Tu B14 Enhanced Polymer Flooding - Reservoir Triggering Improves Injectivity and Eliminates Shear Degradation

Transcription

1 Tu B14 Enhanced Polymer Flooding - Reservoir Triggering Improves Injectivity and Eliminates Shear Degradation W.J. Andrews (NALCO Champion), S.E. Bradley (NALCO), P. Reed (NALCO), M. Salehi* (NALCO Champion) & D. Chappell (BP) SUMMARY This paper describes a successful program of lab and pilot-scale studies qualifying a new shear-resistant, high-injectivity, reservoir-triggered polymer (Polymer) for field trial. The Polymer mitigates two of the major operational and economic challenges facing polymer flooding applications for mobility control, namely, shear degradation during injection and reduced fluid injectivity. Shear degradation of conventional HPAM polymers through injection facilities can result in dramatic losses of up to 7% of viscosity yield. However, this can be eliminated using the new Polymer. This is particularly important in an offshore environment where highly-shearing subsea chokes are required for flow distribution control. The Polymer formulation uses a novel yet inexpensive chemical approach enabling it to inject with nearwater viscosity in a shear-resistant form. The Polymer has been engineered such that it does not viscosify the injection fluid until it is triggered in the reservoir away from the near wellbore region. Higher injection rates and viscosities can therefore be attained than would otherwise be possible with a conventional polymer flood. Methods: The Polymer s triggering performance in porous media under both static and dynamic conditions has been demonstrated. The un-triggered Polymer has been subjected to extremes of shear at both lab and pilotscale to test shear resistance. Injectivity of the Polymer has been assessed through an extensive suite of sand pack and coreflood experiments. Tests have also been conducted to verify the Polymer s suitability for field deployment including surface storage, inversion, and long-term reservoir stability. Results: The Polymer is completely shear-resistant during injection, demonstrated by flowing through a scaled choke with pressure drops exceeding those expected during deployment. The viscosity of the un-triggered Polymer solution has been shown to be almost independent of the Polymer concentration, injecting with a viscosity close to that of sea water and giving excellent injectivity into sand packs and cores. In addition, the Polymer has been demonstrated to inject, propagate and trigger to deliver a pre-determined viscosity in a temperature-controlled 4ft sand pack experiment. The Polymer solution is easily and reliably prepared, out-performing a conventional HPAM in a pilot-scale inversion study, and demonstrates storage characteristics above the industry standard. A 15 month-long stability test performed at reservoir temperature with reservoir fluids showed minimal loss of viscosity. Testing will now proceed to field trial. If successful, this new technology offers a route to overcoming some of the key obstacles to large scale polymer EOR deployment, particularly in the offshore environment.

2 Introduction Dilute solutions of water-soluble polymers, typically partially hydrolyzed polyacrylamide (HPAM), have been used extensively to improve reservoir sweep efficiency (Gogarty, W. B. 1967). By increasing the viscosity of the injection water the mobility ratio between the hydrocarbon phase and the aqueous phase is reduced and a more piston-like displacement of oil results. In addition, there are fractional flow benefits that contribute to the incremental oil. Two of the major operational and economic challenges facing polymer flooding applications for mobility control are shear degradation during injection and reduced fluid injectivity. Shear degradation of conventional HPAM polymers through injection facilities can result in dramatic losses of up to 7% of viscosity yield. In order to achieve the target reservoir viscosity during the polymer flood, more polymer is needed to compensate for the shear loss experienced during injection (Caulfield, M.J. et al. 22, Maerker, J.M. 1975, Sorbie, K.S. 1991, Seright, R.S. et al. 1983, Seright, R.S. 1983). This is particularly important in an offshore environment where highly-shearing subsea chokes may be required for flow distribution control. This paper describes a successful program of laboratory and pilot-scale studies qualifying a new shear-resistant, high-injectivity, reservoir-triggered latex polymer (TX15843, the Polymer ) for mobility control applications. The Polymer mitigates the above-mentioned operational and economic challenges facing polymer flooding applications and is currently being tested at scale in an inter-well field pilot. Theory By constraining the polymer backbone, the Polymer can be injected with near-water viscosity in a shear-resistant form. The Polymer has been engineered such that it does not viscosify the injection fluid until it is triggered in the reservoir away from the near wellbore region. With increasing distance from the well the fluid velocity for a given injection rate rapidly drops, hence most of the pressure drop between injection well and average reservoir pressure occurs close to the injection well. A triggered solution of the Polymer is indistinguishable from HPAM thereby facilitating the targeted delivery of intact HPAM into the reservior. This novel, yet inexpensive, approach means that higher injection rates and in-reservoir viscosities can be attained than would otherwise be possible with a conventional polymer flood. Laboratory studies Polymer Preparation The following experiments were conducted using emulsion polymer TX15834, the Polymer, from Nalco Champion. The properties of TX15834 are described in Table 1. For comparison, the properties of a conventional liquid HPAM formulated for enhanced oil recovery (EOR), TX1567, are also shown. Dilute solutions of the polymers were prepared for testing by inverting the stock water-in-oil emulsion with water to an oil-in-water emulsion by stirring with an overhead mixer and cage stirrer at 8 RPM for a period of 3 minutes at room temperature. Complete latex inversion is necessary to ensure reliable and repeatable testing. A 1, ppm mother solution of polymer was first prepared before dilution with synthetic sea water (SSW) to the final concentration of 1 ppm. The SSW composition used in this study is given in Table 2. Triggering For product characterization and evaluation, the inverted TX15834 polymer solution needs to be triggered before use. When required, the test solution was triggered by aging overnight in an oven at 65 C.

