SAGD with a Longer Wellbore
Authors: Parappilly R. , Zhao L. 
Published: March 2010

This paper investigates the feasibility of bitumen production using SAGD technology with a longer wellbore. A longer wellbore can be used to exploit a greater volume of the reservoir with a single wellpair as opposed to multiple wells; thus, decreasing pad and drilling costs.

The major concern in using a longer wellbore from a reservoir engineering point of view is the potentially non-uniform steam chamber growth over the whole well length due to pressure drop in the horizontal well. Reservoir simulation was performed using Computer Modelling Group's STARS ™?simulator and the QFlow thermal wellbore simulator to address the issue. The pressure drop inside the wellbore was calculated using QFlow. Reservoir simulations were performed using STARS. A sectioned model was constructed, dividing the whole length of the horizontal wellbore into multiple sections, each with its own pressure restriction to match the pressure drop obtained from QFlow. The results showed that with a sufficiently large wellbore size that resulted in a sufficiently low pressure drop, one longer well performed as well as two shorter wells. When a smaller wellbore size was used, production from a longer well was significantly impeded due to the large pressure drop.

Introduction

Since the discovery of oil sands, many techniques have been tried - and improved upon - to recover the highly viscous bitumen for energy consumption. One of the methods of bitumen recovery that is becoming increasingly prevalent is a thermal in situ method called Steam-Assisted Gravity Drainage (SAGD). Butler(1) first proposed this method in the late 1970s and it was field tested in AOSTRA's Underground Test Facility (UTF) in the 1980s. The initial phase of UTF testing produced successful field results(2, 3). Following Phase A, Phase B was started, implementing the use of longer horizontal wells(4, 5). Surface-operated SAGD was studied in Phase D(6). Following these successful pilot tests, a number of commercial projects have been developed, including Petro-Canada's MacKay River project.

One of the factors that needs to be considered in commercial projects is the optimization of the horizontal well length. Typically, the horizontal sections of the SAGD wells are 500 to 700 m long. This paper will examine the effectiveness of using a longer horizontal wellbore in SAGD. There is tremendous potential for economic savings in implementing longer wellbores in a SAGD project.

A longer wellbore has the ability to reach greater lengths within a reservoir with one single wellbore as opposed to multiple wells. When multiple wells are drilled, multiple pads must also be built - one for each group of wells. Thus, to cover a given reservoir, the average pad cost will be lower for longer wellbores.

Another significant cost advantage of the longer wellbore is the fact that fewer riser sections (also known as build sections) need to be drilled and completed. The riser section has a significantly higher cost than the horizontal section because it is generally much larger in diameter. By decreasing the number of wells in a pattern with the use of longer wells, the cost of all the riser sections is decreased significantly.