Recent advances in NMR logging have made it valuable to optimize the acquisition sequence of NMR wireline tools for advanced applications. Of particular interest is the generalization of two main features which keep increasing the complexity of NMR activation sequences:
multidimensional NMR acquisition (enforcing relaxation and/or diffusion contrast between fluids) and
radial profiling of saturations.
This paper recapitulates the main uses for multidimensional NMR applications and introduces new criteria for the optimization of complex acquisition sequences. Namely, logging tool geometry, logging speed and signal-to-noise ratio, all play a role in defining a measure of activation efficiency that can be used to fine-tune acquisition parameters and preferentially select activations. We introduce new pulse sequences that either improve existing T1-T2 or T2-D activations, or generalize them to include multi-shell measurements for different tool designs (both centralized and padded). Finally, we discuss field tests of these new sequences in several continental US fields using both MREX and MRIL-Prime and XL tools.
Most current wireline NMR logging tools have benefitted from two types of improvements over previous tool generations and have offered a totally new array of applications for NMR logging:
They can use a wide variety of echo spacing, wait time and number of echoes, which means that multidimensional NMR logging is now possible with wireline tools from all three major service companies.
These tools can be tuned at different Larmor frequencies sequentially, which means they are able to scan one depth of investigation (DOI) after another and can establish on the same pass an NMR profile of the fluids in the very near wellbore.
Even though the basic NMR logging acquisition sequence often remains the classic Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence, the range of acquisition parameters now available offers numerous choices of NMR logging activations. It is often very difficult to select the right combination of parameters that will yield optimal measurements for a given formation, while respecting operational constraints due to rig time and long open-hole exposure risks. We recognize that defining (and consequently optimizing) the acquisition parameters will mostly stem from three interdependent aspects: 1-Logging objectives and type of fluids present 2-Signal-to-noise ratio (SNR) 3-Logging speed and vertical resolution After recapitulating some background on multidimensional NMR logging and radial saturation profiling, this paper introduces a method to optimize the selection of NMR logging activation sequences. New general pulse sequences for T1-T2 and T2-D NMR acquisition are also introduced in an effort to maximize the value of NMR wireline logs, and field trials of these sequences using several NMR tool types in the continental United States are discussed.
Standard 1D T2 NMR logging is designed for fast and basic NMR logging (bound fluid and light hydrocarbon detection, fast logging involving lower rig time, and less expensive tool) with simple CPMG sequences that stack echoes to improve signal-to-noise ratio.
