FIELD OF THE INVENTION
[0001] The present invention relates generally to subterranean formations. In particular, the present invention relates to shutting-off unwanted water flow in the subterranean formations to increase recovery of a hydrocarbon gas in the subterranean formation.
BACKGROUND OF THE INVENTION
[0002] Excessive water production in hydrocarbon gas reservoirs has always been regarded as one of the most formidable problems as it hampers productivity of the well to a great extent. High water production from the gas wells increases several undesirable costs like production cost, treatment cost and disposal cost. Further, in extreme cases, the water production may also lead to ceasure of the well due to water loading despite the reservoir containing sufficient amount of recoverable reserves.
[0003] The source of water production can be a channel behind casing due to poor cementation, casing leaks, coning, encroachment, water breakthrough and flow through natural and induced fractures. In the process of shutting-off or mitigation of unwanted water production, the identification of source of water inflow in the well is the most important step for choosing a correct technique. However, the problem becomes even more

complex if multiple mechanisms of water invasion are simultaneously active in the well.
[0004] Over the years, numerous techniques have been used to mitigate water invasion in the gas wells. For example, the mechanical techniques provide a seal in the near-wellbore openings. However, most of the time, it is desirable to have a matrix or small fissure penetration of the sealing material which makes the mechanical techniques inefficient. The secondary cementation techniques have also not been reliable due to poor penetration of the cement slurry into the formation. The existing organic and inorganic gel solutions have also been used but with very limited success. Thus, each of these existing solutions have one or more limitations.
[0005] Accordingly, there exists a need for developing a better solution that may be used for water shut-off in the hydrocarbon gas reservoirs so as to increase recovery of the hydrocarbon gas.
SUMMARY OF THE INVENTION
[0006] A process for increasing recovery of a hydrocarbon gas in a subterranean formation invaded with water is provided in accordance with an embodiment of the present invention. The process comprises preparing a plurality of gelant mixtures with varied concentrations. The plurality of gelant mixtures comprise a polymer in a range of 1000 ppm to 8000 ppm, hexamine in a range of 3000 ppm to 5000 ppm, and hydroquinone in a range of 4000 ppm to 6000 ppm.

[0007] The process further comprises placing, in a plurality of stages, the plurality of gelant mixtures in the subterranean formation. The varied concentrations of the gelant mixtures form a plurality of gels that shutoff water in the subterranean formation to increase recovery of the hydrocarbon gas in the subterranean formation. Upon placement, the plurality of gelant mixtures form plurality of gels with varying gel strengths in a predefined interval of time. The plurality of gels swell when in contact of water and shrink when in contact of hydrocarbon gas thereby increasing flow of the hydrocarbon gas into the subterranean formation. Further in an embodiment of the present invention, the predefined interval of time in which the plurality of gelant mixtures form plurality of gels ranges from 3 days to 120 days. Furthermore, the placement of the gelant mixtures comprises four stages. At a first stage, a placement zone is pre-flushed with 1000 ppm of hexamine and hydroquinone in 1% sodium bicarbonate (NaHCC>3) solution. At a second stage, a first concentration of the gelant mixture is placed in the zone of the subterranean formation. The first concentration of the gelant mixture comprises 7000 ppm to 8000 ppm of a polymer with 3000 ppm to 5000 ppm of hexamine and 4000 ppm to 6000 ppm of hydroquinone in 1% NaHCCb. At third stage, a second concentration of the gelant mixture is placed in the zone of the subterranean formation. The second concentration of the gelant mixture comprises 6000 ppm to 8000 ppm of the polymer with 3000 ppm to 5000 ppm of hexamine and 4000 ppm to 6000 ppm of hydroquinone in 1% NaHC03. Finally, at fourth stage, the placement zone is post flushed with 950 ppm to 1000 ppm of the polymer in 1% NaHC03 and again by 1% NaHC03.
[0008] Further, prior to placing them in the subterranean formation, the plurality of the gelant mixtures are evaluated

