Fundamentals of wettability applied to Brazilian Pre-Salt reservoirs and wettability alteration evaluation in low salinity water injection
Abstract
Brazilian Pre-Salt carbonate reservoirs have been one of the greatest oil discoveries
of the past decade. In fields such as Lula, it is expected to recover between five to
eight billion barrels of oil equivalent. To fulfil this, knowledge of reservoir wettability
is fundamental, given that it dictates how fluids (water, oil and, gas) are distributed
within the porous media, and how the multiphase flow occurs in the pore-network.
This information is important in the construction of relative permeability curves which
are input for reservoir simulators. Few works have been published regarding initial
wettability of Pre-Salt carbonate reservoirs in spite of their great oil recovery potential.
In this way, to close the data gap in the literature, this study aimed to investigate
parameters that influence initial wettability of pure rocks (calcite and quartz) and
analogue rocks (coquinas) that partly represent the Pre-Salt reservoirs.
Contact angle experiments were used to study the influence of parameters that are
important to rock wettability, such as rock composition, brine salinity, temperature,
rock ageing and, presence of carbon dioxide in brine. Initial wettability was analysed at
core scale by spontaneous imbibition and core flood experiments, obtaining the Amott
index to water. Through these experiments, it was possible to evaluate the performance
of different brines respective to oil recovery. Furthermore, wettability alteration was
investigated in low salinity water injection as a tertiary injection method. Contact
angles experiments were also used to analyse wettability alteration in clay-less rocks,
focusing on understanding the role of fluid-fluid interactions on low salinity water
injection.
Respective to initial wettability in pure rocks and rock analogues relevant to the
Brazilian Pre-Salt, it was found that at reservoir temperature (60°C) calcite, and quartz
were neutral and weakly oil-wet, respectively. Amott index to water indicated that
at that temperature, coquina rocks were intermediate-wet (mixed-wet). Presence of
carbon dioxide in brine indicated that both calcite and quartz rocks become more
water-wet. Regarding the application of wettability to low salinity water injection,
the results suggested that wettability alteration is not exclusive of clayey rocks, nor
resulting from a change in the ratio of divalent to monovalent cations. They indicated
that crude oil composition is crucial for wettability alteration to happen, and it is
possible that crude-oil/brine interactions dictate the low salinity effect.
In summary, this work generated important data regarding initial wettability of
Brazilian Pre-Salt reservoirs which are scarce in the literature. These results also
instigate further investigation respective to the effects of carbon dioxide in brine on
the initial wettability of rocks considering complex brines (presence of divalent cations
and anions) rather than just brines composed of sodium chloride. This work showed
that crude-oil/brine interactions is important for the low salinity effect.