Quantitative characterization of pore structure in offshore low-permeability reservoirs based on double-peak Gaussian function:a case study of the third member of the Lingshui Formation in the BD21 Gas Field,Qiongdongnan Basin
The proportion of offshore low-permeability gas reservoirs has increased,and the differences in pore structure have affected well productivity,limiting the development of offshore low-permeability gas reservoirs on a larger scale.To characterize the complex pore-throat structure of low-permeability reservoirs,high-pressure mercury intrusion and nuclear magnetic resonance(NMR)experiments were conducted on low-permeability core samples.The double-peak Gaussian function fitting curve method was used to convert the NMR T2 spectrum into pseudo-capillary pressure curves to quantitatively charac-terize pore size and distribution.Furthermore,by analyzing the correlation between the NMR T2 spec-trum and parameters such as large pore volume and medium-large pore porosity,a new parameter that better reflects reservoir flow capacity was constructed for pore-throat evaluation.Lastly,a prediction model for pore-throat and petrophysical parameters was established using multivariate regression based on the response relationship between logging curves and reservoir classification parameters.The results show that the pore-throat distribution of the third section of the Lingshui Formation low-permeability reservoir exhibits bimodal and unimodal patterns,with overlapping regions between small and large pore curves in both types.The overlapping region is smaller in the bimodal pattern and larger in the unimodal pattern.The pore system can be divided into large pores,medium pores,and small pores based on the distribution ranges of the fitted curves for small and large pores and the overlapping region.The medi-um-large pore porosity and large pore porosity effectively evaluate reservoir petrophysical properties and pore structure.The better the large-medium pore system developed,the better the petrophysical prop-erties are,and the higher the movable fluid saturation is.A classification standard for low-permeability reservoirs was established based on comprehensive evaluation parameters primarily focusing on large pores and medium-large pores.The reservoirs were divided into three types.Type Ⅰ and type Ⅱ reser-voirs have better development of large-medium pores and good petrophysical properties,comprehensive evaluation parameter values are greater than 7 and 10 respectively,and relatively high movable fluid sat-uration is generally above 65.75％.Type Ⅲ reservoirs have relatively poor petrophysical properties,smaller values for comprehensive evaluation parameters of large pores and medium-large pores,complex pore-throat structure,and relatively low movable fluid saturation.These results can provide guidance for the economic and effective development of low-permeability gas fields.
low-permeability reservoirspore structure characterizationdouble-peak Gaussian functionpore-throat evaluation parametersprediction modelthe third member of Lingshui FormationQiong-dongnan Basin
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