[0001] The present invention relates generally to conformance control in oil and gas reservoirs. More particularly the present invention relates to a process for preparing a sealing agent with improved mechanical strength, elasticity and thermal stability for enhanced conformance control at high temperatures in oil and gas reservoirs.
Background of the invention
[0002] Oil and natural gas are generally recovered from land and seabed via onshore and offshore platforms. Often, oil and gas extracted from underground reservoirs present in land and seabed is accompanied by water or brine, which is referred to as produced water. The underground reservoirs eventually produce water along with oil and, therefore, water production is an inevitable process during an operational life span of the reservoirs. Although, the produced water is useful in some cases for increasing oil production but it has been observed that after a certain limit is crossed, it is a major source of problem in oil and gas production platforms. The problems include decline in oil productivity, inorganic scale formation, increased corrosion, water coning, increased water handling facilities, increased water treatment and disposal costs etc. Further, with time the oil and gas wells continue to age which causes the flow of unwanted water through fractures, thief zones, high permeability streaks etc. The unwanted water also reduces production quality of oil recovered from the reservoirs and increases overall production cost.
[0003] Conventionally, various types of formulations such as cements, gels and like are used as sealing agents for conformance control in oil and gas platforms. Gel formulations are preferred for effective controlling of unwanted produced water in oil and gas reservoirs. The gel formulations generally used may include

chromium (III) carboxylate or acrylamide-polymer gels, aluminum-citrate acrylamide polymer gels, biopolymer xanthan gels, monomer gels and the like. These gels have inorganic chemical crosslinking agents with certain toxic chemical components which are harmful to use. The crosslinked agents may use polymers having heavy metal crosslinking which are not environmentally benign, have limited penetration depth, gets easily degraded, gets absorbed on the reservoir surface and precipitates under harsh reservoir conditions.
[0004] Furthermore, sealing agents used are largely made of polymer derivatives which are injected in oil and gas reservoirs for reducing flow of unwanted water. Traditionally, polymer gels are based on chromium VI salts, organochromium compounds and like. It has been observed that these compounds have toxic effects on environment and therefore their use has been regulated. Further, polymer gel technologies used for this purpose have certain constrains and limitations, which does not render it to be used up to its full extent for reducing unwanted water flow. Polymer derivative gels, on gelation, using initiators, forms gels that are opaque brittle gels without elasticity and require high material concentrations, having fast gelation time, which are difficult to control. The opaque brittle gel formed, therefore does not provide effective conformance control and casing leak plugging. Furthermore, sealing agents break down on being exposed to high temperatures in oil and gas reservoirs.
[0005] In light of the aforementioned drawbacks, there is a need for a method to provide enhanced water controlling ability to the existing polymer derivative sealing agents. Further, there is a need to provide effective conformance control and casing leak plugging mechanism by reducing or delaying gelation time. Further, there is a need to provide increased mechanical strength and elasticity to sealing agents such as gels, upon delayed gelation. Furthermore, there is a need for improving penetration and increasing adhesive quality of the sealing agent. Yet further, there is a need to provide cost effective and

environment friendly gelation initiators. Furthermore, there is a need to increase the efficiency and production capacity of the oil and gas reservoirs. Yet further, there is a need to enhance the thermal endurance capacity of the sealing agent so as to make it suitable for use at high temperatures.
Summary of the invention
[0006] A process for preparing an improved sealing agent for conformance control is provided. In accordance with various embodiments of the present invention, the process comprises preparing an aqueous alkali metal silicate solution, adding a non-crosslinked polymer derivative to the aqueous alkali metal silicate solution in a pre-determined concentration. The pre-determined concentration of the non-crosslinked polymer derivative is 0.1%. Thereafter, a carboxylic compound organic initiator is added to the aqueous alkali metal silicate solution after adding the non-crosslinked polymer derivative. The carboxylic compound organic initiator is added to the aqueous alkali metal silicate solution in a pre-determined weight ratio to form the sealing agent. The pre-determined weight ratio is ascertained to control the gelation time of the sealing agent. The formed sealing agent is thermally stable, mechanically strong and elastic in nature. The sealing agent is pumped in the gaps between the casing and formation for conformance control. Further, the improved sealing agent is capable of being used at a temperature of more than 150°C.
Detailed description of the invention
[0007] The present invention provides a method for preparing a thermally stable, mechanically strong and elastic sealing agent for effective conformance control in oil and gas platforms. In accordance with various embodiments of the present invention, the invention provides for effectively sealing casing leaks and spaces and preventing premature well plugging and loss of oil and gas reserves. Furthermore, the invention provides for shutting off water by sealing the gaps formed between cement and

