What are the challenges and solutions for NMR and dielectric logging in unconventional reservoirs?

1 NMR logging principles and applications

NMR logging measures the response of hydrogen nuclei in the formation to a magnetic field and a radiofrequency pulse. The response depends on the relaxation time (T1 or T2), which is related to the pore size, fluid type, and wettability of the rock. NMR logging can provide information on porosity, permeability, fluid saturation, viscosity, and bound water in unconventional reservoirs. However, NMR logging also faces some challenges, such as low signal-to-noise ratio, borehole effects, and invasion effects.

2 Dielectric logging principles and applications

Dielectric logging measures the response of the formation to an electric field and a microwave frequency. The response depends on the dielectric constant and the conductivity of the rock and the fluid. Dielectric logging can provide information on water-filled porosity, water saturation, and water salinity in unconventional reservoirs. However, dielectric logging also faces some challenges, such as calibration, environmental corrections, and depth of investigation.

3 NMR logging challenges and solutions

One of the main challenges for NMR logging in unconventional reservoirs is the low signal-to-noise ratio, due to the low porosity and low fluid mobility of the formation. This can affect the accuracy and resolution of the NMR measurements. To improve the signal-to-noise ratio, some solutions are to use longer wait times, higher magnetic field strength, and advanced signal processing techniques. Another challenge for NMR logging is the borehole effects, such as mud properties, tool eccentering, and tool motion. These can cause distortions and artifacts in the NMR data. To minimize the borehole effects, some solutions are to use borehole corrections, tool centralization, and tool stabilization. A third challenge for NMR logging is the invasion effects, which can alter the fluid distribution and relaxation times in the near-wellbore region. To account for the invasion effects, some solutions are to use multiple depths of investigation, invasion models, and core calibration.

4 Dielectric logging challenges and solutions

One of the main challenges for dielectric logging in unconventional reservoirs is the calibration, which is essential for obtaining accurate dielectric measurements. Calibration involves measuring the dielectric response of known materials in a laboratory or a calibration well, and comparing them with the tool response. However, calibration can be affected by temperature, pressure, frequency, and tool wear. To ensure proper calibration, some solutions are to use frequent calibration checks, temperature and pressure corrections, and tool maintenance. Another challenge for dielectric logging is the environmental corrections, which are needed to remove the effects of borehole size, mud properties, casing, and cement on the dielectric data. These effects can cause errors and uncertainties in the dielectric measurements. To apply the environmental corrections, some solutions are to use borehole models, mud models, and casing corrections. A third challenge for dielectric logging is the depth of investigation, which determines how far the dielectric signal penetrates into the formation. The depth of investigation depends on the frequency, the dielectric contrast, and the attenuation of the signal. In unconventional reservoirs, the depth of investigation can be limited by the low dielectric contrast and high attenuation of the formation. To increase the depth of investigation, some solutions are to use lower frequencies, higher power, and directional antennas.

5 NMR and dielectric logging integration and interpretation

NMR and dielectric logging can complement each other and provide a comprehensive characterization of unconventional reservoirs. By integrating and interpreting the NMR and dielectric data, you can obtain more reliable and accurate estimates of porosity, permeability, fluid saturation, fluid type, and fluid properties. For example, you can use the NMR T2 distribution to separate the bound water from the free water, and use the dielectric constant to determine the water salinity. You can also use the NMR permeability to correct the dielectric saturation for capillary pressure effects. You can also use the NMR viscosity to calibrate the dielectric viscosity. Moreover, you can use the NMR and dielectric data to identify different rock types, fluid phases, and flow regimes in unconventional reservoirs.

6 NMR and dielectric logging best practices and recommendations

To optimize the performance and results of NMR and dielectric logging in unconventional reservoirs, you should follow some best practices and recommendations. First, you should plan your logging program carefully, considering the objectives, the formation characteristics, the tool specifications, and the operational constraints. Second, you should acquire your data with high quality and resolution, using appropriate logging parameters, calibrations, corrections, and quality control checks. Third, you should process and analyze your data with rigorous methods, using suitable models, algorithms, and software. Fourth, you should integrate and interpret your data with multiple sources of information, such as core data, petrophysical data, geophysical data, and production data. Fifth, you should validate and update your data with feedback and verification, such as well testing, reservoir simulation, and history matching.

7 Here’s what else to consider

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