Rate-transient analysis (RTA) is a robust technique for evaluating reservoir/stimulation properties and for forecasting production from shale reservoirs. However, knowledge of fluid storage and flow mechanisms, and controlling rock and fluid parameters, is critical for obtaining meaningful information from RTA.
It is common practice to use PVT data measured in laboratories (i.e. bulk fluid properties) for reservoir modeling and production data analysis purposes. These measurement techniques were developed for conventional reservoirs and cannot explain some of the anomalous fluid production behaviors observed for shale gas-condensate wells, such as long-term constant gas/oil ratio (GOR) trends. One explanation for this behavior is that the PVT properties of fluids are affected by confinement in nano-scale pores, and hence deviate from bulk fluid properties.
In order to study the effects of pore confinement on fluid properties in shales, the simplified local density (SLD) model is used. The SLD model can be used to estimate fluid density gradients from pore wall to pore center, and therefore explicitly considers pore geometry in adsorption modeling. This model can also be used to adjust the confined fluid critical properties, phase envelope and viscosity. Significant shifts in phase envelope and fluid properties due to pore confinement are observed in this work. Importantly, the corrected equation-of-state predicts a later onset for condensate dropout in shale reservoirs than for bulk systems. The SLD model is also used to estimate adsorbed layer thickness, which in turn is used to modify flow calculations.
The corrections for fluid properties, adsorbed layer thickness and non-Darcy flow are then analytically incorporated into transient linear flow analysis of nanoporous shale gas-condensate wells. Analysis of simulated cases using the "corrected" (for pore confinement effects) and "uncorrected" RTA is performed to quantify errors associated with the latter.
This study demonstrates that failure to account for pore confinement effects on fluid properties and fluid flow results in errors in linear flow parameter estimation using RTA, but the error depends on the fluid composition, pore size, permeability and pressure. The effects of pore confinement should therefore be considered for proper evaluation of shale gas-condensate reservoirs using analytical or numerical methods.