Page 39 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

22 David A. Wood and Bin Yuan

2.1 INTRODUCTION

Low-salinity water flooding (LSWF) has become an established
enhanced oil recovery (EOR) method (i.e., improving oil recovery by
5 38%, compared to conventional, high-salinity water flooding), as dem-
onstrated in numerous experimental studies and field trials for both
tertiary (residual oil) and secondary (initial water condition) modes of
water flooding (Bernard, 1967; Hourshad and Jerauld, 2012; Behruz and
Skauge, 2013). Early chemical injection studies of the 1960s and 1970s
established that injecting water with low divalent ion concentrations
tended to improve oil recovery. The pioneering work of Bernard (1967)
identified some key attributes of the outcomes of LSWF: “In general, it
appears that water sensitive cores will produce more oil with a freshwater flood than
with a brine flood. However, the fresh-water flood is accompanied by a lowering of
permeability and the development of a relatively high pressure drop”. That work
also highlighted the complexity of the mechanisms at play. Although the
technique has been applied successfully in many core experiments and
field tests, it does not work in all formations being dependent on many
factors; particularly, the initial wetting conditions, formation water
composition, crude oil composition, and clay mineralogy. Also, LSFW
involves multiple mechanisms impacting the reservoir, and there remains
disagreement and lack of understanding as to which of these are dominant
and which are merely effects rather than causes of incremental oil
recovery associated with LSWF process.

2.2 ORIGINS OF LSWF AND IDENTIFICATION OF
RESERVOIR MECHANISM DRIVING INCREMENTAL OIL
RECOVERY

Changes to reservoir wettability caused by LSWF, and the impact of
fines migration on associated oil recovery, were recognized by Tang and
Morrow (1999). Several specific mechanisms have been proposed to
explain how LSWF actually works. These include: the partial stripping of
fines (Tang and Morrow, 1999); reduction of interfacial tension (IFT)