The purpose of this study was to quantify Permeability impairment due to fines migration in extracted core material from the Clearwater Formation of Cold Lake, Alberta. Flow tests were conducted to compare the permeability behaviour of preserved cores with cores prepared from repacked reservoir material when each was exposed to fresh water and brine.
The results of these experiments indicate that the absolute permeability of preserved core, typically between and 3 Darcies, was not significantly affected when brine (NaCl or CaCl2) alternating with fresh water was injected. Repacked tar sands cores, however, were found to have initially much lower absolute permeabilities and were subject to severe permeability damage, resulting in total plugging, when exposed to fresh water. Disturbance of the core material dur1ng repacking possibly results in the reorientation of fines particles and the destruction of naturally occurring flow paths. Repacked core was susceptible to large permeability reductions during flow regardless of whether the tar sand bitumen was extracted prior to or after the repacking operation.
It is recommended that only preserved core be used for laboratory tests in which the results can be influenced by fines migration problems and native core chemistry. It should be noted that only high grade oil sand samples. i.e. about 12 weight % bitumen and 10% fines, From two wells were used in this study. As a result, the data may not be applicable to lower quality reservoirs.
Fines migration has been recognized as a cause of permeability damage In both consolidated and unconsolidated sandstones for many years. The literature available in this area is extensive.1–7 It was generally assumed that permeability damage in highly permeable sands such as Cold Lake tar sands was not likely to be significant. However, some flow tests conducted In the Research Department of Esso Resources Canada Limited brought this assumption into question.
In these tests, abnormally low absolute permeabilities to fresh water of less than 50 ml were measured in extracted, dried and reconstituted Cold Lake tar sands. Since only core material has the necessary reservoir wettability and mineral composition for relative permeability tests, it is important to determine the core handling methods which minimize the effects of fines migration and absolute permeability in tar sand cores. It is also important to identify and avoid the conditions under which such effectsoverwhelm the relative permeability effects. In short, this study was designed to investigate the proper procedures of using tar sand cores to determine relative permeability and to recommend alternate systems in other flow tests. Cold Lake tar sand core materials were chosen because Esso Resources Canada Limited has an extensive in situ thermal recovery operation in this region.
Permeability damage due to fines migration and clay swelling can be caused by both chemical and mechanical effects. Mechanical flow forces can induce particulates to migrate into pore throat constrictions resulting in decreases in absolute permeability. The movement of fines due to these flow forces has been associated with flow velocity5 and is not restricted to fines composed solely of the clay material.