3 Table 1. Properties of TX15834 and TX1567 Property TX15834 ( the Polymer ) TX1567 Product Form Emulsion Emulsion Product Chemistry Modified polyacrylamide formulated for EOR Polyacrylamide formulated for EOR Polymer Hydrolysis Level Polymer molecular weight* Polymer Actives Level (as salt) Product Bulk Viscosity 3 mol % 3 mol % 9-18 M 9-18 M 28% 28% <1 Centipoise <2 Centipoise Product Shelf Life Regulatory Compliance*** 6 months when stored at < 3 C** North Sea (UK sector) compliant 1 year when stored at < 3 C North Sea (UK sector) compliant *= the MW can be adjusted within this range **= Evaluation still ongoing ***=pending product registration Table 2. Synthetic Sea Water Composition Component g/l NaHCO 3.1 MgCl 2 x 6H 2 O CaCl 2 x 2H 2 O 1.57 Na 2 SO NaCl Water Total Hardness Total TDS 179 ppm 3.56 wt.%

4 Filter Ratio The filter ratio test is used as an indication of how well a solution will inject into porous media. Modified from the original API RP63 method (API Recommended Practice 63), the filter ratio tests used in this study were conducted using 1.2 µm pore size filter membranes for both un-triggered and triggered solutions. In this test, 3 ml of polymer solution is placed in a steel bell filter ratio housing and the solution is pushed through the filter membrane at 2 psi Nitrogen overpressure. The calculated filter ratio value is the rate of flow at a later time divided by the rate of flow of an earlier time. A value of <1.2 is considered acceptable. A solution of emulsion polymer which is not completely inverted will fail the filter ratio test described. Both the un-triggered and triggered states of the TX15834 polymer passed the test (Table 3). The values for a typical HPAM are included as a reference. Table 3. Viscosity and Filter Ratio Data For TX15834 and Typical HPAM TX1567 (1 ppm, SSW, 25 C) Rheology Polymer Viscosity (cp) Filter Ratio (1.2µm) TX15834 (Un-triggered) TX15834 (Triggered) TX1567 (HPAM) Table 3 demonstrates that, in its un-triggered form, the inverted TX15834 Polymer in SSW has low, water-like viscosity. The triggered TX15834 Polymer solution shows 7.8 cp when measured at 25 C and ~1 sec -1 which is a similar viscosity yield obtained from an equivalent HPAM reference (TX1567). Figure 1 compares the rheological profiles of un-triggered Polymer, triggered Polymer and a standard HPAM solution vs. shear rate. Figure 1. Viscosity vs. Shear Rate Profile for Un-Triggered and Triggered TX15834 and Conventional HPAM TX1567

