Abstract
Reservoir rock type (RRT) classification is commonly used for the fine characterization of strongly heterogeneous reservoirs. Many research focused on proposing different RRT methods, while few studies focused on the relationship between micropore structure and macroscopic seepage law of fluid transporting through different types of formation. Considering the different diagenesis degrees of rock types in different sedimentary facies, the quadrant unit method and FZI method were applied to classify and evaluate the strongly heterogeneous reservoirs. The corresponding capillary pressure curves and relative permeability curves for different rock types could be obtained. A set of theoretical models that could describe the macroscopic seepage law are proposed, and numerical simulation case studies were carried out to explore the superimposed seepage law in reservoirs under different degrees of heterogeneity. Results show that water cut rises faster in poor-property RRT formation than in good-property RRT formation, with a lower recovery factor under the same conditions. The water cut rising rate increases firstly and then decreases with water cut and recovery degree increasing. The poorer the rock property is, the earlier and higher the peak value of water cut rising rate is. Also, based on numerical simulation case studies, the superimposed seepage law is close to that in the poor-property formation as the heterogeneity degree strengthens. The newly proposed method not only could provide a reference for rock type classification based on micropore structure but also could expound the influence of micropore structure on the change law of macroscopic water cut and elaborates the macroscopic change law after the superposition of different types of reservoirs. The conventional normalization of relative permeability curves is only applicable to relatively homogeneous reservoirs and not applicable to strongly heterogeneous reservoirs, especially when the permeability variation is higher than 0.7. The newly proposed approach is capable of analyzing the effect of micropore structure on macroscopic seepage law and improving the prediction accuracy of the production profile.
Highlights
At present, most of the reservoirs in the world are strongly heterogeneous, and the permeability variation could be very large with poor porosity-permeability relationship
The analytical models presenting the relationship between micropore structure parameters and macroscopic seepage law have been established through theoretical derivation
The classification of capillary curves, relative permeability curves of different Reservoir rock type (RRT), and the macroscopic seepage law could be discussed below taking the X carbonate oilfield in the Middle East as an example
Summary
Most of the reservoirs in the world are strongly heterogeneous, and the permeability variation could be very large with poor porosity-permeability relationship. Many scholars conducted extensive research on RRT classification and proposed many methods such as the capillary pressure method, Leverett dimensionless J-function method, Winland R35 method, reservoir quality factor (RQI) method, and flow zone indicator (FZI) [1,2,3] These studies provide guidance for fine formation characterization, few research has been carried out to study the relationship between micropore structure and seepage law for different RRT, resulting in unclear understanding of seepage law of strongly heterogeneous reservoirs and difficulties in making corresponding adjustment measures in different development stages. Based on the above-mentioned methods, micropore structure could be characterized effectively through introducing parameters such as the pore-throat ratio, coordination number, pore geometry factor, microhomogeneous factor, threshold pressure, pore relative sorting coefficient, throat relative sorting coefficient, and surface volume ratio. The influence law and superposition effect of different RRT on macroscopic seepage law were further discussed
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