Combined spatially resolved and non-resolved 1H-NMR relaxation analysis to assess and monitor wettability reversal in carbonate rocks
Authors: Bortolotti Villiam , Macini Paolo , Mesini Ezio Nicola , Srisuriyachai Falan , Fantazzini Paola , Gombia Mirko 
Published: March 2015

Abstract
Wettability is a petrophysical property that directly affects oil recovery by controlling the location, flow and distribution of fluids inside the reservoir. In this study it has been verified that a polar interaction mechanism to change wettability of a carbonate rock from water-wet to oil-wet by acid adsorption onto the rock surface can be obtained by injecting adequate chemicals, and in particular oleic acid dissolved in non-polar oil, like Soltrol® 170 used for laboratory experiments. The paper deals with the assessment and monitoring of wettability reversal of carbonate rocks used in laboratory studies to evaluate the efficiency of chemical EOR methods, and in particular alkali flooding. The study is performed by combining spatially resolved and spatially non-resolved 1H-NMR information obtained on full-size cores (up to 7cm in diameter). In particular, spatially resolved quantitative NMR imaging maps (or Quantitative Relaxation Tomography, QRT) and spatially-non resolved (or Magnetic Resonance Relaxometry, MRR) analyses have been performed on full-size cores to track the process and to assess the quality of wettability reversal. Changes of relaxation time distributions and of T1 maps obtained in the experiments seem correlated with changes in wettability resulting from the chemical treatment, i.e., the contamination with oleic acid. Although further investigations are needed, MRR and QRT are promising techniques to correlate the wettability index with local values of T1 in internal sections of a rock sample.

Introduction
Enhanced Oil Recovery (EOR) has long been exploited in the field of oil production. EOR methods act on different reservoir aspects to increase the productivity index. In particular, the alkali EOR method deploys an injected alkaline solution that may reverse wettability to a more favourable condition for increasing efficiency of water displacement in the pores. Wettability is a petrophysical property that affects oil recovery by controlling the location, flow and distribution of fluids inside the reservoir, and plays a major role in the production of hydrocarbon, especially in water drive reservoirs. For this reason wettability received a lot of attention in laboratory studies.
The concept of wettability expresses the relative adhesion of two immiscible fluids to a solid surface. In a porous medium containing two or more immiscible fluids, wettability is defined as the preferential tendency of one of the two fluids to wet the solid surface, in the sense that one of the two fluids tends to adhere better than the other one, covering it and spreading itself over the solid surface like a film. In a water-wet porous medium, in the presence of water and oil, the water occupies the smaller pores and wets most of the surface of the larger pores. In areas of higher oil saturation, the oil lies on a film of water in contact with the rock, inside the pores. In a sample of water-wet rock, the water is imbibed by the smaller pores, and may displace the oil if the sample is placed in contact with water and oil. By contrast, if the sample is oil-wet, in the presence of even a small degree of water saturation, the oil wets the rock and displaces the water. A sample saturated in oil is water-wet if it imbibes water, whereas it is oil-wet if it imbibes oil. Generally, the wettability of a rock may vary from being strongly water-wet to strongly oil-wet, depending on the interactions of the water, oil and rock. If the porous medium does not show preferential wettability to one of the two fluids, it is referred to as neutral-wet (or intermediate-wet), meaning that both fluids wet the porous medium equally [1]. Water-wet reservoir rocks can be effectively flooded by water, whereas oil-wet ones require more pore volume of water to be injected during the waterflooding in order to obtain an equivalent ultimate oil recovery [2].