Temporal and spatial development of scale formation: One-dimensional hydrogeochemical transport modeling

Abstract

Abstract Seawater injection is commonly applied for reservoir pressure maintenance even though it may cause scaling. The admixture of injected seawater to reservoir aquifers triggers a series of complex hydrogeochemical water–rock–gas interactions leading to scale formation within the aquifer and also at the location of the production wells. Basically, the fraction of seawater in the produced water depends on the prevailing hydraulic flow conditions, and determines the type and amount of minerals precipitated or dissolved. To quantify such processes with temporal and spatial resolution, we developed a one-dimensional hydrogeochemical transport model that relies on chemical equilibrium thermodynamics and that additionally considers temporal and spatial aspects in contrast to a batch modeling approach. Our test site is the Miller oilfield, UK North Sea, where hydrogeochemical interactions achieve near-equilibrium conditions within a reaction time span of less than 2 years. Our modeling results for the Miller field test site show a fairly good accordance between (1) the modeled and measured temporal compositional development of produced water as well as between (2) the observed and modeled composition of the scale mineral assemblage formed in the production well. This validates that our one-dimensional hydrogeochemical transport model is capable of reproducing the simultaneously occurring and coupled hydraulic (fluid flow) and hydrogeochemical processes (water–rock–gas interactions). Besides identification and quantitative prediction of mineral dissolution and precipitation, the transport model allows us to determine where such processes occur within the reservoir and/or at the location of the wells. In the Miller field test site, primary calcite and microcrystalline quartz dissolve close to the injection well, which could enhance the secondary porosity locally within the reservoir aquifer. In contrast, massive formation of scale minerals (strontium bearing barite is greatly favored over calcite and microcrystalline quartz) occurs close to or in the production well. Moreover, specific scale minerals (mainly strontium bearing barite in the test site) precipitate along the flow path of formation water–seawater mixtures. Additionally, the modeling results demonstrate that batch modeling and calculation of mineral saturation indices based on one original seawater analysis and one formation water analysis are incapable of predicting which scale minerals actually form.