Abstract:

In strongly diagenetically altered low- to medium-permeability sandstone reservoirs, disconnected pores are widely developed, making it difficult for conventional nuclear magnetic resonance (NMR)-based permeability evaluation methods to accurately reflect the true seepage capacity of the reservoir. These methods generally use an empirical T₂ cutoff to partition free fluid volume (FFV) and bound volume irreducible (BVI), and then estimate permeability using the classical Timur-Coates model. However, because they cannot effectively distinguish connected pores from disconnected pores, the predicted permeability is often systematically overestimated. To address this issue, this study proposes a pore-throat-connectivity-constrained method for determining the T₂ cutoff and incorporates it into the Timur-Coates permeability calculation workflow. First, core NMR T₂ spectra and high-pressure mercury intrusion capillary pressure data are jointly utilized and transformed into equivalent pore-size distributions and cumulative distribution curves. By quantitatively analyzing the correspondence between these two types of curves, the volume of disconnected pores and the associated critical pore-size range are identified, thereby determining a free-fluid T₂ cutoff with clear physical significance. Based on this cutoff, FFV and BVI are reclassified and then substituted into the Timur-Coates model to recalculate permeability. A case study from sandstone reservoirs in the Bozhong Sag, Bohai Bay Basin, demonstrates that the proposed method effectively reduces the interference of disconnected pores in FFV estimation and transforms the T₂ cutoff from an empirical selection into a quantitatively determined parameter constrained by pore-throat connectivity. Compared with the conventional empirical cutoff method, the proposed approach yields permeability predictions that are in much better agreement with measured core permeability, with the logarithmic root mean square error reduced from 1.079 to 0.104. These results indicate that the proposed method significantly improves the accuracy and stability of permeability evaluation in reservoirs affected by complex diagenesis, and provides a more geologically meaningful and practically valuable approach for the refined permeability characterization of low-permeability complex sandstone reservoirs.

Key words: pore-throat connectivity, permeability, nuclear magnetic resonance, mercury intrusion capillary pressure, carbonate cementation

摘要：

在强成岩作用改造的中低渗砂岩储层中，非连通孔隙普遍发育，使传统核磁共振(NMR)渗透率评价方法难以准确反映储层真实渗流能力。该类方法通常采用经验T₂截止值划分可动流体体积(FFV)与束缚流体体积(BVI)，并基于经典Timur-Coates模型进行渗透率计算。然而，由于其未能有效区分连通孔隙与非连通孔隙，往往导致渗透率评价结果出现系统性高估。针对这一问题，本文提出一种基于孔喉连通性约束的T₂截止值确定方法，并将其嵌入Timur-Coates渗透率计算流程。首先联合利用岩心核磁共振T₂谱与高压压汞毛管压力实验数据，将二者统一转换为等效孔径分布及累积分布曲线，通过对两类曲线的对应关系进行定量分析，识别非连通孔隙体积及其所对应的临界孔径范围，确定具有明确物理约束意义的可动流体T₂截止值。基于该截止值重新划分可动与束缚流体体积，并代入Timur-Coates模型实现渗透率计算。以渤海湾盆地渤中凹陷砂岩储层为例开展方法验证，新方法能够有效削弱非连通孔隙对可动流体体积估算的干扰，使T₂截止值由经验选取转变为受孔喉连通性约束的定量确定过程。与传统经验截止值方法相比，该方法所得渗透率结果与岩心实测渗透率具有更好的一致性，对数均方根误差由1.079降低至0.104，显著提升了复杂成岩背景下渗透率评价的准确性与稳定性。研究表明，该方法可为复杂成岩背景下低渗砂岩储层渗透率精细评价提供新的技术思路，为低渗复杂砂岩储层渗透率评价提供了更具地质意义和应用价值的技术途径。

关键词: 孔喉连通性, 渗透率, 核磁共振, 毛管压力实验, 碳酸盐岩胶结物

CLC Number: 

• P618.13

• TE19