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The impact of geological and petrophysical heterogeneities on Archie’s exponents: a case study for Sarvak carbonate reservoir, Dezful Embayment, southwest Iran

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Abstract

Calculating the volume of hydrocarbons in place in carbonate reservoirs is associated with a high uncertainty due to the sedimentary texture and diagenetic characteristics. The Archie’s equation is one of the most common methods to measure the volume of reservoir fluids. Petrophysical parameters and geological attributes control the exponents of this equation. If Archie’s exponents (m and a) are considered constant, it can have a high uncertainty in the volume of reservoir fluids. This study investigates the effect of petrophysical parameters (pore type, pore size distribution, and pore throat radius) and geological attributes (sedimentary facies and diagenesis) on the Archie’s exponents. Based on petrography results, seven sedimentary facies related to a carbonate ramp were identified, and the diagenesis processes have caused a great heterogeneity of the studied carbonate reservoir. Archie’s exponents were calculated for four models of reservoir rock types. Finally, a total of five rock types were defined based on the study of thin sections, well logs, scanning electron microscopy images, special/conventional core analysis, and mercury injection testing. Combining pore types is influenced by diagenetic processes and sedimentary texture, so the diversity in the composition of pore types reduce the measurement accuracy of Archie’s exponents and resulted water saturation. The results show that pore type, pore size distribution, and fractures significantly affect the cementation exponent and the tortuosity factor. Accordingly, the determination of the rock types in heterogeneous reservoirs based on the pore size distribution, pore type, pore throat radius and fractures has the most significant impact on the accuracy of the Archie exponents.

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Abbreviations

Φ:

Porosity, fraction

Φe:

Effective porosity, %

a:

Tortuosity factor

CGR:

Compensated gamma ray, API

DT:

Sonic, μs/ft

FRF:

Formation resistance factor, Ω.m

GR:

Gamma ray, API

K:

Permeability, mD

LLD:

Deep laterolog, Ω.m

LLS:

Shallow laterolog, Ω.m

m:

Cementation exponent

MICP:

Mercury injection capillary pressure

MSFL:

Microspherically focused log, Ω.m

NPHI:

Neutron porosity, %

Pc:

Capillary pressure, psi

PEF:

Photoelectric factor log, Ω.m

R35:

Pore throat radius at 35% mercury saturation, μm

RCAL:

Routine core analysis

RFN:

Rock-fabric numbers

RHOB:

Density, g/cm3

Ro:

Resistivity of the 100% saturated core plug, Ω.m

RQI:

Reservoir quality index, μm

RT:

Rock type

Rt:

Resistivity of the formation, Ω.m

Rw:

Resistivity of the formation brine, Ω.m

SCAL:

Special core analysis

SEM:

Scanning electronic microscopy

SW:

Water saturation, v/v

SWE:

Effective water saturation, v/v

VDL:

Velocity deviation log, m/s

Vpreal:

Real synthetic velocity

Vpsyn:

Calculated synthetic velocity

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Acknowledgements

We express our gratitude to the unknown reviewers that helped us improve the quality of this research paper.

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All the authors had good and active cooperation in writing the article and the article has been completed with the participation of all the authors. MM: is the main writer of the manuscript. AK: is supervisor and results interpreter.

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Correspondence to Ali Kadkhodaie.

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Moradi, M., Kadkhodaie, A. The impact of geological and petrophysical heterogeneities on Archie’s exponents: a case study for Sarvak carbonate reservoir, Dezful Embayment, southwest Iran. Carbonates Evaporites 39, 35 (2024). https://doi.org/10.1007/s13146-024-00945-6

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  • DOI: https://doi.org/10.1007/s13146-024-00945-6

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