Use of thin liquid spacer volumes to enhance hydraulic fracturing

A method for enhanced hydraulic fracturing which comprises injecting a proppant laden fracturing fluid into a formation or reservoir at a rate and pressure sufficient to fracture said formation. Next, a thin spacer fluid is injected into the created fracture. Afterwards, a proppant laden fracturing fluid is injected into the formation at a rate and pressure sufficient to hold the created fracture open which allows proppant to be more evenly distributed throughout the created fracture as proppant falls through the spacer fluid thereby avoiding proppant convection in the created fracture while obtaining substantially improved propping of the fracture.

FIELD OF THE INVENTION 
This invention is directed to a method for fracturing a subterranean earth 
formation penetrated by a least one well wherein a fluid spacer is used 
with a slug of proppant laden fracturing fluid. 
BACKGROUND OF THE INVENTION 
Techniques for hydraulically fracturing subterranean formations by 
injecting a fracturing fluid down a well and into the formation under 
sufficient pressure to create fractures in a formation are well known. 
Proppant materials are generally entrained in the fracturing fluid and are 
deposited in the fracture to keep the fracture open. 
After fracturing the formation, hydrocarbonaceous fluids are produced from 
the formation into the well. These produced fluids may carry sand 
entrained therein, particularly when the subsurface formation is as 
unconsolidated formation. Produced sand is undesirable for many reasons. 
It is abrasive to components found within the well, such as tubing, pumps 
and valves, and must be removed from the produced fluids at the surface. 
Further, produced sand may partially or completely clog the well, 
subsequently inhibiting production thereby making necessary an expensive 
workover. In addition, sand flowing from the subsurface formation may 
leave therein a cavity which may result in caving of the formation and 
collapse of a well casing. 
Often after completion of hydraulic fracturing, a steam-flood or other heat 
generating method is used to heat the formation to remove 
hdyrocarbonaceous fluids therefrom after having placed a proppant into the 
created fractures. Proppants utilized in this manner keep the created 
fractures from closing. They also assist in reducing undesired fines from 
being carried from the formation with the produced fluids. Also, proppants 
increase the permeability thereby allowing more intimate contact of the 
heating medium with the formation. Generally sand is used as a proppant. 
Based on recent research concerning proppant slurry transport in hydraulic 
fractures, it has become more apparent that density effects of slurry 
volume are important in the deposition of proppant material used in a 
treatment to hold the fracture open. During the fracturing treatment, a 
neat fluid i.e., fracturing fluid without proppant therein, is used to 
create a hydraulic fracture. 
Under fracturing pressure, following the creation of the initial fracture, 
a fracturing fluid containing a proppant therein, is injected into the 
created fracture. Because the proppant is added to the slurry in 
increasing concentrations, the effective density of the slurry is greater. 
Therefore, proppant has a tendency to fall to the bottom of the created 
fracture. The phenomena related to this type of proppant settling has been 
called "convection."It is described in SPE paper 24825 authored by M. P. 
Cleary and A. Fonseca. This paper was presented at the 67th Annual 
Technical Conference and Exhibition of the SPE. It is entitled "Proppant 
Convection and Encapsulation in Hydraulic Fracturing: Practical 
Implications of Computer and Laboratory Simulations". 
Therefore, what is needed is a method for effective proppant placement 
during fracturing which will diminish the "convection effect" so as to 
allow more effective proppant deposition in the main part of the created 
fracture. 
SUMMARY OF THE INVENTION 
This invention is directed to a method for enhanced hydraulic fracturing. 
In the practice of this invention, a neat fracturing fluid is injected 
into the formation under a pressure and at a rate sufficient to create a 
fracture. Next, a proppant laden fracturing fluid is injected into the 
formation at a pressure and rate sufficient to hold open said fracture 
with said proppant. Thereafter, a thin spacer fluid is injected into the 
created fracture. Afterwards, a proppant laden fracturing fluid is 
injected into the formation or reservoir at a rate and pressure sufficient 
to hold the created fracture open which allows proppant to be more evenly 
distributed throughout the created fracture as proppant falls through the 
spacer fluid. In this manner, the effects of proppant convection in the 
created fracture are enhanced while substantially improved propping is 
obtained in the main part of the fracture. 
It is therefore an object of this invention to enhance proppant convection 
in a created fracture during hydraulic fracturing. 
It is another object of this invention to provide for a method for that 
will allow an even distribution of proppant over the main part of a 
fracture created during a hydraulic fracturing operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the practice of this invention, a fracturing method is utilized to 
induce fractures into a hydrocarbonaceous fluid bearing formation to 
increase its permeability. Prior to fracturing the formation a well is 
cased and then selectively perforated over a one to two foot interval in a 
productive interval of a formation. A hydraulic fracturing technique which 
can be used herein is disclosed in U.S. Pat. No. 4,067,389 which issued to 
Savins on Jan. 10, 1978. Another method for initiating hydraulic 
fracturing is disclosed by Medlin et al. in U.S. Pat. No. 4,378,849 which 
issued on Apr. 5, 1983. Both patents are hereby incorporated by reference 
herein. As is known by those skilled in the art, in order to initiate 
hydraulic fracturing in a formation, the hydraulic pressure applied must 
exceed the formation pressures in order to cause a fracture to form. The 
fracture which forms would generally run perpendicular to the least 
principle stress in the formation or reservoir. 
