Horizontal well treatment method

Methods are provided for isolating segments of a horizontal wellbore drilled through an unconsolidated formation and treating the well to control rates of fluid flow into selected segments of the wellbore. The methods include screen segments separated by blank pipe segments to provide isolation outside the tubulars upon collapse of the surrounding formation around the blank pipe segments.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of treating a well which has been 
drilled through an unconsolidated formation in a substantially horizontal 
direction. In particular, methods are provided for effective flow 
isolation of selected segments in the substantially horizontal portion of 
the wellbore by the use of screens and blank pipe and for treating the 
well to control flow rate of fluids into selected segments of the 
wellbore. 
2. Description of Related Art 
In recent years it has become common to drill wells which penetrate the 
earth in a substantially vertical direction to a selected depth and then 
deviate from vertical to reach a direction which is substantially 
horizontal or which is substantially along the bedding planes of the 
formation being penetrated. The horizontal portion of such wells may 
extend for as much as hundreds of feet in the same formation. Such 
horizontal wells are advantageous for producing hydrocarbons from 
unconsolidated or consolidated formations. Particular advantages may 
accrue in thin formations, heterogeneous formations, formations having 
natural fractures in the vertical direction, and in formations where water 
or gas coning limits the production rate of oil. A horizontal well will 
normally allow production at a higher rate than a vertical well in the 
same formation. 
A serious limitation of horizontal wells has been the means for 
"completing" the well. Such completion means preferably allow for 
controlling or altering the pattern of flow of hydrocarbons and water from 
selected portions of the wellbore. Some horizontal wells have been drilled 
and produced from the open hole. This method of production can be 
practiced in formations where the rock is sufficiently strong for the 
wellbore to remain open with production, but an open hole provides limited 
means to alter the pattern of flow from selected portions of the wellbore. 
In most wells, it is very desirable to be able to alter the pattern of 
fluid entry into the well. When the horizontal well is drilled through 
consolidated rock, the casing of the well may be cemented in place and 
then perforated, which provides a completion means for selectively 
altering the pattern of fluid entry into the well. When the horizontal 
well is drilled through unconsolidated rock, a liner made up of pipe 
containing narrow slots to allow flow into the wellbore (a "slotted 
liner") or a screen liner is commonly placed in the well. The screen has 
openings small enough to prevent flow of grains from the formation through 
the liner and into the wellbore. With the slotted liner or screen liner in 
the wellbore, it is not possible with conventional methods to control flow 
rate into the wellbore at different locations along the liner. 
The method of cemented and perforated casing or liner is not a suitable 
method for completing horizontal wells in unconsolidated formations, 
because the grains of formation rock will flow through the perforations 
and into the liner, where they will accumulate and retard or prevent flow 
from the well. An inside-casing screen and gravel pack can be used, but 
this usually causes an excessively high resistance to flow into the well, 
does not allow for isolation or control of flow patterns along the 
wellbore and is an expensive method of completion. 
A variety of techniques used in completing horizontal wells have been 
summarized in the paper "An Overview of Horizontal Well Completion 
Technology." (SPE Paper No. 17582, Society of Petroleum Engineers, 1988) 
One of the techniques discussed in this paper is the possible use of 
uncemented liners having predrilled holes in segments of the liner, these 
segments being separated by "blank sections." "Blank" sections or "blank" 
pipe refers to pipe having no holes. The possible collapse of the 
formation around the blank sections to provide isolation of the predrilled 
segments is discussed. The method would not be suitable in an 
unconsolidated or poorly consolidated formation, however, as the grains of 
the formation could enter the liner through the predrilled holes. 
Equipment has been developed which is designed to prevent flow of fluids 
outside a liner pipe or casing in horizontal wells. A method for 
completing a horizontal well by employing this equipment, called an 
"external casing packer," is described in U.S. Pat. No. 4,714,117. This 
method employs a casing string composed of alternating casing subs and 
external casing packer subs. The method provides for isolation of discreet 
segments of the casing string to allow for localized production and 
remedial treatments in the horizontal portion of the wellbore. 
To determine the need for selectively controlling the influx of fluids into 
a wellbore, flowmeters to measure the flow rate of fluids in a wellbore 
along the length of the wellbore are available in industry. Also available 
to be employed with the flowmeter or alone are logging instruments (based 
on measurements of density or dielectric constant, for example) to 
determine the relative amounts of gas, water and oil in the flowing stream 
in the wellbore. In a horizontal well, these flowmeters and logging 
instruments to determine relative amounts of different fluids may be 
placed in the well on rigid tubing which can push the instrument along the 
horizontal portion of the wellbore. 
There is a continuing need for an inexpensive method to complete horizontal 
wells drilled into unconsolidated or poorly consolidated formations so as 
to allow selective production of fluids from the wells and selective 
treatment to increase or decrease flow rate from selected segments along 
the horizontal wellbore. 