for their compatibility with the subterranean formation. In an embodiment of the present invention, the subterranean formation is a carbonate gas reservoir. The plurality of the gelant mixtures may be tested using a common qualitative coding method. The testing evaluates gelation rate, gel strength, and thermal stability of the plurality of the gels.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0009] The present invention is described by way of embodiments illustrated in the accompanying drawings wherein:
[0010] FIG. 1 is a flowchart illustrating a process for
increasing recovery of a hydrocarbon gas in a subterranean
formation in accordance with an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0011] The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Exemplary embodiments are provided only for illustrative purposes and various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to

technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
[0012] The present invention would now be discussed in context of embodiments as illustrated in the accompanying drawings.
[0013] FIG. 1 illustrates a process for increasing recovery of a hydrocarbon gas in a subterranean formation in accordance with an embodiment of the present invention. The subterranean formation may be invaded with water. At step 102, a plurality of gelant mixtures with varied concentrations are prepared for producing a plurality of gels with varying gel strengths. In an embodiment of the present invention, the plurality of gelant mixtures are prepared using a polymer in a range of 1000 ppm to 8000 ppm, a first cross-linker hexamine in a range of 3000 ppm to 5000 ppm, and a second cross linker hydroquinone in a range of 4000 ppm to 6000 ppm.
[0014] In an embodiment of the present invention, the polymer is water-soluble, has an anionic character and has a medium to high molecular weight. The polymer is further in the form of a white free flowing powder and has a moisture content of less than 10%.
[0015] Further, the first cross-linker hexamine and the second cross-linker hydroquinone are organic in nature. The hexamine on hydrolysis gives formaldehyde that reacts with the second cross-linker hydroquinone to give tetramethylol hydroquinone. The tetramethylol hydroquinone with the polymer results in a gelant mixture. The gelant mixture, when placed into a zone of the subterranean formation, with passage of time

gets converted into a gel which is 3-dimensional cross-linked structure and is hydrophilic in nature. In an embodiment of the present invention, the gel has a characteristic to swell when it comes in contact with water and to shrink when it comes in contact with the gas and thereby the gel restricts flow of water in the reservoir well and allows hydrocarbon gas to flow into the reservoir. In an embodiment of the present invention, the reservoir is a carbonate gas reservoir.
[0016] After the plurality of gelant mixtures have been prepared, extensive laboratory tests and studies are undertaken to optimize the plurality of gelant mixtures with various polymer samples and organic cross-linkers hexamine and hydroquinone. The tests ascertain the compatibility of the plurality of gelant mixtures with reservoir fluids at anticipated reservoir temperatures to obtain the gel strengths of varying degree. The compatibility studies between reservoir fluids and the plurality of gelant mixtures are critical as they help in predicting whether the plurality of gelant mixtures can be applied successfully or not. The efficiency of the gelant mixture can be reduced if there are precipitation and insoluble particles in the brine present in the zone of the subterranean formation. With this reduced efficiency, the gelant mixture may not be applied successfully. Thus, the compatibility studies or tests are conducted for the optimized gel systems with formation water and crushed pay core samples. The formation water is naturally occurring water within the pores of the rock.
[0017] In an embodiment of the present invention, the gelation process of the prepared gelant mixtures are tested or evaluated qualitatively over an extensive range of polymer and cross-linker concentrations. The evaluations are conducted to measure

gelation rate, gel strength, and thermal stability of the gel as a function of time. In an embodiment of the present invention, the plurality of gelant mixtures are tested using a common qualitative coding method. The common qualitative coding testing method comprises filling of a glass ampoule with 20 ml of the gelant mixture with 1% of sodium bicarbonate (NaHCOs) . The ampoule is then sealed and kept in an oven at 105°C. Thereafter, the cross-linking process is monitored as a function of time, starting from the point when the gelant mixture is put into the ampoule and kept in the oven. The plurality of gelant mixtures kept in the oven are inspected visually at regular intervals to observe the gel formation. When proper thickening of the gelant occurs the time is noted as gelation time. The gel strength is noted in the form of codes that range from No Gel (NG) to Flexible Gel (FG) to Good Flexible Gel (GFG) to Good Hard Gel (GHG). The code NG indicates that the gel appears to have the same viscosity (fluidity) as the original polymer solution and no gel is visually detectable. The code FG indicates that the gel is slightly more viscous (less fluid) than the initial polymer solution. It is a highly flowing gel with barely any gel structure visibly detectable. The code GFG indicates that the gel surface deforms slightly only upon inversion. The GFG structure is visibly detectable. The code GHG indicates a rigid rubbery gel formation. This gel formation may give a tuning-fork-like mechanical vibration upon tapping the ampoule. Table 1 shows readings of the test conducted on the plurality of gelant mixtures in accordance with an embodiment of the present invention.

