Comparisons of Evaluating Fractured Tight Sandstone Reservoirs Pore Structures Based on Borehole Electrical Image and Nuclear Magnetic Resonance NMR Logs

Lijun Guan, Wei Zhang, P. Zhang, Yuqing Yang, Weiping Cui, Y. Li, Kun Meng, Liang Xiao
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Abstract

Tight sandstone reservoirs characterization and evaluation is very difficult based on conventional well log data owing to the extremely low porosity and permeability, and strong heterogeneity. The main accumulation spaces of conventional reservoirs are intergranular pores, and the pore size is the main controlling factor of permeability. However, besides intergranular pores, fractures play much greater important role in accumulating hydrocarbon, improving the pore connectivity and pore structure in tight sandstone reservoirs. Hence, it should be accurately predicted the pore structure dredged by fractures to improve the characterization of tight sandstone reservoirs. Generally, nuclear magnetic resonance (NMR) logging is an effective method to evaluate formation pore structure. However, it cannot be well used in fractured reservoirs because the NMR T2 spectra has no any response for fractures with width <2mm. The borehole electrical image log is usable in characterizing fractured reservoirs. The pore spectrum, which is extracted from the borehole electrical image log, can be used to qualitatively reflect the pore size. Hence, it will play an important role in fractured reservoirs pore structure characterization. In this study, based on the comprehensive analysis of the pore spectra, the corresponding mercury injection capillary pressure (MICP) data and pore-throat radius distributions acquired from core samples, a relationship that connects the 1/POR and capillary pressure (Pc) is proposed. Established a model based on formation classification to transform porosity spectrum into pseudo capillary pressure curve. In addition, a Swanson parameter-based permeability prediction model is also developed to extract fractured formation permeability. Meanwhile, to verify the superiority and otherness of borehole electrical image and NMR log, the model that evaluated reservoirs pore structure from NMR log is also established. Based on the application of the proposed method and models in actual formations, the evaluated pore structure parameters and permeabilities from two types of well log data are compared. The results illustrates that in formations with relative good pore structure, the predicted pore structure parameters and permeabilities from these two types of well log data agree well with the drill stem testing data and core-derived result. However, in low permeability sandstones with relatively poor pore structure, the porosity spectra can be well used to evaluate the pore structure, whereas the characterized pore structure from NMR log is overestimated. With the comprehensive research of reservoirs pore structure and permeability, the fractured tight sandstone formations with development value are precisely identified. This proposed method has greatest advantages that the pore structure of fractured reservoirs can be characterized, and the contribution of fractures to the pore connectivity and permeability can be quantified. it is usable in tight sandstone reservoirs validity prediction.
基于井眼电成像与核磁共振测井评价裂缝性致密砂岩储层孔隙结构的比较
致密砂岩储层的孔隙度和渗透率极低,非均质性强,常规测井资料很难对其进行表征和评价。常规储层主要成藏空间为粒间孔隙,孔隙大小是影响渗透率的主要因素。但致密砂岩储层除粒间孔隙外,裂缝对油气聚集、改善孔隙连通性和孔隙结构的作用要大得多。因此,应准确预测裂缝疏通的孔隙结构,以改善致密砂岩储层的表征。一般来说,核磁共振测井是评价地层孔隙结构的有效方法。然而,由于裂缝宽度小于2mm的裂缝,核磁共振T2谱没有任何响应,因此不能很好地应用于裂缝性油藏。井眼电成像测井可用于裂缝性储层的表征。从井眼电成像测井曲线中提取孔隙谱,可定性反映孔隙大小。因此,它将在裂缝性储层孔隙结构表征中发挥重要作用。本文在综合分析岩心样品孔隙谱、相应的压汞毛细管压力(MICP)数据和孔喉半径分布的基础上,提出了1/POR与毛细管压力(Pc)之间的关系。建立了基于地层分类的孔隙度谱模型,将孔隙度谱转化为拟毛管压力曲线。此外,还建立了基于Swanson参数的渗透率预测模型,用于裂缝性地层渗透率的提取。同时,为验证井眼电成像与核磁共振测井的优越性和差异性,建立了利用核磁共振测井评价储层孔隙结构的模型。将该方法和模型应用于实际地层,比较了两种测井资料评价的孔隙结构参数和渗透率。结果表明,在孔隙结构相对较好的地层中,两种测井资料预测的孔隙结构参数和渗透率与钻杆测试数据和岩心推导结果吻合较好。然而,在孔隙结构相对较差的低渗透砂岩中,孔隙谱可以很好地评价孔隙结构,而核磁共振测井表征的孔隙结构则被高估。通过对储层孔隙结构和渗透率的综合研究,准确识别出具有开发价值的裂缝性致密砂岩储层。该方法最大的优点是可以表征裂缝性储层的孔隙结构,量化裂缝对孔隙连通性和渗透率的贡献。该方法可用于致密砂岩储层有效性预测。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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