Laboratory experiments and numerical modelling of CO2-rich brine injection through sandstone samples: Role of flow rate, brine composition, mixing and spreading effects

Abstract:

The Heletz structure has been selected as a test site for a prospective CO2 reservoir and for the MUSTANG European project injection experiment based on the analysis of the available geological, geophysical and borehole data from various areas of Israel. The Heletz area is located in the Southern part of the Mediterranean Coastal Plain, about 7 km from the sea shore. The target layer is composed of sandstone, has a thickness of around 20 meters and is located at a depth of 1500 m.

Flow-through laboratory experiments of CO2-rich brine were realized to evaluate the chemical processes occurring in the Heletz site.
The Heletz sandstone rock is poorly consolidated and has a high porosity (around 23%) and connectivity.

We performed four flow-through experiments at in situ storage conditions (T = 60ºC, P = 15 MPa, PCO2 =1.8 MPa). The flow rates injection were 0.05 and 0.30 mL.min-1. Two different brine solutions were used, both representative of the Heletz reservoir native water. The first one was a synthetic brine of the Heletz reservoir (closed to seawater). The second one was the first one equilibrated with gypsum.

The results show an increase in permeability for all the percolation experiment whatever the flow rate and the brine solution. This is explained by the dissolution of ankerite, dolomite and feldspar. We observed that the permeability increase is higher and faster for high flow rate injection than for low flow rate. Precipitation of secondary phases is characterized such as gypsum during equilibrated-gypsum brine injection. Secondary clay minerals precipitation is also observed near feldspar dissolution.

Modeling these experiments is a non-trivial task, as some chemical processes are local in space. Reactive transport models have been performed in order to understand and reproduce the observed processes. We have used a water mixing approach which uses the mass mixing fraction between mobile and/or immobile zones. The approach relies in the fact of not transporting species or components, but the containing waters. This allows decoupling chemistry calculation from solute transport modelling.