Method of identifying gas, oil and water zones in a subsurface formation

A method of logging a wellbore to identify oil, gas and water zones in a hydrocarbon-bearing formation that is characterized by acoustic velocities which are maximum when saturated with oil, minimum when saturated with water and intermediate when saturated with gas. The formation is logged with a sonic logging system utilizing a frequency within the range of 0.1 to 10 KHz and logged with a lithologic logging system to account for variations in lithology on the sonic log. Thereafter the sonic log is compared with the known acoustic velocity characteristic of the hydrocarbon-bearing formation to identify the oil, gas and water zones therein.

BACKGROUND OF THE INVENTION 
This invention relates to a method of logging subsurface earth formations 
penetrated by a wellbore and more particularly to a method of identifying 
gas, oil and water zones in a subsurface formation having known acoustic 
characteristics. 
Wellbores are conventionally logged to determine rock and fluid properties 
of subsurface formations that are traversed by the wellbore. Sonic logs, 
also referred to as acoustic logs, acoustic-velocity logs, and continuous 
velocity logs, are well logs of the travel time (transit time) for 
acoustic waves over a unit distance, and hence the reciprocal of the 
longitudinal wave (p-wave) velocity. Measurements are usually in 
microseconds per foot, though the log is normally displayed as velocity in 
feet per second versus depth. Such logs are used for porosity 
determinations as well as for furnishing geophysicists with velocity 
information for use in seismic interpretation. Other logs useful for 
determining porosity of subsurface formations are neutron logs and density 
logs. 
In U.S. Pat. No. 4,131,875 there is described a method and apparatus for 
preferentially exciting and for extracting late arrivals at low 
frequencies in an acoustic investigation of a borehole. It is there 
pointed out that a transmitter spectrum whose frequencies extend with 
significant amplitudes down to about 500 Hz would be particularly 
desirable. Acoustic transmitters for producing sonic pulses containing low 
frequencies have been described in the art for investigating early 
arrivals, particularly in cased holes as is shown in U.S. Pat. No. 
3,909,775. A low frequency transducer for generating acoustic energy above 
75 db level for a bandwidth from 2.4 KHz to 9.6 KHz is described in U.S. 
Pat. No. 3,845,333. 
There is published in THE OIL AND GAS JOURNAL, beginning with the May 15, 
1978 issue, a series of articles entitled "Practical Log Analysis". In the 
fifteenth series, published in May 12, 1979 issue of THE OIL AND GAS 
JOURNAL, it is pointed out that basically three types of porosity logs are 
available: acoustic (sonic), neutrons, and density logging devices. As 
there noted, all porosity logs are primarily responsive to porosity but 
other formation characteristics influence the measurements. Various 
combinations of the three logging measurements can be used to determine 
specific lithologies, porosity, and, under certain circumstances, type and 
amount of fluid in the pore space. 
The ninth series found in the Sept. 25, 1978 issue of THE OIL AND GAS 
JOURNAL, beginning at page 96, is entitled "Neutron Density Log As A 
Valuable Open Hole Porosity Tool". There it is said that the neutron 
density type logs are rapidly becoming the standard porosity device for 
open hole evaluation by wireline techniques. It is further pointed out 
that with the neutron density log one can obtain lithology, porosity, 
hydrocarbon types; locate shaley gas reservoirs; and serve as the basis 
for the advanced computer programs when combined with the basic 
resistivity devices. 
SUMMARY OF THE INVENTION 
This invention is directed to a method of logging a wellbore to identify 
hydrocarbon zones in a formation penetrated by the wellbore. First, there 
is identified a hydrocarbon-bearing formation which is characterized by an 
acoustic velocity which is maximum when the formation is saturated with 
oil, minimum when saturated with water, but not exceeding 90% water, and 
intermediate when saturated with gas. The hydrocarbon-bearing formation is 
logged with a sonic logging system that utilizes a frequency within the 
range of about 0.1 to 10 KHz to produce a sonic log. The 
hydrocarbon-bearing formation is also logged with a lithologic logging 
system to determine porosity and lithologic variations of the formation 
whereby the sonic log may be normalized by being corrected for porosity 
and lithology. Thereafter the sonic log is compared with the characterized 
acoustic velocity profile of the hydrocarbon-bearing formation to identify 
oil, gas and water zones therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention is directed to a method of logging a wellbore that 
penetrates a hydrocarbon-bearing formation and more particularly is 
directed to a method of distinguishing gas, oil and water zones in a 
hydrocarbon-bearing formation which is characterized by an acoustic 
velocity profile which exhibits a maximum velocity in those zones thereof 
having a maximum oil concentration, an intermediate acoustic velocity in 
those zones having a high gas concentration and a minimum acoustic 
velocity in those zones having a high water concentration not exceeding 
90% water saturation. 
