Thermal Behavior of Unconsolidated Oil Sands

ABSTRACT Thermal conductivities of unconsolidated oil sands have been measured and the results correlated with physical properties of the sand-fluid system. Saturation of the wetting fluid has a dominant effect on thermal conductivity values. Water-saturated sands were found to have thermal conductivities six to eight times greater than values for the same sand packs air saturated. For correlation purposes, the porosity of a sand pack is an adequate indicator of matrix structure. Other quantities needed to develop a satisfictory equation for predicting thermal conductivity are the conductivities of the wetting fluid and of the rock solids. The effects of changes in temperature on the thermal conductivity of unconsolidated oil sands are relatively small and can be evaluated with a simple linear equation. The effects of changes in pressure on the thermal conductivity of liquid-saturated unconsolidated sands are also small and for practical purposes can be ignored. Results of the present work are believed to have direct application to calculations relating to thermal processes in underground reservoirs. Core-analysis and well-log data can be used to evaluate the thermal properties of unconsolidated oil sands required for such calculations. INTRODUCTION The most successful thermal recovery operations are those that have been applied to relatively shallow producing formations consisting mostly of unconsolidated sands. Necessary in designing such projects is a knowledge of the thermal properties and behavior of the sand-fluid system under reservoir conditions of saturation, pressure, and temperature. A great deal of literature has been published on the thermal properties of granular materials, and several models and correlations for predicting thermal properties have been proposed.1–5 Unfortunately, most test data have been obtained for systems or conditions much different from those found in petroleum reservoirs. Most models or prediction equations do not cover ranges of variables of importance to subsurface applications and, in addition, often require knowledge of system parameters that normally are not readily available from common sources such as core analyses and well logs. The purpose of this work was to establish relationships between laboratory measurements of thermal properties and other more easily measurable properties of unconsolidated sands. Simple systems consisting of uniform-grain-size quartz sands saturated with single fluids were first studied. The work then progressed to more complex systems, including actual oilfield cores containing substantial portions of their original fluids. RELATIONSHIP OF THERMAL CONDUCTIVITY TO OTHER PHYSICAL PROPERTIES The thermal conductivity of fluid-saturated, unconsolidated sand is strongly dependent upon the saturation and thermal conductivity of the wetting phase fluid. Air- or gas-saturated sands characteristically have low thermal conductivities. This is because the contact areas between grains, through which heat must flow, are small. Introduction of a wetting-phase liquid greatly increases the thermal conductivity by increasing the effective grain contact area and thus enlarging the effective area through which heat can flow. Present experimental results show that the thermal conductivity of brine-saturated unconsolidated sands increases sixfold to eightfold over that of the same sand packs air saturated. This effect is considerably less pronounced in consolidated sandstones; Anand's data6 show a twofold to threefold increase between brine-saturated and air-saturated sand packs.