Optimizing Frac-Pack Completions in Multilayer and Low-Stress-Contrast Zones

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 151870, ’Systematic Approach Used to Optimize Frac-Pack Completions in Multilayer and Low-Stress-Contrast Environment Encountered in the Usan Field,’ by M. Prada, SPE, Chevron; J. Hernandez, SPE, and R. Bouget, Total; and S. Green, SPE, Schlumberger, prepared for the 2012 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 15-17 February. The paper has not been peer reviewed. The deepwater Usan field, approximately 100 km south of Port Harcourt, Nigeria, has multiple oil-bearing turbidite sands of varying thickness and permeabilities that are separated by massive and thin shale intervals, making the selection of completion intervals difficult. Fracture-height confinement is challenging because of the small stress contrast between the sand bodies and the bounding shales and the stress-field variability in a highly compartmentalized environment. Frac-pack completions were used in this field. The main challenges, lessons learned, and improvement opportunities captured during the first 2 years of the Usan completions campaign are presented. Introduction The Usan oil field is offshore Nigeria in Block OML 138, approximately 60 km south of the existing Amenan field (Fig. 1). Water depth ranges from 660 to 820 m. The Usan project is a stand-alone development and will comprise more than 40 subsea wells (oil producers and water/gas injectors) connected to a floating production, storage, and offloading (FPSO) facility. Oil producers are completed as cased-hole frac packs (CHFPs), and the water and gas injectors are openhole expandable-screen (OHES) completions. Some of the oil producers and gas injectors are equipped with surface-operated downhole flow-control valves to enable real-time reservoir management and production optimization. Reservoir Aspect These Usan Tertiary reservoirs have multiple turbidite-sand bodies of 3- to 30-m thickness with permeability up to 3,000 md and are separated by 3- to 20-m-thick shale layers, making them difficult to complete. In a smaller scale, some of the reservoir targets have very-thin (<5 cm) to medium (5–10 cm) laminated shale intercalations that are difficult to predict and exclude from the completion interval. Because of the strong compartmentalization, there are important variations of fluid parameters between reservoir levels and panels. Oil density ranges from 20 to 40°API, and in-situ viscosity ranges from 0.2 to 4 cp. Oil is slightly undersaturated, with an average reservoir pressure 10 to 20 bar above saturation pressure.