Horizontal wellbore stimulation technique

A method of treating a downhole formation in a wellbore having multiple zones of porosity by circulating a carrier fluid downhole to the lowermost zone of porosity, and then flowing an acid soluble blocking agent into the wellbore. The blocking agent is translocated to a location where it is free to flow along the lower end of the borehole and seal the zones of porosity. The blocking agent is a mixture of acid soluble material that is relatively insoluble in the carrier fluid, and is of a size to block the pores of the zones of porosity so that flow into the zones is severely reduced. A solution of acid is spotted downhole within a selected zone of porosity. The solution of acid degrades the blocking agent and the degraded blocking agent becomes admixed with the carrier fluid and the flow restriction of the blocking agent within the zone is destroyed. Next a treatment fluid for stimulating the selected zone is conveyed downhole and forced into the opened selected zone until the zone is stimulated. The residual blocking agent is removed from the borehole by circulating a solution of acid downhole and into contact with the acid soluble material that is blocking the untreated pores of the zones of porosity and opens all of the zones to the borehole.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to a well completion process and more particularly 
to a well completion process in a wellbore which has an extended length 
exposed to a subterranean hydrocarbon bearing formation. 
Historically, wellbores were drilled vertically, exposing the vertical 
cross-sectional height of the hydrocarbon bearing formation to the 
wellbore. Horizontal wells, which are currently being drilled, are drilled 
nearly perpendicular to the vertical axis of the formation to contact a 
greater amount of the formation. The horizontal portion of the wellbore is 
herein referred to as the lateral wellbore. 
Horizontal and deviated wellbores are drilled laterally, penetrating 
thousands of feet of a hydrocarbon bearing formation. These bores 
intersect numerous porous and permeable hydrocarbon bearing zones. These 
intersected zones vary in permeability from secondary porosities such as 
open fractures and vugs which have darcies of permeability, to matrix 
porosities and closed fractures which have millidarcies of permeability. 
Hydrocarbon bearing zones which intersect the borehole vary in 
distribution and length within the wellbore. Some wells exhibit a number 
of small zones (less than 50 feet) randomly distributed in the wellbore, 
while other wells have hydrocarbon shows throughout the entire length of 
the lateral bore, which sometime exceed 5,000" in length. 
The prior art indicates that the Oil and Gas Industry has attempted many 
forms of conventional vertical stimulation techniques in these extended 
horizontal wellbores. Ideally these hydrocarbon bearing zones are isolated 
and treated individually for the best result. A major obstacle and concern 
in the stimulation and treatment of horizontal bores has been the 
management, placement and distribution of treatment fluids into the 
numerous hydrocarbon bearing and non-hydrocarbon bearing zones exposed in 
the wellbore. Inability to isolate a segment of the exposed lateral 
renders many forms of conventional Oil and Gas stimulations ineffective 
because the necessary hydraulic horsepower cannot be delivered to the 
desired portion of the formation due to fluid losses in the remainder of 
the exposed wellbore. Other conventional stimulations which are not 
injection rate dependent are also rendered ineffective by the inability to 
create a path of least resistance into a desired interval or segment of 
the exposed lateral. 
Mechanical isolation using casing, external casing packers, cement, and 
perforations have been mechanically difficult and cost prohibitive. 
Attempts to distribute treatment fluids into the numerous pay zones using 
fluid entrained diverting agents such as polymers, wax beads, rock salt, 
and the like have met with limited success. Dendritic or water fracs 
appear to be successful in some areas, but have also experienced failure 
in other areas. 
The present invention provides a solution to the above problems by 
isolating and diverting treatment fluid in horizontal wellbores in a new 
and novel manner. 
DESCRIPTION OF RELATED ART 
In prior art there are many examples of the use of acid soluble materials 
and the inherent benefits thereof in the drilling and completion of oil 
and gas wells. For example, Robinson U.S. Pat. No. 2,198,563 cites the use 
of crushed calcium carbonate as a mud additive to aid in the subsequent 
acid removal of the mud's filter cake from a producing formation. Barrett 
U.S. Pat. No. 2,739,940 describes the user of acid soluble fibrous rock 
wool or mineral fiber and mica and the subsequent removal of this mud's 
filter cake using acid. Twining U.S. Pat. No. 2,793,995 cited the same 
removal benefits using nylon, rayon, and silk fibers as the acid soluble 
component of drilling mud. Corley and Dorsey U.S. Pat. No. 4,422,948 cited 
the use of combined fiber, flake, and granular acid soluble mud additives 
to prevent lost circulation in a hydrocarbon bearing zone and the 
capability to restore formation permeability through the acid solubility 
of the lost circulation mud additives. 
Many others have cited the effectiveness of combinations of fibers, flakes, 
and particles in sealing permeable formations encountered while drilling. 
Lummus and Randall U.S. Pat. No. 3,375,888 establishes prior art which 
deals with the effectiveness of sealing permeable formations with various 
compositions of fiber, flakes, and granules. Many others have addressed 
these arts. 
