Cold fluid hydraulic fracturing process for mineral bearing formations

A method for hydraulic fracturing a subterranean formation region to control the fracture extent in vertical and horizontal directions characterized by the injection of cold liquid into the formation region to precool the region and reduce the stresses in the formation region so that a hydraulic fracture may be propagated at a lower fluid injection pressure. The shape of the cooled region may be controlled by injection of various quantities of leakoff control agent during injection of the cold liquid and extension of the hydraulic fracture may be carried out simultaneously with the cold liquid flooding or by raising the pressure after the flood front has progressed a desired radial extent from the wellbore. The fracturing operation may be completed by injecting a pad of cold liquid with a high concentration of leakoff control agent to seal the fracture face followed by injection of liquid carrying a sufficient quantity of proppant material to maintain the fracture width and conductivity at the desired level.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to a hydraulic fracturing process for 
subterranean hydrocarbon producing formations which includes injection of 
a cold liquid into the formation to reduce the earth stresses and to 
control the extent of the fracture within the desired mineral producing 
zone. 
2. Background 
When a relatively cold fluid, such as water, is injected into a relatively 
warm subterranean hydrocarbon bearing reservoir an ever increasing region 
of cooled rock is established around the injection well and results in the 
reduction in stresses in the rock which may be on the order of several 
hundred pounds per square inch (psi). This reduction in stresses in the 
rock matrix may be utilized to extend hydraulic fractures to enhance the 
recovery of liquid and gaseous hydrocarbon substances present in the 
formation to be produced. 
Discussions of the effects of thermoelastic stresses in earth formations 
resulting from the injection of relatively cold liquids into relatively 
warm formations are discussed in papers published by T. K. Perkins and J. 
A. Gonzalez entitled "Changes in Earth Stresses Around a Wellbore Caused 
by Radially Symmetrical Pressure and Temperature Gradients", Society of 
Petroleum Engineers Journal, April, 1984, and "The Effect of Thermoelastic 
Stresses on Injection Well Fracturing", Society of Petroleum Engineers 
Journal, February, 1985. These papers present methods to determine the 
effect of the injection of large volumes of liquid into a subterranean 
earth formation and methods for calculating the thermoelastic stresses and 
hydraulic fracturing pressures required to achieve a hydraulic fracture. 
At least some of the assumptions made in the abovementioned publications 
can be utilized in dealing with fracturing lightly consolidated formations 
such as the type found in the West Sak Oil Field in Alaska. It is 
particularly important in developing fields which have relatively low well 
productivity as determined by conventional fracturing methods to improve 
productivity by enhancing the width and size of the fracture without the 
chance of extending the fracture outside of the mineral bearing formation 
or zone which is desired to be produced. 
Conventional hydraulic fracturing, particularly in lightly consolidated 
formations, is difficult to control as regards the extent of the fracture. 
Moreover, in lightly consolidated formations, such as the abovementioned 
oil field, relatively large quantities of fine solid particles are usually 
carried with the flowing oil stream being produced. These formation 
particles are carried into a propped hydraulic fracture and tend to 
significantly reduce fracture conductivity. To prevent the embedment or 
saturation of the fracture proppant by these relatively fine particles, 
smaller sizes of proppant particles might be used. However, the use of 
smaller proppant particles also requires wider fractures to achieve the 
fracture conductivity required to make the well completion economical. 
Under these conditions, the use of conventional hydraulic fracturing 
processes to achieve wide fractures greatly increases the chance of 
fracturing beyond the desired formation boundaries. 
It is an object of the present invention to improve the productivity of 
hydrocarbon bearing reservoirs which may be damaged or degraded by 
uncontrolled hydraulic fractures. It is a further object of the present 
invention to provide an improved process for hydraulically fracturing a 
hydrocarbon bearing formation, including formations which are lightly 
consolidated, so as to increase well productivity. These objects, as well 
as additional objects obtained by the present invention, will be further 
appreciated by those skilled in the art. 
SUMMARY OF THE INVENTION 
The present invention provides an improved process for hydraulic fracturing 
a subterranean hydrocarbon bearing formation wherein pre-cooling of the 
formation is obtained to reduce formation stresses and pressures and to 
provide for a hydraulic fracture which has relatively high conductivity 
but does not extend outside of the desired zone of a formation to be 
produced. 
