Abstract

The incremental demand for oil and gas is driving the industry to search and produce hydrocarbons in more challenging environments. The evolution of Drilling Technologies in recent years has allowed operators to explore high viscous oil in turbidite reservoirs, at shallow burial depth and water depths greater than 1500 m. In such environment, the reservoirs are usually formed by unconsolidated sands with low fracture gradient, high porosity and permeability, and low water saturation at low temperatures.

Although great advances have been made for open-hole completion technologies, optimizing sandface completion for productivity is a big challenge. Issues for a sub-sea well construction can be: reduced drilling fluid weight window available; special well geometries to maximize reservoir drainage; non-invasive drilling and completion fluids for well productivity; mandatory sand control; effective control on water injection and production; completion reliability and intervention requirements, thermal effects due to cold waters like hydrate formation and thermal fractures for injectors.

This paper addresses the issues related to well construction in such environment and presents recent developments for well completion design such as: new approach for sand control system selection for heavy oil that excludes sand but allows fines to pass; minimization of the pressure drop through the sand control system; well segmentation to correct the oil influx profile, improving reservoir drainage, and giving selectivity and control of water production.

Introduction

The incremental demand for oil and gas and the high oil price at the market are driving the industry to search and produce hydrocarbons in more challenging environments. The current exploration efforts are focused in cold waters and deepwater basins such as those in North Sea, Canada, West Africa and Brazil where viscous crude oil has already been found and significant additional discoveries are expected. Viscous crude oil are commonly referred as heavy oil and identified as having API gravity lower than 20 degrees and viscosity at the reservoir conditions higher than 10 cP. In Brazil, Petrobras considers as ultra-viscous crude oil or ultra-heavy oil the crude with API gravity lower than 14 degrees and viscosity at reservoir conditions higher than 100 cP. Offshore heavy and ultra-heavy oil accumulations are estimated to be in the order of 2 trillion of barrels worldwide.

Typically, offshore heavy oil (OHO) projects have higher costs per unit volume of hydrocarbon as compared to lighter oil and these projects tend to have low recovery efficiency and low productivity. Considering costs involved on the well construction and the technical challenges needed to drill, complete and produce these wells, questions related to the reservoir recovery and well design optimization become progressively more important. These tasks are complex and demands extensive efforts towards a detailed reservoir modeling, fluid properties, and investigation of the parameters that affect the productivity as well as the risks and costs involved in each project alternative.

The keys to successful heavy oil exploitation are reservoir exposure, which increases the mobility of high viscosity liquids within the reservoir matrix, and enhanced well completion techniques. Advances in the completion techniques are on the way to directly address the efficient recovery processes, improving productivity and reducing development and operational costs.

Conditions on the Offshore Heavy and Ultra-Heavy Oil Reservoirs in Brazil

Most of the offshore heavy and ultra-heavy oil reservoirs in Brazil are found in Campos and Santos Basins which are in water depths greater than 1500 m. In such environment, the reservoirs are found in turbidite reservoirs at shallow burial depths with significant internal sub-seismic complexity. Usually, these formations are weak and unconsolidated sandstones, with low fracture gradients, normal (or lower) pressure gradients, high porosities and permeabilities, low water saturation, low GOR and low reservoir temperatures. A schematic view of the conditions heavy oil is encountered offshore is presented on Figure 1 and quantified on Table 1.

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