Estimating Ultralow Permeability at Multiple Locations Using Simultaneous-Impulse Tests: A Fit-for-Purpose Pressure-Transient Solution and Its Field Application

Summary One of the major challenges in efficiently developing ultratight or shale reservoirs is to obtain reliable permeability distribution. Ultralow permeability and very strong stratification or heterogeneity in the formations require conducting long-duration tests at multiple locations in a well to attain a complete reservoir characterization. Because of these characteristics, existing tools or methods that work well for conventional reservoirs are usually not applicable for ultralow-permeability formations. An innovative reservoir-monitoring/testing tool system was developed and successfully applied to fields in both the United States and Canada (Zhan et al. 2016). The tool created multiple pressure pulses at targeted locations simultaneously along monitoring wells for zonal in-situ permeability estimations as well as long-term formation-pressure monitoring. Existing pressure-transient solutions are incapable of handling the measured data when potential zonal-pressure interferences appear. A fit-for-purpose pressure solution, the associated optimization algorithm, and a suitable data-interpretation work flow were developed to analyze the data whenever commercial software tools are inadequate. The new solution fully considers all impulses as well as potential interferences among them through suitable superposition. In-situ permeability values at all involved zones can be obtained through a combination of individual-impulse analyses and systematic multiple-impulse inversions. The new pressure-transient solution was verified through comprehensive synthetic cases using numerical simulations. The results show that the solution can handle either fully or partially open wellbores in each zone and properly considers crossflow between formation layers. In addition, it allows any number of perforated intervals and formation layers where the number of perforated intervals is fewer than that of the formation layers. Field examples demonstrate the applicability of the new solution and the data-interpretation work flow to formation permeability down to tens of nanodarcies. The detailed zonal-permeability distribution in the formation enables a more-representative reservoir model for better hydraulic-fracturing design and field-development optimization.