In accordance with one aspect of the present invention, a relatively wide 
and propped formation fracture is obtained by injecting a large volume of 
cold liquid such as water into the formation to create a region of reduced 
stress adjacent to a wellbore. When the desired size of the reduced stress 
region within the formation and the stress condition therein has been 
achieved, a pad of cold fluid containing a relatively high concentration 
of leakoff control agent is injected to seal the fracture faces to 
minimize leakoff of fracturing fluid and proppant bearing fluid. 
In accordance with another aspect of the present invention, there is 
provided an improved hydraulic fracturing process wherein after injection 
of a relatively large volume of cold fluid to reduce the stresses in a 
particular formation to be fractured, extension of the fracture is carried 
to a desired limit, and then a relatively cold or viscous fluid is 
injected at a relatively high rate and with high proppant concentration to 
widen and prop the fracture in the widened condition. 
The overall process of the invention provides for improved hydrocarbon 
fluid production from formations which are lightly consolidated, in 
particular. 
Additional superior features and advantages of the present invention will 
be recognized by those skilled in the art upon reading the detailed 
description which follows in conjunction with the drawing.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The drawing figures comprise a somewhat schematic illustration of a typical 
well completion into a subterranean formation which has been determined to 
have economically recoverable mineral deposits therein, such as 
hydrocarbon fluids. Referring to FIG. 1 of the drawing, there is 
illustrated an earth formation 10 into which a well 12 has been drilled 
and provided with a suitable casing 14, a conventional wellhead structure 
16 and an elongated fluid injection tube 18 extending through the casing. 
The tube 18 is open into a lower portion of a wellbore 20 which is sealed 
from the remainder of the wellbore by a packer 22. The casing 14 is 
provided with suitable perforations 23 which open into a region or zone 24 
of the earth formation 10 which has been determined to have recoverable 
quantities of hydrocarbon fluids, for example. The formation region 24 is 
bounded by regions 26 above and 28 below, which may or may not be 
desireable for eventual fracturing to release minerals or fluids contained 
therein. Typically, for example, a region above or below a produceable 
formation or region may contain quantities of water or brine 27 which, if 
the formation region is fractured, would be released to flow into the 
wellbore 20 through the formation region 24, thereby damaging the 
producibility of the region 24 or create unwanted separation problems with 
respect to any fluids produced by the well 12. 
For purposes of the discussion herein and by way of illustration, only the 
well 12 is illustrated as being suitably connected to a source of cold 
fluids such as treated sea water, not shown, which may be pumped into the 
tubing 18 by way of a suitable high pressure pump 28 connected to a 
conduit 29. A second pump 30 may also be connected to the conduit 29 which 
is in flow communication with the tubing 18. The pump 30 may be 
selectively connected to a source 34 which includes a leakoff control 
agent and a source 36 which includes a proppant material. The arrangement 
illustrated for pumping fluid into the wellbore 20, as shown in FIG. 1, is 
exemplary and the arrangement of pumping apparatus and sources of material 
such as leakoff control agents and proppant materials may be modified in 
one of several ways. 
When the well 12 has been drilled and the vertical extent of the formation 
region 24 determined, perforations 23 are formed in the casing 14 to 
provide for conduction of fluid between the wellbore 20 and the formation 
region or zone 24. Depending on the depth of the formation region 24, a 
significant temperature differential may exist between the temperature of 
the formation region and the surface ambient temperature, including 
possibly the temperature of a source of cold fluids such as a nearby lake 
or ocean. It is, for example, not unusual to experience subterranean 
hydrocarbon reservoir or formation region temperatures in the range of 
150.degree. to 200.degree. F. and greater. Sources of large volumes of 
"cold" water rarely exceed ambient temperatures higher than 70.degree. to 
80.degree. F. The injection fluid can, of course, be artificially 
refrigerated if desired. Accordingly, a significant temperature 
differential can exist between the formation being flooded and the 
temperature of the injection fluid itself. As discussed in the 
aforementioned publications, significant lowering of formation stresses 
can be achieved by injecting relatively large volumes of cold fluid into a 
particular zone or region and, consequently, the pressures required to 
extend a hydraulic fracture in the region of reduced stress may also be 
significantly lowered. This reduction in fracture extension pressure can 
have significant effects on the costs of hydraulic fracturing and can 
permit greater fracture extension and conductivity thereby resulting in a 
higher yield of recoverable substances from the fractured formation. 
