Page 284 - Introduction to Petroleum Engineering

272 RESERVOIR PERFORMANCE
or the development of a world‐class reservoir. The reservoir flow model must include
a representation of the reservoir, which often is based on reservoir characterization
using flow units.

14.1.1 Flow Units
Geological characterization of the reservoir begins by defining flow units. A flow
unit has been defined as “a volume of rock subdivided according to geological and
petrophysical properties that influence the flow of fluids through it” (Ebanks, 1987).
Geologic properties include the texture of the rock, mineral content, sedimentary
structure, location and type of bedding contacts, and location and distribution of per‑
meability barriers. Petrophysical properties include porosity and permeability
distributions, formation compressibility and moduli, and the distribution of fluid sat‑
urations. Ebanks and his colleagues refined the definition to state that a flow unit is
“a mappable portion of the total reservoir within which geological and petrophysical
properties that affect the flow of fluids are consistent and predictably different
from the properties of other reservoir rock volumes” (Ebanks et al., 1993, page 282).
They identified several characteristics of flow units.
A specified volume of the reservoir contains one or more rock types that are
reservoir quality and contain commercial fluids. The specified volume can also
include rock types that are not reservoir quality. For example, the reservoir volume
may include sandstone and conglomerate formations with layers of embedded shale.
Compared to sandstone and conglomerate permeabilities, the permeability of shale is
insignificant. Flow unit zones can be correlated and mapped at a scale that is
comparable to the distance between wells. Flow units in the specified volume may be
in communication with other flow units.
The initial identification of flow units in the reservoir is based on static data. Static
data is considered data that does not change significantly with time and includes the
structure of the reservoir. By contrast, dynamic data is data that changes with time
and includes flow rates and pressure. Flow units identified using static data can be
validated by comparing the identified flow units to dynamic data such as actual flow
measurements in flow tests. It may be necessary to modify the set of flow units to
assure consistency between static and dynamic data.

14.1.2 Reservoir Characterization Using Flow Units
Flow units can be used to characterize the reservoir. The goal is to produce field‐wide
maps of flow units and geologic regions. A stratigraphic and structural framework
provides a 3‐D representation of rock layers. Modern maps are prepared using
geologic modeling software. The resulting geologic or static reservoir model includes
a structure of the reservoir and rock properties distributed throughout the structure.
Rock properties are distributed by subdividing the 3‐D volume into many smaller
volumes called grid blocks, grid cells, or simply blocks or cells. Reservoir and
rock properties include elevations or structure tops, gross thickness, net to gross