SUMMARY OF THE INVENTION 
There is provided a method of controlling flow rate into a horizontal well 
along selected segments of the horizontal portion of the wellbore drilled 
through an unconsolidated or poorly consolidated formation. In one 
embodiment, screens are joined to blank liner pipe segments and placed in 
the wellbore. Sufficient time is allowed for the surrounding formation to 
close around the blank liner segments and screen. A selected segment of 
screen in the wellbore is isolated by packers in blank pipe segments and a 
treatment fluid is injected into a screen segment. In another embodiment, 
a horizontal well containing segments of screen separated by blank pipe is 
treated by placing packers in blank pipe segments and injecting a treating 
fluid into the screen segment. In another embodiment, the flow resistance 
outside blank pipe segments is measured before the treating fluid is 
injected.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIG. 1, horizontal well 10 has been drilled in a vertical 
direction through the earth and then deviated to penetrate the productive 
formation 12 for a substantial distance more than the thickness of the 
formation 12. Whether the productive formation 12 is in a horizontal 
direction or not, if the wellbore is drilled at an angle so as to 
penetrate the productive formation for a distance substantially more than 
the thickness of the formation, the well is defined herein as a 
"horizontal well." Casing 14 has been placed in the well and cemented in 
place by cement 16 before the horizontal section of the well was drilled. 
An open hole exists below the shoe of the casing 14. Fluids could not be 
produced through the open hole 20 because the formation 12 would collapse 
to close the hole; but even if fluids could be produced from the well 
through the open hole 20 penetrating the formation 12, there would be no 
means to control the flow pattern of the fluids entering the open hole. 
In FIG. 2, screen segments 30(a), 30(b) and 30(c) and blank pipe segments 
32(a) and 32(b) are shown in wellbore 10, drilled in a horizontal 
direction through formation 12. The screen segments and blank pipe 
segments are attached to a liner 34, which has been placed in an open 
hole, such as the open hole 20 of FIG. 1, using conventional techniques. A 
liner hanger 36 seals the annulus between the liner 34 and the casing 14 
in the well. The same numerals are used to denote the same components in 
all figures. 
The screen segments 30(a), (b) and (c) are available in different sizes 
from Baker Sand Control Company, Howard Smith Company, Houston Well Screen 
Company and other companies. Conventional metal screens or slotted liners 
may be used, but preferably the screen segment is a prepacked screen. A 
prepacked screen contains particles which are consolidated into a 
permeable filter. Such a screen is available, for example, from Baker Sand 
Control Company of Houston, Tex. The screen may be made from sintered 
materials. The diameter of the screen or liner is selected based on the 
size of the hole drilled through the formation. The size of openings in 
the screen or the slot width may be selected based on samples of the 
formation rock, if such samples are available, using conventional 
techniques well-known in industry for sizing the openings of screens to 
exclude particles of known dimensions. For application of our invention, 
the formation rock 12 is unconsolidated or poorly consolidated, such that 
the rock will collapse around the pipe segments and limit or prevent flow 
outside the pipe segments. 
In FIG. 3, the formation rock 12 has collapsed around the liner 34, the 
screen segments 30 and the blank pipe sections 32. The collapse of the 
formation rock 12 is caused by stresses in the earth and the lack of 
consolidation of the grains of the rock. Production of fluids through the 
screen segments 30 may decrease the time required for the rock 12 to 
collapse around the tubular members 34, 30 and 32, but collapse may occur 
in the absence of production from the well. 
The formation 12 is packed against the outside walls of the tubular members 
and serves to prevent excessive flow of fluids along the outside surface 
of the tubular members. Thus, effective flow "isolation" outside the 
tubulars of the separate screen sections 30(a), 30(b) and 30(c) is 
obtained, although a hydraulic seal is not present outside the tubular 
members. For example, if fluid having a viscosity of 1 cp is injected at a 
rate of 2 barrels per minute into a screen only 1 foot in length, and the 
permeability of the formation rock around the screen is 2 Darcies, the 
radial pressure drop in the formation opposite the screen will be about 
1400 psi. Approximating flow along blank pipe as linear flow, the pressure 
drop along a blank pipe with the same permeability formation in a 
thickness of 11/2 inches around the blank pipe would be about 4000 psi per 
foot. To provide effective flow isolation between screen segments, the 
blank pipe between screen segments should have a minimum length of about 
10 feet, and this length is preferably at least 20 feet. Thus, the blank 
pipe 32 will, after collapse of the formation around the pipe, provide 
isolation between screen segments 30 for injection of fluids into selected 
screen segments or production of fluids from selected screen segments. It 
may be desirable to inject treating fluids into selected screen segments 
after the formation 12 has collapsed around the tubulars and before the 
well is produced. 
FIG. 4 shows additional equipment which has been placed in well 10 for 
isolating segments of the wellbore by packers and selectively injecting 
fluids into one of the screen segments 30 or producing fluids from the 
selected segment. The additional equipment consists of a workover string 
or tubing 40 having attached to it a retrievable packer 44. A bridge plug 
42, which serves as a packer, has been placed in the well below the 
selected screen segment 30(b). The packer 44 has been placed above the 
selected screen segment 30(b) and set by conventional methods known in 
industry. Optionally, a straddle packer mounted on the work string 40 is 
set in blank pipe segments on each side of a screen segment to isolate a 
screen segment. 