Referring now to FIG. 2 also, there is illustrated in somewhat schematic 
form the development of a typical fracture radially outwardly from the 
well 12 within the formation region 24. Since the stresses exerted in the 
horizontal direction are typically much lower than those in a vertical 
direction, earth formation fractures induced by hydraulic fracturing, for 
example, typically extend vertically and propagate perpendicular to the 
minimum horizontal stress. FIGS. 1 and 2 illustrate an extended fracture, 
generally designated by the numeral 40 having opposed generally 
symmetrical wing portions 42 and 44. These fracture wings 42 and 44 extend 
in an idealized manner generally equally radially outwardly from the 
central longitudinal axis 13 of the well 12. 
FIG. 2 further illustrates the assumed zones of the region 24 for which the 
temperature of the earth formation has been significantly lowered due to 
flooding of the formation by a relatively cold liquid such as treated sea 
water. In a substantially homogeneous earth formation, such as typically 
is found in unconsolidated sands, it can be assumed that the injected 
fluid migrates radially outwardly from the well axis 13 uniformly in all 
directions, thereby forming a generally cylindrical boundary of the cooled 
region as defined by the dashed line 46 in FIG. 2. Depending on the 
selected injection rate of formation cooling fluid, the region of cooled 
rock or earth substance may continue to be defined by a generally 
cylindrical boundary having its central axis coincident with the wellbore 
axis 13. 
However, due to the significant lowering of stresses in the formation 
region 24 during injection of the cold fluid in the area that has been 
cooled, a fracture may be initiated and begin propagating radially 
outwardly from the well 12. The formation of the fracture 40, for example, 
as it grows radially will tend to alter the shape of the zone or region of 
cooled rock to become somewhat elliptical as indicated by the boundary 
lines 48, 50 and 52. The boundary lines 48, 50 and 52 show the progressive 
growth in the area of the cooled region, as viewed in the horizontal 
plane, as the opposed ends of the fracture 40 extend radially outwardly. 
Thus, at least two fracture forming conditions can exist and may be 
controlled by design, knowing the formation characteristics of porosity, 
and the existing temperatures and stresses prior to injection of the cold 
fluid. If the fluid injection rate is sufficiently low and the injection 
pressure maintained sufficient to avoid reaching fracture initiation and 
extension pressures, the flooded region may grow to maintain a generally 
cylindrical boundary with respect to the well 12. However, this injection 
rate may be somewhat time consuming and uneconomical. If the injection 
rate or pressures are increased above the calculated horizontal stress in 
the region being cooled, a vertical two-winged fracture will likely be 
initiated and propagated radially outwardly to change the shape of the 
cooled region from one having a generally cylindrical boundary to the 
generally elliptical boundaries indicated by the boundary lines 48, 50 and 
52 as the fracture extends radially away from the well. 
One major advantage of initiating a fracture by pre-cooling the formation 
region to be fractured is that control over the fracture extension in a 
vertical direction as well as the horizontal direction may be enhanced. In 
the arrangement illustrated, for example, it may be highly desired to 
avoid extending the fracture 40 into either the region 26 or 28. Since it 
can be reasonably assumed that injection of cold liquid into the region 24 
will be confined vertically to this region and not extend substantially 
vertically above or below the perforations 23, then fluid injection 
pressures into the formation 24 may be controlled to avoid the possibility 
of extending the fracture vertically into either the regions 26 or 28. In 
this way the fracture 40 avoids breaking into areas in which large 
quantities of water or other fluids are disposed and which are not 
desireable to be produced through the well 12. 
Accordingly, the fracturing process of the present invention is initiated, 
upon completion of the well 12, and determination of the physical 
properties of the formation region 24, by commencing the injection of 
relatively cold liquid such as water through the conduit 18 and the 
perforations 23 into the formation region 24 at a controlled rate so as 
not to exceed the maximum hydraulic fracture extension pressure desired. 
Depending on formation characteristics, the injection rate may be 
relatively slow so as to essentially waterflood a generally cylindrical 
region, or the injection rate may be increased to the hydraulic fracture 
extension pressure of the cooled region so that the outer limits of the 
flooded portions of the region 24 tend to become elliptical. The extent of 
the ellipse defining the boundary of the cooled region with respect to the 
length of the minor axis may be selectively controlled by injecting a 
leakoff control agent into the cold injection liquid to partially seal the 
fracture faces. 
