Revisiting Petrophysical and Fluid Characteristics of a Mature Smectite-Rich Shally Sand Reservoir for EOR Screening in the Sultanate of Oman
Authors: AL-Harrasi Arwa , AL-Mjeni Rifaat , Al-Yaarubi Azzan , Al-Battashi Fathiya , Al-Jabri Ahmed 
Published: November 2018

This paper reviews the petrophysical evaluation of a major oil field in the South of the Sultanate of Oman. The current recovery factor is 15% with over 400 vertical and horizontal well penetrations. The reservoir is in the shaly-sand Mahwis formation. This reservoir is characterized by high porosity, averaging around 30pu, and permeabilities between 200 to 2000mD. The hydrocarbon is of variable viscosity ranging from 250 to 2000 cP closer to water contact. The formation water is of low salinity around 5000ppm NaCl. The mineralogy is composed of quartz, feldspars and clays that include large proportion of smectite. Smectite has the highest cation exchange capacity–CEC-among all clay types. The varying distribution of smectite in the field led to a varied resistivity response across the reservoir. Consequently, the resistivity-based saturation models overestimated water saturation in some parts of the field. That coupled with historical production has led to ambiguity in the current saturation distribution of the reservoir.

A petrophysical workflow is devised to overcome saturation ambiguity and provide essential information for current infill and future field developments. The evaluation utilizes modern logging tools including elemental spectroscopy, dielectric dispersion and multi-dimensional NMR. The Spectroscopy data is useful in determining representative mineralogical composition. The results of this analysis is verified against side-wall-core samples. The mineralogy data is used as an input into a multi-mineral-volumetric-solver to compute accurate porosity and matrix dielectric permittivity. Subsequently, these are used as inputs into dielectric dispersion analysis that outputs total water and hydrocarbon volumes at the tool’s depth of investigation. The integration of NMR and dielectric logs help to partition measured T2 distribution into hydrocarbon, bound and free fluids. This information is then used to determine formation saturation and in-situ oil viscosity. Additionally, Carbon-Oxygen data, which outputs resistivity-independent saturation was acquired after setting the casing and allowing sufficient time for mud filtrate to dissipate. The comparison of saturation from the analysis behind casing and shallow reading tools (i.e NMR and dielectric) allowed to determine intervals with mobile hydrocarbon and accurately determine the onset of the transition zone.

The study presented a methodology to accurately separate NMR signals of the heavy oil, bound and free fluids. Subsequently, these datasets were used as inputs into proven correlations to estimate saturation and in-situ oil viscosity. The results were in good agreement with laboratory data performed on core samples acquired in the same well. In addition, the integration of the elemental capture spectroscopy and dielectric permittivity logs resulted in a good quantitative estimate of the hydrocarbon saturation. The result of this study contributed to delineating sections of the field where the current saturation parameters needed to be re-evaluated to reflect the mineralogy of the field. This in turn forms an integral component of the optimization, planning and execution of ongoing field development operations. This result is also of considerable importance of ongoing EOR screening initiatives aimed to increase the field’s recovery factor.