Abstract

The application of the steam assisted gravity drainage (SAGD) process using insulated concentric coiled tubing (ICCT ™) as the steam injection string, is experiencing increased popularity for the production of heavy oil. The thermal and fluid mechanic performance of the tubing is an important aspect to enable the SAGD process to operate efficiently. In addition, optimizing oil production, maintaining sustained production rates and operating at maximum thermodynamic efficiency is dependent upon the steam injection program.

This paper presents the latest results of thermal performance analyses and measurements made with full scale ICCT test sections in the laboratory and temperature profile measurements of a full length ICCT steam injection string operating in a single well SAGD (SW-SAGD) application. Field measurements of this temperature profiles and various other SW-SAGD process parameters are compared with two-phase flow computer modeling predictions, using HOWSCAT ™, to confirm the validity of the numerical modeling. Pressure drop predictions are also compared with those obtained using Fritzsche's formula and monograph for steam pipes. The horizontal well steam circulation analysis tool (HOWSCAT) is then used to analyze the thermal and fluid mechanic characteristics of larger sized insulated tubing strings to accommodate the higher steam injection rates required as the steam chamber size grows with time and/or for the production of thicker pay zones.

Also presented are some heavy oil reservoir thermal response characteristics which are used to further improve the accuracy of the numerical simulations and assess some of the steam injection program variables which can be controlled for achieving improved efficiencies.

Introduction

Considerable understanding has been gained and information produced by Luft et. al.1 on the thermal performance of the new insulated concentric coiled tubing which had originally been developed for the single-well steam assisted gravity drainage (SW-SAGD) process2. As with the development of conventional jointed thermal tubing, the thermal and fluid mechanic performance of ICCT focused heavily on achieving the lowest possible thermal conductivity of the insulating jacket and the most favorable steam flow characteristics. For example, favorable steam flow parameters included reduction of the pressure drop along the string and minimizing the steam injection pressures and temperatures while retaining maximum enthalpy delivery to the reservoir. For the SW-SAGD application, a steam injection flow in the range of 100 to 150 m3/day CWE was required and a maximum effective thermal conductivity, keff, of 0.174 W/m-C was considered acceptable as reported by Nzekwu et. al3. Numerical simulations using HOWSCAT ™ showed that an ICCT size of only 2-7/8 " OD X 1-3/4 " ID could satisfy these criterion and for typical SW-SAGD well bore lengths of 2000 m TMD, the maximum wellhead steam injection temperature and pressure were specified as 350C and 17 MPa respectively. It is recognized that using smaller steam injection tubing, while eliminating threaded connections along with their external upsets, enables smaller casing completions without sacrificing pump or tubing size. This is a second major cost advantage of the SW-SAGD process using ICCT, along with the primary cost saving of eliminating a second horizontal well which is required for the DW-SAGD process.

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