Dolomite reservoir characteristics and permeability evaluation methods:An example from the Paleogene,Mesopotamian basin,Iraq
In the Paleogene,Mesopotamian basin,Iraq,the diversity of dolomite reservoir space makes it very difficult to quantitatively evaluate the permeability.In order to solve this problem,firstly,based on cast thin sections and CT scans,the reservoir spaces of dolomite reservoirs are analyzed.Secondly,based on the array acoustic logging and electrical image logging,microfractures can be accurately characterized and pore reservoirs as well as microfracture-pore reservoirs can be classified.Finally,combined data-driven with knowledge-driven,the static permeability of dolomite reservoirs can be accurately calculated,and the conversion relationship between dynamic and static permeability of pore dolomite reservoirs and microfracture-pore dolomite reservoirs can be established respectively.The research results show that the average width of fractures of dolomite reservoirs in the study area is 50 μm,which belongs to the level of microfractures,and the microfracture porosity is basically less than 1.5％.When the anisotropy of the dolomite reservoir is less than 3.1％,it is the pore reservoir;when the anisotropy of the dolomite reservoir is greater than or equal to 3.1％,it is the microfracture-pore reservoir.The static permeability evaluation can be gradually transformed into complex,high-dimensional,non-linear classification problem from the perspective of data-driven.Additionally,the introduction of knowledge-driven can make up for the blindness and inefficiency of data-driven.Finally,the depth of the established classification model of decision tree for dolomite reservoirs in the study area is 16 layers and the accuracy is 84.06％,so that the static permeability of dolomite reservoirs can be accurately calculated,and the evaluation accuracy of the static permeability is improved by about 70％.The correlation coefficients R2 between static and dynamic permeability of pore dolomite reservoirs and microfracture-pore dolomite reservoirs are 0.86 and 0.90,respectively.The establishment of an accurate conversion relationship between dynamic and static permeability lays an important foundation for the production prediction of dolomite reservoirs.
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