A subsurface formation surrounding a borehole is flushed with at least one fluid having a density difference from that of the water content of the formation. Borehole gravity logs are recorded. Porosity is also recorded. From the density, gravity and porosity indications, water and oil saturation of the formation is determined.
A borehole logging system includes a borehole gravity meter and a resistivity logging tool. A borehole gravity log and a resistivity log are produced over a select depth interval in a borehole. The bulk density of the subsurface formation surrounding the borehole over the select depth interval is measured from the gravity log. The bulk resistivity of the formation surrounding the borehole over the select depth interval is measured from the resistivity log. The bulk density and bulk resistivity measurements are combined to derive a measurement of the residual oil saturation of the subsurface formation surrounding the borehole for the select depth interval.
A borehole gravimety system employs a pair of pressure transducers for lowering into a borehole along with a borehole gravity meter. The pressure transducers are spaced apart vertically within a pressure sonde for producing a pressure differential measurement of the wellbore fluid. A pressure differential measurement and a gravity reading is taken for each of a plurality of vertical locations within the borehole as the borehole gravimetry system is advanced through the borehole.
For monitoring drilling mud circulation there is incorporated in the supply mud a known amount or concentration of at least one tracer ion which is substantially non-interactive with the other mud components and with the strata drilled, and the return or circulatory mud is analyzed at the rig site (preferably by ion chromatography) for the non-interactive ion(s) to give an indication or annular dispersion. Preferably at least two tracer ions (e.g. a tracer compound providing a non-interactive cation and non-interactive anion) are used, these being analyzed independently in the return or circulatory mud to give separate checks on annular dispersion. Preferably the tracer is injected discontinuously into the supply mud, in spaced discrete relatively concentrated doses, and the residence time distribution of the tracer is plotted for each dose.
A borehole gravimetry survey is carried out by making gravity measurements at a plurality of measurement stations along an interval of the borehole. A moving average gradient is determined for gravity measurements taken from consecutive pairs of such measurement stations over the borehole interval. Each moving average gradient is inverse filtered to reduce the effect of the weighted averaging on the gravity measurements, thereby increasing the vertical resolution of the data.
The invention disclosed herein is a method for producing oil from a reservoir after predetermining its residual oil saturation, S.sub.orm. Such a method would displace a hydrocarbon fluid in a subterranean formation using a substantially non-aqueous displacement fluid after a waterflood. The non-aqueous displacement fluid can be introduced as a single bank, in alternating sequence with water, or by other means. The method predetermines S.sub.orm using the difference in the solubility parameters between the hydrocarbon liquid in the formation and the displacement fluid to be used for displacing the hydrocarbon fluid. The solubility parameter of the hydrocarbon liquid can be determined by a variety methods including, but not limited to, (1) using the hydrocarbon liquid's energy of vaporization, .DELTA.E.sub.vap, and its molar volume, V, (2) using the volume-fraction-weighted average of the solubility parameter values for model compounds representing components of the hydrocarbon liquid, or (3) using the hydrocarbon liquid's average molecular weight and the temperature of the subterranean formation. The solubility parameter of the and a constant, .rho.r(liq) for a displacement fluid that is a pure compound, or (2) using the volume-fraction-weighted average of the solubility parameter values for each component of the displacement fluid. Residual oil saturation for a secondary displacement fluid also can be determined by a number of methods including, but not limited to, (1) using the displacement fluid's critical pressure, P.sub.c, ratio of density to critical density, .rho.r, gasflood, Sorg, may also be predetermined using a substantially similar procedure.