With the configuration of equipment shown in FIG. 4, it is possible to 
determine if the formation 12 has collapsed around a blank pipe section. 
For example, brine or oil can be injected down the workover string 40 at a 
known pressure and flow rate. The pressure at the lower end of the 
workover string, which will be the injection pressure into screen segment 
30(b), can be calculated for that flow rate. The pressure in the annulus 
outside the workover string 40 at the location of the screen segment 30(a) 
can be calculated from the return flow rate through the annulus between 
the workover string 40 and the casing 14. The difference in pressure 
between screen 30(a) and 30(b) will indicate the resistance to flow 
outside the blank pipe 32(a), which will indicate if the formation 12 has 
collapsed around the blank pipe. 
If it is desired to decrease the flow rate of fluids entering the well 10 
through the selected screen segment 30(b), the treatment fluid is a 
plugging liquid which is injected through the workover string 40 and out 
through the screen segment 30(b). The plugging liquid may be a water 
solution of a polymer, a cement slurry made from very fine particles or 
other materials. If it is desired to increase the flow rate of fluids 
entering the well 10 through the selected screen segment 30(b), the 
treatment fluid is a stimulation fluid. Similarly, the stimulation fluid 
is injected through the workover string 40 and out through the screen 
segment 30(b). Suitable stimulation fluids are acids, surfactants, 
solvents (including water), or mixtures of these materials. Such 
stimulation fluids may contain particulate diverting materials which are 
sized to pass through the screen or slotted liner and deposit in the 
surrounding formation to divert flow of injected fluid more evenly into 
the formation along the screen segment. 
To determine the flow rate of fluids into the different screen segments 
while the well is being produced, which will indicate which screen 
segments should be decreased in flow rate and which segments should be 
stimulated, a flow meter can be run into the well while the well is being 
produced. Techniques are available for running such flowmeters in 
horizontal wells by attaching the flowmeters to coiled tubing or other 
tubing, such that the tools can be pushed through the horizontal portion 
of the well. Such techniques are well-known in industry. Also, logging 
devices can be run either alone or in combination with a flow meter to 
determine the composition of the fluid entering the wellbore at each 
distance along the wellbore. Such devices, normally based on density, 
dielectric constant, or electrical resistivity measurements, are 
well-known in industry. 
EXAMPLE 1 
An offshore field is to be developed with several horizontal wells drilled 
from a platform. The primary purpose of having horizontal wells rather 
than vertical wells is to delay entry of water production coming from a 
cone of water rising from the oil-water contact. Water coning would occur 
quickly around vertical wells when produced at the high rates necessary to 
make the field commercial. The formation which is productive of oil is 
unconsolidated or has very low mechanical strength. Therefore, it will be 
necessary to complete the wells with provisions to prevent sand particles 
from the formation entering the wellbores along with the produced fluid 
from the formation. 
An analysis of options for completing the horizontal wells indicates that 
the most effective completion will be a prepacked screen in the open hole 
without a gravel pack outside the screen. There is concern, however, that 
water will break through prematurely into a part of the screen because of 
heterogeneous rock permeabilities along the length of the horizontal 
portion of the well. It is decided to separate 20-foot long segments of 
the prepacked screen with 40-foot long blank pipe segments. Five screen 
segments and five blank pipe segments are attached to the bottom of a 
liner and placed in the open hole drilled below the cemented casing in the 
vertical portion of the well. When the well is produced, there is time for 
the formation around the horizontal section of the wellbore to collapse 
around the liner, screen and blank pipe segments before water breaks 
through into the horizontal well. When water breakthrough occurs, a 
logging instrument which measures fluid density and dielectric constant is 
run in the well while it is being produced to determine through which 
screen segment the water is being produced. A flowmeter is run in 
combination with the logging instrument to determine the flow rate through 
each screen segment. 
After it is determined which screen segment is producing the unwanted water 
into the well, the well is shut-in and killed by injecting a dense fluid, 
the production tubing is pulled from the well and a workover tubing string 
is used to place a bridge plug in the blank pipe section below the screen 
segment where water entry is occurring. Then the workover string with 
packer attached is run into the well and the packer is set in the blank 
pipe just above the screen segment where water entry is occurring. A 
cement slurry made of very fine cement particles, sold as "Matrix Cement" 
by Halliburton Company of Duncan, Okla., is injected down the workover 
string and through the selected screen segment. The workover string is 
flushed to remove remaining cement slurry and removed. After allowing the 
cement to cure in the formation, the well is placed back on production. 
Water production from the well is greatly decreased. 
The invention has been described with reference to its preferred 
embodiments. Those of ordinary skill in the art may, upon reading this 
disclosure, appreciate changes or modifications which do not depart from 
the scope and spirit of the invention as described above or claimed 
hereafter.