The effect of rock type on natural water flooding and residual oil saturation below free water level and oil water contact: A case study from the Middle East

Abstract

The presence of residual oil below the present day free water level (FWL) and oil water contact (OWC) is common in many fields of the Middle East. This residual oil is seen in reservoirs prior to the start of production.A rock typing study was carried out to investigate the effect of rock types on the presence and distribution of residual oil below the free water level (FWL) and oil water contact (OWC) and potential recovery from ROZ's following water flooding.The rock typing study was carried out by integration of different data sources i.e. core description, thin section study, conventional core analysis (CCAL), special core analysis (SCAL) and petrophysical logs. Six major rock types with unique reservoir quality and behaviour were classified in cored wells and subsequently predicted in un-cored wells. Reservoir quality increases from rock type 1 to 6.The residual oil saturation (Sor) of each rock type was defined and it was concluded that all rock types except for rock type 1 contain residual oil below the present day FWL and OWC. The absence of residual oil in rock type 1 was related to the fact that it is unlikely that oil migration into it ever occurred and consequently no residual oil is seen below the present day FWL and OWC. It was also revealed that rock type 6, with better porosity and permeability, includes the highest residual oil saturation. This was attributed to its bimodal pore throat size distribution and the effect of this on water flooding and hence potential recovery.