Analysis of fractures in the Dalan and Kangan carbonate reservoirs using FMI logs: Sefid-Zakhur gas field in the Fars Province, Iran

Abstract

Fracture evaluation is very important in the carbonate reservoirs, particularly fractured reservoir, due to the high impact of fractures on the reservoir permeability. Assessment of fracture parameters of reservoirs is necessary to optimize oil production. Image logs are regarded the best methods for this aim. This study aims to use electrical image logs in the Dalan and Kangan Formations reservoir in the Zagros Basin, SW Iran. This paper presents the new findings of analysis of the effective structures on reservoir quality of the Dalan and Kangan Formations in one of the Iranian gas fields by using FMI image logs. Formation micro-imager (FMI) was taken along with petrophysical logs from depth 4677–5253. FMI data were processed and evaluated. Through this assessment, different types of natural structures such as the fracture (open, close) and bedding planes were identified. The data analysis presented different fracture sets that have a different orientation to the anticline axis. All identified bed contacts were used to determine the structural dip. The general structural dip is interpreted to be 40 degrees toward N15E, with a corresponding strike of N75W-S75E. The total fractures encountered are in three categories: major-conductive, medium-conductive, and minor-conductive fractures. Fracture apertures with values more than 0.01 cm can be the highest value of fracture aperture with a value of more than 0.1 cm, which can be observed at 5013–5015 m, 5023 m, and 5065–5070 m. The range of fracture porosity from conductive fractures is computed to be 0–0.6%. Fractures with porosity range of more than 0.4% can be observed at 4953–4960 m, 4966 m, 4973–4980 m, 5020 m, 5113 m, and 5153 m (Lower Dalan). The highest fracture porosity can be observed at 4954 and 5020 m (close to 0.6%). The main productive zones appear to be 5160–5110 m, 5030–5000 m, 4980–4910 m, and 4830–4770 m. In this section of the reservoir, the high density, porosity, and aperture of the natural conductive/open fractures at certain intervals may indicate high productivity zones at these particular places. Considering the conductive/open fracture apertures, a production profile for a reservoir can be predicted from the fracture-enhanced permeability, with the assumption that the production of the reservoir is fracture-controlled. With the same assumption for the reservoirs within the 5.875-in. section of the well, a predictive production profile was constructed by integrating the fracture apertures.