Single Horizontal Well in Thermal Recovery Processes
Authors: Oballa Viera , Buchanan W. Lloyd 
Published: December 2007

Abstract
Horizontal wells have been used in conjunction with thermal enhanced recovery for several years now, and various well configurations have been applied. In the popular Steam Assisted Gravity Drainage (SAGD) process, a horizontal injector is located above a horizontal producer. The producer below collects and drains away the mobilized oil and water (condensed steam). Often the injector contains tubing for delivering steam to the well toe, while the annulus directs the steam to the formation and produces the excess for circulation. Recently, there has been interest in heavy oil reservoirs in the application of a single dual-stream horizontal well, where the annulus assumes the role of the producer.
In this paper, we examine the use of a single horizontal well using new simulation technology (hybrid grid surrounding a discretized wellbore). We evaluate two different operating strategies, SAGD and cyclic steam injection, as well as the influence of some key reservoir fluid characteristics on production. This paper will help the reader to understand better the interactions between the single-well configuration, the recovery process and the reservoir.

Introduction
Horizontal well technology has been in use for several decades, with various injector-producer configurations. In some fields steam is injected into an existing vertical well and fluids are produced from a horizontal well. In other applications a pair of horizontal wells is used in a Steam Assisted Gravity Drainage (SAGD) process, for which the most readily available information is from the Underground Testing Facility. Recently, ELAN Energy started producing oil at Cactus Lake from a single horizontal well whose success has prompted more companies to consider this technology. Therefore, a numerical study of the single dual-stream horizontal well was conducted at Computer Modelling Group, to identify potential problems and to better understand the mechanisms involved in this complex technology. This paper summarizes the results of the study.

Process Description
Single-well technology applied in the oil industry uses dual stream well with tubing and annulus: steam is injected into the tubing and fluid is produced from the annulus. The tubing is insulated to reduce heat losses to the annulus. Not only does this increase the quality of steam discharged to the annulus, it also avoids high temperatures and liquid flashing at the heel of the wellbore.
Tubing and annulus communicate along the well only by heat transfer. Along the entire well length, except perhaps for uncompleted sections, the annulus communicates with the reservoir by fluid and heat flow. Reservoir fluid is heated by conduction, convection and the release of latent heat. There are four possible scenarios of effective annulus-reservoir communication, depending on the relative magnitudes of the viscous, gravity and capillary forces.
Scenario 1. Along its entire length the well is a co-current injector, that is, both gas and liquid phases flow from annulus to reservoir. Steam goes into the reservoir but no reservoir fluid is produced. Reservoir pressure will increase and there is a possibility of reservoir fracturing.
Scenario 2. Along its entire length the well is a co-current producer, that is, both gas and liquid phases flow from reservoir to annulus. Reservoir fluid is produced, but all the injected steam returns to the surface through the annulus. Reservoir fluid and rock is heated by conduction only. Reservoir pressure may drop quickly depending on fluid mobility.
Scenario 3. Part of the well serves as a co-current injector and part as a co-current producer.
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