Scale formation and deposition in the subsurface and
surface facilities
have been recognized as a major cause of flow assurance issues in
the oil and gas industry. Sulfate-based scales such as sulfates of
calcium (anhydrite and gypsum) and barium (barite) are some of the
commonly encountered scales during hydrocarbon production operations.
Oilfield scales are a well-known flow assurance problem, which occurs
mainly due to the mixing of incompatible brines. Researchers have
largely focused on the rocks’ petrophysical property modifications
(permeability and porosity damage) caused by scale precipitation and
deposition. Little or no attention has been paid to their influence
on the surface charge and wettability of calcite minerals. Thus, this
study investigates the effect of anhydrite and barite scales’
presence on the calcite mineral surface charge and their propensity
to alter the wetting state of calcite minerals. This was achieved
vis-à-vis zeta-potential (ζ-potential) measurement. Furthermore,
two modes of the scale control (slug and continuous injections) using
ethylenediaminetetraacetic acid (EDTA) were examined to determine
the optimal control strategy as well as the optimal inhibitor dosage.
Results showed that the presence of anhydrite and barite scales in
a calcite reservoir affects the colloidal stability of the system,
thus posing a threat of precipitation, which would result in permeability
and porosity damage. Also, the calcite mineral surface charge is affected
by the presence of calcium and barium sulfate scales; however, the
magnitude of change in the surface charge via ζ-potential measurement
is insignificant to cause wettability alteration by the mineral scales.
Slug and continuous injections of EDTA were implemented, with the
optimal scale control strategy being the continuous injection of EDTA
solutions. The optimal dosage of EDTA for anhydrite scale control
is 5 and 1 wt % for the formation water and seawater environments,
respectively. In the case of barite, in both environments, an EDTA
dosage of 1 wt % suffices. Findings from this study not only further
the understanding of the scale effects on calcite mineral systems
but also provide critical insights into the potential of scale formation
and their mechanisms of interactions for better injection planning
and the development of a scale control strategy.