Probing Wettability Reversal in Carbonatic Rocks by Spatially Resolved and Non-Resolved 1H-NMR Relaxation Analysis
- Villiam Bortolotti (U. of Bologna) | Paolo Macini (U. of Bologna) | Ezio Nicola Mesini (U. of Bologna) | Paola Fantazzini (U. of Bologna) | Mirko Gombia (U. of Bologna) | Falan Srisuriyachai (U. of Bologna)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 19-22 September, Florence, Italy
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 1.2.3 Rock properties, 1.6.9 Coring, Fishing, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 2.5.2 Fracturing Materials (Fluids, Proppant), 5.4.10 Microbial Methods, 4.3.4 Scale, 5.2 Reservoir Fluid Dynamics, 5.5.2 Core Analysis, 5.1 Reservoir Characterisation, 5.7.2 Recovery Factors, 5.8.7 Carbonate Reservoir, 4.1.2 Separation and Treating
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The paper is addressed to help designing alkali/surfactant flooding Enhanced Oil Recovery methods. In particular, it has been verified that a polar interaction mechanism to change wettability of a carbonate rock from water-wet to oil-wet can be obtained by injecting adequate chemicals, and in particular a solution of oleic acid and non-polar oil, like dodecane used for laboratory experiments. It is worth recalling that wettability is a petrophysical property that directly affects oil recovery by controlling the location, flow and distribution of fluids inside the reservoir rock.
The study has been performed by combining spatially resolved and spatially non-resolved 1H-NMR (Nuclear Magnetic Resonance) 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 wettability changes measured after the chemical treatment, i.e., the contamination with a solution of oleic acid and dodecane. Although further investigations are needed, the combination of MRR and QRT seems a promising technique to correlate the traditional Amott wettability index with local values of T1 in internal sections of a rock sample.
In particular, the paper deals with the assessment and monitoring of wettability reversal of carbonate rocks used in laboratory studies to evaluate the efficiency of chemical Enhanced Oil Recovery methods, and in particular of alkali/surfactant flooding. The combination of 1H-NMR relaxation analysis, characterized by high temporal resolution, with quantitative maps, characterized by high spatial resolution, permits a better knowledge and the monitoring of the processes occurring inside the pore space leading to wettability reversal.
Enhanced Oil Recovery (EOR) is a general term indicating a wide range of techniques utilized to improve the recovery factor from hydrocarbons reservoirs. These techniques have been developed over many decades and nowadays they can be categorized in various types according to the injected fluids (which are absent in reservoir formations) and their effects on the ultimate oil recovery. In particular, alkaline solutions show significant effects to improve the oil recovery factor. Alkali flooding was utilized for the first time in 1917, and is still nowadays the most important technique involving wettability effects . The injected alkaline solution reacts spontaneously with the natural organic acids present in the oil phase, producing the in-situ surfactants that reduce the interfacial tension between oil and water contained inside the pore space. Moreover, the alkaline solution can also reverse rock wettability to a more favorable condition for oil recovery [2, 3], as in the case of oil wet reservoir rocks that can be turned into moderately water wet condition.
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