Method for Determining Fluid Saturation in a Subsurface Formation

This invention relates to a new and improved method for determining residual oil saturation in a subsurface formation surrounding a borehole by carrying out borehole gravity measurements in accordance with a log-inject-log operation.

The log-inject-log operation has been utilized with thermal neutron capture logs of subsurface formations to determine residual oil saturation. U.S. Pat. Nos. 3,748,474; 3,757,575; 3,812,353 and 3,825,752 all disclose such use. Theoretically, such residual oil saturation may be represented by the following expressions:

where

.SIGMA. represents macroscopic absorption cross section,

S represents saturation (or volume fraction of a particular fluid in phase), and

.phi. represents porosity.

In many instances the macroscopic absorption cross section of the rock matrix (.SIGMA..sub.r) cannot be known or estimated with good accuracy. Strongly absorbing trace elements are usually too abundant in the rock matrix to permit an accurate determination of .SIGMA..sub.r from the rock's major constituents. The log-inject-log technique has been utilized to overcome this problem.

In accordance with the present invention there is provided a method for carrying out a log-inject-log operation in a formation surrounding a borehole. The formation is firstly flushed with a fluid having a density greater than that of the water present in the formation. A first borehole gravity log is then recorded of the flushed formation. The formation is secondly flushed with a fluid having a density greater than that of the water present in the formation and differing from that of the first fluid. A second borehole gravity log is then recorded of the flushed formation. The porosity of the formation is also measured. From the density, borehole gravity and porosity factors, the water and oil saturation of the formation is determined. However, in the event the pore water density is known or measurable prior to the log-inject-log operation, only a single flushing of the formation will be required with a borehole gravity log being recorded before and after the fluid injection of the single flushing operation.

The fluids injected to flush the formation may be brines of higher density then the formation water contents or immiscible hydrocarbons which do not affect the oil in place in the formation.

In accordance with the present invention, there is provided a new and improved method for determining the fluid saturation characteristics of a subsurface formation from a log-inject-log operation. The method utilizes borehole gravity measurements made before and after injection into the formation of a fluid with density (i.e., specific gravity) different from that of the existing water in the formation.

The injected fluid may preferably be a brine of high salt content to yield a higher density fluid compatible with reservoir characteristics, such as reservoir fluids and clay surfaces. For example, an alternate fluid might be an immiscible hydrocarbon, that is, one which does not affect the oil in place in the reservoir. The formation is first flushed with a brine of known density. A first borehole gravity log is then recorded of the flushed formation. The formation is thereafter flushed with a brine of differing density. A second borehole gravity log is recorded. If both flushings are sufficient to achieve residual hydrocarbon saturation in all zones of interest, the two borehole gravity logs can be expressed generally as follows:

where,

.phi.=fractional porosity

.rho..sub.b =formation density

.rho..sub.ma =rock matrix density

.rho..sub.w =pore water density

S.sub.w =fractional water saturation

.rho..sub.o =pore oil density

S.sub.o =fractional oil saturation

Letting S.sub.o =(1-S.sub.w) and subtracting the first and second borehole gravity logs yields:

Since the borehole gravity log represents measurements in the tens of feet beyond the borehole, many pore volumes of injected brines, W1 and W2, are required to insure that formation water is completely displaced.

The borehole gravity log is a measure of the vertical component of the earth's gravitational field at selected depths in the borehole. The borehole gravity tool is stopped at each depth while gravity is measured. Knowing the distance between measurements, the formation bulk density is determinable as follows: ##EQU1## where

F=free air gradient.

.DELTA.g=gravity difference between depth measurement points.

.DELTA.Z=vertical distance between measurements.

G=universal gravitational constant.

Various logging tools and services are available in the industry for carrying out the borehole gravity measurements. One such logging service is provided by Exploration Data Consultants of Denver, Colo. This service of Exploration Data Consultants provides a radius of investigation approximately five times the distance between depth measurements points. For example an investigation depth of 100 feet will be provided for depth measurement points with 20 foot spacing. Consequently borehole effects, the influences of drilling fluid invasion and other normally detrimental factors become insignificant in borehole gravity meter logging. Further, the borehole gravity meter bulk density is generally more representative of the formation than that given by other logs.

In an alternate embodiment of the invention, only one formation flushing will be required if the pore water density of the formation surrounding the borehole is known or can be measured. Consequently, the density expression .phi..sub.w1 of equation (5) above will relate to the known or measured pore water density and the density expression .phi..sub.w2 of the same equation will relate to the pore water density after the single formation flushing with a fluid having a density differing from that of the known or measured pore water density. As before, such flushing fluid may be a brine with salt content so as to yield a higher density fluid than the exiting water content or an immiscible hydrocarbon which does not affect the oil in place in the formation.

It is to be understood that further modifications to the disclosed embodiment, may become apparent to one skilled in the art without departing from the scope and spirit of the invention as hereinafter defined by the appended claims.