Characterization of mineral precipitation in artificially fractured chalk during flooding experiments for IOR research

Abstract

Natural fractures in carbonate reservoirs are essential to effectively transport hydrocarbon in an otherwise low permeable matrix. However, they can reduce the wanted effect of water injection and Improved Oil Recovery (IOR), as the injected water mainly runs through the fractures. We have investigated chemical and textural alteration in and adjacent to an artificial fracture with an aperture of 2.25 (±0.05) mm in Upper Cretaceous chalk from the St. Vaast Formation (Mons Basin, Belgium). The sample has been injected mainly with 0.219 M MgCl₂ under reservoir conditions over two months. The fracture aperture reduced averagely by 76%. The analysis performed by Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) show that, magnesite is the dominating precipitate in the fracture with sizes ranging from 0.5 to 5 μm. The abundance of magnesium phases in the fracture fill was also confirmed by Scanning Electron Microscopy (SEM) coupled with Energy-dispersive X-ray spectroscopy (EDS) and bulk geochemistry (Inductively Coupled Plasma Mass Spectrometry ICP-MS). Besides magnesite, phyllosilicates were observed by TEM and Helium Ion Microscopy combined with Secondary Ion Mass Spectrometry (HIM-SIMS). The newly formed material, precipitates, and possible reworked minerals are characterized by magnesite crystals being covered occasionally by phyllosilicates. This material has formed a distinct area bounded to the surrounding matrix. The final diameter of the precipitate measures about 2.75 (±0.15) mm, which indicates an incremental widening of the original fracture aperture. Calcite along fracture wall dissolved. Effluent concentration supports dissolution by an increase in Ca2⁺ and precipitation of magnesite by the loss of Mg2+. As either silica or aluminium is a component of the fluids, the origin of these elements is necessarily from the matrix of the chalk. It is yet unknown if phyllosilicates precipitated or was reworked, or both. Formation and concentration of magnesite and phyllosilicates in reservoir fractures can reduce fracture permeability over time and lead to subsequent propagation through the matrix of the reservoir rock. This in-depth mineralogical study of artificial fracture changes has the utmost importance for the implementation of techniques in fractured reservoirs.