This invention differs from the prior art primarily because the invention 
is not devoted to the sealing effectiveness or the protection of sealing 
permeable hydrocarbon bearing zones. Instead, this invention concerns the 
discrete stimulation and treatment of multiple hydrocarbon bearing zones 
exposed in a single wellbore. The present invention further comprehends 
the prevention and restriction of stimulation or treatment fluid entry 
into unwanted permeable zones exposed to the wellbore. The term "discrete" 
herein pertains to the creation of a path of least resistance into a 
selected zone or zones for the placement of treatment fluids and thereby 
restricting the flow of fluids into the remaining zones not selected which 
are exposed in the wellbore. 
SUMMARY OF THE INVENTION 
The present invention relates to the use of a chemically removable blocking 
agent, such as flexible fiber, flakes, and rigid granular particles, for 
injection into a wellbore communicated with a subterranean formation to 
seal permeable formations. The blocking agents used in this invention are 
relatively insoluble in both water and hydrocarbons, and are solubilized 
in the presence of acid. A carrier fluid is pumped or flowed into the 
wellbore and a mixture of acid soluble material, or blocking agent, is 
mixed with a transporting liquid or carrier fluid, to form a continuous 
stream, or as a high concentration "slug" or "pill", and injected into the 
wellbore as a slurry. The blocking agent is relatively insoluble in the 
transporting liquid. The initial purpose of the slurry is to seal or 
severely restrict the flow of fluids into all permeable formations exposed 
to the wellbore. Upon stopping or reducing fluid losses into the 
formation, a solution of acid is "spotted" or placed through a suitable 
conduit in a segment or window of the wellbore at a location which the 
operator desires to treat or stimulate. Upon placement of this acid, the 
components of the blocking material dissolve and the selected segment's 
permeability is restored. Accordingly the unblocked segment of the 
wellbore can be stimulated or treated in any of several manners known to 
those skilled in the art, while the remainder of the wellbore remains 
blocked. Upon completion of stimulation or treatment of the segment, it is 
once again sealed or plugged using the fluid entrained acid soluble 
blocking materials initially used to seal the wellbore. The process of 
opening another segment of the wellbore is repeated and again treated or 
stimulated. This process is repeated until all desired segments have been 
treated or stimulated. When desired, after the completion process is 
complete, the wellbore can be circulated with acid and the blocking 
material dissolved and removed from the wellbore in a liquid form. 
A primary object of the present invention is the provision of a method of 
stimulating one of a plurality of zones of porosity of a production 
formation in a wellbore. The invention enables the sequential treatment of 
a plurality of individual zones, through the aforementioned ability to 
discretely treat one zone at a time. 
Another object of the invention is to limit or eliminate the loss of 
stimulation fluids into non-productive porosity zones of a formation 
exposed to the wellbore during the stimulation process. 
A further object of this invention is to disclose and provide a more 
economical process with which extended wellbores can be stimulated. 
A further object of this invention is to disclose and provide a method of 
effectively treating a selected plurality of zones simultaneously while 
reducing or eliminating fluid losses to non-productive zones of porosity 
exposed to the wellbore. 
A further object of this invention is to disclose and provide a cost 
effective process with which a selected plurality of zones can be 
stimulated simultaneously in a wellbore which is exposed to a plurality of 
porous zones. 
A further object of this invention is to disclose and provide a method of 
selectively opening a segment of a wellbore that enables the more 
efficient delivery of hydraulic horsepower, by restricting fluid losses to 
the remainder of the bore, and therefore more effectively treat the 
aforementioned segment of the wellbore. 
These and various other objects and advantages of the invention will become 
readily apparent to those skilled in the art upon reading the following 
detailed description and claims and by referring to the accompanying 
drawings. 
The above objects are attained in accordance with the present invention by 
the provision of a method for stimulating one of a plurality of zones of 
porosity of a production formation of a wellbore in a manner substantially 
as described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The blocking agent or fiber component of this invention must be 
acid-soluble deformable-like material. Fibrous materials which are 
commonly available for this blocking agent are rock wool, also known as 
mineral fiber, and nylon and rayon industrial waste which have been 
processed for use in pumping equipment. The preferable blocking agent is 
the mineral fiber. A typical composition by weight of the desired mineral 
fiber is CaO-36%, MgO-9%, Al2O3-9%, and SiO2-42%. The composition can be 
varied in the spin extrusion manufacturing process, but for the purposes 
of this invention any composition that is relatively insoluble in the 
transporting fluid, and which remains acid soluble can advantageously be 
used. Laboratory tests have shown manufactured samples of some mineral 
fibers to be 99.85% soluble in 7.5% hydrochloric acid (HCL). 
When using flakes in addition to fiber, the flake component of this 
invention should be flexible and acid soluble. A flake material that is 
suitable for the process of this invention is flaked aluminum. Aluminum 
exhibits the flexibility necessary for sealing and is very acid soluble in 
HCL acid. Once the aluminum has been dissolved, the pH of the carrying 
fluid must be controlled to prevent reprecipitation of aluminum salts. 