formation due to poor cementation. Furthermore, the invention provides for controlling the gelation time of the sealing agent.
[0008] The compositions, formulations and methods discussed herein are merely illustrative of specific manners in which to make and use this invention and are not to be interpreted as limiting in scope. While the compositions and methods have been described with a certain degree of particularity, it is to be noted that many modifications may be made in the details of the construction and the arrangement of the systems and processes without departing from the spirit and scope of this disclosure. It is understood that the compositions and methods are not limited to the embodiments set forth herein for purposes of exemplification.
[0009] In various embodiments of the present invention, a method for preparing an improved sealing agent for effective conformance control is provided. The sealing agent is prepared using an improved formulation comprising alkali metal silicate solution in combination with a polyacrylamide derivative, and an organic initiator. The polyacrylamide derivative is added to alkali metal silicate solution prior to adding the organic initiator. Consequently, the sealing agent formed from such composition exhibits a surprising effect of improved elasticity and less brittleness, increased mechanical strength, increased thermal stability, increased penetration capacity, and is environment friendly.
[0010] In particular, in an embodiment of the present invention, the sealing agent comprises an alkali metal silicate. The alkali metal silicate is a silica derivative of inorganic compounds composed of silica (SiC^) and alkali metal oxide. The silica and alkali metal oxide may be used in various proportions. In an exemplary embodiment of the present invention, the proportion of silica and alkali metal silicate may be represented as (Si02)n : (alkali metal) 2O, wherein n < 4. The alkali metal silicate includes alkali metals such as, but not limited to sodium or potassium or the like, and combinations thereof. In an exemplary embodiment of the present invention, an alkali metal silicate is

solid sodium silicate (Na2Si03) which is water soluble i.e. the sodium silicate is used in aqueous form. The aqueous sodium silicate solution is prepared by adding solid sodium silicate to technical water or normal water. The prepared aqueous sodium silicate solution is alkaline in nature. The sodium silicate solution has a pH in the range of 11-13 and is capable of showing gelation and/or polymerization reactions when mixed with water.
[0011] In an embodiment of present invention, an initiator is used for causing delayed gelation of sodium silicate solution. The initiator is an acidic chemical. In an exemplary embodiment of the present invention the initiator is an organic initiator which is environmentally benign. A carboxylic acid compound may be used as an organic initiator. The carboxylic acid compound may include, but is not limited to, citric acid, acetic acid or the like. The organic initiator is added to the sodium silicate solution. The initiator is a source of protons in formulation of the sealing agent, which lowers the pH of the silicate solution. Upon delayed gelation of sodium silicate solution, using the initiator, a very opaque brittle gel is formed.
[0012] In an embodiment of the present invention, a polymer derivative is added to the gel before adding the initiator and a soft and elastic gel is obtained upon delayed gelation of sodium silicate solution. In an exemplary embodiment of the present invention, the polymer derivative used may include, but is not limited to, polyacrylamide derivative or the like. The polyacrylamide derivative has a coil like structure and is pH neutral. In another exemplary embodiment of the present invention, the polyacrylamide derivative may include, but is not limited to, a partially hydrolyzed polyacrylamide (PHPA). The PHPA used in formulation of sealing agent is without any crosslinking. In an exemplary embodiment of the present invention, the polyacrylamide derivative used is a low and high molecular weight polymer. The usage concentration of the low and high molecular weight polyacrylamide derivative is 0.1%. Further, PHPA is added to sodium silicate solution for enhancing softness and elasticity of the gel sealing agent upon gelation.

[0013] In various embodiment of the present invention, a method for formulation of the sealing agent is provided. The sodium silicate solution is acidified to a pH of less than about 10, by adding an initiator. On acidification, sodium silicate solution is converted partially to silicic acid. Silicic acid is a weak acid at these pH values. Following reactions occur during formation of silicic acid:
H2Si04 —►H3Si04 —► H4Si04 —► H5Si04 (I)
(Silicic acid)
Silicic acid formed remain hydrated, forming a three-dimensional network. The three-dimensional network traps water present in solvent and does not form a precipitate of silica (SiCk) . This network is a gel since both phases are continuous. Slightly lowering the pH value of the sodium silicate alkaline solution brings about radical changes in gel time. The gel time is difficult to control and forms lumps from local acid concentrations during large scale mixing.
[0014] Further, the pH levels of the sodium silicate solution can be varied for gelation processes. At low pH, H3Si04- and H4Si04, as in reaction (1), are dominant ions. These ions reacts with each other to initiate a condensation process of the sodium silicate solution. The condensation process is represented by following reaction:
H3Si04 + H4Si04 —► (OH) 3 Si-O-Si (OH) 3 + OH- (2)
In this reaction two Silanol (SiH40) groups form a new bond in which an oxygen atom bridges the silicon atoms to produce Siloxane (Si-O-Si) . Further, a hydroxyl ion (OH-) is released as a result of condensation reaction. The released hydroxyl ion leads to increase in pH of the sodium silicate solution with time, making the sodium silicate solution alkaline.
[0015] The condensation reactions thereafter results in a dimer. The resultant dimer polymerizes and reacts with the H3Si04- to form trimers in alkaline solution. This polymerization reaction

