Decreasing total fluid flow in a fractured formation

A process for decreasing total fluid flow through a large natural or induced fracture where smaller multiple fractures are created. These multiple fractures each have a smaller aperture than said natural or induced fracture. The combined fluid flow through said multiple fractures is sufficiently less than the total fluid flow through said large fracture, thereby decreasing fluid bypass and improving sweep efficiency.

FIELD OF THE INVENTION 
This invention is directed to a method for decreasing total fluid flow from 
a formation into a wellbore where fracture apertures are reduced. 
BACKGROUND OF THE INVENTION 
Several articles and publications have attempted to deal with the 
engineering of a fractured reservoir. Among these is a book authored by E. 
S. Romm entitled "Fluid Flow in Fractured Rocks", and published by Nedra 
Publishing House, Moscow, Russia in 1966. The author cites laboratory 
research done in Russia, Europe, and the United States through 1966 on 
fluid flow through fractures. This book deals mainly with the theoretical 
formulation of fluid flow equations using Romm's own work and that of 
others. All topics concerning single-phase, two-phase flow, gas flow, and 
water imbibition are discussed. The book also presents some laboratory 
work done on fluid flow in very tight fractures (0.25 to 2.9 microns) and 
some two-phase systems. 
In a text book published in 1982 by Elsevier Scientific Publishing Co., New 
York, N.Y., entitled "Fundamentals of Fractured Reservoir Engineering", T. 
D. Van Golf-Racht tries to integrate existing published material into a 
complete book on fractured reservoir engineering. His aim is to present 
new analytical tools to engineers and geologists for examining fractured 
reservoirs, since conventional reservoir evaluation techniques do not 
apply to fractured reservoirs. 
A book written in 1980 by L. H. Reiss entitled "The Reservoir Engineering 
Aspects of Fractured Formations" is a primer on fractured reservoir 
engineering. This book was published by Gulf Publishing Co., Houston, Tex. 
It covers the basic problems for fractured reservoir evalution, but only 
presents published solutions without substantial theoretical or 
experimental backup. It is a good first book to read for fractured 
reservoir engineering. 
Another book, written in 1980 by R. Aguilera, entitled "Naturally Fractured 
Reservoirs", is an organized collection of papers without any conclusions. 
This book was published by the Petroleum Publishing Co. located in Tulsa, 
Okla. It presents the data and correlations without expanding on the 
ideas. This book does have a good summary of pertinent papers. 
A new book written by R. A. Nelson entitled "Geologic Analysis of Naturally 
Fractured Reservoirs", published in 1985, was written to supplement the 
other books listed above. Nelson wrote this book to present a rock data 
approach to fractured reservoir evaulation. This book was published by the 
Gulf Publishing Co., Houston, Tex. 
None of these publications have resolved the problem of fingering caused by 
large fractures in oil fields during secondary recovery operations. This 
problem occurs because large fractures can become conduits from injection 
to production wells which cause an inefficient sweep to occur. Therefore, 
what is needed is a method to decrease fluid bypass so better sweep 
efficiency can be obtained. 
SUMMARY OF THE INVENTION 
Fluid bypass is decreased in reservoirs containing at least one 
substantially large natural or induced fracture. In the practice of this 
invention, at least one substantially large fracture is located which 
fracture causes "fingering" in said reservoir. Thereafter, a fracturing 
method is used to create smaller multiple fractures. These smaller 
fractures are created by applying a force sufficient to create said 
multiple fractures whihc have smaller apertures that produce a total lower 
flow rate than said large fracture. The resultant lower flow rate occurs 
because of the cubed root dependence of the flow rate to fracture 
aperture. 
Creation of the smaller multiple fractures substantially closes said large 
fracture and decreases the overall fluid flow which previously emitted 
from said large fracture. Reducing the fluid flow decreases fluid bypass 
thereby providing for a more efficient sweep efficiency. In the case of a 
hydrocarbonaceous fluid containing reservoir, this more efficient sweep of 
the reservoir affords for the increased recovery of hydrocarbonaceous 
fluids. 
