Drain hole drilling

A method for enhancing the recovery of liquid products from a wellbore having at least one laterally extending drain hole wellbore extending therefrom wherein a portion of the earth around the primary wellbore and a part of the drain hole wellbore is treated to render same essentially gas impermeable so that gas cannot prematurely cone into the primary wellbore, thereby enhancing the liquid recovery by way of the drain hole wellbore before gas reaches the drain hole wellbore.

BACKGROUND OF THE INVENTION 
Heretofore, various types of deviated wellbores have been drilled from a 
primary wellbore. One particular type of deviated wellbore, known as a 
drain hole, is drilled from a primary wellbore through a sharp radius of 
curvature so as to extend laterally away from the primary wellbore. 
Normally, although not necessarily, the primary wellbore is essentially 
vertical and the drain hole, after passing through its sharp radius of 
curvature extends essentially horizontally away from the primary wellbore 
out into the producing geologic formation. Drain holes, and the method for 
drilling same, are fully and completely disclosed in U.S. Pat. Nos. 
3,349,845 and 3,398,804. 
BRIEF SUMMARY OF THE INVENTION 
Often the drain hole is deliberately drilled into a liquid, e.g. crude oil, 
producing formation or strata to maximize the recovery of liquid 
therefrom. Such a formation or strata sometimes has adjacent thereto a 
gas, e.g. natural gas, producing formation or strata overlying or 
otherwise adjacent the liquid producing formation. In those cases, the 
potential is present for producing both gas and liquid from the drain hole 
into the primary wellbore for recovery of both gas and liquid at the 
surface of the earth. 
It has been found that in some such situations, the the gas may 
preferentially sweep into the drain hole, particularly in the area of the 
radius of curvature of the drain hole, thereby reducing the amount of 
liquid produced from the drain hole. According to this invention, a method 
for drilling drain hole wellbores is provided which enhances liquid 
production from the drain hole by first treating a significant portion of 
the earth around the primary wellbore in the gas and liquid producing area 
to render the treated portion essentially impermeable to gas. Thereafter, 
the drain hole is drilled through the thus treated portion of the earth 
out into untreated, liquid producing portions of the earth. This way, in 
order for gas to reach the drain hole, it has to push through the bulk of 
the liquid producing formation, thereby forcing liquid out of the 
formation to a maximum extent before the gas reaches the drain hole. This, 
in turn, substantially enhances the recovery of liquid by way of the drain 
hole. 
Accordingly, it is an object of this invention to provide a new and 
improved method for increasing the productivity of new or old primary 
wellbores. It is another object to provide a new and improved method for 
drilling for and producing hydrocarbonaceous fluids from the earth. It is 
another object to provide a new and improved method for enhancing the 
recovery of fluids by way of a drain hole wellbore when gas is closely 
associated with a liquid. 
Other aspects, objects and advantages of this invention will be apparent to 
those skilled in the art from this disclosure and the appended claims.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows the surface of the earth 1 with drilling rig 2 set over a 
primary wellbore 3 which extends essentially vertically downwardly into 
the earth 4. In area A of primary wellbore 3 are two fluid producing 
geologic formations, for example, an upper formation 5 which produces 
natural gas, and a lower formation 6 which produces liquid crude oil. A 
drain hole wellbore 7 has been drilled laterally from wellbore 3 and, 
after passing through a radius of curvature portion B, extends essentially 
horizontally away from wellbore 3 out into oil producing formation or 
strata 6. Wellbore 7 thereby enhances the flow of oil from formation 6 
into drain hole 7 for production to the earth's surface 1 in a 
conventional manner by way of primary wellbore 3. 
Wellbores 3 and 7 can be either cased or uncased, cemented or uncemented, 
as far as the application of this invention goes. Wellbore 3 can be a 
newly drilled well or an old well that is being worked over for drain hole 
purposes. The invention will be described hereinafter, only for sake of 
simplicity, as though both the wellbores were newly drilled and not cased 
or cemented. However, it should be understood that this invention also 
applied to cased and/or cemented wellbores, work overs, and the like. 
FIG. 2 shows the situation of oil and gas production into primary wellbore 
3 after production has been carried out for a while. What sometimes occurs 
in such a situation is that gas, because of its greater mobility in the 
earth, will cone downwardly toward drain hole 7 as indicated by dotted 
line 8 and arrows 9 and 10, so that gas enters primary wellbore 3 ahead of 
liquid oil, as represented by arrows 10 through 13, inclusive. Gas coning 
into the drain hole prematurely decreases the amount of liquid produced by 
way of the drain hole. This is disadvantageous because, ideally, all 
liquid is produced from reservoir 6 first taking advantage of the pressure 
drive from the gas in reservoir 5 to help drive the oil out of reservoir 
6. However, if gas prematurely escapes to primary wellbore 3 by coning, 
the gas cap can be depleted and its assistance in removing oil from 
reservoir 6 reduced before the optimum amount of oil has been recovered 
from reservoir 6 by way of drain hole 7. 