keeps repeating and finally forms a gel network used as a sealing agent.
[0016] In another embodiment of the present invention, the pH value of sodium silicate solution is further lowered. Lowering of pH causes the sodium silicate solution to increase to acidic ranges. Therefore, acidic sodium silicate solution is formed. In the formed acidic sodium silicate solution, H4SiC>4 and HsSiO,j, in the reaction (1), react with each other to form a different dimer. This dimer further polymerizes and thereafter forms an acidic sealing agent. The acidic sealing agent formed are considerably more stable. However, the sealing agent formed in the foregoing formulations is opaque and brittle.
[0017] In an embodiment of the present invention, for providing mechanical strength and elasticity to the formed sealing agent, the polyacrylamide derivative, such as PHPA, is added to the sodium silicate solution prior to adding organic initiator, such as citric acid. The sealing agent formed is of varied types and range from soft gels to very hard ringing gels with gel times of minutes to days. The sealing agent formed in this manner is therefore, elastic in behavior like a polyacrylamide gel and do not exhibit the characteristic brittleness of sodium silicate gels with the same initiator. Initiator is added after adding polyacrylamide derivative and initiator being a source of protons, thereby, allows the sodium silicate solution to form a sodium silicate hydrogel in the presence of polyacrylamide coils. The polyacrylamide derivatives are pH neutral which does not react with organic initiator and sodium silicate does not react with polyacrylamide. The sodium silicate hydrogel formed may be weakly bound by hydrostatic forces to the polyacrylamide molecules. A weak crosslink is formed between the sodium silicate hydrogel and polyacrylamide derivative. The weak crosslink is formed when the hydrostatic forces between the sodium silicate hydrogel and polyacrylamide derivative wrap around the polyacrylamide and also bind two or more polymer chains in a weak crosslink. As gelation occurs on adding initiator to the solution of sodium silicate and PHPA, the pH of the said solution drops

from 12 to the range of about 10-7, thereby forming a stable and strong sealing agent.
[0018] In an embodiment of the present invention, the sealing agent is formed using a specific weight ratio of sodium silicate 5 aqueous solution to the organic initiator along with very less concentration of high molecular weight and low molecular weight polyacrylamide derivative. In an exemplary embodiment of the present invention, the concentrations of sodium silicate aqueous solution to organic initiator is in a weight ratio which ranges
10 from 3/2.2 to 4/2.8. The concentration of high molecular weight and low molecular weight polyacrylamide derivative, such as PHPA, is added in very small quantity which is around 0.1%. The small quantity of PHAP added aids in achieving the desired mechanical strength and improved elasticity of the sealing agent. Further,
15 at room temperature the improved sealing agent exhibits viscosity similar to water.
[0019] In an embodiment of the present invention, the formulated improved sealing agent can be implemented in the oil and gas reservoir and pumped in the gaps between casing and formation,
20 or can be used in any conformance control requirements. The improved formulated sealing agent helps in profile control in injecting wells by sealing or partially blocking high permeability/oil depleted zones, allowing flood water to be directed to lower permeability/oil rich zones. Further, the
25 sealing agent can alternatively be used in a dehydrated gel pill form to plug the perforations or formation interval losing the drilling fluid.
Example 1
[0020] The sealing agent was made by mixing a solution containing
30 37.85% solids of sodium silicate (Na2Si03) and 29.90% of silica
(Si02) . Citric acid was used as an initiator. The polymers used
were polyacrylamide from Alcoflood 254S as Lower molecular weight
(LMW) and Alcoflood 935 as High molecular weight (HMW). Technical
water was used for making respective solutions. Sodium silicate,
35 initiator and the polymer concentrations were varied by weight
9

by weight ratio. Further, after the required gel vial solution is formed, it was tested at temperatures of 75°C and 90°C and its setting time was observed. The sealing agent was also tested to observe its bonding with sand. The results of the formation of 5 sealing agent with different compositions of the components are provided in the Table 1 as illustrated herein below.
Table 1

