Method for monitoring flood front movement during water flooding of subsurface formations

A method is disclosed for monitoring the flood front movement during enhanced recovery operations wherein flooding fluids are pumped into a number of injection wells forcing residual oil movement toward a production well. A plurality of monitoring wells located between the injection wells and the producing well are logged to establish base logs functionally related to oil saturation and water salinity. Periodically during the water flood operation, the monitoring wells are relogged to detect changes in oil saturation and water salinity. By comparison of the base logs with the series of later derived logs it is possible to accurately monitor the flood front movement including detecting high-permeability zones and monitoring of the flood front profile.

BACKGROUND OF THE INVENTION 
This invention relates generally to methods for monitoring flood front 
movement during secondary and tertiary oil recovery and more specifically 
to methods for monitoring salinity and oil saturation changes and 
directional flood front movement of water injected into subsurface 
formations. 
In the production of oil from subsurface locations it is well known that 
frequently primary production methods are ineffective in recovering 
substantially all the oil within a reservoir. The depletion of the 
reservoir energy before the depletion of the recoverable oil leaves a 
portion of the oil in the ground without a natural propulsive energy to 
move it. After the reservoir energy is about exhausted, and the production 
is approaching its economic limit, much of the remaining oil may be 
recovered by supplying a new energy source. One such outside source of 
energy utilized in secondary and tertiary recovery operations is the 
injection of water into the subsurface formations. 
Water flooding depends on the ability of injected water to displace the oil 
remaining in the reservoir in the same manner it displaces oil in the 
primary production of a water-drive reservoir. Water is injected into the 
reservoir through a number of intake wells located at spaced intervals. As 
the injected water enters the reservoir, it moves toward the area of lower 
fluid potential and, as it moves, drives the oil left behind during the 
primary recovery phase. An increased oil saturation develops ahead of the 
moving water and finally reaches the production wells. 
In performing a water flooding operation it is important to monitor the 
progress of the flood front to determine the lateral movement thereof. Due 
to formation characteristics, the flood front does not move in uniform 
fashion from the injection wells toward the production well. Further, 
subsurface formations may contain high-permeability streaks which allow 
injected water to break through the oil into the production well. The 
result of such a breakthrough is the production from the well of water 
while significant oil may remain in the formations. 
In the prior art, various methods have been utilized to monitor the 
progress of the flood front in secondary and tertiary oil recovery 
operations. One such method as disclosed in U.S. Pat. No. 3,874,451, 
issued to Jones et al, detects the arrival of the flood front by 
monitoring the pressure change in boreholes. A requirement of Jones et al 
is that the boreholes used for pressure monitoring must be uncased. In a 
production reservoir this can require the removal of casing already 
present in the boreholes or the drilling of new, uncased boreholes. 
U.S. Pat. No. 4,085,798, issued to Schweitzer et al discloses a method for 
monitoring the flood front profile during water flooding by adding a 
tracer element having a characteristic gamma ray emission energy to the 
flood fluid. The tracer element may be unlike any element nomrally found 
in the formation, or it may be an element similar to elements normally 
present in the formation. It is recognized as a serious disadvantage to be 
required to add tracer elements to the flood fluid prior to injection. 
Additionally, since the Schweitzer method is only directed to detecting 
elements in the injection fluid it does not provide an indication of flood 
front movement until the fluid flood front reaches or nearly reaches the 
monitor boreholes. 
Accordingly, the present invention overcomes the deficiencies of the prior 
art by providing a method for monitoring the flood front movement through 
cased boreholes without alteration of the injection fluid. 
SUMMARY OF THE INVENTION 
A high energy neutron source is caused to traverse a borehole located 
between the injection wells and a production well. The source is 
periodically pulsed to thereby irradiate the formations with neutrons. A 
detector system detects radioactivity resulting from inelastic scattering 
and neutron capture. The detected signals are coupled to a surface 
electronics which processes the signals to derive a first measurement 
representative of oil saturation and a second measurement representative 
to formation water salinity. At intervals during the flooding operation 
the borehole is relogged to monitor changes in oil saturation and water 
salinity. By establishing a time-series of oil saturation and water 
salinity logs the progress of the flood front through the formation can be 
monitored. 
Accordingly, it is a feature of the present invention to provide a method 
for monitoring the progress of a flood front through an oil production 
formation. 
it is still another feature of the invention to provide a method for 
monitoring the flood front profile from cased boreholes. 
It is yet another feature of the present invention to provide a method for 
detecting high-permeability zones in an oil producing formation. 
It is still another feature of the present invention to provide a method of 
monitoring flood front progress by time-series monitoring of oil 
saturation and water salinity. 
