Impact of Operational Parameters and Reservoir Variables during the Start-up Phase of a SAGD Process

Abstract

This paper highlights how numerical simulation can be used as a tool to optimize the start-up phase of a SAGD process.During start-up, the main objective is to create a uniform communication path between the two wells by first circulating steam in both the injector and the producer well and then imposing a differential pressure between them.The dynamics of this process leads to important temperature and pressure transients that should be carefully considered when developing a start-up strategy. Usually, this start-up strategy aims at minimizing the time in which the well pair can be converted to full SAGD operation without causing any adverse effects on the long-term process performance.

A fully coupled wellbore/reservoir thermal simulator was used to conduct a sensitivity analysis, in which the effects of steam circulation rate, tubing diameter, tubing insulation and bottom hole pressure were investigated.The effects of the pressure differential between the wells, and the timing of imposing such pressure differential, were also looked at.To better account for the interaction between the processes happening in the wellbore and in the reservoir, the discretized wellbore was placed inside a hybrid reservoir grid. Aiming at investigating the influence of vertical and horizontal permeability, reservoir pressure, initial oil/water saturation and fluid properties, the start-up strategy was examined for three different cases representing the main heavy oil production areas in Alberta, Canada: Athabasca, Cold Lake and Peace River.

Introduction

Vast quantities of heavy and extra heavy oil are trapped in shallow and easily accessible reservoirs in Western Canada, but due to the intrinsic high-viscosity nature of the crude, producers face significant challenges in recovering oil from these reservoirs.Using steam-based in-situ recovery methods, coupled to horizontal well technology, has emerged as an economic and efficiently way to produce those reservoirs[1,2]. Currently, one of the most promising of these methods is the so-called Steam Assisted Gravity Drainage (SAGD) process.The most common implementation consists of two parallel horizontal wells, the first drilled near the bottom of the reservoir and the second located a short distance above it, typically 5 to 10 m.The top well provides continuous steam supply into the reservoir and the lower one allows for continuous production of oil, gas and condensed water.

Experiments carried out by the Alberta Department of Energy in the early 1990's showed recovery factors in the zone between the wells of more than 60%[3], which increased the producers' interest for the SAGD process and motivated them to try its implementation in actual field applications. Nowadays, a huge expansion of SAGD in commercial applications in the Alberta oil sands seems to be inevitable.

In these reservoirs, the cold oil is most of the times immobile initially. Therefore, before converting the well pair to full SAGD operation, it is necessary to preheat the reservoir and create an effective thermo-hydraulic communication between the two parallel wells.The start-up phase consists of three steps, as illustrated by Vincent et al.[4].First, steam is circulated in both wells, and the heat transfer within the reservoir occurs mainly by conduction.In the second step, a pressure differential is imposed between the wells, adding a convection component to the heat transfer process in the reservoir.In the third step, the well pair is converted to full SAGD operation.Steam is injected continuously through the top well and rises within the reservoir, developing a steam chamber.The injected steam heats up the cold oil around the chamber, and this now mobile oil, along with any condensed water, flows down by gravity in the reservoir and is drained by the lower well, in which the fluids are continuously being produced.https://www.onepetro.org/conference-paper/SPE-97918-MS?sort=recent&star…

 

Author: Vanegas, J. W., Cunha, L. B., & Alhanati, F. J. S.

Publisher: SPE International Thermal Operations and Heavy Oil Symposium, 1-3 November, Calgary, Alberta, Canada

Year: 2005