Sampling procedure for polymer-based solutions used in underground formations

Abstract

This invention concerns a sampling procedure for an aqueous hydrosoluble polymer solution flowing in a main circuit, enabling a sample to be collected to undergo at least one analysis under ambient air giving at least one property of the hydrosoluble polymer characterised in that a stabilizing solution is added to the aqueous hydrosoluble polymer solution, according to a discontinuous addition method, before or after sampling from the main circuit, so as to obtain a sample comprising a mixture of aqueous hydrosoluble polymer solution and stabilizing solution in which the hydrosoluble polymer is protected against attacks it may undergo in an atmosphere containing at least 10% by volume of oxygen.

Claims

1. A sampling procedure for an aqueous hydrosoluble polymer solution flowing in a main circuit, enabling a sample to be collected to undergo at least one characterisation under ambient air comprising: obtaining the sample by adding to the aqueous hydrosoluble polymer solution a stabilizing solution, according to a discontinuous addition method: the discontinuous addition method comprising: adding the stabilizing solution, either after sampling, or, before sampling, to the aqueous hydrosoluble polymer solution circulating in the main circuit, and obtaining a sample comprising a mixture of aqueous hydrosoluble polymer solution and stabilizing solution, wherein the aqueous hydrosoluble polymer is protected against attacks it may undergo, in the absence of a stabilizing solution, in an atmosphere containing at least 10% by volume of oxygen; wherein the stabilized solution is added in a predetermined quantity, based on a viscosity of the sample and over a predetermined time based on a flow rate of the aqueous polymer solution circulating in the main circuit, to the collected sample so that the sample contains a mixture of the stabilizing solution and the aqueous hydrosoluble polymer solution.

2. Sampling procedure according to claim 1 characterised in that the at least one characterization under ambient temperature is the measurement of viscosity and the added stabilizing solution enables the measured viscosity of the sample to be maintained approximately constant, when said viscosity is measured in ambient air, for a period of at least 1 hour.

3. Sampling procedure according to claim 1 characterised in that the sampling procedure comprises a prior determining of the volume of stabilizing solution to be added to the aqueous hydrosoluble polymer solution in which the volume of stabilizing solution added is varied and the change in measured viscosity of the mixtures obtained is studied over time.

4. Sampling procedure according to claim 1 characterised in that the sampling procedure comprises an analysis step under ambient air for the sample comprising a mixture of the aqueous hydrosoluble polymer solution and the stabilizing solution.

5. Sampling procedure according to claim 1 characterised in that the volume of stabilizing solution in the sample is less than 25% of the total sample volume.

6. Sampling procedure according to claim 1 characterised in that the stabilizing solution contains at least one stabilizing agent chosen from deoxygenating agents, precipitating agents, free radical scavengers, complexing agents, H.sub.2S-absorbing agents and sacrificial agents.

7. Sampling procedure according to claim 1 characterised in that the stabilizing solution contains at least three stabilizing agents chosen from deoxygenating agents, precipitating agents, free radical scavengers, complexing agents, H.sub.2S-absorbing agents and sacrificial agents.

8. Sampling procedure according to claim 1 characterised in that the aqueous hydrosoluble polymer solution circulates in a main circuit (II) used in enhanced oil recovery, the aqueous hydrosoluble polymer solution either on the injection side or on the production side and the sample is collected downstream and/or upstream of the oil reservoir.

9. Sampling procedure according to claim 1 characterised in that the sampling procedure includes a sampling step from a volume of aqueous hydrosoluble polymer solution in a sampling tank (1) using a sampling pipe (3) fitted with a non-shearing sampling closure (6) and a step adding into the sampling tank (1) a volume of stabilizing solution (300), sampling steps being carried out under hermetically-sealed conditions.