s.
No. Composition (mg/L) Observation Remarks

Polymer HA HQ Type of Gel Gel time, davs

1 5000 3000 4000 . No Gel 3 Stable for > 120 days

FG 4-6

GFG 7-120

2 6000 3000 4000 No Gel 3 Stable for > 120 days

FG 4-6

GFG 7

GFG/GHG 8-120

3 7000 3000 4000 No Gel 2 Stable for > 120 days

FG 3-4

GFG 5-7

GHG 8-120

4 8000 3000 4000 No Gel 2 Stable for > 120 days

FG 3

GFG 4-7

GHG 8-120

Table 1
[0018] Table 2 further shows readings of the tests conducted on plurality of gelant mixtures at 105°C in accordance with an embodiment of the present invention.

s.
No Composition (mg/L) Observation Remarks

Polymer Hexamine Hydroquinone Type of Gel Gel time, days

1 5000 3000 4000 No Gel 2 Stable for > 120 days

FG 3-6

GFG/GHG 7-120

2 6000 3000 4000 No Gel 2 Stable for > 120 days

FG 3-4

GFG 5-7

GFG/GHG 8-120 Stable for > 12 0 days
3 7000 3000 4000 No Gel 2

FG 3

GFG 4-7 Stable for > 120 days

GHG 8-120

4 8000 3000 4000 No Gel 2

FG 3 Stable for > 120 days

GFG 4-7

GHG 8-120

Table 2

[0019] Based on the evaluation of the plurality of gelant mixtures, the compatible compositions of the gelant mixtures are identified for shutting-off water in the one or more zones of the subterranean formation. Thereafter, the plurality of gelant mixtures are placed in a multi-stage process for shutting-off water in the reservoir well. The shutting-off water results in an increased recovery of the hydrocarbon gas in the subterranean formation. At step 104, a pre-flush solution is pumped into the zone of the subterranean formation. In an embodiment of the present invention, the pre-flush solution comprises 1000 ppm of cross-linkers hexamine and hydroquinone in 1% NaHC03 solution. At step 106, a gelant mixture with a first or higher concentration is pumped into the zone of the subterranean formation. In an embodiment of the present invention, the higher concentration gelant mixture comprises 7000 ppm to 8000 ppm of the polymer with 3000 ppm to 5000 ppm of hexamine and 4000 ppm to 6000 ppm of hydroquinone in 1% NaHCC>3. Thereafter, at step 108, a second or a lower concentration gelant mixture is pumped into the zone of the subterranean formation. In an embodiment of the present invention, the lower concentration gelant mixture comprises 6000 ppm to 8000 ppm of the polymer with 3000 ppm to 5000 ppm of hexamine and 4000 ppm to 6000 ppm of hydroquinone in 1% NaHC03. Finally at step 110, a post flush job is carried out in the zone of the subterranean formation. In an embodiment of the present invention, the post flush job comprises 950 ppm to 1000 ppm of polymer in 1% NaHCC>3 and again followed by 1% NaHCC>3. In an embodiment of the present invention, the multi stage placement process of the gelant material, with varying gel strengths at different distances from the zone of the subterranean formation, results in a pancake formation. As the gel is hydrophilic in nature, the pancake formation swells after coming in contact with water and blocks the flow of water from

