Three-Dimensional Studies of In-Situ Combustion-Horizontal Wells Process With Reservoir Heterogeneities
Authors: Greaves M. , Al-Honi M. 
Published: March 2010

Abstract
The effect of reservoir heterogeneity on the stability, sweep, and fluids production during in situ combustion of medium heavy West of Shetlands Clair oil has been investigated in a three-dimensional combustion cell. The experimental conditions simulated a post-waterflood state, and the process of air injection involved a producer well arrangement, such that the combustion front propagation and hence displacement of oil occurred in a toe-to-heel manner. This is called the "Toe-to-Heel" -Horizontal Wells Process (THHW). Five experiments were conducted, using two base homogeneous sandpacks. The heterogeneous sandpacks involved two dual permeability layers, one with the higher permeability in the top layer and the other with the higher permeability layer in the bottom layer. The third heterogeneity type was a central high permeability streak layer, sandwiched between two homogenous sand layers.
The results show that the steam and combustion gases downstream of the combustion front tend to channel through the high permeability layer causing some measure of gas override. This effect is less exaggerated when the high permeability layer is at the bottom of the sandpack. The presence of a high permeability streak layer promotes the channeling and bypassing of the injected air around the combustion front. In all cases, stability was enhanced by the gravity assist mechanism created by the drawdown of steam and combustion gases into the exposed section of the horizontal producer well, ahead of the combustion front. The increased stability and control achieved by the THHW process enabled the propagation of the combustion front to be sustained. Very high oil recoveries were achieved, except for the case involving the high permeability streak layer.

Introduction
The study of air injection into oil reservoirs requires specific investigation of kinetic parameters using differential or adiabatic calorimetry, fuel and combustion parameters using a combustion tube or oxidation tube, and sweep and fluids production using a three-dimensional combustion cell. Studies of in situ combustion using 3D geometries are very limited, because of the extra complexity and cost. Nevertheless, the effort in this direction is worthwhile because of the valuable understanding to be gained from a physical scaled model of the process. Ultimately, it should be possible to achieve a full understanding of the process by virtue of the rigorous validation of a 3D numerical model. The key to this is matching the scaled model parameters, especially the dynamic combustion front temperature and also the fluid production rate. In the presence of reservoir heterogeneities, and this is always the case in practice, they are site specific, i.e., they respond to local matrix property conditions-rock composition, permeability. This dual response constraint, in three dimensions, therefore represents an extremely tight condition to be solved for. The solution thereby obtained, for a reasonably scaled model, should provide a good platform for detailed reservoir studies.
Binder et al.(1) carried out 3D model studies of dry and wet oxygen in situ combustion. This was done in two cylindrical cells, one having a volume 325 times greater than the other.