Characterizing and Simulating the Non-Stationariness and Non-Linearity in Unconventional Oil Reservoirs: Bakken Application

Abstract

Abstract This paper presents an innovative integrated methodology and working procedure for characterizing and simulating the strong non-linearity and non-stationariness caused by changes in confined pressure-volume temperature (PVT) properties over time related to pore-throat size, the pressure-dependent permeability, and the intervened multiple porous media created by multi-stage fracture stimulation. The complicated physics behind the observed phenomena are explored. More specifically, this paper demonstrates and discusses the following: 1) a new rate-transient analysis (RTA) procedure to infer the stimulated reservoir volume (SRV) and fracture parameters; 2) the impact of the non-stationary feature, compaction effect, and pore-throat related PVT properties on the flow regime and well performance; 3) how to incorporate the non-stationary and non-linear features into the reservoir model; 4) the integrated procedure for history matching, performance forecast, and recovery assessment; 5) several field examples in the Bakken to illustrate the procedure. The proposed procedure has been successfully applied for the following: 1) constructing the non-stationary, non-equilibrium, and highly non-linear simulation models; 2) facilitating the history matching by addressing permeability reduction and PVT property variations caused by compaction and capillary pressure; 3) and ensuring more reliable performance forecasts and recovery assessments. The study shows that the reduction of the bubblepoint pressure could be several hundred psi in the typical Bakken rock; moreover, such reduction continues following the depletion via the compaction effect. The compaction effect could impair the matrix permeability by up to one order of magnitude. The study reveals the following: 1) the confined PVT properties could widen the favored operation window, whereas the compaction effect could significantly impair the ultimate recovery of the wells; 2) the RTA-inferred SRV-related parameters are the key input for capturing the non-stationary features; 3) the impact on recovery could be over 50% without addressing the aforementioned non-stationary and non-linear issues. This paper explores several unique phenomena in unconventional oil reservoirs which have not previously been published. The proposed analysis and assessment procedure greatly enhances the understanding of the unconventional assets and we feel will improve the accuracy of long-term rate and recovery forecasts.