Sca 2001-62.pdf

The fluid distribution as a function of height in transition zones is often very complex. This may be due to movement of water-oil contact, tilting of the reservoir at some point in time, leak of fluid out of the reservoir zone or complex inflow during secondary migration. The resultant fluid distribution seen in saturation logs may be difficult to model. In this paper we address the changes in fluid distribution versus height, inferred by changes of fluid distribution due to the movement of water-oil contact only. The experimental procedures for determining capillary pressure are based on fluid saturation monitoring by gamma absorption from centrifuge experiments. An analytical capillary pressure-saturation model was fit to the bounding imbibition capillary pressuresaturation data. The drainage-imbibition hysteresis curves were then constructed assuming that these curves have similar shape to that of the bounding imbibition curve. The imbibition hysteresis model proposed may be used to calculate fluid saturation in the reservoir due to the movement of water-oil contact. We also proposed necessary auxiliary equations to solve the new linear and four-parameter (sigmoidal type) initial-residual fluid saturation equations. Thus once the shape of the bounding-imbibition capillary pressuresaturation curve and maximum non-wetting fluid saturation are known one can easily construct any imbibition hysteresis curves that may be required. INTRODUCTION The potential and importance of transition zones are discussed in a number of recent and old papers. Oil transition zone is a zone above the water-oil contact where the water saturation (Sw) is above the irreducible water saturation. In many cases recovery from transition zones are not economical, however for a low permeability reservoir, entire reservoir or a substantial part of it might become a potential target for recovery. In this paper, we present a new initial (Snwi) -residual (Snwr) non-wetting fluid correlation that can be used to construct imbibition capillary pressure (Pc) hysteresis curves. No attempt has been made to model imbibition relative permeability (kro/krw) curves. However if no data is available, a first estimate can be obtained by using Corey-Burdine correlation. No data has been collected for secondary drainage. Further capillary pressure hysteresis has been reported to be a closed loop type hysteresis curves. Tan notes that Killough's Pc hysteresis model is suitable for specific reservoir types where the imbibition and drainage curves meet at the same residual (non-wetting) fluid saturation. Kleppe et al criticised the formulation of Killough's model since it is based on Land's initial-residual non-wetting fluid correlation.