The granular component of this invention incorporates many of the alkaline 
earth carbonates. The material of choice is granulated or flaked calcium 
carbonate which undergoes a rapid solubilizing reaction when contacted 
with HCL acid. 
This invention provides a method of improving the control of the placement 
of treating fluids in boreholes in a cost effective manner. Three 
hypothetical examples of the steps taken to achieve this goal are as 
follows. In each example it will be assumed that evaluation of prior 
drilling and completion data and information is performed by those skilled 
in the art to delineate the portions or segments of the wellbore it is 
desired to stimulate and treat. For the purposes of the following examples 
a horizontal wellbore is depicted in FIG. 1. The wellbore has been formed 
through the surface 1 of the ground. A casing string 2 has been cemented 
into place as indicated by numeral 3. The lower end of the borehole 
extends horizontally and terminates at numeral 4. The horizontal portion 
of the wellbore described in the preceding sentence is also referred to as 
the "lateral". For descriptive purposes the "lowermost" point of the 
wellbore is the end of the bore furthest from the surface location as 
depicted in FIG. 1 by numeral 4. The wellbore has been evaluated and the 
zones labeled "A", "C", and "E" have been found to be hydrocarbon bearing. 
Also the evaluation has determined that zones "B", "D", and "F" are water 
bearing. 
EXAMPLE #1 
A blocking agent is admixed with a water to form a slurry. The blocking 
agent is a fibrous acid soluble material which is depicted by numeral 6 in 
FIG. 2. The slurry is pumped downhole. The material is allowed to 
differentially block the formation. Then the formation is pressured to 
determine the amount of fluid leak off of the fibrous seal. If fluid leak 
off is too great to achieve the desired rate of fluid injection in the 
selected target zone, additional acid soluble blocking components of 
fiber, granular, and flake material are injected into the wellbore until 
the fluid losses through the blocking material are reduced to an 
acceptable value. At this point the segment of the wellbore which is 
targeted for treatment, zone "E", is opened by placing acid in the 
wellbore adjacent to this zone by pumping acid through a suitable conduit 
7, as shown in FIG. 3. The terminal end of the conduit 7 is placed 
adjacent to zone "E". The size of the opening in the selected zone can be 
controlled through manipulation of the conduit, as depicted in FIG. 3, and 
acid volumes. Once the blocking material of the selected zone is 
solubilized, injection into the opened zone "E" can be achieved both by 
pumping through a conduit 7 and the annular area 8 between the conduit and 
casing. The rate through the annular area will be approximated by the rate 
established by the previous fluid leak off injection test of the entire 
wellbore prior to break down of the target zone. In most cases the 
treatment will be conducted through a conduit 7 to protect the remaining 
blocking material from acids used in the treatments as shown in FIG. 4. 
Some stimulations which do not incorporate the use of acid as a component 
of the treatment can be achieved at this this by injection through the 
conduit 7, annulus 8, or both, if desired. Upon the completion of the 
stimulation of zone "E", blocking material is once again pumped into the 
wellbore to block the treated zone "E". Zones "A" and "C" can now be 
sequentially treated using the same above method previously used to treat 
zone "E". This process of segmenting the borehole can be repeated as many 
times as necessary to sequentially treat all the desired zones of a 
wellbore, regardless of the number of zones. The type of stimulation may 
be varied to enhance treatment of different segments of the wellbore, if 
desired. Upon completion of stimulation of the last segment of the 
wellbore, the entire wellbore can be circulated with a weak solution of 
HCL acid whereupon all of the blocking material is solubilized and removed 
by circulation of the borehole. 
EXAMPLE #2 
Another method of treatment would be to place the blocking material 6 
throughout the entire wellbore as illustrated in FIG. 2. The placement of 
acid is achieved using conduit 7 to open multiple windows by sequentially 
solubilizing the blocking material 6 in zones "A", "C", and "E" of the 
wellbore as shown in FIG. 5. The multiple zones can now be treated 
simultaneously, as shown in FIG. 6, using any number of conventional 
treatments which do not incorporate a low pH solution. Once the treatment 
is complete, the wellbore can be circulated with a weak acid to remove the 
remaining blocking material. 
EXAMPLE #3 
Another method would be to use the above segmenting process to treat zone 
"E" of example #1 and not re-block the already treated zone "E". The 
pressure artificially induced in the treatment of zone "E" is used to 
differentially create a path of least resistance into zone "C" when zone 
"C" is opened using the segmenting process. Subsequently zone "A" is 
differentially treated using the same procedure, once the pressures of 
zones "E" and "C" have been elevated by the placement of treatment fluids. 
The procedures of these examples are not meant to limit the mechanical 
methods of opening and stimulating zones of a wellbore. Any number of 
operational methods can be used by those skilled in the arts.