Depth-Related Oil Viscosity Variation In Canadian Heavy Oil Reservoirs

Abstract The natural variability of oil viscosity in the Clearwater B heavy oil formation in east central Alberta has been examined. Oil samples taken from preserved cores from various depths vary in viscosity by as much as tenfold. The data show a consistent trend of increased viscosity with increased depth of origin of the sample. Furthermore, over an area of 16 sections oil viscosity appears to correlate with structural elevation. Samples from similar absolute elevations have similar viscosity. In location where the reservoir is structurally high, the viscosity-depth profile shifts toward less viscous oil, and, in structural lows, the profile shifts to more viscous oil. Distillation curves of selected samples confirm that the oil viscosity variations coincide with compositional differences. Single sets of samples from three other heavy oil reservoirs, the McMurray and Wabiskaw in the Caribou Lake area, and the Waseca sand at Pikes Peak in west central Saskatchewan, show similar trends to increased viscosity toward the bottom of the reservoir sand. These observations are compared with others reported in the literature, and possible causes of these compositional variations and of their significance to thermal recovery processes are discussed. Introduction Variations in oil viscosity and composition were first observed during studies of oil produced from wells in a thermal project near Lloydminster, Saskatchewan. Although much of the variation could be attributed to alteration resulting from the thermal recovery process, some of the compositional data suggested that the unaltered "native" oil may not be as homogenous as was assumed. Other data (density, viscosity and distillation) for samples produced under primary conditions from other reservoirs in the Lloydminster area, also gave indication that significant viscosity variations occur from well to well within the same reservoir. Because variability of oil viscosity within a reservoir could impact on the success of a recovery process, and could also affect assessment of the value and amount of recoverable oil, it was of interest to conduct a more thorough study to confirm the extent of variability in heavy oil reservoirs. The opportunity to conduct such a study in a virgin reservoir area came in 1987 when Husky Oil and Alberta Energy Company carried out extensive drilling and coring of an area in the Caribou Lake block of east central Alberta. This paper reports on a detailed examination of oil variability within the Cleanwater ‘B’ reservoir, in particular in the Caribou Lake area held jointly by Alberta Energy Company and Husky Oil Operations. Literature Review Although the subject has received little attention in Canada, oil variability within individual reservoirs has been observed in many reservoirs throughout the world. Hunt(l) sites a number of examples, including the Coalinga field in California, the Lagunillas field in Venezuela, the Sakhalin field in the U.S.S.R., and the Burgan field in Kuwait which show dramatic variations in oil viscosity and density with location and/or depth. An early discussion of the Kuwait oil fields and the implications of oil heterogeneity was given by Hetherington and Horan(2).