10. Sampling procedure according to claim 1 characterised in that the sampling procedure includes providing a sampling device connected to the main circuit (II) in which the aqueous hydrosoluble polymer solution (200) to be sampled in circulating, comprising: a first vessel (1), called the sampling tank, capable of containing the sample (100) collected, including: an inlet (5) for aqueous polymer solution to be sampled, and a sampling pipe (3) connecting the inlet (5) to the main circuit (II), the said sampling pipe (3) being fitted with a non-shearing sampling closure (6) and capable of being connected to the main circuit (II) and an outlet (8) and outlet pipe (7) fitted with an outlet closure (9) and connected to the outlet (8), a second vessel (2), called the treatment tank, capable of containing a stabilizing solution (300), comprising an outlet (10) for the stabilizing solution (300), a connecting pipe (4) connected to the outlet (10) for the stabilizing solution and fitted with a treatment closure (11) and, the connecting pipe (4) providing, at least in part, the connection between the treatment tank (2) and the sampling tank (1), and where the sampling tank (1) is connected hermetically to the main pipe, and is isolated hermetically when the sampling closure (6), outlet closure (9) and treatment closure (11).

Description

(1) The detailed description below, by reference to the appended Figures, gives a better understanding of the invention.

(2) FIGS. 1 and 2 present drawings of different variants of implementation of the procedure according to the invention.

(3) FIG. 3 shows the impact of concentration on the viscosity of a poly(acrylamide-co-acrylic acid) solution measured using a Brookfield viscometer, UL Spindle, 6 rpm, at 30 C.

(4) FIG. 4 shows the change in residual viscosity of a poly(acrylamide-co-acrylic acid) solution (viscosity being measured with a Brookfield viscometer, UL Spindle, 6 rpm, 24 C.) in different sampling situations.

(5) FIG. 5 shows the impact of concentration on the viscosity of a poly(acrylamide-co-acrylic acid) solution measured using a Brookfield viscometer, UL Spindle, 6 rpm, at 46 C.

(6) In the case illustrated in FIG. 1, stabilizing solution 300 is added directly into main circuit II in which the aqueous hydrosoluble polymer solution 200 to be sampled is circulating. In the embodiment illustrated, it is added to the previously-constituted aqueous solution, but it would also be possible to introduce it into the polymer dissolution water before its introduction, or even at the same time as introducing the polymer into the dissolution water, or with the addition of surfactants or other additives when these are present in the aqueous solution.

(7) As illustrated in FIG. 1, a treatment tank 2 can be used to contain the stabilizing solution 300 and which includes an outlet 10 connected to the main circuit II by a treatment pipe 4 fitted with a treatment closure 11. Given the pressures generally existing in the main circuit, the treatment closure will frequently be a pump. In fact, the aqueous solution may be at high pressure inside the main circuit. In general, the pressure in the main circuit pipe is greater than 0.2 MPa (2 bars) and most frequently between 0.2 MPa and 80 MPa (2 and 800 bars).

(8) The treatment tank 2 may take the form of an open-topped vessel for refilling or a closed vessel as illustrated in FIG. 1, in which case it is refilled using a refilling pipe 12 connected to an inlet 13 and fitted with a refilling closure 14.

(9) Then, the sample comprising a mixture of stabilizing solution and the aqueous hydrosoluble polymer solution can be collected using a sampling tank 1 connected to the main circuit II, through a sampling pipe 3 connected to an inlet 5. This sampling pipe 3 is fitted with a non-shearing sampling closure 6 that, when open, allows the sample to pass through. The fact that this closure is non-shearing ensures that the sampled solution has not undergone mechanical degradation. As an example of a non-shearing closure that can be used as part of this invention, there are ball valves, progressive-cavitation or internal gear pumps. The sampling tank is also connected to an outlet pipe 7 at its outlet 8. This outlet pipe 7 is fitted with an outlet closure 9 that may be shearing or non-shearing. Such an outlet closure 9 must be non-shearing if the sample must subsequently be collected through it, to be submitted for characterisation. To fill the sampling tank 1, it is necessary for both the sampling closure 6 and outlet closure 9 to be open, given that the sampled fluid is incompressible. Before the sampling starts, the sampling tank 1 may contain liquid, air or, non-ideally, an inert gas like nitrogen or argon. When the sampling closure 6 and outlet closure 9 are opened, the aqueous polymer solution circulating under elevated pressure in the main circuit II will enter the sampling tank 1 by chasing the air or gas present in the sampling tank 1 through the outlet closure 8. So that the sample 100 that is going to be stored in the sampling tank does not come into contact with the air or inert gas present in the sampling tank 1, and in order to avoid contamination or degradation in the sampling tank, a flush is ideally performed first using the aqueous polymer solution 200 or a mixture of the aqueous polymer solution and stabilizing solution.

