Residual Oil Saturations Determined by Core Analysis

Abstract

ABSTRACT Residual oil saturation determined by several laboratory techniques on core plug specimens from a variety of lithologies in the Asia-Pacific region have been correlated to porosity, initial water saturation and permeability. To some extent, residual oil decreased with increasing initial water saturation and decreasing permeability. For the same mean hydraulic radius () residual oil left after gas-drive was much higher than that for a water-drive. On the average, residual oil saturation fron steady-state water-oil relative permeability data were higher than those fron the unsteady-state floods. This is an apparent reflection of the fact that steady-state floods are typically conducted at lower flow-rates than unsteady-state floods. INTRODUCTION The principal techniques for determination of residual oil as outlined below have been adapted a summary given elsewhere1:Measurement in-situ with logs.Measurements on cores taken with pressure core barrel in a depleted section of the reservoir.Measurements on non-pressured cores.special core analysis tests. Residual oil saturations determined by methods 1, 2 and 3 above are influenced by the degree of flushing during drilling which is in turn subject to numerous factors2,3,4, some of which are difficult to control. Saturations derived from method 3 are additionally affected by pressure drop within the core on the way to the surface. Thus these methods often produce values of residual oil saturations which are considered too low5,6,7. The main purpose of this paper is to investigate residual oil saturation data obtained by method 4. Although it has long been recognised that core measurements in the laboratory may be affected by the coring process8, core handling9, sample preparation10,11 and test technique12,13, such data are widely used in the industry. This is because if sufficient care and scrutiny are applied at each stage of the process14,15,16,17,18,19, results obtained can be successfully integrated with other data needed for interpretation and development of the reservoir20,21,22,23,24,25. PARAMETERS INVESTIGATED Test Type Residual oil saturations obtained from the following test procedures are compared:Gas-Oil Relative Permeability (Unsteady-state)Water-Oil Relative Permeability (Unsteady-State)Waterflood-Room Conditions (Unsteady-State)Water-Oil Relative Permeability (Unsteady-State)Waterflood-Reservoir Conditions (Unsteady-State)Gas-Oil Relative Permeability (by Centrifuge)Inbibition Water-Oil Capillary Pressure (by Centrifuge) The tests vary according to the production mechanism they represent. the type of data produced and the degree to which they attempt to simulate actual reservoir conditions. Test Type Residual oil saturations obtained from the following test procedures are compared:Gas-Oil Relative Permeability (Unsteady-state)Water-Oil Relative Permeability (Unsteady-State)Waterflood-Room Conditions (Unsteady-State)Water-Oil Relative Permeability (Unsteady-State)Waterflood-Reservoir Conditions (Unsteady-State)Gas-Oil Relative Permeability (by Centrifuge)Inbibition Water-Oil Capillary Pressure (by Centrifuge) The tests vary according to the production mechanism they represent. the type of data produced and the degree to which they attempt to simulate actual reservoir conditions.