It is another feature of the present invention to provide a method of 
monitoring the formation fluid and injection fluid mixing factor to 
determine salinity changes related to injection fluid movement.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now to the drawings in more detail, particularly to FIG. 1, there 
is illustrated a section of the earth formations 10 in which secondary or 
tertiary recovery is undertaken to enhance the amount of recoverable oil. 
The earth formations 10 are penetrated by a plurality of injection wells 
11a and 11b, a production well 12 and a plurality of monitoring wells 13a 
and 13b, located between the injection wells 11a and 11b and the 
production well 12. It should be understood that the number of injection 
wells and monitoring wells illustrated is exemplary only, and that the 
actual number will differ in accordance with the size of the reservoir to 
undergo water flooding. 
Injection wells, 11a and 11b, and production well 12 are cased with 
perforations at the level of the formations where primary production has 
occurred. Monitoring wells, 13a and 13b, are cased and may or may not be 
perforated. Located at the surface are injection pumps 14a and 14b to 
which are attached tubings 15a and 15b, respectively. Tubings 15a and 15b 
extend from surface pumps 14a and 14b into injection wells 11a and 11b, 
respectively. Production tubing 16 is disposed within production well 12, 
terminating at surface pump 17. Attached to pump 17 is pipe 18 which 
carries oil pumped from production well 12 to storage facilities (not 
shown). 
In accordance with the present invention, suspended within monitoring well 
13a is subsurface logging instrument 19. Cable 20 suspends instrument 19 
within monitoring well 13a and contains the required conductors for 
electrically connecting the subsurface instrument 19 with the surface 
electronics 21. The cable is wound on or unwound from drum 22 in raising 
and lowering instrument 19 to traverse the well. Electrical signals 
transmitted to surface electronics 21 from instrument 19 are processed by 
circuitry within surface electronics 21 and recorded on recorder 23, as 
will be fully explained hereinafter. 
In secondary or tertiary recovery operations, surface pumps 14a and 14b are 
supplied with water from the most convenient source available. The water 
source can be surface pools, area lakes, surrounding seas, or wells 
drilled into water bearing formations. The water source to be utilized is 
chosen as being the most ecologically safe and economically available 
source. It should be appreciated that the chemical characteristics of the 
injection water will vary greatly from one water source to another. 
Pumps 14a and 14b pump water from the surface to within injection wells 11a 
and 11b through tubings 15a and 15b, respectively. The injection water is 
forced through the perforations located in the casing of injection wells 
11a and 11b into the permeable formation which was the source of primary 
oil production. The flood front expands radially from injection wells 11a 
and 11b driving the residual oil in the producing formations toward 
producing well 12. The advancement of the flood front, shown generally at 
numerals 24 and 25, causes an area of increased oil saturation to develop 
ahead of the moving water. Additionally, as the injection water flood 
front advances there is a change in the salinity of the water as the 
injection water contaminates the water located within the production 
formations. 
As previously mentioned, to monitor the progress of the flood fronts 24 and 
25 through the permeable production zone logging instrument 19 is caused 
to traverse the cased monitoring well. Electrical signals are generated 
indicative of oil saturation and the change in water salinity of the 
subsurface formations. The logging instrument is run in each monitoring 
well located between the injection wells and the producing well in order 
to obtain a complete profile of the water flood front. 
Referring now to FIG. 2, there is illustrated in greater detail the logging 
operation of FIG. 1. Injection well 13a penetrates the earth's surface. 
Disposed within injection well 13a is subsurface instrument 19 of the well 
logging system. Subsurface instrument 19 comprises a pulsed neutron source 
26, a detecting system 27, a subsurface electronics package 28 and a 
neutron shield 29 located between the source 26 and the detector 27. As 
previously stated, cable 20 suspends instrument 19 within injection well 
13a and contains the required conductors for electrically connecting 
instrument 19 with surface electronics 21. 
In making a radioactivity log of the injection well 13a, instrument 19 is 
caused to traverse the well. Thereby high energy neutrons from source 26 
irradiate the formations surrounding the borehole and radiations 
influenced by the formations are detected by the detecting system 27. The 
resultant signals are processed by subsurface electroncis 28 and are sent 
to the surface electronics 21 through cable 20, where the signals are 
further processed and recorded on recorder 23. Recorder 23 is driven in 
coordination with the movement of the subsurface instrument 19 within 
injection well 13a. 