Typical leakoff control agents could include vegetable gums or quartz 
flour, for example, or other conventional leakoff control agents depending 
on the type of formation structure being fractured. If, for example, the 
overall length of the fracture 40 radially away from the axis 13 was to be 
extended to a certain limit and the amount of injection fluid minimized, 
increasing amounts of leakoff control agent could be mixed with the 
injection liquid to prevent or reduce the migration of fluid generally 
normal to the plane of the fracture 40 itself, thereby reducing the length 
of the minor axes of the elliptical boundaries 48, 50 and 52. Accordingly, 
two discrete steps according to the improved process may be initially 
performed upon completion of the well 12. For example, cold liquid may be 
injected into the formation region 24 at a rate which will maintain 
pressures lower than the reduced stress in the region resulting from 
cooling of the formation rock so that the boundary of cooled rock grows 
substantially radially outward to maintain a generally cylindrical shape. 
Alternatively, at some point in the injection process, the pressure may be 
increased to a value which will initiate the fracture 40 and the radial 
extent of the fracture may be controlled by the injection rate and 
pressure or by introduction of a leakoff control agent into the injected 
fluid to at least partially seal the faces of the fracture wings 42 and 
44, which faces are designated in FIG. 2 by the numerals 43, 45, 47 and 
49, respectively. 
After the radial extent of the fracture 40 has been carried to its desired 
length, one or the other of the pumps 28 and 30 is activated to inject a 
pad of cold fluid into the fracture 40, which fluid contains a 
significantly higher concentration of leakoff control agent than 
previously used in the fracturing process. This pad of cold fluid is 
injected without reducing the pressure in the lower portion of the 
wellbore 20 and in the fracture 40 to thereby prevent closing the 
fracture. The introduction of the pad of cold fluid with the high 
concentration of leakoff control agent and sealing of the fracture face is 
carried out to minimize the quantity of injected fluid required to 
maintain the fracture propped open until the injection of a suitable 
proppant can be initiated. Accordingly, following the injection of the pad 
of cold fluid containing leakoff control agent, and without reducing the 
fracture extension pressure, a second injection process would be initiated 
immediately using the pump 30 and the source of proppant 34 by injecting a 
cold or relatively viscous fluid at a relatively high rate and with a 
relatively high concentration of proppant material, preferably in a 
proppant size range which would maintain the fracture propped open to the 
desired width without significantly reducing fracture conductivity. 
After injection of the propant material in sufficient quantity to fill the 
fracture wings 42 and 44, the fluid pressure in the wellbore 20 and the 
formation region 24 could be relieved to permit the flow of recoverable 
mineral fluids toward the wellbore. 
Thanks to the overall process of fracturing the formation region 24 by 
initially cooling the region within an envelope which extends radially 
outwardly from the well 12, hydraulic fractures may be extended within the 
region without extending the fracture into undesired portions of the earth 
formation 10 such as the regions 26 and 28 above or below the region which 
is desired to be produced. In like manner, the horizontal and vertical 
extent of the fracture may also be controlled through the process of 
preflooding of the region 24 with cold fluid at a rate which would 
significantly cool the region without initiating a fracture, or at some 
point in the injecting and cooling process selectively raising the 
injection pressure to exceed the horizontal stress to thereby initiate a 
fracture. By measuring the quantity of injected fluid during the 
precooling or fracture initiation process, the radial outward extent of 
the fracture may be controlled and to a great extent the formation region 
24 may be controllably fractured without extending the fracture into an 
area generally outside the vertical confines of the region 24 which it may 
be desirable to avoid. 
Although a preferred embodiment of an improved hydraulic fracturing method 
has been described herein, those skilled in the art will recognize that 
various substitutions and modifications to the basic method or process may 
be made without departing from the scope and spirit of the invention as 
recited in the appended claims. The physical characteristics of the 
formation region 24 may be determined in accordance with conventional 
methods known to those skilled in the art and the calculations required to 
determine the fracture extension pressure and other injection conditions 
may be obtained in accordance with the teaching of the publications 
referenced hereinabove.