A New Process Combining Cyclic Steam Stimulation and Steam-Assisted Gravity Drainage: Hybrid SAGD
Authors: Coskuner G. 
Published: March 2010

A new thermal recovery scheme is proposed that utilizes Steam-Assisted Gravity Drainage (SAGD) well pairs as well as Cyclic Steam Stimulation (CSS) wells placed in between the SAGD well pairs. The wells are operated in CSS mode until the steam chambers are in contact with each other and then switched to SAGD operation. It is shown that the new process recovers greater amounts of bitumen with lower injected steam in shorter operation time than is achieved with SAGD, Fast-SAGD and CSS.

Introduction

Alberta's major oil sands deposits, with an estimated 1.7 trillion bbls of bitumen-in-place, account for approximately 40% of the world's bitumen resource(1). The estimated remaining recoverable bitumen from this resource is 170 billion bbls, which dwarfs the remaining conventional reserves of crude oil in Alberta by more than two orders of magnitude(2). While shallow oil sands reserves can be extracted by mining, most of these reserves (82%) are accessible through in situ thermal processes only(2). Two commercially-applied in situ production methods are Steam-Assisted Gravity Drainage (SAGD) and Cyclic Steam Stimulation (CSS). These methods are technically effective but can be rendered uneconomic due to steam requirements.

Although the first commercially applied thermal recovery process was CSS, there are a number of commercial and smaller SAGD projects under way at present in Alberta(3). The main attraction of SAGD compared to CSS is that higher recoveries in excess of 50% can be obtained because of the efficiency and effectiveness of the gravity drainage process. However, a challenge for SAGD is to try to promote the lateral and downward expansion of the steam chamber(4). Recently, a new process called Fast-SAGD has been proposed to overcome this problem(5–10).

Fast-SAGD Process

In the Fast-SAGD process, additional single offset horizontal wells are drilled in between and parallel to the SAGD well pairs. The offset wells are placed at the same elevation as the SAGD producers and can be 50 to 80 m away from the SAGD well pairs. The concept relies on operating the SAGD wells until the steam chamber reaches the top of the formation and then starting a CSS operation at the offset wells at considerably higher pressure than the SAGD wells. The purpose of injecting steam into the offset CSS well is to accelerate growth and propagation of the steam chamber laterally. Once the inter-well area between the SAGD well pairs is heated enough, ideally when the two steam chambers come into contact, the offset well is converted into a producer and the SAGD operation continues.

Authors of Fast-SAGD articles reported the results of thermal numerical simulations to initially analyze the response in a Cold Lake-type reservoir and later in Athabasca- and Peace River-type reservoirs(5–10). Their numerical models were generic, two dimensional (2D) homogeneous layer cake models. Results reported were quite impressive in these idealized models where bitumen production rates increased significantly and energy efficiency, as measured by steam-oil ratio (SOR), was better compared to conventional SAGD. Given the encouraging results, it was suggested that a field test should be carried out.