Page 56 - Handbook Of Multiphase Flow Assurance

Fluid characterization 51

Extreme cases
In rare cases the bicarbonate concentration may reach nearly 10,000 mg/L in some reser-
voirs which have high CO 2 content. This indicates that reservoir water is saturated with CO 2 .
Reservoir water chemistry may change if seawater is introduced. CO 2 will dissolve in seawa-
ter lowering its pH and making it acidic. Acidic seawater can dissolve carbonate minerals in
the reservoir and as pressure drops in or near the wellbore, dissolved minerals can precipitate
out of water solution causing solid scale deposition.

Drilling and wellwork fluids formulation and safety

Salts make the drilling muds and completion or wellwork fluids heavy in order to use hy-
drostatic pressure of the mud to counteract the potentially high pressure of reservoir fluids.
Overbalanced and underbalanced drilling are not in the scope of this work. Salts also help
control hydrate formation which can plug flow paths and stop circulation during wellwork.
All reservoirs differ in their many parameters. The two parameters we review in this sec-
tion are rock consolidation and pressure.
Consolidation defines how strong is the rock and how much pressure it takes to fracture it.
Consolidation of mudstone or sandstone may be weak.
Pressure in some known reservoirs can be very high, up to 29,000 psi or almost 2000 bar.
When well and reservoir pressure dictate the need to prevent hydrate formation yet the
consolidation of the reservoir rock is weak, then the use of salt may create a wellwork fluid
which is too heavy and could lead to an uncontrolled fracture of the weakly consolidated
rock. In extreme cases this is manifested as hydrocarbon seeps at seabed around the wellbore.
To avoid such events, the wellwork fluid has to be formulated with both salts and other hy-
drate inhibitors to maintain both the desired mud weight and the hydrate inhibiting proper-
ties. A specialized series of lab measurements at up to 30,000 psi (2000 bar) were completed at
the Colorado School of Mines (Hu et al., 2017a,b) which provide hydrate stability conditions
with a variety of salts and inhibitors commonly used in wellwork fluids.

Fluid characterization

After the sample quality is verified, a flow assurance specialist can use the PVT report in-
formation to characterize the fluid for predictive use with an equation of state.
The objective for the fluid characterization process is to find the parameters for the selected
equation of state which will accurately predict properties of vapor, liquid and undersaturated
fluid phases along the range of temperature and pressure of interest for the subsequent mul-
tiphase flow assurance analysis. Key predicted parameters which must match the laboratory
measurements closely include density, saturation pressure, gas oil ratio and viscosity.

Fluid properties and measurements
Selection of the equation of state most appropriate for fluid characterization would de-
termine the accuracy of fluid properties calculation. Equations of state are cubic equations
relating pressure (P), volume (V) via compressibility (Z) and temperature (T) in a mixture of