Who Invented Gravity?
Authors: Donnelly J.K. 
Published: March 2010

Abstract
It has been 40 years since Pirson, Wong and Ramsey(1) proposed combining in situ thermal processes with gravity drainage to recover heavy crude oils and bitumen. In their paper, they presented the results obtained from scaled laboratory models for Athabasca Oil Sands and a Texas heavy oil. Two processes were tested: Steam Injection with Gravity Drainage (SIGD) and Air Injection with Gravity Drainage (AIGD). The well configurations that they tested and proposed were all based on vertical wells and horizontal fractures. However, the concepts they presented are essentially the same as Steam Assisted Gravity Drainage (SAGD) with the only significant difference being the use of horizontal wells.
Farouq Ali(2) in his article "Is There Life After SAGD?" suggests that there is an original pristine form of SAGD. If there was a pristine form, it exists in theory, not in practice nor application. Pirson's form of gravity drainage is just as original and pristine as Butler's(3) even though they are not the same. Reservoir models, physical, analytical or numerical exist to enable engineers to predict future field performance. None of these modelling methods can fully describe the actual processes going on in the ground. However, they may describe enough of what is going on to provide an accurate prediction of the key process parameters. For in situ steam projects, the key parameters are steam injection rate, oil production rate and water production rate. No matter which modelling method is used, assumptions are made and physical parameters are required as input. Production data from field operations is required to confirm any model. Since Farouq Ali prepared his article, several SAGD pilots have been started using parallel horizontal wells drilled from the surface. Included are the Suncor Burnt Lake Project, the AEC Primrose Project, and the Black Rock Hilda Lake project which are all claiming initial success.

Vertical Permeability and Geological Variation
Kisman and Yeung(4) reported sensitivities of numerical models to various input parameters. Vertical permeability is one parameter which is shown to have a strong influence on SAGD performance. Vertical permeability is generally not known to any degree of certainty because it is controlled by thin discontinuous streaks of silt, clay, shale or other impermeable materials for which the areal extent is not known. These impermeable barriers are usually saturated with water which when exposed to high temperature will create internal stresses which will break down the barrier. This means that steam will create vertical permeability. This phenomenon is not included in any model.
Initial oil saturation and horizontal permeability can be determined at each point a delineation well is drilled, cored, and logged. In many formations there is considerable areal variation in these parameters. The Hilda Lake project(5) is an exception. In spite of there being little areal variation, significantly different predictions are obtained when the geological parameters from four different wells are used with a numerical simulator as shown in Figure 1. Comparison with early time production from the pilot is shown in Figure 2.