潮坪相白云岩储层核磁共振扩散耦合效应——以四川盆地龙门山山前带中三叠统雷四上亚段为例

NMR diffusion coupling effect in tidal-flat dolomite reservoirs:A case study of upper Lei-4 Member of Middle Triassic in the Longmenshan piedmont zone,Sichuan Basin

受扩散耦合效应影响,采用核磁共振资料评价白云岩储层物性及含气性面临以下难点:①目前尚无白云岩储层扩散耦合效应影响机理的研究;②对于扩散耦合效应的研究主要是针对T_(2),极少有对T_(1)的研究;③目前缺乏考虑钻井液滤液侵入条件下残余气对扩散耦合影响的先例。鉴于上述问题,选取四川盆地龙门山山前带中三叠统雷口坡组四段上亚段(以下简称雷四上亚段)白云岩岩心,通过实验观测雷四上亚段潮坪相白云岩储层扩散耦合效应对物性的敏感程度,并在不同离心力条件下进行T_(2)和T_(1)实验,比较T_(1)扩散耦合效应与T_(2)的差异;最后通过观测气信号及水信号的二维谱响应特征,分析位于中等孔隙中的残余气对扩散耦合效应的影响程度。研究结果表明:①扩散耦合效应在白云岩中普遍存在,扩散耦合效应强度与岩性、物性等因素有关;②不同物性的岩心扩散耦合效应程度存在差异,T_(1)的扩散耦合效应略强于T_(2);③冲洗带残余气对氢核在大、小孔隙之间的扩散起阻碍作用。结论认为,扩散耦合效应导致潮坪相白云岩核磁共振弛豫谱偏离其储层孔隙结构分布,其形态特征与偏离程度受物性因素控制,且冲洗带残余气的存在进一步增大了两相流体的核磁共振弛豫谱的复杂程度;由于弛豫谱对大、小孔隙的分辨能力模糊化,使得T_(2)截止值在计算白云岩束缚水饱和度时不再适用。

In light of diffusion coupling effect,evaluation by NMR on the physical and gas-bearing properties of dolomite reservoirs faces the following difficulties.First,there is no special study on the mechanism of diffusion coupling effect in dolomite reservoirs.Second,researches on diffusion coupling effect focus mainly on T_(2) but rarely on T_(1).Third,there is no precedent to consider the influence of residual gas on diffusion coupling under the condition of mud filtrate invasion.Under this background,the dolomite cores from the upper part of the fourth member of Leikoupo Formation(upper Lei-4 Member)of the Middle Triassic in the Longmenshan piedmont zone,Sichuan Basin,are selected for experiment to observe the sensitivity of diffusion coupling effect in the tidal-flat dolomite reservoirs in the upper Lei-4 Member to the physical properties of the reseervoirs,and for T_(2) and T_(1) experiments under different centrifugal forces to identify the difference of diffusion coupling effect between T_(1) and T_(2).Finally,by observing the two-dimensional(2D)spectral responses of gas signal and water signal,the influence of residual gas in medium pores on diffusion coupling effect is analyzed.The results show that the diffusion coupling effect is common in dolomites,and its intensity is related to lithology,physical properties and other factors.The cores with different physical properties are different in diffusion coupling effect,and the diffusion coupling effect of T_(1) is slightly stronger than that of T_(2).The residual gas in the flushing zone hinders the diffusion of hydrogen nuclei between large and small pores.It is concluded that the diffusion coupling effect makes the NMR relaxation spectrum of tidal-flat dolomite deviate from the distribution of reservoir pore structure,and its morphological characteristics and deviation degree are controlled by physical factors.The existence of residual gas in the flushing zone further complicates the NMR relaxation spectrum of two-phase fluid.Because the resolution of the relaxation spectrum to large and small pores is fuzzy,the T_(2) cut-off is no longer applicable to the calculation of irreducible water saturation of dolomite.