Page 114 - Geology and Geochemistry of Oil and Gas

86 WATER

4. Formation water. Usually, a well producing 1–2% of water is called ‘‘water free;’’
100% water-free wells are rare. Formation water has diverse origins, potential
energy, and chemistry. If this water is fresh or has very low salinity, its origin
could be due to condensation out of the gas phase (if present), when the
formation pressure or the bottomhole pressure drops. Formation water may have
been earlier isolated in hydrodynamic traps (small interbeds, lenses, etc.) or may
have been present throughout the reservoir. When the water coning or fingering
occurs, such water can substantially accelerate the water encroachment process.
5. Intermediate water. The intermediate water is associated with thin interbeds
within an oil/gas-producing interval. When the volume of such water is low it is
possible to assign it to the previous category.
6. Upper water. The upper water is associated with independent overlying aquifers.
7. Lower water. The lower water is associated with independent underlying aquifers.
The identification of water category encountered or expected in an accumulation is
very important. It affects the field development strategy, well pattern, production
techniques, reservoir management, drilling, and workover. These issues are solved
taking into account hydrogeologic conditions not just in a single accumulation but in
the entire field. Especially important is the information about changing hydro-
geologic environment in that field. The information about the pre-development
hydrogeologic (hydrodynamic and hydrochemical) environment is crucial.
Hydrogeologic studies during the petroleum exploration are also very important
and data for solving the aforementioned problems should be collected during this
stage.
Significant issues during the exploration stage include:
(1) the identification of independent hydrogeologic systems in the basin being
studied;
(2) hydrodynamic drives occurring in each one;
(3) their hydrodynamic and geochemical evolution;
(4) the identification of hydrodynamic and geochemical anomalies as indicators of
cross-flows between various horizons;
(5) the distribution of the normalized pressure within the identified hydrogeologic
stages, in the section and laterally;
(6) the identification of locations for possible hydrocarbon traps and patterns in
their distribution; and
(7) analyzing the concepts of hydrocarbon migration in the basin and the formation
and destruction of hydrocarbon accumulations.
As previously mentioned, the saturation pressure (elastic pressure) is an important
exploration indicator of gas fields. If the gas-saturation pressure is close to the
reservoir pressure, any uplifted area of the reservoir may be gas-bearing.