Electrical Downhole Heaters for Faja Heavy-Oil Reservoirs

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 117682, "Feasibility of Using Electrical Downhole Heaters in Faja Heavy-Oil Reservoirs," by Raul Rodriguez, SPE, Jose Luis Bashbush, SPE, and Adafel Rincon, SPE, Schlumberger, originally prepared for the 2008 SPE International Thermal Operations and Heavy Oil Symposium, Calgary, 20-23 October. The paper has not been peer reviewed. The full-length paper examines use of downhole heaters in thick sands stimulating vertical and horizontal wells, evaluates temporary application of downhole heaters in horizontal wells for a limited period of time, and covers basic economic analyses. Introduction The Orinoco belt (Faja) in Venezuela contains one of the largest resources in the world of heavy and extraheavy oil. Because of the production decline of conventional light crude, projects must focus on increasing the recovery of heavy and extraheavy oils by use of thermal and nonthermal methods. Steam-based thermal-recovery processes are more efficient in low-pressure reservoirs; however, because of their depth, the initial pressures of the reservoirs in the Faja are relatively high, ranging from 600 to 1,500 psi with viscosities typically greater than 2,000 cp. For these reasons, it is important to decrease the pressure of the reservoirs with primary-production techniques to facilitate the economical implementation of steam-injection-based methods. The initial production of heavy and viscous oils can be accelerated by use of downhole heaters that, by providing energy to the vicinity of the well, decrease oil viscosity and increase the oil-production rate. A consequential advantage of using downhole heaters as a prelude to a steam-injection process is that they accelerate early production and reservoir-pressure depletion. Electrical Downhole Heaters. Electrical heating consists of providing electrical currents to generate heat and increase the temperature near the wellbore. There are two kinds of downhole heaters in use by the oil industry: inductive heaters, generating heat in accordance with Maxwell's law, and resistive heaters that generate heat in accordance with the Joule effect. In the latter case, the heat transfer is by conduction and it requires an extended time period to heat the reservoir. In this study, the resistive heater is modeled with the aid of a numerical thermal simulator. This type of heating process stimulates oil recovery primarily by reducing the oil viscosity in the near-wellbore region and secondly by thermal expansion of reservoir fluids. The key parameters to be understood in this process are the heavy-oil-viscosity variation with temperature and the rate of heat provided by the heater associated with the generated temperature gradient in the volume around the well. Heat output from resistive heaters typically ranges from 14 to 730 W/ft (1,100 to 60,000 Btu/D/ft). A maximum-exposure temperature normally is imposed on the heaters.