The fracturing fluid which is used herein to hydraulically fracture the 
formation comprises a viscous gel. Ungelled fluids can also be used. The 
viscous gel can include a water-based hydroxypropyl guar and (HPG) 
hydroxyethyl cellulose (HEC), carboxymethylhydroxyethyl cellulose (CMHEC), 
guar or oil-based diesel oil, and kerosene gel with aluminum phosphate 
esters (e.g., Halliburton Services' "MY-T OIL II" gel, 
Dowell/Schlumberger's "YF-GO" gel, B. J. Titan's "ALLOFRAC" gel, and The 
Western Company of North America's "MAXI-O" gel). 
The proppant concentration in the viscous gel should be from about one to 
about 18 pounds per gallon. In those situations where high temperatures 
are encountered a fused refractory proppant can be used and should be in 
the amount of about 10 to about 18 pounds per gallon. These proppants 
include silicon carbide, silicon nitride or a garnet proppant and mixtures 
thereof. These proppants are particularly preferred when high temperature 
effects of steam are encountered. 
In carrying out a hydraulic fracturing treatment as is shown in FIG. 1, it 
is a desired practice to first inject a fluid pad or neat "frac" fluid 
lacking a proppant therein into the formation to initiate the fracture. 
Thereafter, a very low concentration of propping agent or fused refractory 
material along with the "frac" fluid is injected into the fracture to 
ensure that the fracture has taken the propping agent or fused refractory 
material. As the fracture propagates into the formation and a greater 
fracture area is created, increased amounts of proppant 18 are added to 
the fracturing or "frac" fluid. This is illustrated in FIG. 1. Here as is 
shown the neat fluid enters wellbore 10 and exits the wellbore by 
perforations 12. The neat fluid 16 enters the formation and creates a 
vertical fracture 14. 
During the conventional fracturing operation, as is shown in FIG. 2, 
proppant 18 from the injected fracturing fluid has fallen to the bottom of 
fracture 14 due to gravitational or "convectional" effects on the 
proppant. This is an undesired situation since nearly all of the proppant 
has settled to the bottom and the main part of fracture 14 still lacks 
sufficient proppant therein to hold fracture 14 open. 
In the practice of this invention, after creating the initial fracture, as 
is shown in FIG. 3, a gel "frac" fluid with proppant therein is injected 
into wellbore 10 whereupon it enters the formation via perforations 12 
which causes fracture 14 to form vertically. While the fracturing pressure 
remains on the wellbore and fracture 14, a thin spacer fluid is next 
injected into the fracture via perforations 12 in wellbore 10. The thin 
spacer fluid which is utilized comprises the fracturing fluid which has 
been diluted in a manner so as to allow the proppant from a subsequently 
injected fracturing fluid containing proppant to fall therethrough. This 
initial increment of proppant laden "frac" fluid 22 falls to the bottom of 
fracture 14. Thereafter, a second proppant laden fracturing fluid 24 is 
directed into the wellbore and out through perforations 12. 
Upon entering fracture 14 proppant from the second stage "frac" fluid 24 
falls through the thin spacer fluid thereby obtaining a more even 
distribution of the proppant in the fracture. Afterwards, a third 
increment of proppant or stage of proppant laden "frac" fluid 26 is 
directed into the wellbore and out through perforations 12 where it causes 
the proppant to settle out above the second increment or stage of "frac" 
fluid 24 which was previously placed into the fracture. Thereafter, if 
required another slug of thin spacer fluid can be directed into fracture 
14 via perforations 12. Should it be necessary, an additional increment or 
stage of proppant laden fracturing fluid can be directed into the fracture 
so as to fall through the thin spacer fluid and obtain a more even 
distribution of proppant over the main area of the fracture. 
By repeating the steps as necessary, a more even distribution of proppant 
within the main area of the fracture can be obtained thereby avoiding 
proppant convection in the fracture and obtaining a substantially improved 
propping of the fracture. As each increment of proppant laden "frac" fluid 
is directed into the fracture, the concentration of proppant contained in 
each subsequent increment or stage is less than the prior increment or 
stage of "frac" fluid. At the conclusion of the fracturing operation, 
wellbore 10 is shut in to allow the fracture to close. The thin fluid 
spacer is effective because it dilutes a portion of the proppant slurry in 
the fracture and causes the proppant to drop into the created fracture. 
Since the fluid spacer is thin, it has high fluid leak off properties 
compared to the gel "frac" fluid which carries the proppant. Thus, it 
allows some intermittent closure of the fracture to an extent sufficient 
to trap slurry proppant before it falls. 
Obviously, many other variations and modifications of this invention as 
previously set forth may be made without departing from the spirit and 
scope of this invention as those skilled in the art readily understand. 
Such variations and modifications are considered part of this invention 
and within the purview and scope of the appended claims.