It is therefore an object of this invention to increase the recovery of 
hydrocarbonaceous fluids from a formation by creating smaller multiple 
fractures in lieu of a large fracture to reduce steam flow bypass during a 
steamflood. 
It is another object of this invention to increase the recovery of 
hydrocarbonaceous fluids from a formation by creating smaller multiple 
fractures in lieu of a large fracture to reduce carbon dioxide bypass 
during a carbon dioxide recovery process. 
It is a yet another object of this invention to increase the recovery of 
hydrocarbonaceous fluids from a formation by creating smaller multiple 
fractures in lieu of a large fracture to reduce water bypass during a 
waterflood recovery process.

DESCRIPTON OF THE PREFERRED EMBODIMENT 
In the practice of this invention, one substantially large fracture is 
located which causes "fingering" of fluids from a reservoir or formation. 
This "fingering" generally results when a large natural or induced 
fracture dominates fluid flow. This problem may exist when attempting to 
remove fluids from a formation, particularly oil, water, gas or steam. It 
is of concern in oil fields when a secondary recovery operation is 
utilized because a large "fingering" fracture may become a conduit from an 
injection well to a production well thereby decreasing sweep efficiency. 
The location of this large "fingering" fracture can be determined by those 
skilled in the art such as geologists. 
In one embodiment of this invention, a controlled pulse or high energy 
fracturing method is used to create the smaller multiple fractures. The 
drawings are illustrative of said controlled pulse or high energy 
fracturing. Hydraulic fracturing can also be used to create the smaller 
multiple fractures. 
FIG. 1. is a plane view which depicts the large "fingering" fracture 16 
interconnected with wellbore 12 in formation 10. Inside wellbore 12 is 
propellant device 14. After fracture 16 has been located, in one 
embodiment, a propellant device 14 contained in a cannister is suspended 
from the ground level in wellbore 12. This device is located near fracture 
16. The opening or aperture of fracture 16 can be determined, preferably 
before the propellant device 14 is suspended in wellbore 12. After the 
aperture or opening of fracture 16 is determined, the size and number of 
fractures to be created are determined in order to obtain the desired 
reduction in flow. The number and size of the desired fractures are 
determined by using Boussinesq's cubic law formula for steady-state 
isothermal, laminar flow between two parallel plates. This equation is 
##EQU1## 
where Q=flow rate (L.sup.3 /T) 
W=width of fracture face (L) 
p=density (M/L.sup.3) 
g=acceleration of gravity (L/T.sup.2) 
b=fracture aperture (L) 
.DELTA.h=hydraulic head of water (L) 
L=length of fracture (L) 
The total fluid flow is governed by the third root of the fracture 
aperture. If, for example, fracture 16 has an aperture of 0.1 inches, 
reducing it to three smaller fractures of 0.034 inches, each will reduce 
the total fluid flow by 88%. This is explained further when it is 
ascertained that Wpg(b).sup.3 .DELTA.h/L12.mu. is a constant or "C". 
Therefore, Q=Cb.sup.3 where b, the fracture aperture, becomes the 
variable. The total fluid flow in 0.1 inch "fingering" fracture 16 is 
determined to be 0.0010, i.e., Q=C(0.1).sup.3 =C(0.0010). When three 
smaller fractures are created in lieu of "fingering" fracture 16, each 
smaller fracture would have an aperture of 0.034. Thus, the total fluid 
flow through all three apertures would be C(0.000118) i.e., 
Q=3[C(0.034).sup.3 ]=C(0.000118). Reduction of flow is equal to 0.001 
-0.000118/0.0010 .times.100=88%. 
In order to create the required number of multiple fractures with the 
desired apertures, it is preferred to use a propellant as disclosed by 
Godfrey et al. in U.S. Pat. No. 4,039,030 which issued on Aug. 2, 1977. 