Laboratory studies have been carried out concerning the acoustic velocities 
of cores taken from subsurface formations penetrated by wellbores. These 
studies have shown that the addition of water or oil to gas saturated rock 
causes large changes in bar velocity in certain types of sandstones. They 
also show that the bar velocity is close to the p-wave velocity for liquid 
saturations below 90%. These effects are not observed in limestones. 
It has been found that the general acoustic characteristics of many 
sandstones are similar to that of Berea sandstone. Spring-mass resonance 
measurements have been carried out on Berea sandstone cores using bar 
lengths between 10 and 30 cm., bar diameters between 0.9 and 2.0 cm. and 
masses between 2000 and 20,000 g. 
Measurements of resonant frequencies have been used to compute Youngs 
Modulus bar velocity. The latter is the velocity a wave would have 
travelling through a thin prismatic bar of Berea sand provided it behaved 
like a perfectly elastic material. For small values of Poisson Ratio the 
bar velocity can be taken, as a first approximation, to be equal to the 
p-wave velocity in the rock material. In most sands the Poisson Ratio is 
small. Bar velocities computed from Berea resonance measurements are 
compiled in TABLE 1 below. 
TABLE I 
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SPRING-MASS RESONANCE MEASUREMENTS 
BEREA SANDSTONE 
Bar Length = 13.5 cm. 
Bar Diameter = .889 cm. 
Mass = 5885 g. 
Saturation Resonant Frequency 
Computed Bar Velocity 
Conditions (Hz) (Ft/Sec) 
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50% 
Gas/50% Water 
111.7 3990 
100% Gas 162.0 5800 
50% 
Gas/50% Oil 
202.0 7200 
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There it is seen that completely dry, unconfined Berea samples give bar 
velocities of about 6000 feet per second. The addition of water to the dry 
samples reduces this velocity to about 3800 feet per second and the 
addition of a light oil (viscosity of 0.7 cp.) produces little change. The 
addition of a heavier oil (viscosity of 60 cp.) increases the velocity to 
about 7200 feet per second. These data show that an almost twofold change 
in bar velocity occurs between gas/water and gas/oil saturation provided 
the oil is a heavy oil. We can expect similar changes in the p-wave 
velocity measured by logging tools at low frequency. Increased confining 
pressure produces a general increase in bar velocity. It also reduces the 
velocity separations associated with differences in gas, water and oil 
saturations. This is illustrated for gas-water saturations in FIG. 1. The 
effect of confining pressure causes the bar velocity characteristics of 
sandstones of different saturation conditions to tend to disappear with 
depth. 
Measurements of pulse travel times at ultrasonic frequency do not show the 
large velocity changes of FIG. 1. This is demonstrated by the work of A. 
R. Gregory, "Fluid Saturation Effects on Dynamic Elastic Properties of 
Sedimentary Rocks", Geophysics, Vol. 41, pp 895-913, (1976). Evidently the 
effects are limited to sonic frequencies for reasons that are not well 
understood. The actual upper frequency limits which could be used in a 
sonic logging tool in carrying out the present invention could be 
determined by conducting lab measurements. 
Conventional logging methods often do not provide good estimates of gas, 
oil and water saturations in shallow reservoirs. This invention offers a 
new technique of logging shallow sandstone reservoirs to determine gas, 
oil and water saturations. 
