基于核磁共振标定的高压压汞孔喉分布定量评价方法

Quantitative Evaluation Method of HPMI Pore-throat Distribution Based on NMR Calibration

孔喉分布是控制低渗-致密砂岩储层物性的关键因素,其评价依托于各种储层微观特征测试方法,需要综合多方法各自优势进行孔喉分布定量表征。本文提出基于核磁共振标定的高压压汞孔喉分布定量评价的方法,即通过核磁共振离心前后横向弛豫时间T_2谱图对比,依据流体赋存状态重新划分三孔隙度组分百分比法的T_2值界限T_(21)和T_(22),对应将孔喉划分为束缚流体孔喉、过渡流体孔喉和可动流体孔喉,再结合T_2值与孔喉半径的关系将T_2值界限转化为孔喉半径界限r_1和r_2,最终依据高压压汞统计的不同流体赋存状态的孔喉含量S_1、S_2和S_3进行孔喉分布定量评价。该方法综合了核磁共振有效表征孔喉流体赋存状态和高压压汞有效表征孔喉大小的优势。将此方法应用于西湖凹陷花港组低渗-致密砂岩储层孔喉分布评价,建立了T_2值与孔喉半径平均转化系数C为0.0079,求取r_1和r_2为60 nm和160 nm,依据各类孔喉含量及其相互关系将孔喉分布划分为四类,从而为储层评价提供新的参数和思路。

Pore throat distribution is an important factor controlling the physical properties of low-permeability and tight sandstone reservoirs. The evaluation of pore throat distribution is based on the analytical methods of micro features of reservoirs, and it is necessary to synthesize multiple methods to quantify the pore throat distribution.Quantitative evaluation method of High Pressure Mercury Injection （HPMI ） pore throat distribution was developed based on Nuclear Magnetic Resonance （ NMR ） calibration. Fluid states can be identified by thecomparison of transverse relaxation time T2 spectrum before and after centrifugation, which enables the redefining of T2 boundaries （ T21 and T22 ） of three porecomponents of NMR. Furthermore, pore throat is divided into irreducible fluid pore throat, transition fluid pore throat and movable fluid pore throat accordingly. T2 boundaries are then converted to pore throat radius boundaries r1 and r2 by the correlativity between T2 value and pore throat radius. Finally, pore throat distribution is evaluated quantitatively by statistics of the content of various types of pore throat （ Sj,S2 and S3 ）. Thismethod combines the advantages of HPMI describing fluid states and NMR characterizing pore throat size. The method was applied to evaluate the pore throat distribution oflow-permeability and tight sandstone reservoirs in the Huagang Formation of Xihu Depression. The average conversion coefficient C between T2 value and pore throat radius is 0.0079, and pore throat radius boundaries r1 and r2 are 60 nm and 160 nm,respectively. Pore throat is classified into four types according to the content ofvarious types of pore throat,which provides new parameters and thought for further reservoirs evaluation.