Estimation of Matrix Flow Contribution in Naturally Fractured Reservoirs

Justin Brand Ferrell

Research output: Book/ReportPh.D. thesis

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Naturally fractured carbonate reservoirs (NFRs) account for a majority of the world’s proven oil reserves (~60%), and a significant portion of proven gas reserves (~40%), as well. However, the petroleum industry as a whole is quite far behind in its understanding of these reservoir systems especially in comparison to single porosity reservoirs with or without induced fracturing/stimulation, i.e. a more recent industry trend with the onset of shale/tight formation development and production. NFRs are most often modelled as dual porosity systems with a separate matrix and fracture porosity and their each own associated characteristics. NFRs can then be categorized by types (1-4 with 1 being almost entirely fracture storage and 4 being a majority in matrix) in accordance to the Shell 1955 classification system, and this dual porosity definition. This is a static oil/gas storage categorization only, and does not account for dynamic behaviour, i.e. how the field behaves in development/production scenarios. A majority of the reservoirs currently in development are of type 1 or 2 meaning that their static classification would indicate that a majority of the oil or gas is in the fracture system with low matrix porosity (< 5% avg). Traditional porosity vs permeability plots from core samples would utilize a porosity cut-off based on 10-30 °API oil type that would negate almost the entire matrix system resulting in little to no recoverable volumes from the reservoir matrix. The matrix stock tank oil initially in place (STOIIP) even for low matrix porosities are still significant, and can comprise 50-80% of the overall STOIIP for a type 1 NFR. The current industry approach would then be to model the entire reservoir system as a fracture system only (single porosity), artificially increase fracture porosity to offset this eliminated matrix STOIIP, and ignore the matrix flow contribution. Of course, this often leads to scenarios of overestimated production performance, and underestimated ultimate recovery given this fracture only approach to NFRs. Many attempts have been made throughout the decades with little success to properly model the actual dual continuum behaviour of dual porosity NFR systems including with transmissibility terms, discretized shape factors, etc., however, with little success in industrial application. The issue remains as to how to estimate the recoverable resources/reserves from the matrix of NFRs, and to also be able to do this for all stages of field life (appraisal to production). This gap is a significant industrial issue in that any improvement or new methodology to help estimate matrix contribution can have significant material impacts on NFR development planning, reserves/asset value, reservoir and production management, etc. The goal of this PhD project is to provide an appropriate method that takes into account the dual continuum behaviour of the matrix and fractures in the application of reserves estimation and development planning.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages120
Publication statusPublished - 2020

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