Research On Reservoir Pore Structure And Crude Oil Production Law Of Ahdeb Oil Field In Iraq

This paper takes the Kh2 segment,which consists of carbonate rocks and constitutes the principal production formation in Ahdeb oil field,Iraq,as target,and studies the characteristics of pore structure and their effect on oil movability.The methods of cast thin section,scanning electron microscope,high pressure mercury injection,constant-rate mercury injection,X-CT scanning and others are used to characterize qualitatively and quantitatively the micro-pore throat in Kh2.Wettability test,low field NMR and two-phase relative permeability test are also applied to evaluate the movability of crude oil.The findings are as follows:(1)In this segment,Layer Kh2-1-2 is primarily composed of sand-clastic limestone while Layer Kh2-3 algal-biodetritus limestone.Sand-clastic limestone has well-devefoped primary intergranular pores and dissolved intergranular pores,which lead to good connectivity between pore throats,while algal-biodetritus limestone developed dissolved algal pores,visceral foramen,etc.,which means poor connectivity.A specialized digital core software is used to analyze the pore structure parameters in the CT images of limestone,and it is found out that there is an effective connectivity of>90%and 6.14 of pore-throat ratio in sand-clastic limestone and that an effective connectivity of 3.2%and 0.16 of pore-throat ratio in algal-biodetritus limestone.(2)The pcrosity of sand-clastic limestone averages 19%and the permeability 85mD;the porosity of algal-biodetritus limestone averages 23%and the permeability 20mD.A poor correlation occurs between porosity and permeability.But a relatively good correlation exists between permeability and average pore throat radius,radius homogeneity,displacement pressure.Constant-rate mercury injection tests show that throat size and pore-throat ratio are key parameters for controlling the reservoir permeability.(3)According to the geological characteristics of the Ahdeb oil field in Iraq,the experimental conditions for the nuclear magnetic resonance(NMR)to measure the fluid saturation and for nuclear magnetic resonance imaging are optimized,and the T2 cutoff value method and the spectral coefficient method are used to measure the saturation of the movable fluid.The T2 cutoff value method indicates that the average value of the movable fluid saturation of the sand-clastic limestone and biodetritus limestone is 48.44%and 52.29%respectively.(4)An analysis of residual oil in the sealed frozen core of AD1-22-1H shows that the residual oil saturation averages 35.73%.The movable oil saturation in sand-clastic limestone by water drive is 33.2%,while it is 29.12%in biodetritus limestone.Residual oil is left over in throats measuring<1?m in radius.The percentage of residual oil varies in different layers and for different lithology.(5)Nuclear magnetic resonance imaging shows that the uneven distribution of compact biodetritus in the rock leads to the poor continuity of throats in Kh2 segment and strong micro-heterogeneity.There are locally dominant channels in sand-clastic limestone;the throats of algal detritus limestone is distributed in stripes or patches,which means poor connectivity where flooded water cannot reach;this is one of the major causes for the generally low recovery and high-porosity but low-permeability in the oil field.(6)Self-absorption method is used to decide on the wettability of Kh2 and yields the result that kh2 segment is predominantly lipophilic or strongly lipophilic,resulting in capillary force to be displacement resistance,one of the major elements that restrict crude oil recovery in Ahdeb oil field.Oil-water and oil?gas relative permeability tests reveal that gas drive performs better than water drive in terms of recovery.