Pressure and rate transient analyses are reliable methods to estimate effective formation permeability of unconventional reservoirs. The results of such analyses are used to validate or fine-tune the numerical simulation models both for the primary and enhanced oil and gas recovery methods.
Pressure-rate transient test applications in unconventional reservoirs are somewhat different from similar tests in conventional reservoirs because of the extremely tight nature of target formations and the use of horizontal wells as an integral part of well completion and reservoir stimulation. Accordingly, this paper presents theory, mathematical formulation, and application of the numerical and analytical well testing solutions to characterize unconventional reservoirs. The mathematical models pertain to the single-porosity and dual-porosity concepts. In the latter we focused on the issues surrounding the unsteady state (USS) and pseudo steady state (PSS) flow. The models studied in this paper were a 1-D analytical model and a hybrid 3-D numerical model that combines the USS and PSS dual-porosity formulations; a trilinear analytical model and a MINC (Multiple Interacting Continua) numerical model. The 3-D numerical model was compared with the trilinear analytical and MINC numerical models.
In unconventional reservoirs the dual-porosity environment emanates from the auxiliary effect of multi-stage hydraulic fracturing for which the unsteady-state mathematical solutions are more appropriate. The benefits of such solutions are supported by field results, which we will present and discuss. Furthermore, the results of pressure-transient and rate-transient analyses of two oil wells in an unconventional reservoir were presented.