(10) The stabilizing solution is added for the needs of sampling and is therefore performed in a discontinuous fashion. In other words, just before a sample is taken, a known quantity of stabilizing solution is added over a given time. This addition is adjusted, depending on the flow rate of the aqueous polymer solution circulating in the main circuit, so as to obtain the desired volume of stabilizing solution in the resulting mixture, and depending on the distance separating the addition point for the polymer solution and the sampling point, so ensuring that a sample contains a mixture of stabilizing solution and polymer solution. Between two samples collection, addition of stabilizing solution is stopped.

(11) According to a preferred embodiment illustrated in FIG. 2, the procedure according to the invention uses a device enabling the stabilizing solution to be introduced into a sample of aqueous solution, and not directly into the main circuit, so enabling the consumption of stabilizing solution to be reduced. For the purposes of simplicity, the numbering used in FIG. 1 has been retained in FIG. 2 for the common items.

(12) In FIG. 2, the treatment tank 2 is connected to the sampling tank 1. They are connected using a treatment pipe 4 directly linking the outlet 10 for stabilizing solution 300 located on the treatment tank 2 to an inlet 30 for the stabilizing solution 300 on the sampling tank 1. This treatment pipe 4 is fitted with a treatment closure 11, for example in the form of a high pressure pump, such as those used for liquid chromatography such as HPLC, which may be shearing or non-shearing. In devices according to the invention, closures may be valves or pumps, in particular. If the treatment closure 11 is a valve, the pressure in the treatment tank 2 will be greater than the pressure in the sampling tank 1. Conversely, when the treatment closure 11 is a pump, the pressures in the tanks may be independent, ie. identical or different.

(13) For the rest, the two tanks and pipe used in the device presented in FIG. 2 are identical to those in FIG. 1. When the treatment closure 11 located on the treatment pipe 4 is open, the stabilizing solution can enter the sampling tank 1. The stabilizing solution may be added to the sampling tank 1 before or after, preferably after sampling from the aqueous hydrosoluble polymer solution. Flushing steps on the sampling tank will preferably be performed. The volumes introduced may be determined by any appropriate system, by determining the volumes coming out after outlet 8, or by using a flow rate measuring device in pipes 3 and/or 4.

(14) Whatever the device variant I used, the sampling tank 1, connections, pipes and closures are selected so that the sampling tank 1 can be hermetically sealed from the outside and its connection to the main circuit II is achieved hermetically. In devices according to the invention, the sampling tank 1, and possibly the treatment tank 2, is hermetically sealed. In particular it may be gas bottles or cylinders. Preferably, the tanks and also the various pipes are made from austenitic stainless steel. Similarly, the various closures used will preferably be made from austenitic stainless steel.

(15) In order to perform the analysis, the sample 100 located in the sampling tank 1 may be collected directly from the sampling tank 1, directly through the outlet 8, or through another outlet dedicated for the purpose, not shown. It is also possible that the sampling tank 1 may be separated from the rest of the equipment after each sample is collected, in which case the sample may be collected from the sampling tank 1 through the non-shearing inlet closure 6, the outlet closure 9 then being either shearing or non-shearing.

(16) The various closures in devices according to the invention can be controlled either manually or automatically. In this case, a command unit will be provided enabling the various closures to be actuated.

(17) The different advantages of this invention can be illustrated by the following examples, which are not limiting in nature.

Example 1

Sampling at Injection Under Off-Shore Conditions

(18) A viscous polymer solution is prepared by dissolving an inverted emulsion containing a copolymer of 70 mol % acrylamide/30 mol % acrylic acid, partially neutralised using caustic soda. The average mass molecular weight of the copolymer is 18 million. The commercial name for such emulsion is Flopaam EM 533 EOR at SNF.

(19) The water used comes from a water treatment unit in which traces of oxygen are eliminated by adding a 50 ppm ammonium bisulphite solution. The brine water used is composed as described below (Table 2) and contains about 10 ppm ferrous iron and 0 ppb O.sub.2.

(20) It is known that such water will not degrade the viscosity of the polymer solution, if and only if the solution remains completely free of oxygen. In fact, reintroducing any oxygen in the presence of iron and excess reducing agent will have an immediate effect by degrading the molecular weight of the polymer.

