || Evaluate the relationship between fracture occurrence and lithology<ref name=pt06r100>Pitman, J. K., Sprunt, E. S., 1986, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0200/0221.htm Origin and distribution of fractures in Lower Tertiary and Upper Cretaceous rocks, Piceance basin, Colorado, and their relation to the occurrence of hydrocarbons] in Spencer, C. W., Mast, R. F., eds., Geology of Tight Gas Reservoirs: AAPG Studies in Geology Series 24, p. 221–234.</ref>

| Reservoir performance is dictated by origin and distribution of natural fractures || Evaluate the relationship between [[fracture]] occurrence and lithology<ref name=pt06r100>Pitman, J. K., Sprunt, E. S., 1986, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0200/0221.htm Origin and distribution of fractures in Lower Tertiary and Upper Cretaceous rocks, Piceance basin, Colorado, and their relation to the occurrence of hydrocarbons] in Spencer, C. W., Mast, R. F., eds., Geology of Tight Gas Reservoirs: AAPG Studies in Geology Series 24, p. 221–234.</ref>

|-

|-

| Fracture mineralization impacts reservoir performance. || Determine the nature, origin, and timing of mineralization through petrographical and stable isotope techniques<ref name=pt06r100 />

| Fracture mineralization impacts reservoir performance. || Determine the nature, origin, and timing of mineralization through petrographical and stable isotope techniques<ref name=pt06r100 />

! Type|| Problems

! Type|| Problems

|-

|-

| rowspan = 4 | Type 1. Fractures provide essential porosity and permeability || Rapid decline rate

| rowspan = 4 | Type 1. [[Fracture]]s provide essential porosity and permeability || Rapid decline rate

|-

|-

| Often limited reserves

| Often limited reserves

==Evaluation==

==Evaluation==

When evaluating a fractured reservoir, the analyst must follow these steps:<ref name=pt06r95 />

When evaluating a [[fracture]]d reservoir, the analyst must follow these steps:<ref name=pt06r95 />

* 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, [http://archives.datapages.com/data/specpubs/fieldst4/data/a010/a010/0001/0050/0082.htm Reservoirs in fractured rock] in Stearns, D. W., and Friendman, N., Stratigraphic Oil and Gas Fields--Classification, Exploration Methods and Case Histories: AAPG Memoir 16, p. 82–100.</ref>

* 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, [http://archives.datapages.com/data/specpubs/fieldst4/data/a010/a010/0001/0050/0082.htm Reservoirs in fractured rock] in Stearns, D. W., and Friendman, N., Stratigraphic Oil and Gas Fields--Classification, Exploration Methods and Case Histories: AAPG Memoir 16, p. 82–100.</ref>

==Data acquisition==

==Data acquisition==

The data needed to evaluate a fractured reservoir are commonly obtained from detailed observation of reservoir cores (including their associated well logs) and relevant outcrops. Cores and core-calibrated well logs give the one-dimensional depth and lithological variation in fracturing in one position in the reservoir. Outcrops give appropriate two-dimensional variation of the reservoir in map view within individual layers. Together, a three-dimensional representation of the reservoir can be approximated and correlated with [[production histories]].

The data needed to evaluate a [[fracture]]d reservoir are commonly obtained from detailed observation of reservoir cores (including their associated well logs) and relevant outcrops. Cores and core-calibrated well logs give the one-dimensional depth and lithological variation in fracturing in one position in the reservoir. Outcrops give appropriate two-dimensional variation of the reservoir in map view within individual layers. Together, a three-dimensional representation of the reservoir can be approximated and correlated with [[production histories]].

==Core and well log analysis==

==Core and well log analysis==

[[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 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 fractures 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).

==Outcrop analysis==

==Outcrop analysis==

When collecting fracture data from outcrops, one should look for rocks similar to the reservoir rocks of interest on similar outcropping structures. Look at more of the rock section than just the prospective reservoir rock, including potential sealing beds, and pay attention to relative fracture intensity between layered units.

When collecting [[fracture]] data from outcrops, one should look for rocks similar to the reservoir rocks of interest on similar outcropping structures. Look at more of the rock section than just the prospective reservoir rock, including potential sealing beds, and pay attention to relative fracture intensity between layered units.

* Select outcrop areas having representative fracture patterns for specific structural positions. Record strike and dip data, or at least strike data. Additional sites can be chosen for one unit at different positions within the structure.

* Select outcrop areas having representative fracture patterns for specific structural positions. Record strike and dip data, or at least strike data. Additional sites can be chosen for one unit at different positions within the structure.

[[file:evaluating-fractured-reservoirs_fig4.png|300px|thumb|{{figure number|4}}Total reservoir pore volume due to fractures plotted as a function of fracture width, fracture spacing, and matrix porosity. © Nelson, 1985; courtesy of Gulf Publishing Co.]]

[[file:evaluating-fractured-reservoirs_fig4.png|300px|thumb|{{figure number|4}}Total reservoir pore volume due to fractures plotted as a function of fracture width, fracture spacing, and matrix porosity. © Nelson, 1985; courtesy of Gulf Publishing Co.]]

In doing core and outcrop analyses of fractures to determine reservoir properties and reservoir type, it is often difficult to judge the relative effect of the fracture system. Two crossplots can be used to alleviate this problem ([[:file:evaluating-fractured-reservoirs_fig3.png|Figures 3]] and [[:file:evaluating-fractured-reservoirs_fig4.png|4]]). These plots are the percentage of total reservoir permeability ([[:file:evaluating-fractured-reservoirs_fig3.png|Figure 3]]) and porosity ([[:file:evaluating-fractured-reservoirs_fig4.png|Figure 4]]) as a function of fracture width and fracture spacing for various orders of magnitude of matrix values. Assumptions can be made for width of the fractures at depth. Matrix properties are determined from core analyses, thin sections, etc. and the relative contribution of the fracture system for various spacings can then be read off the graph. Ideally, ranges in values for width and spacing of fractures are used and a box or area created on the graph within which the actual value is likely to occur. These figures assume one set of regularly spaced fractures, hydraulic apertures, and parallel plate laminar flow.<ref name=pt06r95 />

In doing core and outcrop analyses of [[fracture]]s to determine reservoir properties and reservoir type, it is often difficult to judge the relative effect of the fracture system. Two crossplots can be used to alleviate this problem ([[:file:evaluating-fractured-reservoirs_fig3.png|Figures 3]] and [[:file:evaluating-fractured-reservoirs_fig4.png|4]]). These plots are the percentage of total reservoir permeability ([[:file:evaluating-fractured-reservoirs_fig3.png|Figure 3]]) and porosity ([[:file:evaluating-fractured-reservoirs_fig4.png|Figure 4]]) as a function of fracture width and fracture spacing for various orders of magnitude of matrix values. Assumptions can be made for width of the fractures at depth. Matrix properties are determined from core analyses, thin sections, etc. and the relative contribution of the fracture system for various spacings can then be read off the graph. Ideally, ranges in values for width and spacing of fractures are used and a box or area created on the graph within which the actual value is likely to occur. These figures assume one set of regularly spaced fractures, hydraulic apertures, and parallel plate laminar flow.<ref name=pt06r95 />

==Checklist for sequence of study==

==Checklist for sequence of study==