Temperature as Key Factor for Porosity Development in Brine Flooded Chalk at Realistic Reservoir Stress

Abstract

Seawater is injected into the high temperature North Sea chalk reservoirs to improve oil recovery with great success. Recently, a series of laboratory studies at elevated temperatures were performed to test chalk cores while injecting different brines at realistic reservoir effective stress. Four chalk cores sampled from Aalborg (Denmark) were selected for hydrostatic tests above yield point with a following creep phase in standard tri-axial cells. Two chalk cores were tested at 130 °C and flooded with 0.219 M MgCl2 and 0.657M NaCl brines, respectively. Two other samples were flooded with 0.219 M MgCl2 at temperatures of 92°C and 60 °C, respectively. At 130 °C, the MgCl2 flooded core has much higher total strain and more mass loss (16.3%, 4.15 g) compared with the NaCl flooded core (6.63%, 1.51 g). When testing at 92 and 60 °C, the MgCl2 flooded cores have less total strain and mass loss (92 °C: 9.1%, 2.24 g; 60 °C: 6.14%, 1.50 g) compared to 130 °C. The density of the NaCl flooded core is unchanged after the experiment. Densities of the MgCl2 flooded cores, however, are elevated; the density increase correlates with the increase in temperature. Scanning Electron Microscope (SEM) and Mineral Liberation Analysis (MLA) studies show no mineralogical change for the NaCl flooded core. In contrast, magnesium-bearing minerals were formed in the open pore space of both foraminifers and the matrix when flooded with MgCl2. For the MgCl2 flooded cores, it had also been observed that chemical reaction activity decreases with decreasing test temperatures. The porosity calculations are based on bulk volume, core mass and core density after testing. The calculations show that the porosity losses of the MgCl2 flooded cores decrease as temperatures decrease (130 °C: 6.6%, 92 °C: 3.44%; 60 °C: 2.74%). The porosity of the NaCl flooded core (130 °C) decreased by 2.95%, which is comparable to the MgCl2 flooded core at 60 °C. We can state that: 1) Precipitation of magnesium-bearing minerals and calcite dissolution had happened in flooding experiments on Aalborg chalk cores exposed to MgCl2 brine, while there was no mineralogical change in a core flooded with NaCl. 2) Flooding fluid and test temperature affect the porosity development of Aalborg chalk samples at realistic reservoir effective stress. Due to less chemical reactivity at lower temperatures, the porosity development observed for cores exposed to MgCl2 is getting closer to that observed for the one exposed to NaCl.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015