Fluid typing is one of the greatest challenges in Sichuan Basin, China. Due to the shortcomings of conventional logs, various advanced borehole measurements such as nuclear magnetic resonance, dielectric, and formation testing are introduced by operators. However, these logging tools often fall short as well in the tight carbonate reservoirs of late Precambrian Dengying Formation owing to their sensitivity to high porosity, mud invasion and heterogeneity in the near borehole environment. Besides, the production performance was not fully correspondent with the conventional log responses due to the limited resolution of conventional logs and strong heterogeneity of carbonate reservoirs. This study will demonstrate an integrated workflow for reservoir and fluid characterization by introducing high-resolution electrical image logs.
Dengying carbonate reservoirs have secondary pore in the form of vugs, caves and fractures that account for most of the porosity seen in core. However, distribution of the dissolution features and fractures is not uniform. Borehole images can be used to clearly identify and analyze these features to determine hydrocarbon storage potential. Various attributes of visible porosity can be extracted from image logs, including type, orientation and how open and well connected the dissolution vugs or fractures may be. This information can then be used to rank zones in terms of hydrocarbon storage potential. High-resolution electrical image logs from more than 60 wells and available core data were used to define a secondary porosity facies scheme, where 7 dissolution facies were identified based on core calibrated electrical image logs: cave, alveolate vug, stratiform vug, fracture-vug, isolated vug, massive dense, and interbedded dense facies.
By detailed analysis of open hole logs and well test data, we found that the resistivity log was significantly affected by the rock framework, apart from the fluid type. The resistivity of massive framework like alveolate vug facies is apparently higher than that of interbedded framework like stratiform vug facies. The previous lower limit of resistivity for gas pay was 200 ohm.m, which leaded to many misinterpretation of the fluid type. Now the lower limit could be only 100 ohm.m in the reservoir of interbedded dissolution facies. Besides, the reservoirs with different dissolution facies types show various production performance, the productivity of reservoirs dominated by alveolate vug or cave facies is much higher than the reservoirs dominated by stratiform vug facies with the same porosity.
In this case study we present how to identify different dissolution facies by integrating high-resolution electrical image logs with calibrated core data, and how to establish the fluid typing chart based on deep resistivity logs and formation structures. Our methodology noticeably lowers the uncertainty for fluid typing and gas production forecast in this heterogeneous carbonate reservoirs.