(21) TABLE-US-00002 TABLE 2 composition of brine for example 1 for 1,000 g NaCl 15.4 g CaCl.sub.2, 2H.sub.2O 2.54 g MgCl.sub.2, 6H.sub.2O 2.1 g NaHCO.sub.3 0.62 g

(22) In order to dissolve the polymer rapidly and avoid contamination with oxygen, the emulsion containing 30% active polymer is firstly inverted using a static mixer to obtain a 10,000 ppm polymer solution, then this solution is immediately diluted with the same brine water to obtain a polymer solution containing 3,000 ppm polymer, which according to the graph presented in FIG. 3 should generate a viscosity of 50 cps at 30 C., using a Brookfield viscometer with UL module at a speed of 6 rpm. Details of this procedure are described in EP2283915.

(23) A sample is collected downstream of the polymer dissolution unit using a device comprising a sampling cylinder to which is connected a set of non-shearing valves, and a cylinder that may be used to inject a previously-prepared stabilizing solution. The construction of the device is represented schematically in FIG. 2.

(24) The stabilizing solution is a mixture of 25% dimethyl thiourea, 25% HTPO (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) 25% glycerine and water, the percentages being % by weight given for the total weight of the stabilizing solution.

(25) 1-A A first sample is collected. The solution corresponding to the sample collected in the sampling cylinder is de-pressurised into a beaker under ambient air and after de-foaming, its viscosity is measured at 24 C. The loss of viscosity is immediate and the value measured in only 27 cps.

(26) 1-B A second sample is collected. The cylinder containing the sample collected is then disconnected from the rest of the device, transferred by helicopter to a measuring laboratory, and finally introduced into a glove box, an enclosure rendered inert by nitrogen purging so as to have less than 50 ppb of oxygen in the box. Therefore the measurement is made 2 days after collecting the sample. The viscosity is then 51 cps, which is the expected value. This value therefore confirms that the dissolution line remains strictly anaerobic and that no mechanical degradation occurred. This also confirms the quality of polymer dissolution and the accuracy of its concentration. However, it requires significant and slow measuring resources that do not enable real time or frequent checks of the quality of fluid injected.

(27) 1-C A third sample is collected. 100 ppm of stabilizing solution is then injected into the sampling cylinder using overpressure and is therefore added to the sample collected. After waiting 10 minutes mixing time, the cylinder is de-pressurised into a beaker under ambient air and after de-foaming the viscosity is measured at 24 C. The viscosity is 43 cps, equivalent to a degradation of about 14% compared to example 1-B, due to inadequate short stopping of the degrading reactions.

(28) 1-D A fourth sample is collected. 500 ppm of stabilizing solution is then added to the sample by injection into the sampling cylinder using overpressure. After waiting 10 minutes mixing time, the cylinder is de-pressurised into a beaker under ambient air and after de-foaming the viscosity is measured at 24 C. The viscosity is 51 cps and is therefore in perfect agreement with the expected value. This value therefore confirms that the dissolution line remains strictly anaerobic and that no mechanical degradation occurred. This also confirms the quality of polymer dissolution and the accuracy of its concentration. This technique requires minor and rapid measuring resources that enable real time and frequent checks of the quality of injected fluid.

(29) 1-E A fifth sample is collected. 1,000 ppm of stabilizing solution is then injected into the sampling cylinder using overpressure. After waiting 10 minutes mixing time, the cylinder is de-pressurised into a beaker under ambient air and after de-foaming the viscosity is measured at 24 C. The viscosity is 54 cps, a little above the expected value. This value is explained by overdosing the stabilizing solution, which modifies the viscosifying power of the polymer in this fluid.

(30) FIG. 4 shows the change in residual viscosity as a function of time, for samples 1A, 1C, 1D and 1E. It appears that the measured viscosity remains constant, only in case 1D where viscosity matches the expected value exactly.

(31) The residual viscosity is the ratio of viscosity measured over time/viscosity at t.sub.0 (obtained just after de-pressurisation and therefore corresponding to the viscosity of the polymer before any possible degradation given the preparation method), multiplied by 100.

