On Estimating Irreducible Water Saturation in Tight Formations Using Nuclear Magnetic Resonance Relaxometry

ABSTRACT

Most of the Nuclear Magnetic Resonance (NMR) log based permeability models require the estimation of the irreducible water saturation (S_wirr). Several methods are available for calculating this parameter using NMR relaxometry. The most straightforward method with the lowest accuracy is to consider a fixed relaxation time (T₂) value. It has been suggested to use a T_2-cutoff equal to 10 ms for tight reservoirs. Another traditional experimental method involves centrifuging core plugs to S_wirr. In this paper, an additional approach to separate free and bound water using NMR relaxation time is introduced. This method involves the area under the amplitude-T₂ relaxation time graph.

A series of experiments were conducted on 81 core plugs. These samples are mainly from the Western Canadian Sedimentary Basin. Core plugs are from Montney, Nordegg, Mist Mountain, Red Beds, Doig, Killam, Lathom, York River, Wapiti, Teslimkoy, Kesan, and Ordivician Quartz formations. NMR measurements were obtained initially on the dry cores to establish the presence of any liquids that were not cleaned or any isolated porosity. The air permeability was measured using an in-house permeameter. The cores were then brine saturated in two steps of spontaneous imbibition followed by forced imbibition under vacuum. The Archimedes principle was used to measure the sample pore volumes. Porosity was subsequently calculated. NMR relaxation data were then acquired on the brine-saturated cores. Then the core plugs were centrifuged under air to an expected irreducible saturation. NMR relaxation times were obtained on all cores at S_wirr.

NMR porosity, T_2gm, Irreducible Bulk Volume (BVI), and Free Fluid Index (FFI) were calculated. S_wirr was calculated with the three aforementioned methods. Excel Visual Basic for Applications (VBA) programming language was employed for analyzing the relaxation times. The Timur-Coates model was applied for permeability calculation using all the aforementioned S_wirr estimation methods. Data were analyzed, and discrepancy analysis was conducted.

The implemented area analysis method has been used previously in reservoir typing based on formation types and also as a factor in one permeability model. However, this is the first time this approach is used in calculating FFI/BVI exclusively. This method is faster than conventional estimators, and it is the only method that can implement Timur-Coates based permeability models for logging tools. From the experimental point of view, only a single NMR measurement is needed. Centrifuging the cores is not necessary. The possibility of cracking these cores due to spinning is eliminated. This new approach is less computationally demanding, and calculations are easier to perform. It is proven that the fast peak area method is more accurate than the fixed T_2-cuttoff and in some cases the centrifuge method.

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