In accordance with this invention there is provided a method of logging a 
wellbore to identify the hydrocarbon and water zones of a 
hydrocarbon-bearing formation and more specifically to identify the gas, 
oil and water zones thereof. A shallow hydrocarbon-bearing formation is 
identified that has characteristic acoustic velocities that are maximum at 
high oil saturations, minimum at high water saturations, not exceeding 90% 
and intermediate at high gas saturations. Thereafter the well is logged 
adjacent the hydrocarbon-bearing formation with a sonic logging system 
utilizing a frequency within the range of about 0.1 to 10 KHz to produce a 
sonic log. The well is also logged adjacent the hydrocarbon-bearing 
formation with a lithologic logging system to determine the porosity and 
lithologic variations of the formation whereby the sonic log may be 
normalized by correcting for porosity and lithology. The sonic log is then 
compared with the characterized acoustic velocity of the 
hydrocarbon-bearing formation to identify the hydrocarbon and water zones 
thereof. 
Suitable lithologic logging systems for use in carrying out this invention 
are density and gamma logging systems. 
The identification of the hydrocarbon-bearing formation by acoustic 
velocity characteristic may be done by obtaining cores from at least one 
wellbore that penetrates the hydrocarbon-bearing formation and conducting 
laboratory resonance measurements. 
By conducting the laboratory resonance measurements on cores having high 
gas saturations, high oil saturations, and high water saturations, the 
hydrocarbon-bearing formation may be characterized by the bar velocity of 
the gas zone, the oil zone, and the water zone, such that the sonic log 
may be compared against these acoustic velocity characteristics to 
identify the gas and oil zones as well as the water zone. Further 
measurements may be made at different oil, gas and water saturations to 
produce a more complete acoustic velocity profile of the formation from 
which the cores were taken. 
Suitable laboratory measurement techniques for characterizing the 
hydrocarbon-bearing formation by acoustic velocity include spring-mass 
resonance methods and bar resonance methods. Reference is made to G.H.F. 
Gardner, "Effects of Pressure & Fluid Saturation on the Attenuation of 
Elastic Waves in Sands" Jour. Pet. Tech. Feb. 1964, pp. 189-198 for 
description of the bar resonance method. 
With reference again to FIG. 1 there are shown plots of bar velocity versus 
gas and water saturation at 0 psi, 500 psi, and 1000 psi, confining 
pressure. These plots show that the bar velocity of the Berea sandstone 
increases with increasing gas saturation in the range above 70% gas. At 
increasing gas concentrations from 20% it is seen that the increase in bar 
velocity with increasing gas saturation is most pronounced at 0 psi 
confining pressure and diminishes with increasing confining pressure. The 
same trends should be observed in low frequency p-wave velocities measured 
by a logging tool. Thus, FIG. 1 illustrates that the method of this 
invention is particularly applicable for distinguishing hydrocarbon zones 
in shallow subsurface formations. It is seen that the increase in bar 
velocity with increasing gas saturation at 1000 psi is clearly 
sufficiently pronounced to identify gas from water zones. It is considered 
that this invention is applicable for logging formations at depths of at 
least 3000 to 5000 feet and possibly deeper. As previously noted there is 
a particular need for a technique which can distinguish gas, oil and water 
zones in shallow formations. The present invention provides such a 
technique. 
With reference to FIG. 2 there is shown a hypothetical series of logs, 
namely gamma, sonic and density logs which have been run adjacent a 
hydrocarbon-bearing formation containing a gas zone, an oil zone, and a 
water zone. The gamma and density logs show no big variations in the 
porosity of the hydrocarbon-bearing formation. Thus there is no need to 
normalize the sonic log to correct for porosity changes in the formation. 
Such techniques for normalizing a sonic log are readily available, 
however, should a need exist. An examination of the sonic log shows three 
distinct acoustic velocities adjacent the hydrocarbon-bearing formation. 
By comparing the acoustic velocity of the formation as represented by the 
sonic log against the characteristic acoustic velocities of the formation 
being logged as illustrated by TABLE I, one can identify the gas zone as 
having the intermediate acoustic velocity, the oil zone as having the 
maximum acoustic velocity, and the water zone as having the minimum 
acoustic velocity.