Cross-frequency band petrophysical experiments and understandings on Taiyuan Formation bauxite in the Longdong area of the Ordos Basin
Breakthroughs have been achieved in natural gas exploration in the Permian Taiyuan Formation bauxite in the Longdong area of the southwestern Ordos Basin,with several exploratory wells obtaining high-yield industrial gas flows.This has made bauxite natural gas exploration of this area a hot field.So far,however,the basic studies have not been conducted on the dispersion characteristics of bauxite in this area.To further investigate the seismic wave dispersion characteristics of bauxite and their implications for oil and gas exploration,this paper carries out cross-frequency band experiments to analyze the petrophysical properties of the Taiyuan Formation bauxite,and clarifies the dispersion characteristics of bauxite.In addition,the experimental results are verified using the improved Gurevich jet flow model.And the following research results are obtained.First,the main mineral components of bauxite are diaspore and clay minerals,and the storage space types mainly include intragranular dissolution pores,intercrystalline dissolution pores and microfractures,which constitute fractured-porous type reservoirs with the porosity ranging from 0.2%to 28.7%and averaging 10.7%,and the permeability ranging from 0.004 mD to 38.550 mD and averaging 4.040 mD,indicating favorable physical properties of reservoirs.Second,the value of porosity is one of the primary factors determining the dispersion characteristics of aluminiferous rocks.Both water-saturated bauxite and bauxitic mudstone exhibit dispersion phenomena.The dispersion characteristics of bauxite samples with higher porosity are significantly stronger than those with lower porosity,particularly in the 1-10 Hz range.Third,porosity is one of the main factors affecting the pressure sensitivity of rocks.With the increase of pressure,the measured ultrasonic compression wave velocity of high-porosity bauxite samples is close to the velocity calculated using low-frequency stress-strain,and its change range exceeds 700 m/s,which is significantly greater than that of low-porosity samples.Fourth,water saturation and ultrasonic velocity are in the relationship of oscillating change with multiple local extreme points.In conclusion,for high-porosity bauxite samples,the velocity measurement results are consistent with the calculation results of the dispersion theoretical model,while for low-porosity samples,the experimental results of ultrasonic frequency band are not in line with the calculation results of the dispersion theoretical model,with the dispersion mainly appearing above 10 Hz.What's more,cross-frequency band experiment is carried out to explore the dispersive petrophysical characteristics of bauxite,which lays a theoretical foundation for applying the dispersion theory to predict the distribution and gas content of bauxite reservoirs.
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