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Petroleum geologists and hydrologists have long recognized the need to define quasi-geological/petrophysical units to formalize their descriptions of rock strata as storage containers and conduits for flow of fluids. Maxey<ref name=pt06r83>Maxey, G. B., 1964, Hydrostratigraphic units: Journal of Hydrology, v. 2, p. 124–129., 10., 1016/0022-1694(64)90023-X</ref> even proposed the introduction of the term ''hydrostratigraphic unit'' into the Code of Stratigraphic Nomenclature to fulfill this need. Other terms that have been introduced include ''reservoir facies'' (Langston and Chin, 1968), ''reservoir unit''<ref name=pt06r98>Pettijohn, F. J., Potter, P. E., Siever, R. 1973, Sand and Sandstone: New York, Springer-Verlag, 618 p.</ref>, ''flow unit''<ref name=pt06r51>Hearn, C. L., Ebanks, W. J. Jr., Tye, R. S., Ranganathan, V. 1984, Geological factors influencing reservoir performance of the Hartzog Draw field: Journal of Petroleum Technology, v. 36, Aug., p. 1335–1344., 10., 2118/12016-PA</ref><ref name=pt06r31>Ebanks, W. J., Jr., 1987, Flow unit concept—integrated approach to reservoir description for engineering projects, abst.: AAPG Bulletin, v. 71, n. 5, p. 551–552.</ref>, and ''lithohydraulic unit''<ref name=pt06r67>Krause, F. F., Collins, H. N., Nelson, D. A., Mochemer, S. D., French, P. R., 1987, Multiscale anatomy of a reservoir— geological characterization of Pembina-Cardium pool, west-central Alberta, Canada: AAPG Bulletin, v. 71, p. 1233–2260.</ref>.

Petroleum geologists and hydrologists have long recognized the need to define quasi-geological/petrophysical units to formalize their descriptions of rock strata as storage containers and conduits for flow of fluids. Maxey<ref name=pt06r83>Maxey, G. B., 1964, Hydrostratigraphic units: Journal of Hydrology, v. 2, p. 124–129., 10., 1016/0022-1694(64)90023-X</ref> even proposed the introduction of the term ''hydrostratigraphic unit'' into the Code of Stratigraphic Nomenclature to fulfill this need. Other terms that have been introduced include ''reservoir facies,''<ref name=Langston_and_Chin_1968>Langston, J. R., and G. E. Chin, 1968, Rainbow Member facies and related reservoir properties, Rainbow Lake, Alberta: AAPG Bulletin, v. 52, n. 10, p. 1925-1955.</ref> ''reservoir unit,''<ref name=pt06r98>Pettijohn, F. J., Potter, P. E., Siever, R. 1973, Sand and Sandstone: New York, Springer-Verlag, 618 p.</ref> ''flow unit,''<ref name=pt06r51>Hearn, C. L., Ebanks, W. J. Jr., Tye, R. S., Ranganathan, V. 1984, Geological factors influencing reservoir performance of the Hartzog Draw field: Journal of Petroleum Technology, v. 36, Aug., p. 1335–1344., 10., 2118/12016-PA</ref><ref name=pt06r31>Ebanks, W. J., Jr., 1987, Flow unit concept—integrated approach to reservoir description for engineering projects, abst.: AAPG Bulletin, v. 71, n. 5, p. 551–552.</ref> and ''lithohydraulic unit.''<ref name=pt06r67>Krause, F. F., Collins, H. N., Nelson, D. A., Mochemer, S. D., French, P. R., 1987, Multiscale anatomy of a reservoir— geological characterization of Pembina-Cardium pool, west-central Alberta, Canada: AAPG Bulletin, v. 71, p. 1233–2260.</ref>

This chapter outlines one approach to zonation of a reservoir for modeling and prediction of performance—the flow unit concept. The subdivision of a reservoir into flow units provides a practical means for reservoir zonation that makes use of both geological and petrophysical data representing heterogeneity observed at several scales (see [[Geological heterogeneities]]).

This article outlines one approach to zonation of a reservoir for modeling and prediction of performance—the flow unit concept. The subdivision of a reservoir into flow units provides a practical means for reservoir zonation that makes use of both geological and petrophysical data representing heterogeneity observed at several scales (see [[Geological heterogeneities]]).

