Elemental geochemistry of the Upper Cretaceous reservoir and surrounding formations applied in geosteering of horizontal wells, Lebăda Field – Western Black Sea

Ionuţ Mihai Prundeanu,Ciprian Chelariu,David Rafael Contreras Perez

doi:10.2516/ogst/2020083

Abstract

The precise landing and steering of horizontal wells using conventional mudlogging and Logging While Drilling (LWD) data is a particular challenge for the Lebăda Field, offshore Romania. The use of a new technique of elemental geochemistry analysis (or chemosteering) became an option for the identification of Cenomanian, Turonian–Coniacian–Santonian, Campanian and Eocene strata. This has enabled more accurate placement of the horizontal development wells within the desired reservoir target interval. Geochemical data enabled the identification of chemostratigraphic zones C1, C2, C3 and zone R that correspond to the reservoir section. The application is a result of the geochemical zonation performed using elements and ratios that are sensitive to depositional environment, sea level change, heavy mineral concentrations and siliciclastic input namely: Sr/Ca, Zr/Th, Si/Zr and Si/K. In ascending stratigraphic order, the ratio thresholds of zone C3 are Zr/Th > 11, Sr/Ca > 1.1, Si/Zr < 22 and Si/K < 19, while zone R corresponds to 5.5 < Zr/Th < 11, Sr/Ca < 1.1, Si/Zr > 22 and Si/K > 19. C2 zone is defined by Zr/Th < 5.5, Sr/Ca > 1.1, Si/Zr < 22 and Si/K < 19 and C1 zone is characterized by Si/Zr > 22 and Si/K > 19. The selected geochemical ratios indicate a strong geochemical zonation. In the case of offset wells, 85.9% of the data confirmed the proposed classification and 89.4% for the real-time application case. The zone R shows a strong contrast with the surrounding formations facilitating critical decisions during well placement and geosteering, increasing the reservoir exposure by 28%. The quantitative approach delivered very valuable results, providing a solid foundation to define correlation and well landing intervals. Simultaneously, the cost of the method represents a fraction of the LWD cost and 0.15% of the total project cost, making it very cost effective and a standard approach for future projects.

Highlights

IntroductionFor the landing and steering of wells, the industry uses lithology, micropaleontology, real-time petrophysics interpretation from Logging While Drilling (LWD), and gas chromatography data
For well placement purposes, cuttings elemental composition is rarely used
Chemostratigraphy involves the identification of the geological characteristics based on the spatial-temporal

Summary

Introduction

For the landing and steering of wells, the industry uses lithology, micropaleontology, real-time petrophysics interpretation from Logging While Drilling (LWD), and gas chromatography data. In the Lebăda Field, offshore in Black Sea (Fig. 1), none of these methods enables clear contrast between the reservoir and overlying/underlying sediments (Fig. 2). In order to land horizontal wells for further field development, alternative solutions are necessary. Chemostratigraphy involves the identification of the geological characteristics based on the spatial-temporal. I.M. Prundeanu et al.: Oil & Gas Science and Technology – Rev. IFP Energies nouvelles 76, 1 (2021)

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