Application of the RTAPK Core Analysis Method to Address Problems Facing Field-Based Characterization and Evaluation of Unconventional Reservoirs

The rate-transient analysis (RTA), porosity, and permeability (“RTAPK”) core analysis method reproduces conditions under which wells completed in unconventional reservoirs are operated in the field with the data analyzed similarly. As with RTA methods applied to wells in the field, RTAPK data analysis involves (1) identification of flow regimes and (2) application of models to extract parameters of interest. RTA of field data has faced many challenges when applied to unconventional reservoirs, including (1) nonunique flow regime interpretations, which affect model selection; (2) uncertain reservoir/fracture property estimates as a result of (1); and (3) accounting for the effects of interwell communication. RTAPK can be used to address some of these challenges in the laboratory. To address challenges (1) and (2), low-permeability reservoir samples with different degrees of permeability heterogeneity (parallel to flow) caused by bedding/laminations were studied using RTAPK to assess the impact on flow-regime signatures and permeability estimates. Low to moderate levels of permeability heterogeneity resulted in a flow-regime sequence similar to hydraulically fractured wells completed in homogeneous reservoirs. However, a high permeability heterogeneity sample yielded a flow-regime sequence resembling field signatures previously interpreted to be caused by fracture complexity. The laboratory results suggest an alternative interpretation for the latter. To address challenge (3), interwell communication in the lab was simulated using RTAPK. A new apparatus allowing flow from both sides of the core plug was constructed for this purpose. The results, as analyzed with flow-regime identification and contacted fluid-in-place plots for the parent well, resembled simulated cases of parent–infill well communication. Finally, relevant to the energy transition, RTAPK was used to study water production from a low-permeability sandstone core plug, applicable to the evaluation of enhanced geothermal systems, and hydrogen production from a coal core plug, applicable to the evaluation of hydrogen storage in deep subsurface coal reservoirs.