S. No. Temperature (°C) Weight ratio
(Silicate/Ci
trie Acid) PHPA (Low
and High
mol. Wt. %) Gel times (Hrs) Sealing
agent Strength
1. 75 3/2.2 0.1+0.1 7 Weak
2. 75 3/2.4 0.1+0.1 7 Strong
gel but
breaks on
shaking
3. 75 3/2.6 0.1+0.1 6 Strong
4. 75 4/2.8 0.1+0.1 5 Very
strong
plug
5. 90 9.6/20 Low mol. Wt. 4 Ringing gel
[0021] It was observed, as illustrated in Table 1, that best 10 results in terms of mechanical strength and elasticity of sealing agent were obtained with concentrations in specific weight ratios of 4/2.8 (Silicate to citric acid weight ratio) and 0.1+0.1 (PHPA Low and High mol. Wt. %) at a temperature of 75°C, used in formulation of improved sealing agent. The improved sealing agent 15 is used to fill gaps between casing and formation i.e. channels developed by poor cementation in shaly area or by defective cementing. Further, the improved sealing agent is used to fill
10

the channels or gaps, to stop water excursion through the channels or gaps left because of poor cementation.
[0022] In various embodiments of the present invention, advantageously the sealing agent and method of formation provides 5 environmentally friendly and cost effective solution for controlling flow of excess produced water. The sealing agent formed using a polyacrylamide and an organic initiator in respective weight ratios imparts various advantages to the existing silicate sealing agents. The sealing agent is a 10 mechanically strong and elastic gel which can be used for in-depth placement in oil and gas reservoirs.
[0023] Further, the sealing agent produced, in accordance with various embodiments of the present invention, provide flexibility to the silicate gel binders with improved penetration through
15 small opening and spread over large area thereby effectively plugging unwanted flow. The improved sealing agent have long term stability which work under low temperatures and also under high temperatures of more than 150°C in harsh reservoirs environments where most organic polymers are not suited. Further, there is no
20 issue with regard to handling of the produced sealing agent as it do not use any harmful chemical compositions during formation processes. The gelation and/or polymerization reactions of alkali metal silicates are used for conformance control and casing leak plugging for extending the life of the oil field
25 well.
[0024] Further, the gelling time of the sealing agent can be controlled to set in minutes to hours by controlling the type and concentration of the initiators used, depending upon the requirements of the particular reservoir system and the amount 30 of unwanted water present in the oil and gas reservoir. Further, on delayed gelation the sealing agent formed is a soft and elastic gel and provides enhanced lubrication capabilities in the oil and gas reservoir system.
[0025] While the exemplary embodiments of the present invention 35 are described and illustrated herein, it will be appreciated that
11

they are merely illustrative. It will be understood by those skilled in the art that various modifications in form and detail may be made therein without departing from the scope of the invention except as it may be described by the following claims.

We claim:

A process for preparing an improved sealing agent for
conformance control, the process comprising the steps of:
a) preparing an aqueous alkali metal silicate solution;
b) adding a non-crosslinked polymer derivative to the aqueous alkali metal silicate solution in a pre-determined concentration, wherein the pre-determined concentration of the non-crosslinked polymer derivative is 0.1%; and
c) adding a carboxylic compound organic initiator to the aqueous alkali metal silicate solution in a predetermined weight ratio to form the sealing agent.
2. The process as claimed in claim 1, wherein the step of
preparing the aqueous alkali metal silicate solution comprises:
a) combining silica (SiCb) and alkali metal oxide in a proportion of (Si02)n : (alkali metal) 2O, wherein n < 4, to form a solid alkali metal silicate; and
b) adding the solid alkali metal silicate to technical water to form the aqueous alkali metal silicate solution.

3. The process as claimed in claim 1, wherein the pH of the aqueous alkali metal silicate solution is in the range of 11-13.
4. The process as claimed in claim 1, wherein the predetermined weight ratio of the alkali metal silicate solution and the carboxylic compound organic initiator is in a range of between 3/2.2 and 4/2.8.

5. The process as claimed in claim 4, wherein the step of adding carboxylic compound organic initiator to the aqueous alkali metal silicate solution comprising the pre-determined weight ratio is ascertained such that the gelation time of the sealing agent is controlled.
6. The process as claimed in claim 1, wherein the non-crosslinked polymer derivative is a low and high molecular weight polyacrylamide derivative.
7. The process as claimed in claim 6, wherein the non-crosslinked polymer derivative includes a partially hydrolysed polyacrylamide (PHPA).
8. The process as claimed in claim 1, wherein after steps a, b, and c a sodium silicate hydrogel is formed and a weak crosslink is formed between the sodium silicate hydrogel and the non-crosslinked polymer derivative.
9. The process as claimed in claim 1, wherein the sealing agent is pumped in the gaps between casing and formation for conformance control.
10. The process as claimed in claim 1, wherein the sealing agent
is prepared to be used at temperatures of more than 150°C.