The Impact of Oil Viscosity Heterogeneity on the Production Characteristics of Tar Sand and Heavy Oil Reservoirs. Part II: Intelligent, Geotailored Recovery Processes in Compositionally Graded Reservoirs
Authors: Gates I.D. , Adams J. , Larter S. 
Published: March 2010

Abstract
Compositional and fluid property gradients are common and documented in conventional heavy oilfields and in super heavy oil occurrences such as oil sand reservoirs. In the severely biodegraded oils of both Athabasca and Peace River oil sand reservoirs, highly non-linear vertical and lateral chemical compositional and fluid viscosity gradients are common and have been shown to dramatically impact existing generation recovery processes such as SAGD and CSS. The fluid and geological heterogeneities at a variety of spatial scales in heavy oil and bitumen reservoirs, combined with the dynamic evolution of produced fluids during solvent or thermal recovery, should be integrated into recovery methods tailored to each reservoir which are operated using time-variant production strategies informed by produced fluid composition, flow rates and detailed history matching. We describe here, two new approaches of a new generation of transitional and initial thermal recovery processes that take advantage of mobility gradients: JAGD (J-well and Gravity Drainage) and gSAGD (mobility ratio optimized SAGD), which demonstrate significant improvements in recovery, economics and, thus, carbon dioxide emissions over existing thermal methods. Well configurations tailored to specific reservoir geometries and properties as well as fluid property distributions for primary thermal recovery increase initial production by 50 to 100%. Substantial cost savings are achieved in transitional cold primary to thermal secondary recovery methods (JAGD) by using a production J-well placed below what is initially a CHOPS production well, which is then later used to inject steam, as in SAGD. Three-dimensional reservoir simulations predict 25% more oil recovery with up to a 50% decrease in cumulative steam-oil ratio compared to standard SAGD in an identical reservoir. The JAGD process has many similarities to SAGD, such as steam trap control and potential for low pressure and solvent-assisted operation. Such geotailored processes (processes tailored, operated and optimized to reservoir fluid and geological heterogeneities) are expected to outperform conventional 'off the shelf' well placement designs and operating strategies.

Introduction
The bulk of the world's petroleum resources are stored in heavy oil and oil sand reservoirs. While some of this resource can be recovered by geotolerant (tolerant of unfavourable geology) recovery processes such as mining, these procedures are only suitable for shallow resources, are very costly and have high carbon dioxide emissions and other environmental penalties. Most of the world's heavy oil and bitumen resources are too deep to mine and so in situ recovery methods predominate. In situ recovery of viscous and poor quality oils currently relies on either high pressure primary production, as in cold heavy oil production, or thermal and/or solvent-based methods to mobilize the oil by reducing its viscosity. Average recoveries from heavy oil and oil sand reservoirs are typically low ranging from 5 to 15% for cold heavy oil production and from 30 to 85% for steam-based in situ processes. However, such processes are not very geotolerant. Also, profit margins are small because of high capital and operational costs.