Referring now to FIG. 3, a portion of surface electronics 21 is shown in 
greater detail. The detected radiation signals represent the radioactivity 
resulting from inelastic scattering and the measurement of neutron capture 
caused from the pulsing of the neutron source 26. The input terminal 30 in 
the illustrated portion of surface electronics 21 receives electrical 
pulses representative of the detected radiations. The pulses are coupled 
into a conventional sync and signal separator circuit 31. The sync or 
timing pulse is coupled out of sync and signal separator circuit 31 by 
conductor 32 to multichannel analyzer 33. The detector signals are coupled 
from sync and signal separator 31 by conductor 34 into multichannel 
analyzer 33. Multichannel analyzer 33 has seven outputs which are each 
connected into four address decoders, identified by numerals 35-38, 
respectively. The outputs of address decoders 35 and 36 are coupled into 
ratio circuit 39. The outputs of address decoders 37 and 38 are coupled 
into ratio circuit 40. The output of ratio circuits 39 and 40 are coupled 
into recorder 23. 
The operation of the multichannel analyzer and the address decoders is 
explained in greater detail in U.S. Pat. No. 4,013,874, issued to R. B. 
Culver on Mar. 22, 1977. In accordance with the present application, 
address decoder 35 is configured to measure pulses in the 3.17 Mev to 4.65 
Mev band of the capture gamma ray spectrum. Address decoder 36 is 
configured to measure pulses in the 4.86 Mev to 6.62 Mev band of the 
capture gamma ray spectrum. Address decoder 37 is configured to measure 
pulses in the 3.17 Mev to 4.65 Mev band of the inelastic gamma ray 
spectrum and address decoder 38 is configured to measure pulses in the 
4.86 Mev to 6.62 Mev band of the inelastic gamma ray spectrum. The windows 
for address decoders 35-38 are graphically illustrated in FIG. 4 which 
shows a typical thermal neutron capture curve 41 following a neutron burst 
and a typical inelastic scattering curve 42. 
In the operation of the portion of the surface circuitry shown in FIG. 3, 
it should be appreciated that address decoders 35-38 provide information, 
respectively, with regard to silicon, calcium, carbon and oxygen windows. 
Thus, ratio circuit 39 provides a silicon/calcium ratio and ratio circuit 
40 provides a carbon/oxygen ratio, each of which is recorded on surface 
recorder 23. 
Referring now to FIG. 4, there is illustrated graphically a plot of 
radioactivity counts versus energy showing both a capture spectrum and 
also an inelastic spectrum, in addition to the energy windows used for 
obtaining a Si/Ca ratio and a C/O ratio. As shown, the silicon capture 
window is coincident with the inelastic carbon window and the calcium 
capture window is coincident with the oxygen inelastic window. 
FIG. 5 graphically illustrates data which was derived using the windows 
illustrated with respect to FIG. 4. It has been found that using the 
described windows the silicon/calcium capture ratio is highly sensitive to 
water salinity. As shown in FIG. 5, with a known reservoir porosity the 
silicon/calcium ratio will vary in accordance with changes in reservoir 
salinity. In monitoring flood front movement in a water flood operation 
the monitoring wells are first logged to establish a base log of oil 
saturation, as represented by the carbon/oxygen ratio. Simultaneously, a 
base log of water salinity is established as indicated by the 
silicon/calcium ratio. The base logs should be run prior to commencement 
of water flooding. 
As water is injected into the subsurface formation through injection wells 
13a and 13b an area of increased oil saturation will preceed the water 
flood fronts 24 and 25. By logging the monitor wells in accordance with 
the description hereinbefore described the early progress of the flood 
front can be detected and monitored by repeating the logging operations in 
the prior logged monitor wells and comparing the later carbon/oxygen ratio 
logs with the base logs to determine increase in oil saturation. 
Simultaneously with the carbon/oxygen ratio log there is obtained a 
silicon/calcium ratio log, as previously explained. By comparing the base 
silicon/calcium ratio logs with the later derived silicon/calcium ratio 
logs there is provided a method of monitoring salinity variations caused 
by the mixing of the known initial formation water salinity and the 
salinity of the injection water. By monitoring both oil saturation and the 
salinity mixing factor one can monitor the directional radial movement of 
the flood front within a permeable zone and detect any high-permeability 
streaks where the injected water moves faster than in the remainder of the 
permeable formation. 
Thus, there has been described and illustrated herein a method for 
monitoring the movement of a flood front from within a cased borehole 
without the addition of elements to the injected water. However, obvious 
variations will occur to those skilled in the art. For example, rather 
than pulsing the neutron source a continuous source of neutrons can be 
employed, as from an isotopic americium-beryllium source. Further, 
injection water could be pumped into a central well to force oil radially 
to a series of outer production wells.