Analysis and Modeling of Flow Processes During Sponge Coring
Authors: van Lingen P.P. , Schermer P.J.C. , Barzandji O.H.M. , van Kruijsdijk C.P.J.W. , Sonet P. 
Published: December 2007

Abstract
Sponge coring is a relatively cheap, direct, method to measure local oil saturations. Although the basic concept is simple - oil which flows from the core is collected in the enclosing sponge material- the actual flow processes which take place are complicated, and have not been fully analyzed to date.
In this paper, we develop a practical model of the sponge coring process, which is ideally suited for engineering purposes. The model combines the spontaneous imbibition process with the simultaneous movement of the sponge relative to the core. All parameters are combined in dimensionless scaling groups, which make it possible to quickly estimate the resulting oil distribution in the sponge given the characteristic field conditions. More importantly, the initial oil distribution in the core can be calculated by solving the inverse problem.
In addition, we report the results of a laboratory study, in which we measured the water-oil capillary pressure curve of the sponge, and residual oil saturation in the presence of both water and gas.

Introduction
Sponge coring was developed between 1977 and 1980, and is commercially available since 1981. The objective of sponge coring is to measure the interstitial oil saturation directly. Sponge coring therefore is an alternative to pressure coring which is more costly, complicated and time consuming. Sponge coring is most widely applied to measure the remaining oil saturation.
The basic concept of sponge coring is simple. Oil which leaks from the core during the coring process is collected in the enclosing sponge material. However, the actual flow processes which take place are complicated, and have not been thoroughly analyzed to date.
In this work, we model part of the flow processes which are important during sponge coring; the capillary driven exchange of fluids between the core and the sponge. Although this flow is in the horizontal direction, it may also cause vertical redistribution of the oil due to the simultaneous movement of the sponge relative to the core. We model the theoretical oil distribution which is caused by this process.

Sponge Coring Procedure
The sponge coring technique uses a sponge-sleeve modification to a conventional core barrel. Figure 1 shows a schematic picture of the sponge inside the aluminum liner.
Prior to the coring operation, the sponge is saturated with water with the reservoir connate water properties. As the core is being drilled, the sponge liner is designed to fit tightly around the core. Oil which leaks from the core during coring and core retrieval is thus collected in the sponge material. After retrieval, the core is cut into slabs, which are analyzed for their oil content. The amount of oil in the core and the sponge is converted back to reservoir conditions by use of the formation volume factor.

Sponge Properties.
The sponge sleeve is made of polyurethane material, with a porosity higher than 80 % and a reported air permeability in the order of 2 Darcy The sponge material was verified to be chemically inert and stable under most coring conditions. Capillary pressure curve. In this study, we have measured the capillary pressure curve for a sponge material sample, which we found to be water-wet. The drainage curve, shown in Fig. 2, was measured using double distilled water and decane, with an interfacial tension of 0.028 N/m.
Three Phase Residual Oil Saturation. Three phase residual oil saturations can vary significantly from the 2-phase values due to spreading of the oil phase on the water-gas interface.
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