The accurate identification of the different components of unconventional tight oil reservoirs is fundamental to determining reservoir quality (RQ). It is important to determine these components because, whereas some components, such as movable light oil, are positive RQ indicators, others, such as kerogen and bitumen, may have negative RQ characteristics. Whereas the existing methodologies for the identification and separation of these components from cores in unconventional shale are both time-consuming and destructive, high-field NMR (nuclear magnetic resonance) is not and may be a viable solution. Low-field NMR downhole logging is also useful in that it can assist with a subset of measurements based on 2D NMR T1-T2 maps. These T1-T2 maps reflect molecular mobility due to their sensitivity to motions at multiple frequencies aiding log and/or advanced core analysis, especially of the fast-relaxing components. Diffusion measurements also play a unique role in the identification of the producible fluid having relaxation times of several tens of milliseconds.
A comparison of 2D NMR T1-T2 measurements on the Upper Bakken source rock with the Middle Bakken and Three Forks intervals at 2-MHz and 400-MHz Larmor frequencies reveals clear differences between the T1/T2 ratios of the different fluids. The main differences are between the bitumen plus bound water, the light hydrocarbon, and the fluids in the inorganic porosity. Furthermore, the bitumen T1/T2 response is sensitive to its environment, varying in value from 20 to 26 in the kerogen-rich Upper Bakken interval to about 2 to 6 for the Middle Bakken and lower Three Forks at 2-MHz Larmor frequency. Such measurements can be carried out downhole, allowing for bitumen identification and T2-based cutoffs for aiding the determination of the potentially movable fluids. The presence of the light hydrocarbon in the Middle Bakken and the Three Forks can also be identified from the 2D NMR T1-T2 maps, at a T2 value above 500 μs and a T1/T2 ratio between 1 and 3.
One of the main challenges for low-field NMR T1-T2 relaxometry is the insufficient contrast between the bitumen and bound-water signals. High-field NMR T1-T2 maps at 400-MHz Larmor frequency enable separating the kerogen/bitumen and the clay-associated water component by using the frequency dependence of the relaxation times. This makes quick, nondestructive fluid identification of the different components of tight oil reservoirs and tight oil organic shale possible. This separation is enabled by the high T1/T2 ratios of the kerogen and the bitumen of more than a thousand in comparison with that of clay-associated water, which has a relatively lower T1/T2 ratio, on the order of a few hundred.
The oil and water in the inorganic porosity relaxing at a few tens of milliseconds have insufficient T1/T2 contrast to enable their identification through NMR relaxometry. Diffusion measurements can fill this gap in fluid typing by helping identify this phase. Diffusion measurement made in both the middle Bakken and the Three Forks intervals are useful for identifying the long relaxing fluid in the inorganic pores as oil. It is interesting to contrast this with measurements of water in these inorganic pores in other shale samples. The measurements are also complexed due to the existence of very small pores and pore throats with diameters of tens of nanometers or less.