This patent is hereby incorporated by reference. To accomplish this said 
cannister containing a propellant is suspended into a wellbore. This 
cannister is placed downhole next to the "fingering" fracture 16. 
The propellant in the cannister can belong to the modified nitrocellulose 
or the modified and unmodified nitroamine propellant class. Suitable solid 
propellants capable of being utilized include a double-based propellant 
known as N-5. It contains nitroglycerine and nitrocellulose. Another 
suitable propellant is a composite propellant which contains ammonium 
perchlorate is a rubberized binder. The composite propellant is known as 
HXP-100 and is purchasable from the Holex Corporation of Hollister, 
California. N-5 and HXP-100 propellants are disclosed in U.S. Pat. No. 
4,039,030. 
A M-5 solid propellant was utilized by C. F. Cuderman in an article 
entitled "High Energy Gas Fracturing Development", Sandia National 
Laboratories, SAND 83-2137, October 1983. This article is also 
incorporated by reference. High energy gas fracturing or controlled pulse 
fracturing is a method used for inducing multiple radial fractures around 
a wellbore or borehole. Via this method a solid propellant-based means for 
fracturing is employed along with a propellant composed to permit the 
control of pressure loading sufficient to produce multiple fractures in a 
borehole at the oil or hydrocarbonaceous fluid productive interval. A peak 
pressure is generated which is substantially above the in-situ stress 
pressure but below the rock yield stress pressure. 
After placing the propellant means for creating multiple fractures downhole 
near the "fingering" fracture 16, it is ignited. Ignition of the 
propellant means for creating the multiple fractures causes the generation 
of heat and gas pressure. As is known to those skilled in the art, the 
amount of heat and pressure produced is dependent upon the kind of 
propellant used, its grain size and geometry. Heat and pressure generation 
also depends upon the burning rate, weight of charge and the volume of 
gases generated. 
Subsequently, the heat and pressure are maintained for a time sufficient to 
allow fluid penetration and extension of fractures. As is known, heat 
generation and pressure maintenance are dependent upon the nature of the 
formation and the depth it is desired to extend the fractures into the 
formation. After the heat and pressure have been maintained for a time 
sufficient to promote the desired fracturing, the heat and pressure 
dissipate into the formation surrounding the wellbore. In order to more 
precisely direct the force and direction of the energy released, the 
cannister device 14 containing the propellant can be molded into a desired 
pattern or shape. A method and apparatus which can be used is disclosed by 
Keller in U.S. Pat. No. 4,018,293 which issued on Apr. 19, 1977. This 
patent is hereby incorporated by reference. However, as mentioned above, 
it is preferred to use a propellant in lieu of Keller's explosive. 
As disclosed by Godfrey et al., the propellant can be tailored to create 
the desired number and width of vertical fractures. By utilizing a 
cannister of the desired shape, the released force can be directed in a 
manner so as to create additional fractures sufficient to reduce the total 
fluid flow to the volume desired, thereby decreasing fluid bypass and 
obtaining a better sweep of the reservoir. The direction of the released 
energy should be such that existing fracture 16 would be substantially 
closed, or reduced to an aperture no greater than the newly created 
fractures 18 shown in FIG. 2. 
This cubic law equation utilized herein, may have deviations when used in 
extremely tight fractures (aperture under 50 microns), at least in 
non-porous granite but this only affects the "C" constant in the equation. 
The same reduction of flow is seen even if the "C" constant changes. As 
will be understood by those skilled in the art, flow inhibitors can be 
used to additionally reduce flow when combined with this inventive method. 
These flow inhibitors include foams, surfactants, and polymers. Exemplary 
flow inhibitors which can be used herein include, but not limited to, 
LTS18, K-Trol, and Celogen AZ, which are purchasable from Shell, 
Halliburton, and Uniroyal, respectively. 