Example 2

Sampling at Injection on Land

(32) A viscous polymer solution is prepared by dissolving a powder made of 70 mol % acrylamide and 30 mol % acrylic acid copolymer, partially neutralised using caustic soda. The average mass molecular weight of the copolymer is 18 million. The commercial name for this powder is Flopaam 3630S at SNF.

(33) The water used comes from a water treatment unit in which traces of oxygen are eliminated by adding a 30 ppm ammonium bisulphite solution.

(34) The brine water used is composed as described below (cf. Table 3) and contains about 100 ppm hydrogen sulphide H.sub.2S and 0 ppb O.sub.2.

(35) It is known that such water will not degrade the viscosity of the polymer solution, if and only if the solution remains completely free of oxygen. In fact, reintroducing any oxygen in the presence of H.sub.2S and excess reducing agent will have an immediate effect by degrading the molecular weight of the polymer.

(36) TABLE-US-00003 TABLE 3 composition of brine for example 2 for 1,000 g NaCl 3.115 g KCl 0.054 g CaCl.sub.2, 2H.sub.2O 0.096 g MgCl.sub.2, 6H.sub.2O 0.093 g Na.sub.2SO.sub.4 0.237 g NaHCO.sub.3 1.31 g

(37) In order to dissolve the polymer rapidly and avoid contamination with oxygen, the powder is first dissolved using a PSU unit as described in patent application WO2008107492 to obtain a 15,000 ppm polymer solution, then this solution is diluted after two hours maturation time with the same brine water to obtain a polymer solution containing 1,000 ppm polymer, which according to the graph presented in FIG. 5 should generate a viscosity of 18 cps at 46 C. using a Brookfield viscometer with UL module at a speed of 6 rpm.

(38) A sample is collected downstream of the dissolution and dilution unit using a device comprising a sampling cylinder to which is connected a set of non-shearing valves, and a cylinder that may be used to inject a previously-prepared stabilizing solution. The construction of the device is represented schematically in FIG. 2.

(39) The stabilizing solution is a mixture of 10% 1,3,5 triazine, hexahydro-1,3,5-trimethyl 10% diethyl thiourea, 10% HTPO or 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, 10% pentaerythritol and water, the percentages being % by weight given for the total weight of the stabilizing solution.

(40) 2-A A first sample is collected. The solution sample contained in the sampling cylinder is de-pressurised into a beaker under ambient air and after de-foaming, its viscosity is measured at 35 C. The loss of viscosity is immediate and the value measured in only 5 cps.

(41) 2-B A second sample is collected. The cylinder containing the sample collected is then disconnected from the rest of the device, taken to a measuring laboratory, and finally introduced into a glove box, an enclosure rendered inert by nitrogen purging so as to have less than 50 ppb of oxygen in the box in the box. Therefore the measurement is made 6 hours after taking the sample. The viscosity is then 18 cps, which is the expected value. This value therefore confirms that the dissolution line remains strictly anaerobic and that no mechanical degradation occurred. This also confirms the quality of polymer dissolution and the accuracy of its concentration. However, it requires significant and slow measuring resources that do not enable real time or frequent checks of the quality of fluid injected.

(42) 2-C A third sample is collected. 100 ppm of stabilizing solution is then added to the sample collected by injection into the sampling cylinder using overpressure. After waiting 10 minutes mixing time, the cylinder is de-pressurised into a beaker under ambient air and after de-foaming the viscosity is measured at 35 C. The viscosity is 9 cps, equivalent to a degradation of about 50% compared to example 2-B, due to inadequate short stopping of the degrading reactions.

(43) 2-D A fourth sample is collected. 500 ppm of stabilizing solution is then added to the sample collected by injection into the sampling cylinder using overpressure. After waiting 10 minutes mixing time, the cylinder is de-pressurised into a beaker under ambient air and after de-foaming, the viscosity is measured at 35 C. The viscosity is 18 cps and is the expected value. This value therefore confirms that the dissolution line remains strictly anaerobic and that no mechanical degradation occurred. This also confirms the quality of polymer dissolution and the accuracy of its concentration. This technique requires minor and rapid measuring resources that enable real time and frequent checks of the quality of injected fluid.