==Definition and characteristics of flow units==

==Definition and characteristics of flow units==

There is no universally applicable set of rules by which to define flow units. Dividing a reservoir into flow units requires an integration of stratigraphic, sedimentological, structural, petrographic, petrophysical, and field performance data. The process is summarized as follows (Figure 1):

There is no universally applicable set of rules by which to define flow units. Dividing a reservoir into flow units requires an integration of stratigraphic, sedimentological, structural, petrographic, petrophysical, and field performance data. The process is summarized as follows (Figure 1):

* Identify the major lithofacies, vertical sequences, and deposirional environments from available core. Relate lithofacies, at the whole-core scale, to their mineralogical, textural, and pore level properties and to permeability, [[porosity]], fluid saturations, and capillarity as measured on core plugs. Establish consistent relationships between rock properties and petrophysical properties.

* Identify the major lithofacies, vertical sequences, and depositional environments from available core. Relate lithofacies, at the whole-core scale, to their mineralogical, textural, and pore level properties and to permeability, [[porosity]], fluid saturations, and capillarity as measured on core plugs. Establish consistent relationships between rock properties and petrophysical properties.

* Determine what lithofacies, or associations of lithofacies, are probable flow units based on petrophysical properties, changes in texture, cementation, fracture density, differences in sedimentary structures or bedding styles, and/or separations by prominent shales or other features that may bear on fluid distribution and flow.

* Determine what lithofacies, or associations of lithofacies, are probable flow units based on petrophysical properties, changes in texture, cementation, fracture density, differences in sedimentary structures or bedding styles, and/or separations by prominent shales or other features that may bear on fluid distribution and flow.

* Calibrate wireline log response to major rock types in as much detail as possible and with appropriate depth shifting of core to logs, in order to detect changes quantitatively in flow unit quality and to correlate major flow units to uncored wells. If cores are not available, cuttings, sidewall cores, patterns of textural change inferred from log signatures, cementation or shales detected on logs, downhole images of the borehole wall, microscanner logs, or other such information must be used in place of core (see Part 4 on Wireline Methods).

* Calibrate wireline log response to major rock types in as much detail as possible and with appropriate depth shifting of core to logs, in order to detect changes quantitatively in flow unit quality and to correlate major flow units to uncored wells. If cores are not available, cuttings, sidewall cores, patterns of textural change inferred from log signatures, cementation or shales detected on logs, downhole images of the borehole wall, microscanner logs, or other such information must be used in place of core (see [[Wireline methods]]).

* Establish the three-dimensional distribution of flow units by correlation of calibrated wireline logs. Knowledge of environments of deposition of the reservoir sequence is important to interpreting the style of correlation to be used and the expected patterns of external and internal geometry of any flow unit (see “Lithofacies and Environmental Analysis of Clastic Depositional Systems”). During correlation, the flow unit zonation established in individual cored wells may change somewhat. Tying correlation horizons around a loop is critical because individual correlation sections alone can be deceptive.

* Establish the three-dimensional distribution of flow units by correlation of calibrated wireline logs. Knowledge of environments of deposition of the reservoir sequence is important to interpreting the style of correlation to be used and the expected patterns of external and internal geometry of any flow unit (see [[Lithofacies and environmental analysis of clastic depositional systems]]). During correlation, the flow unit zonation established in individual cored wells may change somewhat. Tying correlation horizons around a loop is critical because individual correlation sections alone can be deceptive.

* Test the validity of flow units established by consideration of production logs (see Part 9), flow tests of small intervals, oil and water geochemistry (see Part 5), repeat formation tester (RFT) surveys (see Part 4), injectivity logs, tracer surveys, and any available data on patterns of production through time. Modify the flow unit definitions as needed to accommodate the physical measurements of flow, if a rationale can be found for the differences.

* Test the validity of flow units established by consideration of production logs (see [[Production engineering methods]]), flow tests of small intervals, oil and water geochemistry (see [[Laboratory methods]]), repeat formation tester (RFT) surveys (see [[Wireline methods]]), injectivity logs, tracer surveys, and any available data on patterns of production through time. Modify the flow unit definitions as needed to accommodate the physical measurements of flow, if a rationale can be found for the differences.

[[file:flow-units-for-reservoir-characterization_fig1.png|thumb|{{figure number|1}}Major types of geological and petrophysical data applied to flow unit zonation of a well. Here the flow units are delineated on the basis of permeability contrasts due to lithofacies changes and to the presence of a laterally continuous barrier to vertical permeability (flow unit 4). According to the definition of flow unit, it is permissible to define a flow unit that exhibits only weak flow or no flow through it. This property of flow units makes it possible to use a single numbering system for identifying both obvious flow units and probable permeability barriers that can be mapped at the same scale as reservoir quality flow units.]]

