Abstract

In-situ saturation monitoring during a coreflood with X-ray CT scanner confirmed that the waterflood performance of a mixed-wet core is totally different from that of a water-wet core. Namely, it was observed that the brine injected in the mixed-wet core preferentially invaded into the most porous lamina, causing early water breakthrough at an outlet end. This result gives a significant insight into understanding the mechanism of unexpected early water production that has frequently been reported from some giant carbonate reservoirs in the Middle East. Many efforts have been made to grasp how such phenomenon took place. One of the most widely accepted explanations at present ascribes it to reservoir heterogeneities such as the existence of super permeability streaks. A drawback of this super-k concept is, however, that neither cores nor conventional logs could detect their locations. Thus, based upon the experimental result above, this study aims at associating the localized wett ability conditions of the core sample with the early water breakthrough. For this purpose, a numerical study was carried out, using 3-D images of the in-situ saturation distribution obtained from X-ray CT scanning. Then, the waterflood performance of the mixed-wet core was successfully reproduced in the computer model. It was concluded from this numerical study that in spite of the small variation in porosity and permeability, the local variation in wett ability can occur within the Berea sandstone core after it has been in contact with crude oil. As a result, the brine is preferentially imbibed in and runs through the more water-wet laminae, causing the finger-shaped water encroachment and, finally, the early water breakthrough.

Introduction

Pressure support into many of the giant carbonate reservoirs in the Middle East is being achieved either water injection or natural water influx from an aquifer. In order to maximize waterflood recovery efficiency from these reservoirs, the field-wide water encroachment pattern is carefully monitored and every effort is made to control water breakthrough at each of the production wells. In some of them, however, breakthrough was observed significantly earlier than had originally been predicted.1, 2, 3 Many studies have been conducted to understand how such a phenomenon took place. One of the most reliable explanations, which is hence widely accepted at present, ascribes it to reservoir heterogeneities such as the existence of high permeability, or so called super-permeability (super-k), streaks.3 These studies indicate that the field performance is completely dominated by such super-k streaks although they are very localized within a reservoir. The only drawback of the super-k concept is that neither cores nor conventional logs could detect their locations. As a result, both the existence and distribution of such streaks has to be inferred in some way.

On the other hand, many oil companies have reviewed their historic core data to improve the understanding of the underlying causes of early water breakthrough. Some of them concluded that one of the key uncertainties governing the ability to predict waterflood performance was the quality of capillary pressure and relative permeability data.1, 4 Especially, it was found to be a crucial issue whether samples in a laboratory were at a wett ability state that was representative of the reservoir. Wett Ability is defined as the tendency of one fluid to spread on, or adhere to, a solid surface in the presence of other immiscible fluids.5 Thus, wett ability is a major factor controlling the location, flow and distribution of fluids in a reservoir.6, 7 Recent studies have shown there is growing evidence that many reservoir rocks do not have uniform wett ability.4, 8 In other words, different regions of a given reservoir rock may have different local wetting conditions; e.g., the high permeability portions may have one type of wett ability and the low permeability portions may have another.9 In this study, these localized wett ability conditions are referred to as mixed-wet, irrespective of their scales.

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