Pingo Almada, M. B. (Shell Global Solutions International B.V) | Pieterse, S. G. J. (Shell Global Solutions International B.V) | Marcelis, A H. M. (Shell Global Solutions International B.V) | van Haasterecht, M. J. T. (Shell Global Solutions International B.V) | Brussee, N. J. (Shell Global Solutions International B.V.) | van der Linde, H. A. (Shell Global Solutions International B.V.)
Low salinity flooding (LSF)- decreasing ionic strength to enhance oil production- is an Enhanced Oil Recovery (EOR) process currently being evaluated in industry and academia with first deployment beginning. A wettability modification is assumed to take place when decreasing the ionic strength. In this work we explore the effects of varying salinities from formation water down to very low salinity on brine permeability and on effluent composition. The following effects have been investigated: the presence and absence of oil in the core, the cation exchange capacity (CEC), mineral dissolution and cation stripping.
The experimental component of this investigation consisted of continuous permeability measurements during flooding at various salinity steps and simultaneous collection of the effluent. The effluent was analyzed using Inductively Coupled Plasma (ICP elemental analysis). The CEC's of the rock exposed to the different salinities have also been measured. Scanning Electron Microscope (SEM) visual investigations have also been carried out.
During the flooding with several different brines, permeability variations were observed. The variation of the ionic composition of the effluent has allowed for:
• identification and characterization of the temporary divalent cation stripping process
• the framing of hypotheses about other possible mechanisms taking place in the core during LSF, such as:
o ion exchange between injected brine and the clays as the salinity decreased
o The role that CEC plays in the re-equilibration with the new salinity
o the CEC variation throughout the experiment at Sor
o mineral dissolution and clay deflocculation.
The comprehensive suite of tools and techniques used here has given more insights into the mechanisms taking place when decreasing the ionic strength and their use can serve to improve the deployment of the technology, including the prevention of formation damage.
The role of the injection brine composition in oil recovery efficiency has been increasingly investigated in recent years. In several cases there is evidence of incremental oil after injection of a lower salinity brine in sandstones (Vledder et al, 2012, Seccombe et al, 2010), or when modified salinity brine is injected in carbonates (Yousef et al,2012; Romanuka et al , 2012); while in other cases no effect has been observed (i.e. Snorre Field, Skrettingland et al., 2011). The reason for the wide response is still to be elucidated but it is clear that it is related to the several factors underlying the microscopic mechanisms. It is clear that the low salinity brine needs to have a salinity low enough to yield extra recovery and at the same time high enough to prevent any formation damage (FD) by clay swelling. Typical low salinity concentrations are in the ranges of 1000 to 2000 mg/l TDS while the specific ranges are investigated individually per field.
This paper addresses salinity ranges below 6000 ppm TDS that are relatively low. Extensive work on the relationship between clay swelling/deflocculation and brine composition was carried out by Scheuerman et al. in the 1990's, which led to Injection-Water guidelines based on the compatibility of injection water and formation clays. A FD prevention criteria was outlined based on the fraction of total divalent cations needed in injection water to avoid clay deflocculation and hence permeability impairment.