Although it is preferred to use this method in the recovery of 
hydrocarbonaceous fluids from a formation containing at least one 
injection well, this method can also be utilized in reducing the flow of 
other fluids from a formation including water, gas, and steam. When one 
injection well is used when recovering hydrocarbonaceous fluids, that 
injection well can also serve as a production. As is understood by those 
skilled in the art, any number of injection and production wells may be 
utilized herein. 
While it is preferred to use this method in those applications where the 
"fingering" fractures (16) have been produced naturally, this method can 
also be used in applications where fractures 16 have been induced by 
fracturing methods commonly utilized so long as said fractures are not 
propped. Several fracturing methods which can be used appear below. 
U.S. Pat. No. 3,863,709 issued to Fitch on Feb. 4, 1975 discloses a method 
and system for recovering geothermal energy from a subterranean geothermal 
formation having a preferred vertical fracture orientation. At least two 
deviated wells are provided which extend into the geothermal formation in 
a direction transversely of the preferred vertical fracture orientation 
and a plurality of vertical fractures are hydraulically formed to 
intersect the deviated wells. A fluid is injected via one well into the 
fractures to absorb heat from the geothermal formation and the heated 
fluid is recovered from the formation via another well. This patent is 
hereby incorporated by reference herein. 
Savins in U.S. Pat. No 4,067,389 issued Jan. 10, 1978, discloses a 
technique of hydraulically fracturing a subterranean formation wherein 
there is used a fracturing fluid comprised of an aqueous solution of an 
interaction product of a polysaccharide and a galactomannan. This patent 
is incorporated by reference herein. 
Another fracturing technique is disclosed by Medlin et al. in U.S. Pat. No. 
4,415,,035, which issued on Nov. 15, 1983. Here, a well casing penetrating 
a plurality of subterranean hydrocarbon-bearing formations is perforated 
adjacent select ones of such hydrocarbon-bearing formations that are 
expected to exhibit at least a minimum pressure increase during fracturing 
operations. A fracturing fluid is pumped down the well through the 
perforations, and into the formations so as to fracture each of the select 
formations during a single fracturing operation. This patent is 
incorporated by reference herein. 
Where it is desired to obtain increased sweep efficiency, this invention 
can be used with several enhanced oil recovery methods. 
One method where this invention can be utilized is during a waterflooding 
process for the recovery of oil from a subterranean formation. After 
creating the multiple smaller fractures of this invention, a waterflooding 
process can be commenced, U.S. Pat. No. 4,479,894, issued to Chen et al, 
describes one such waterflooding process. This patent is hereby 
incorporated by reference in its entirety. 
Steamflood processes which can be utilized when employing the invention 
described herein are detailed in U.S. Pat. Nos. 4,489,783 and 3,918,521 
issued to Shu and Snavely, respectively. These patents are hereby 
incorporated by reference herein. 
The invention described herein can also be used in conjunction with a 
cyclic carbon dioxide steam stimulation in a heavy oil recovery process to 
obtain greater sweep efficiency. Cyclic carbon dioxide steam stimulation 
can be commenced after creating the smaller multiple fractures in the 
reservoir with this invention. Another suitable process is described in 
U.S. Pat. No. 4,565,249 which issued to Pebdani et al. This patent is 
hereby incorporated by reference in its entirety. Increased sweep 
efficiency can be obtained when the subject smaller multiple fractures are 
used in combination with a carbon dioxide process by lowering the carbon 
dioxide minimum miscibility pressure ("MMP") and recovering oil. Carbon 
dioxide MMP in an oil recovery process is described in U.S. Pat. No. 
4,513,821 issued to Shu which is hereby incorporated by reference. 
Although the present invention has been described with preferred 
embodiments, it is to be understood that modifications and variations may 
be resorted to without departing from the spirit and scope of this 
invention, as those skilled in the art will readily understand. Such 
modifications and variations are considered to be within the purview and 
scope of the appended claims.