FIG. 3 shows primary wellbore 3 after a radial area therearound and 
extending vertically over area A has been treated in accordance with this 
invention. That is to say, prior to the drilling of drain hole 7 outwardly 
from wellbore 3, a cylindrical volume 20 was established which extends a 
substantial radial distance away from wellbores 3 in both gas producing 
formation 5 and oil producing formation 6. Zone 20 has been produced by 
injecting into the earth, outwardly from wellbore 3, a material which 
fills the pores of the earth in zone 20 and with time, heat and the like 
hardens to render zone 20 essentially impermeable to gas from formation 5. 
After such treatment of zone 20 around wellbore 3, drain hole 7 is then 
drilled so that at least part of the radius of curvature part B of drain 
hole 7 is in the treated area 20. The radius of curvature referred to 
herein is the curved part of drain hole 7 which stops when drain hole 7 
reaches an essentially horizontal position in the case where primary 
wellbore 3 is essentially vertical, or, in other situations, where drain 
hole 7 reaches approximately a perpendicular attitude with respect to the 
primary wellbore. Although any significant treatment of zone 20 is helpful 
in the prevention of gas coning, it is presently preferred that the area 
of treatment cover essentially all of the radius of curvature of drain 
hole 7. It should be understood, however, that such treatment can cover 
less than the radius of curvature B or can extend beyond such radius of 
curvature if desired by the operator and economical. 
It can be seen from FIG. 3, that by establishing a relatively gas 
impermeable zone 20 in area A around wellbore 3, gas from formation 5 is 
prevented from prematurely coning into drain hole 7 as shown in FIG. 2. 
Because of this invention, gas must move downwardly directly toward drain 
hole 7 as indicated by dotted arrows 21 and 22, thereby pushing a maximum 
amount of oil ahead of it into drain hole 7 as shown by arrows 23, 24 and 
25. Thus, gas does not prematurely enter wellbores 3 or 7 and, by the time 
the gas does reach drain hole 7, it has pushed essentially all of the oil 
ahead of it into drain hole 7 for maximum enhanced recovery of oil from 
formation 6. This invention provides a more efficient recovery of liquid 
from a reservoir which has adjacent thereto an in-situ gas source. 
FIG. 4 shows primary wellbore 3 passing through gas and oil producing zones 
5 and 6. In this embodiment of the invention, after primary wellbore 3 is 
drilled, the drain hole wellbore 7 is started from wellbore 3 but drilled 
only through the radius of curvature portion B. When the drain hole is 
drilled to point 31 where it essentially levels out horizontally and will 
thereafter be passing directly away from wellbore 3, drilling of drain 
hole wellbore 7 is terminated. Thereafter, the earth surrounding wellbore 
3 in area A and the earth surrounding the radius of curvature path B of 
drain hole wellbore 7 is treated to produce the desired essentially gas 
impermeable zone 30 around wellbore 3 and radius of curvature B of drain 
hole wellbore 7. After this treatment, wellbore 3 is re-entered with 
drilling equipment and drain hole wellbore 7 extended beyond its radius of 
curvature terminating at point 31. This extension drilling goes as far out 
into oil producing strata 6 as desired, as shown by dotted line 7'. By 
practicing this embodiment of the invention, less material can be used to 
produce a desirable gas impermeable zone 30 than was used to produce gas 
impermeable zone 20 of FIG. 3. Thus, the desired result of the production 
of liquid oil, represented by arrows 32, 33, 34 and 35, due at least in 
part to pressure from gas travelling downwardly towards the untreated 
portion of drain hole wellbore 7, represented by arrows 36 and 37, is 
accomplished. The gas forces essentially all of the oil out of strata 6 
before the gas reaches the untreated portion 7' of drain hole wellbore 7 
since premature coning of the gas into primary wellbore 3 and radius of 
curvature portion B of drain hole wellbore 7 is prevented by zone 30. This 
may be achieved by the use of a minimum amount of treating material. For 
example, if radius of curvature B extends 15 ft. away from primary 
wellbore 3, and it is desired to treat the earth in accordance with this 
invention to render the entire radius of curvature distance relatively gas 
impermeable, by using the approach of FIG. 3, treating material will be 
squeezed outwardly from wellbore 3 a distance of 15 ft. for the treatment 
described in FIG. 3. However, for the treatment described in FIG. 4, the 
distance of treatment away from wellbore 3 and the radius of curvature 
part of drain hole wellbore 7 to establish zone 30 in FIG. 4 could be on 
the order of 1 to 3 radial ft. Obviously, if less than the entire radius 
of curvature portion B of drain hole wellbore 7 is to be treated, the 
approach of FIG. 3 will be more practical. 