Example 3

Sampling at Production on Land

(44) The viscous polymer solution as prepared in example 1 is injected through an injector well into the oil reservoir, which has a permeability of 1.2 D. The distance between the injector well and the producer well where the recovered hydrocarbons exit is such that the polymer takes 200 days to sweep the reservoir. During this propagation, the aqueous solution rises to 46 C. and takes up H.sub.2S to a level of 250 ppb. During this period, the acrylamide groups of the polymer are partially hydrolysed, the polymer concentration falls through adsorption and dilution, and the molecular weight is reduced by chemical degradation. The fluid is then pumped back to the surface using a horse-head type, non-shearing system. The materials of construction lead to the contamination of the fluid with iron at a level of 2 ppm. The reducing nature of the swept rock maintains the oxygen content at zero.

(45) It is known that such water will not degrade the viscosity of the polymer solution, if and only if the solution remains completely free of oxygen. In fact, introducing any oxygen in the presence of H.sub.2S and iron and excess reducing agent will have an immediate effect by degrading the molecular weight of the polymer.

(46) A sample is collected downstream of the pumping unit using a device comprising a sampling cylinder to which is connected a set of non-shearing valves, and a cylinder that may be used to inject a previously-prepared stabilizing solution. The construction of the device is represented schematically in FIG. 2.

(47) The stabilizing solution is a mixture of 10% 1,3,5 triazine, hexahydro-1,3,5-trimethyl 10% diethyl thiourea, 10% HTPO or 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, 10% pentaerythritol and water, the percentages being % by weight given for the total weight of the stabilizing solution.

(48) 3-A A first sample is collected. The solution contained in the sampling cylinder is de-pressurised into a beaker under ambient air and after defoaming the viscosity is measured at 35 C. The measured viscosity is only 2 cps, corresponding to a polymer concentration of 375 ppm if the molecular weight and anionicity remain identical to those that were injected.

(49) 3-B A second sample is collected. The cylinder containing the sample collected is then disconnected from the rest of the device, taken to a measuring laboratory, and finally introduced into a glove box, an enclosure rendered inert by nitrogen purging so as to have less than 50 ppb of oxygen in the box. Therefore the measurement is made 6 hours after collecting the sample. The viscosity is then 14 cps. The sample is then removed from the glove box and measurements of molecular weight, anionicity and polymer concentration were made and the measurements obtained are presented in Table 4 below.

(50) TABLE-US-00004 TABLE 4 Injected fluid Produced fluid Anionicity (colloidal titration) 30 mol % 43 mol % Molecular weight (IV) 18 million 3 million Daltons Daltons Concentration (starch iodine 1,000 ppm 830 ppm API concentration) Viscosity (Brookfield) 18 cps at 46 C. 14 cps at 35 C. in BAG

(51) There is an apparent inconsistency between viscosity measured in a glove box and the measured molecular weight. Remeasuring the viscosity of the aqueous solution shows that rapid degradation took place when the solution was exposed to the air, viscosity falling to 3.5 cps. The molecular weight measurement is therefore incorrect and no conclusion can be drawn about the mobility ratio between the oil phase and the aqueous phase dilution phase actually established when sweeping the reservoir.

(52) 3-C A third sample is collected. 500 ppm of stabilizing solution is then added to the sample collected by injection into the sampling cylinder using overpressure. After waiting 10 minutes mixing time, the cylinder is de-pressurised into a beaker under ambient air and after de-foaming, the viscosity is measured at 35 C. The viscosity is 14 cps, consistent with the value in 3-B. The sample is then subject to measurements of molecular weight, anionicity and polymer concentration and the measurements obtained are presented in Table 5 below.

(53) TABLE-US-00005 TABLE 5 Injected fluid Produced fluid Anionicity 30 mol % 43 mol % Molecular weight 18 million 13 million Daltons Daltons Concentration 1,000 ppm 830 ppm Viscosity 18 cps at 46 C. 14 cps at 35 C.

(54) The molecular weight and viscosity are entirely consistent and enable easy and rapid checks to be made of the fluid viscosity that has swept the reserve and thus to correlate data for the increase in oil recovery with the actual mobility value, to monitor polymer breakthrough, to prevent uncontrolled degradation of the polymer and therefore, if necessary, to decide the injection of stabilisers.