Pore-scale modelling of in situ geochemical reactions

Abstract

The key objective of this work is to develop a framework to predict, design and interpret the chemistry and physics relating to rock geochemistry and scale precipitation in porous media. The interest is to understand the impact of mineral scale deposition on the produced reservoir fluid compositions and on the petrophysical properties of the rocks they occur in. In the bid to achieve these aims, the first step adopted is to account for the propagation/transport of chemical species within a pore network model used in modelling the reservoir rock. Afterwards, chemical interactions amongst the species are incorporated into the model. Subsequent steps look at mechanisms of the fluid flow at the pore scale and the impact of mineral deposition on subsequent flow distribution and on composition of chemical species downstream of the flow. A pore network simulator capable of predicting mineral scale deposition in various conditions was developed to achieve the goals of this study. Transport of tracers is incorporated into the simulator and the improved model is validated by comparison with related observations in the literature and other sensitivity studies. The sensitivity studies also provide valuable insights into the physics of scale deposition on pore walls of reservoir rocks. Furthermore, in situ reactions and permeability decline due to barite scale deposition were investigated using the developed model. Raw and de-sulphated seawater injections were simulated. The pattern of permeability impairment with time was monitored. Additionally, the effluent chemical concentration profile was analysed. A novel ‘Delay Factor’ was developed to help understand the retardation of the breakthrough of chemical species at the production well due to ion stripping taking place in situ. Finally the impact of rock heterogeneity on the process of in situ reaction and mineral scale deposition was explored. The degree of accuracy in scale prediction hinges partly on sound understanding of rock structural parameters and their corresponding controls on fluid flow. Results underscore the fact that the damage deep in a reservoir due to one pore volume displacement of formation water with seawater is negligible. The reduction in permeability only becomes significant when several pore volumes of brine containing barium are flowing simultaneously and mixing with the sulphate rich brine. Results also suggests that rock parameters, such as pore connectivity, may impact on the extent of stripping taking place in situ when they are considered in networks with narrow pore size distributions. In addition, analysis of the saturation ratios suggests that risk of downstream scaling may be higher in rocks with permeability heterogeneity.