Either approach represented by FIG. 3 or FIG. 4, or combination of those 
approaches or other similar approaches will be obvious to those skilled in 
the art based upon this disclosure and such combinations of approaches and 
obvious similar approaches are all within the scope of this invention. For 
example, another approach that could be used in the practice of this 
invention is to drill primary wellbore 3. Thereafter drill the entire 
drain hole wellbore 7 and after completing drilling of the entire drain 
hole wellbore, putting a bridge plug or other packoff means any place 
desired in drain hole wellbore 7, such as at point 31 of FIG. 4. 
Subsequent treatment of the primary wellbore 3 in area A and the radius of 
curvature portion B of drain hole wellbore 7 up to the bridge plug can be 
accomplished without treating the entire length of the fully drilled drain 
hole wellbore 7. This way, the results of FIG. 4, i.e. establishment of 
zone 30, are obtained. 
Generally, any material which is not soluble to a substantial degree in the 
gas, oil or other gaseous or liquid fluids to be produced from the 
formations adjacent the wellbores can be used to form impermeable zones 20 
and 30 so long as those materials can be made sufficiently liquid to be 
pumped down wellbore 3 and forced outwardly therefrom by pressure into the 
earth to form zone 20 and 30, and thereafter solidified to a sufficient 
extent to render zones 20 and 30 essentially gas impermeable. A number of 
materials are known in the art which can be injected into the pores of a 
geologic formation as a liquid which will then, with time, heat, catalyst, 
and the like, essentially solidify to plug the formations pores against 
gas incursion. Such materials will be readily obvious to one skilled in 
the well drilling art and cover a wide variety of materials. Particularly 
useful materials include polyacrylamides, particularly partially 
hydrolyzed polyacrylamides, polyacrylates, natural gums such as guar gum 
and xanthum, and derivatives of natural gums. Inorganic materials such as 
silicates, particularly alkali metal silicates such as sodium silicate can 
also be employed to form zones 20 and 30. Of particular use are materials 
in the foregoing classes of materials which are water soluble so that they 
can be injected into the wellbore as a readily flowable liquid and which 
then, after being pressured out into the formation to form zones 20 and 
30, harden in-situ in the pores of the formation. Suitable materials are 
fully and completely disclosed in U.S. Pat. Nos. 3,011,547 and 3,827,977, 
the disclosures of which are incorporated herein by reference. The amount 
of such materials employed will vary widely depending upon the desired 
radial extent of zones 20 or 30 away from wellbores and the permeability 
of the geological formations to be plugged. Therefore, such amounts will 
vary widely and are not critical to the practice of this invention so long 
as a sufficient amount is employed to render zones 20 and 30 essentially 
gas impermeable. For example, in the case of U.S. Pat. No. 3,827,977, the 
use of larger amounts of the inhibiting agent than contemplated by that 
patent can result in plugging of the pores of the formation to produce 
relatively impermeable zones 20 and 30. Thus, the same materials can be 
used in the same manner but just in more substantial amounts so as to plug 
the pores rather than deposit a small, non-plugging amount in the pores 
for scale inhibition purposes. One skilled in the art, once apprised of 
this invention and the advantages therefor, can readily devise numerous 
ways to form impermeable zones 20 and 30 prior to drilling drain hole 7, 
and all such approaches to establishing zones 20 and 30 are within the 
purview of this invention. 
EXAMPLE 
A mixture of sixty barrels of lease water containing 8,000 to 10,000 ppm 
calcium, one hundred gallons of 15% hydrochloric acid, and one hundred 
sixty-five gallons of a 15-18% sodium polyacrylamide, molecular weight 
1,000-8,000 is injected by conventional squeeze treatment into the 
formation in area A surrounding wellbore 3 using a sufficient volume of 
the mixture so as to equal one pore volume of the surrounding formation 
which will extend six feet radially from wellbore 3. The mixture, after 
being squeezed into the pores of the formation, is allowed to sit until 
the calcium reacts with the polyacrylamide to form calcium cross linked 
polyacrylamide deposits in the pores, thereby plugging the pores of the 
formation radially around wellbore 3 for a distance of six feet and 
throughout height A of formations 5 and 6. A relatively gas impermeable 
zone 20 is thereby formed around wellbore 3. 
Thereafter, drain hole 7 is drilled from wellbore 3 in a conventional 
manner so as to penetrate impermeable zone 20 and extend a substantial 
distance out into oil producing formation 6 for maximum recovery of oil 
from formation 6 by way of drain hole 7 before gas from formation 5 
reaches drain hole 7. Drain hole 7 is drilled near the bottom of formation 
6 and as far away from formation 5 as possible in order to maximize liquid 
recovery before gas reaches drain hole 7. 
Reasonable variations and modifications are possible within the scope of 
this disclosure without departing from the spirit and scope of this 
invention.