Carbonate Low Resistivity Pay Formation Evaluation Workflow

Abstract

Objectives/Scope Complex carbonate reservoirs have several formation evaluation challenges because of more than one pore type, and the main challenges in this kind of formation is resulted from the presence of microporosity which is affecting the log measurements and has large impact on the fluid flow characteristics of reservoir. Therefore, the formation evaluation results and the expectation of the reservoir fluid flow are far from accurate using conventional approach. Excessive presence of Microporosity suppresses the resistivity and Neutron measurement, and the complex pore size distribution affects the reservoir fluid flow. Formation evaluation workflow will be presented. The key element of this workflow is identifying the complex carbonate pore type distribution then correct the logging measurements, which is affected by the excessive micro pores presence and leading to the optimization of well testing and the production. The presented workflow in this paper is mainly utilizing the NMR (Nuclear Magnetic Resonance) logging measurements, Dielectric measurements and tensor 3DEX resistivity and apply the formation water salinity correction on neutron measurement. Microporosity is holding a saline formation water due to the capillarity forces. Excessive micro-pores is reflecting more irreducible saline water, this saline water is suppressing the resistivity to shortcut the resistivity response, resulting high water saturation in the oil zone which is not confirmed by the water cut during the production. In addition, the high saline water has chlorine, which has high capture cross section affecting the neutron values and resulting overestimation for the porosity. Microporosity developed in carbonate rocks is mainly due to micrite and Micritization diageneses. Carbonate pore type distribution from NMR logging measurements has been validated by special core analysis in several studies, by quantifying the different carbonate pore type Micro, Meso and Macro porosities will help to understand the nature of the rock and predict the rock Petrophysical properties. So getting water fraction in the formation-testing sample while it is located above the 100% oil sample can be explained using this carbonate pore type quantification. Fluid typing from NMR and Dielectric are resistivity and Archie parameters independent process. Micritization is creating a micro pores in layering format so the micro pores and large pores are behaving like the thin laminated Shaly sand reservoir, and so tensor 3DEX resistivity to get Rh (Horizontal resistivity) and Rv (vertical resistivity) is used to enhance the formation evaluation using the improved formation resistivity. The integrated Petrophysical workflow for the complex Micritic carbonate reservoir has enhanced the formation evaluation results, accurate porosity, improved water saturation and fluid flow prediction. Utilizing this workflow will help to avoid the bypassed oil in the reservoir and clear identification for the fluids contacts inside the reservoir and better reservoir management. Conventional formation evaluation is limited in such complex Micritic carbonate reservoir as the results is misleading the interpreter so the presented workflow will help to overcome these challenges and provide a simple technique with accurate results and no more bypassed oil.

AAPG Datapages/Search and Discovery Article #90356 ©2019 AAPG Middle East Region Geoscience Technology Workshop, Low Resistivity Pay, Muscat, Oman, October 7-9, 2019