A primary role of logging in vertical pilot wells in tight oil shale plays is to determine the reservoir quality, and use this information to choose the optimum lateral landing point. The reservoir quality (RQ) of tight oil reservoirs is characterized by several quantities such as hydrocarbon storage and its ease of production, porosity, total organic carbon (TOC), fluid saturations, wettability, permeability etc. It is now well recognized that among the different components constituting the TOC, the kerogen and bitumen components adsorb a fraction of the light oil and reduce the producible volume of light oil and, also, permeability by clogging pore throats. These components therefore can be considered negative reservoir quality indicators. The light hydrocarbon component is distributed between a lower mobility fraction in the oil wetting kerogen pores and a relatively higher mobility fraction in the mixed wet inorganic matrix porosity. The water phase is also distributed between the clay associated bound water fraction and the more mobile water in the mixed wet pores of the inorganic matrix. Identifying these different components and their environments is vital for determining the reservoir quality.
Recently, a new metric namely the reservoir producibility index (RPI) that treats the light hydrocarbon (oil) as a positive reservoir quality (RQ) indicator and kerogen, bitumen as negative RQ indicators has been introduced for tight oil organic shales. The measurement of the RPI requires quantitative fluid typing of the different components of organic carbon and water phases, downhole. In this paper we demonstrate measurement of RPI logs through the application of 2D NMR T1-T2 for determining the light hydrocarbon, in combination with spectroscopic measurements for TOC.
The 2D NMR T1-T2 logs separate the different fluid fractions using its sensitivity to molecular mobility. Using this in combination with spectroscopy logs, RPI is obtained as a continuous depth log in tight oil pilot wells. To determine the production potential along the well, the total liquid porosity was compared with the liquid porosity of cores with the difference reflecting the fluids that escape core retrieval. This quantity which is a good proxy for the producible fluid fractions is shown to be positively correlated to the RPI measured from the logs.
In conclusion, we demonstrate the combination of 2D NMR T1-T2 and spectroscopy logging to obtain a continuous log of the RPI in tight oil pilot wells, to help determine the zones for landing the laterals. This method is driven by the ability of 2D NMR T1-T2 technique to quantitatively identify the fluid fractions and their confining environments, especially the light oil in the organic kerogen and inorganic porosities. We demonstrate the value of the RPI in a tight oil well in the Eagle Ford shale by showing a relatively strong correlation with estimated oil producibility.
