Page 198 - Well Control for Completions and Interventions

Completion, Workover, and Intervention Fluids 191

exceed formation pressure by the required margin. That margin (overbal-
ance) is typically between 200 and 300 psi (14 20 bar).
To enable brine density to be calculated, both the vertical depth
(TVD) to the reservoir and the reservoir pressure must be known.
Eq. (5.1) is used to find the brine density necessary to overbalance reser-
voir pressure by the desired amount.

ReservoirpressureðpsiÞ1requiredoverbalanceðpsiÞ
DensityðppgÞ5
0:0523verticaldepthðfeetÞ
ReservoirpressureðbarÞ1requiredoverbalanceðbarÞ
DensityðSGÞ510:23
verticaldepthðmÞ
(5.1)

5.3.1 Adjusting brine density for wellbore
temperature and pressure

Having calculated a surface brine density, it must be adjusted to correct for
temperature and pressure in the wellbore. Brine expands when heated,
lowering the density. Since the wellbore temperature is, almost without
exception, higher than ambient surface temperature, this density decrease
must be calculated and surface brine weight increased to compensate.
Pressure has the opposite effect; increased pressure in the wellbore com-
presses the brine, increasing density. Since the density decrease due to
increased temperature is significantly more than any density increase due
to pressure, the net result is always to increase brine weight to adjust for
wellbore conditions. To enable density loss downhole to be calculated, it is
necessary to know the wellbore average temperature and pressure, as well
as the brine expansion and compression coefficients. Expansion and com-
pression coefficients vary considerably and are a function of brine compo-
sition and weight, as well as temperature and pressure. Brine vendors are
usually able to supply the user with accurate expansion and compression
coefficients for any combination of brine weight and brine composition
across a wide range of temperatures and pressures. These vendor supplied
coefficients are based on empirical tests and theoretical models.
In addition to vendor supplied data, expansion and compression coef-
ficients are widely available from society of petroleum engineers (SPE)
papers and technical publications. Two of the most widely quoted exam-
ples are listed in Tables 5.2 and 5.3. These list expansion and compression