Known gas well water shut-off processes include the"Unocal"alternating polymer-nitrogen injection process which has been used with a degree of success.

The"Unocal"process uses a cross-linking polymer to produce a gel block to water production with nitrogen injection forming channels for gas flow. Testing of the"Unocal"process under laboratory conditions raised doubts regarding the reliability and durability of the process. It is also known to use relative permeability modifiers, i. e. chemicals that reduce the reservoir permeability to water with minimum effect on hydrocarbon permeability and while these rel. perm modifiers represent a low risk in terms of impairment of gas production they are not wholly effective as water shut-off means in addition to which their durability is somewhat limited.

The perceived difficulty in the known processes was the establishment of a permanent water shut-off mechanism by use of a durable polymer system. It was therefore realised that there could be unforeseen advantages over prior art processes viz. the"Unocal"process or processes using the rel. perm modifiers, in combining the attributes of the"Unocal"system with the reliable"Maraseal"gel system and using the combination of processes to selectively place a reliable and durable water shut-off in a water zone of a gas well. It is, therefore, an object of the present invention to mitigate or overcome the shortcomings of the prior art processes referred to herein.

Thus. the present invention conveniently provides a process for reducing water flow from a hydrocarbon production well comprising the steps of :- (a) injecting a first chemical composition into the hydrocarbon and water zones of a reservoir; (b) injecting a second chemical composition into said hydrocarbon and water zones; (c) effecting a"shut-in"for a reaction period of the first chemical composition; (d) producing the second chemical composition out from the hydrocarbon zones prior to reaction of the second chemical composition; and (e) maintaining the production of the first and second chemical compositions from the hydrocarbon zones to facilitate the creation of a passage for hydrocarbon flow therefrom whilst enabling the second chemical composition to react in the water zones to form a water shut-off.

Preferably step (b) of the process is effected immediately upon the completion of step (a) thereof.

In one preferred arrangement the first chemical composition is effective as a relative permeability modifier.

Conveniently, the first chemical composition comprises a cationic polyacrylamide and the second chemical composition comprises a cross-linkable polyacrylamide-chromium acetate system.

Preferably, any reaction period for the first chemical composition is less than the delay period prior to reaction of the second chemical composition.

The present invention also provides a process for forming a water shut-off including the injection of a relative permeability modifying chemical composition and a flow blocking chemical composition into hydrocarbon and water zones of a reservoir; characterised in that. the process also includes the production of the chemical compositions from the hydrocarbon zones concommitantly with the reaction of the flow blocking chemical composition in the water zones whereby a water shut-off is formed in the water zones.

Conveniently, the production of the chemical compositions from the hydrocarbon zones is facilitated by the reservoir pressure.

By way of example of the present invention, a novel water shut-off process will now be described in detail with reference to the accompanying drawings in which:- Figure 1 illustrates schematically a gas reservoir accessed by a gas production well; Figure 2 illustrates schematically a first step of a novel process for establishing a water shut-off; Figure 3 illustrates schematically a second step of the novel process; Figure 4 illustrates schematically a third step of the novel process; Figure 5 illustrates schematically a fourth step of the novel process; and, Figure 6 illustrates schematically a fifth step of the novel process.

In overcoming the drawbacks of prior art water shut-off processes, the process provided by the present invention utilises the benefits derived from using rel. perm modifiers in combination with a reliable and durable gel blocking polymer.

A reservoir 2 to be accessed is comprised of two zones, i. e. an upper gas zone 6 and a lower water zone 8 located between upper and lower stratum 10 and 12.

After accessing the gas reservoir via the well bore 4, see Figure 1, the novel process of forming a water shut-off is effected in five steps as shown schematically in figures 2 to 6.

In the first step of the process a relative permeability modifier 14. viz."Floperm 500 (RTM) is injected through the well bore 4 into the gas zone 6 and water zone 8 as indicated by the arrows in figure 2.

"Floperm 500"is a cationic polyacrylamide polymer including a cross-linking agent.

Immediately upon completion of step one of the process, step two is effected by injecting a blocking gel 16, viz."Maraseal"through the well bore 4 into the gas zone 6 and water zone 8 behind the relative permeability modifier 14 as indicated by the arrows in Figure 3.

"Maraseal"is a polyacrylamide-chromium acetate system having a low viscosity (-15cp at 25°C), controllable gel times and cross-links to a"ringing gel"of proven durability.

Step three of the novel process involves a shut-in for the reaction period of the rel. perm modifier 14. The reaction period is typically of 16 to 24 hours duration for "Floperm 500" ; however, the reaction period must be compatible with the delayed gellation times of the polymers utilised in the process.

In step 4 the polymers, i. e., the"Floperm 500"and the"Maraseal"are back produced from the gas zone 6 utilising the inherent pressure of the reservoir 2 to leave the pre-treatment rel. perm modifier 14 and the blocking gel 16 in the water zone 8, see Figure 5.

Back production of the polymers from the gas zone 6 is maintained into continuous gas production as indicated in Figure 6, by which time the"Maraseal" gels to a block in the water zone 8 to provide an effective and durable water shut-off.