A Laboratory Study Of Permeability Damage To Cold Lake Tar Sands Cores

Abstract

Abstract 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 distilled water and brine. The results of these experiments indicate that the absolute permeability of preserved core, typically between 1 and 3 Darcies, was not significantly affected when brine (NaCl or CaCl2) alternating with distilled 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 distilled water. Disturbance of the core material during repacking possibly results in the reorientation of fines particles and the destruction of naturally occurring flow paths. Repacked core was susceptible to large permeability reduction 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. Introduction 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(1–2).Permeability damage due to fines migration and clay swelling can be caused by both chemical and mechanical effects. Detailed discussions of these effects are beyond the limited scope of this study. Various aspects of the effects of flow velocity on fines migration are discussed in Gruesbeck and Collins(5). Chemical interactions often acts in concert with mechanical forces to mobilize fines. The movement of fines and clay particles in ionic solutions are governed by particle surface electrical charges, solution ion concentration and other aspects of colloidal chemistry. This phenomenon is further discussed in Veley(2) and Reed(8). It was generally assumed that permeability damage in highly permeable sands such as Cold Lake tar sands was not likely to e 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 distilled water of less than 50mD were measured in extracted, dried and reconstituted Cold Lake tar sands. This study is a follow-up of the above preliminary investigation. The main objective of this work is to identify the proper methods) of preparing tar sand core material so that meaningful relative permeability tests can be conducted while minimizing finesmigration effects. In other words, the relative merits of using loose tar sands material, preserved cores and extracted tar sands are being examined. Tar sands core material is typically prepared by three different methods in permeability tests. First, loose tar sand material can be extracted and dried and repacked into a core holder system. Second, bulk tar sand material can be repacked with bitumen in place into a core holder and thenextracted while under triaxial loading.