Developing Carbonate Low Resistivity Pay Using Advanced LWD Ultra-Deep Resistivity with Nuclear Magnetic Resonance and High-Resolution Micro-Resistivity Imaging, A Case Study in a Mature Field, Onshore Abu Dhabi
Authors: Fares Wael , Dey Swapan , New Paul , Albreiki Warda , Bin Abd Rashid Atiqurrahman , Lopez Gabriela , Ghobashy Ahmed , Mubeen Muhammad , Clegg Nigel 
Published: October 2023

One of the major challenges in low resistivity pay zones (below 1 ohm.m) is to map the surrounding boundaries and the reservoir thickness. This has a significant impact on the understanding of the subsequent hydrocarbon production. This case study presents an effective approach in two recent wells where effective well placement and formation evaluation were achieved using Multiple Logging While Drilling (LWD) sensors, including Ultra-Deep Azimuthal Resistivity (UDAR) mapping technology in low resistivity carbonate zone.

Extensive pre-well modeling analysis performed for both wells using all available nearby offset wells has concluded that the UDAR technology would be sensitive enough to identify the low resistivity zones in two 6-in. laterals in two wells. In Well 1, 4 ¾-in. Gamma Ray, Wave Propagation Resistivity, Density/Neutron and UDAR Logging-While-Drilling (LWD) sensors were used to drill approximately 3,200 feet. Well 2 was planned as a 6-in. lateral with Maximum Reservoir Contact (MRC) up to 8,466 feet, intersecting two carbonate layers. The Bottom Hole Assembly (BHA) included an even larger suite of LWD sensors, comprising UDAR, Gamma Ray, Wave Propagation Resistivity, Laterolog Micro-Resistivity Imager, Density/Neutron and Nuclear Magnetic Resonance (NMR). 1-Dimensional (1D) and 3-Dimensional (3D) inversions of the UDAR data in real-time allowed a precise mapping of formations and fluid boundaries at great distance from the wellbore.

Wells 1 and 2 were drilled successfully through their respective defined target zones. In Well 1, the UDAR 1D inversion identified and confirmed the low resistivity zone below the trajectory and helped to keep the well in target zone till well TD. In Well 2, the UDAR real-time 1D and 3D inversions assisted in precisely mapping complex carbonate reservoir boundaries. Furthermore, the integration of the UDAR 1D and 3D mapping with the NMR measurements allowed identifying the bound water intervals and the direction. In addition, Laterolog Micro-Resistivity Imaging confirmed the presence of vugs and fractures along the wellbore. While moving up to the second target layer, the UDAR 1D inversion successfully mapped the reservoir boundaries, despite the low resistivity of the reservoir (0.7 – 1 Ohm.m). The array of resistivity sensors provided real-time information on the oil saturation, which helped in maximizing reservoir contact by placing the well strategically. Furthermore, the improved reservoir understanding, and insight provided by the integration of UDAR for boundary mapping, NMR for permeability and porosity readings, and Micro-Resistivity Imaging for aperture identifications assisted in optimizing the completion design to isolate unproductive intervals along the well path and delay possible water influx.

Successful placement of these laterals in the productive zones despite the low resistivity normally limiting formation and boundary detection brought increased reservoir understanding and optimization of the completion design, confirming the LWD solution adequacy for such environment.