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Abstract

The sedimentary structure and wettability of clastic rocks at the laminaset scale may have an important influence on oil recovery by waterflooding Previous work from this laboratory reported results for oil displacements in water wet laminated systems. In this paper, results from a full cycle of drainage-imbibition floods in a cross-laminated rock slab are reported. The distribution of fluid saturation was monitored using CT scanning techniques. Results from a water wet laminated system are compared with those where the wettability of the slab was changed by ageing the system with a "synthetic crude oil", The main purpose of this work is to investigate the effects of both the laminaset heterogeneity and wettability alteration on the displacement efficiency and oil trapping mechanisms for such systems.

In these experiments, the recovery efficiency significantly increased, due to the slab becoming more oil-wet. An analysis-of the remaining oil saturation shows that (a) no significant oil trapping occurred in this flood, and (b) there was a higher remaining oil in the low permeability regions. Numerical simulations were performed to help our understanding and interpretation of the wettability changes. An excellent match of experiment with numerical simulation was obtained for a particular model case with the lower permeability laminae being more oil-wet than the higher permeability ones. We have termed this a heterogeneously wet (or het-wet) system. To our knowledge, this is the first time that such behaviour has been demonstrated and analysed in detail.

Introduction

The sedimentary structure of clastic rocks at the laminaset scale may have an important influence on the amount and nature of "remaining oil" after a waterflood. This has been found in previous experimental, simulation and theoretical work from this laboratory. The laminar trapping of oil in waterflood displacements depends on a number of factors such as viscous/capillary ratio, the magnitude of the laminar permeability contrast and the wettability of the fluid/rock system. The consequences of this behaviour are important not only for "remaining oil" estimations but also for estimating the anisotropy of two phase transport parameters (directional relative permeability) and for upscaling two phase flow in such structured geological facies.

In our previous work, experimental results were presented from drainage-imbibition floods in strongly water-wet laminated rockslabs. It is the current consensus that most clastic reservoir sands are not strongly wetted by either phase; they tend to be intermediate or mixed-wet. A strongly water-wet substrate (quartz and clays) may be rendered intermediate/mixed-wet by contact with crude oil, even in the presence of some level of initial water saturation. Indeed, the wetting state depends on several aspects of the crude oil/brine/rock system. For example, the composition of the oil is important since certain polar components within it may adsorb onto the mineral surface and hence change the contact angle/wettability of the mineral surface. In this work, a "synthetic crude oil" is used which contains many of the "candidate" polar species which may change the wetting condition of the rock surface. The mineral surface is also important since the structure and charge of this surface (which also depends on the contacting brine), will determine the nature of the species from the oil which may (or may not) be adsorbed. The brine mediates the polar crude oil component/mineral surface interaction by determining the surface charge on the rock, Hence, the entire crude oil/brine/rock system is coupled together in this manner and all parts of this interaction must be considered when we are trying to understand wettability alterations.

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