the reservoir. The blockage of water results in flow of gas from the reservoir. Thus, the gel acts as a relative permeability modifier near the zone of the subterranean formation by improving relative permeability of the gas after injection of gel.
[0020] After, the gelant mixture is placed in multiple stages, the well is kept closed for a predefined period of time for the gelation process. In an embodiment of the present invention, the predefined interval of time ranges from 4 days to 7 days. During the gelation process, the gelant material converts into gel. After gelation, perforations in the desired depth intervals are created for hydrocarbon gas production. In an embodiment of the present information, the gelant may be injected in structurally lower part of the formation as naturally occurring water enters from bottom part of the wellbore. The pancake formation may restrict this entry of the water in wellbore. The selective placement of gelant in lower part of the formation supported by selective perforation in upper part of formation facilitates gas production from the wellbore.
[0021] Thus, based on various tests conducted on the gelant material it may be apparent to a person of ordinary skill in the art that the plurality of gelant materials of the present invention provide an effective solution to shut-off water in the carbonate gas reservoirs to increase the recovery of the hydrocarbon gas in the subterranean formation. In an exemplary embodiment of the present invention, the gelant material of the present invention was tested in predefined gas well and it gave amazing results. The well had a water flow of 444 m3/day with no hydrocarbon gas recovery. After treating the well with the unique compositions and placement technique of the gelant material, the well started producing 65000 m3/day of hydrocarbon gas with only

48 m3/day water flow. The well continues to deliver 5 6000 m3/day of hydrocarbon gas with only 60 m3/day of water flow.
[0022] While the present invention has been shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from or offending the spirit and scope of the invention as defined by the appended claims.

We claim:
1. A process for increasing recovery of a hydrocarbon gas in a
subterranean formation, the process comprising:
preparing a plurality of gelant mixtures with varied concentrations; and
placing, in a plurality of stages, the plurality of gelant mixtures in the subterranean formation, wherein the varied concentrations of the gelant mixtures form a plurality of gels that shutoff water in the subterranean formation to increase recovery of the hydrocarbon gas in the subterranean formation.
2. The process as claimed in claim 1, wherein the plurality of gelant mixtures form plurality of gels with varying gel strengths in a predefined interval of time.
3. The process as claimed in claim 2, wherein the predefined interval of time ranges from 3 days to 120 days.
4. The process as claimed in claim 1, wherein the plurality of gelant mixtures comprises:
a polymer in a range of 1000 ppm to 8000 ppm; hexamine in a range of 3000 ppm to 5000 ppm; and hydroquinone in a range of 4000 ppm to 6000 ppm.
5. The process as claimed in claim 1, wherein placing of the
gelant mixtures in the plurality of stages comprises:
pre-flushing, in a first stage, a placement zone with 1000 ppm of hexamine and hydroquinone in 1% sodium bicarbonate (NaHC03) solution;

placing, in a second stage, a first concentration of the gelant mixture in the zone of the subterranean formation, wherein the first concentration of the gelant mixture comprises 7000 ppm to 8000 ppm of a polymer with 3000 ppm to 5000 ppm of hexamine and 4000 ppm to 6000 ppm of hydroquinone in 1% NaHCC>3;
placing, in a third stage, a second concentration of the gelant mixture in the zone of the subterranean formation, wherein the second concentration of the gelant mixture comprises 6000 ppm to 8000 ppm of the polymer with 3000 ppm to 5000 ppm of hexamine and 4000 ppm to 6000 ppm of hydroquinone in 1% NaHC03; and
post-flushing, in a fourth stage, the placement zone with 950 ppm to 1000 ppm of the polymer in 1% NaHC03 and again by 1%
NaHC03.
6. The process as claimed in claim 1 further comprises testing of the plurality of gelant mixtures prior to placing them in the subterranean formation, wherein the testing facilitates evaluation of compatibility of the gelant mixtures with the subterranean formation.
7. The process as claimed in claim 6, wherein the plurality of the gelant mixtures are tested using a common qualitative coding method.
8. The process as claimed in claim 7, wherein the testing evaluates gelation rate, gel strength, and thermal stability of the plurality of the gels.

9. The process as claimed in claim 1, wherein the plurality of
gels swell when in contact of water and shrink when in contact
of hydrocarbon gas thereby increasing flow of the hydrocarbon
gas into the subterranean formation.
10. The process as claimed in claim 1, wherein the subterranean
formation is a carbonate gas reservoir.