Remaining Oil Investigation in a High Recovery Oilfield
Authors: Burke Michael James , Bremeier Martin , Shebani Mohamed , Xian ChengGang , Dowley Joseph Edward 
Published: February 2016

Residual oil estimations are mainly based on special analysis of representative core samples (SCAL). In high recovery oil fields, where remaining oil saturations approach residual oil saturations, it is possible to test these estimations using Pulsed Neutron Decay (PND) logging to monitor water saturation changes. Such monitoring techniques can identify inconsistencies, leading to possible adjustments in recovery strategies and eventual improvements in ultimate recovery.

The recovery strategy for As Sarah oilfield in Libya has been based on SCAL. PND logging in producing wells has generally confirmed forecast saturations1 and only slight adjustments to the initial strategy have been necessary to achieve a recovery factor greater than 40% at the end of the plateau phase. However, recent infill drilling allowed for the first time an investigation of recovery efficiency in swept, but previously un-drilled, parts of the reservoir. Higher than expected, the remaining oil saturations lead to the suspicion that cased hole saturation logs in producing wells may underestimate overall reservoir oil saturations.

In order to enhance understanding of the recovery process a research program was undertaken to investigate the true, post aquifer sweep, remaining oil saturation. Included in this program are: special core investigations using preserved and restored state core material; analysis of pore size distribution and fluid typing through nuclear magnetic resonance (NMR) measurements; surface and downhole analysis of oil composition verses depth; and finally, a program of production and injection testing to monitor oil saturation changes in a new well, aquifer swept, zone.

The planning and first results of this program are presented in this publication, including available field data. It is expected that the program, once complete, will lead to a better understanding of the depletion mechanism and help improve ultimate recovery of As Sarah field. Some of the methodologies described in this paper may be of general interest in the effort to optimise the recovery of hydrocarbons.

Field Description

The large onshore As Sarah oil field, discovered in 1989, is located in the Sirte basin of eastern Libya. As Sarah field is estimated to contain over 1Billion bbl STOIIP. Present production from As Sarah is around 90,000 BOPD and is nearing the end of its plateau phase.

As Sarah field structure is a northward dipping fault-block, bounded by a major fault to the south. Internal fault compartments are common throughout the field and are in communication. A roughly north-south trending major fault divides the field into eastern and western zones, each with distinct saturation pressures. Observed only in the western part of the field are "tar-mats", comprising 10–20ft thick, highly viscous, oil layers; simulation has confirmed that these tar-mats adversely affect vertical reservoir connectivity. The reservoir comprises clean, fluvial reservoir sands with over 1000ft thickness in places. Porosity ranges from 10% to 14% while permeability ranges up to 1000mD, with an average of approximately 100mD. As Sarah produces under a strong aquifer-drive, directed from the northern flanks and bottom of the field. The low mobility-ratio of oil and water phases (0.47) and generally homogeneous, thick reservoir sequences, indicate oil is swept by a favorable gravity-dominated displacement. As Sarah oil is a light crude of 38° API and formation water has total chlorides in the region of 148,000 mg/l.

SCAL has been performed on a number of occasions throughout As Sarah field's life and these tests have confirmed the reservoir to be more oil than water wet. The results of oil-water relative permeability and capillary pressure measurements have given residual oil saturations ranging between 7% to 38% and irreducible water saturations in the region of 5%. The present As Sarah simulation model uses oil-water relative permeability curves calculated from porous plate capillary pressure measurements using Purcell's method2. It is thought that the ultimate overall recovery factor of As Sarah field will exceed 50%.