[[file:flow-units-for-reservoir-characterization_fig1.png|thumb|{{figure number|1}}Major types of geological and petrophysical data applied to flow unit zonation of a well. Here the flow units are delineated on the basis of permeability contrasts due to lithofacies changes and to the presence of a laterally continuous barrier to vertical permeability (flow unit 4). According to the definition of flow unit, it is permissible to define a flow unit that exhibits only weak flow or no flow through it. This property of flow units makes it possible to use a single numbering system for identifying both obvious flow units and probable permeability barriers that can be mapped at the same scale as reservoir quality flow units.]]

The distribution of petrophysical properties such as porosity and permeability can be mapped within flow units using well control only or by applying geostatistical procedures to create stochastic realizations of these distributions “conditioned” on the well data (see Part 6). Geostatistical techniques that have a strong stochastic component are consistent with, and complementary to, the flow unit concept, which is itself mostly deterministic.

The distribution of petrophysical properties such as porosity and permeability can be mapped within flow units using well control only or by applying geostatistical procedures to create stochastic realizations of these distributions “conditioned” on the well data (see [[Geological methods]]). Geostatistical techniques that have a strong stochastic component are consistent with, and complementary to, the flow unit concept, which is itself mostly deterministic.

==Examples of application of flow units==

==Examples of application of flow units==

|   Hartzog Draw, Wyoming

|   Hartzog Draw, Wyoming

| Flow units applied to reservoir characterization of shelf sandstones for an EOF) project

| Flow units applied to reservoir characterization of shelf sandstones for an EOF) project

| Hern et al., 1984

| <ref name=Hearn_et_al._1984>Hearn, C. L., W. J. Ebanks Jr., R. S. Tye, and V. Ranganathan, 1984, Geological factors influencing reservoir performance of the Hartzog Draw field: Journalof Petroleum Technology, v. 36, Aug., p. 1335-1344.</ref>

|-

|-

|   Pembina, Alberta

|   Pembina, Alberta

| Spraberry Trend, Midland basin, Texas

| Spraberry Trend, Midland basin, Texas

| Facies architecture and petrophysical properties of submarine fan reservoirs

| Facies architecture and petrophysical properties of submarine fan reservoirs

| <ref name=pt06r43>Guevara, E. H., 1988, Geological characterization of Permian submarine fan reservoirs of the Driver Waterflood Unit, Spraberry Trend, Midland Basin, Texas: The Univ. of Texas Bureau of Economic Geology Report of Investigations, n. 172, 44 pp.</ref> ; <ref name=pt06r143>Tyler, N., Gholston, J. C., 1988, Heterogeneous deep-sea fan reservoirs, Shakelford and Preston waterflood units, Spraberry Trend, West Texas: The Univ. of Texas Bureau of Economic Geology Report of Investigations, n. 171, 38 p.</ref>

| <ref name=pt06r43>Guevara, E. H., 1988, Geological characterization of Permian submarine fan reservoirs of the Driver Waterflood Unit, Spraberry Trend, Midland Basin, Texas: The Univ. of Texas Bureau of Economic Geology Report of Investigations, n. 172, 44 pp.</ref><ref name=pt06r143>Tyler, N., Gholston, J. C., 1988, Heterogeneous deep-sea fan reservoirs, Shakelford and Preston waterflood units, Spraberry Trend, West Texas: The Univ. of Texas Bureau of Economic Geology Report of Investigations, n. 171, 38 p.</ref>

|}

|}

[[file:flow-units-for-reservoir-characterization_fig2.png|thumb|{{figure number|2}}Some examples of lithofacies and flow unit subdivisions of clastic and carbonate reservoirs. (a) Lithofacies and (b) flow unit subdivision of the Shannon Sandstone body in the Hartzog Draw field, Powder River basin, Wyoming. (Modified from <ref name=pt06r51 />.) (c) Lithofacies and (d) reservoir facies (flow unit) subdivision of the Rainbow Lake reef reservoir (“A” Pool), Alberta, Canada. (Modified from Langston and Chin, 1968.)]]

[[file:flow-units-for-reservoir-characterization_fig2.png|thumb|{{figure number|2}}Some examples of lithofacies and flow unit subdivisions of clastic and carbonate reservoirs. (a) Lithofacies and (b) flow unit subdivision of the Shannon Sandstone body in the Hartzog Draw field, Powder River basin, Wyoming. (Modified from <ref name=pt06r51 />.) (c) Lithofacies and (d) reservoir facies (flow unit) subdivision of the Rainbow Lake reef reservoir (“A” Pool), Alberta, Canada. (Modified from <ref name=Langston_and_Chin_1968 />.)]]

==See also==

==See also==