Application of image logs to improve cementation exponent of the Asmari reservoir in an oilfields of Dezful Embayment

Document Type : Original Article

Authors

1 Department of Geology, Faculty of Earth Sciences, Kharazmi University, Tehran, Iran

2 Research Institute of Petroleum Industry, Tehran, Iran

Abstract

Introduction
Determination of petrophysical properties is the most important part of hydrocarbon reservoirs characterization. Accordingly, by obtaining total and effective porosity, the volume of shale, and the saturation of the fluids, the reservoir regions are determined. Identification of fluid saturation as a petrophysical characteristic has an important effect in hydrocarbon reservoir studies. The exact calculation of this factor depends on the cementation exponent (m) which is commonly considered as a constant number. Due to the heterogeneous structure of carbonate rocks, the use of constant values can lead to a large error in the fluid saturation calculations.
In this study, in order to improve the cementation exponent and to accurately calculate the saturation of the fractured Asmari reservoir in one of the wells south of Dezful Embayment oilfields, petrophysical, core and image log data (FMI) were used.
Material & Methods
Petrophysical evaluation was performed once with a conventional method, taking into account the constant value for m and again with the variable m value specified by image logs. In this regard, firstly, the petrophysical interpretation of full set logs of the studied well were made, using the Multimin model in Geolog software. Then, the FMI image logs of the well were used to extract various structural phenomena, especially fractures. Finally, the variable cementation exponent was calculated by taking into account the density of fractures of different zones and the petrophysical evaluation was again considered with consideration of its effect.
Results and discussion
Based on image log analysis in studied reservoir, three major fractures, including open and semi-open fractures, filled fractures and breakout fractures -due to borehole collapse- have been identified. These fractures are more commonly observed in the lower part of the Asmari carbonate reservoir in the studied well. Image log analysis showed that with increasing the open and semi-open fractures’ density, variation of cementation exponent also increased. So, the effect of this exponent on petrophysical evaluation is important. Also, the results of petrophysical evaluation showed that the amount and pattern of fractures are important factors controlling the cementation exponent of the studied reservoir. As the fractures increase, the cementation exponent decreases and as a result, the water saturation decreases.
Conclusions
Comparison of petrophysical evaluation with conventional and image log methods and correlation with the core data indicate that despite the changes in cementation exponent, porosities of Asmari reservoir did not change significantly in the studied oil field. On the contrary, small changes in the cementation exponent, can lead to large variations in the saturation factor in this reservoir.
Therefore, the correct estimation of the cementation exponent in each zone is necessary for an accurate estimation of the saturation factor in the reservoirs. Therefore, using image logs to evaluate petrophysical properties in order to improve the evaluation of reservoir quality can be a precise and relatively rapid method in reservoir studies.

Keywords

References

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