5 It can be seen that the un-triggered Polymer is unaffected by shear. Once triggered, the Polymer backbone is extended and indistinguishable from HPAM and therefore demonstrates the same shearthinning behaviour as HPAM. In addition, the viscosities of triggered solutions of TX15834 Polymer and HPAM across various concentrations correlate well (Figure 2). 6 Viscosity vs. Concentration (SSW, 25 C, 1 s -1 ) Viscosity (cp) Un-triggered TX15834 Triggered TX15834 TX Concentration (ppm) Figure 2. Comparison of TX15834 Polymer and HPAM Viscosity Yields Figure 2 indicates that the viscosity of un-triggered TX15834 Polymer is independent of concentration in SSW. Even concentrated solutions of 1, ppm demonstrate water-like viscosity making it significantly easier to pump than the equivalent concentration of HPAM. Triggered TX15834 and HPAM are chemically indistinguishable. Therefore, in essence, the TX15834 is a tool for in-depth delivery of an un-sheared HPAM. Shear Degradation Tests For any EOR process that requires the use of polymers, knowing the polymer solution shear sensitivity as it travels through the injection lines and into the well completion is necessary for the optimum design of the process. In most cases, the long polymer backbone is susceptible to shear degradation from extensional forces that lower the viscosity yield; to achieve a target reservoir viscosity, a higher concentration of polymer is needed to compensate for the shear losses. A modified API RP 63 method was developed to test the shear sensitivity of the TX15834 Polymer in its un-triggered form and the results were compared against a conventional HPAM baseline (TX1567). The shearing device utilizes a capillary tube with an internal diameter (ID) of 125 µm, through which polymer solution is injected at controlled rates. The apparatus enables a precise measurement of the shear rate to which the polymer is subjected. The level of degradation was measured by the viscosity loss induced by the passage through the capillary tube. A 1, ppm mother solution was prepared and then diluted with SSW to the final concentration of 2 ppm. Having passed through the apparatus, the un-triggered Polymer showed no indication of shear degradation up to a shear rate of 1, sec -1 (Figure 3). In order to confirm the effect of shear on the integrity of the Polymer, the solutions were then triggered by aging overnight at 65 C. After triggering, all solutions exhibited identical viscosity yields regardless of shear rate indicating that the

6 Polymer, TX15834, is shear-resistant in the un-triggered form. By way of comparison, when the same experiment was conducted with a standard HPAM EOR polymer, TX1567, an irreversible 5% loss of viscosity after exposure to 1, sec -1 shear rate was observed. In addition, when the triggered Polymer is tested in this way, it performs similarly to the HPAM because the polymer backbone is extended and exposed. Shear Effects: TX15834 Polymer vs. Standard EOR Polymer 2 ppm Actives in SSW 3 65 C) Viscosity at 25 C and 1 Sec -1 (cp) TX15834 Un-triggered Standard EOR Polymer: 5% reduction in viscosity Shear Rate (Seconds-1) TX15834 triggered: % reduction in viscosity Figure 3. Shear Sensitivity of Un-Triggered TX15834 vs. Standard HPAM EOR Polymer (TX1567) Latex Stability for TX15834 The stability of the TX15834 Polymer has been tested against a conventional HPAM latex polymer. Upon standing, emulsion polymer products can phase-separate/split. This results in a concentration gradient through the emulsion that could lead to variability in the delivered polymer concentration. An oil split study was conducted in which TX15834 and conventional HPAM emulsions were allowed to stand for 6 weeks at various temperatures. The percent oil split (oil layer thickness total emulsion thickness) over time confirmed that the TX15834 emulsion is just as storage stable as an industry standard HPAM through this period (Figure 4).

7 % Oil Split at 25 C 6. % Oil Split TX15834 HPAM Number of days Figure 4. Oil Split Study Comparing Stability of TX15834 and HPAM Emulsions The triggering characteristics and viscosity yield of the TX15834 Polymer are unchanged through at least six monts when the emulsion is stored below 3 C. This evaluation is ongoing and shelf life is expected to be one year at a minimum. Table 4 shows that after nine weeks of storage at 4 C, the polymer still displays a water-like viscosity once made down in sea water, and then reaches a typical HPAM viscosity after triggering. Table 4. Trigger Stability of The Polymer After Product Storage for Nine Weeks at 4 C Made Down Viscosity (cp) of 1 ppm Polymer in Product Storage Time at 4 C C, ULA, 1 s Weeks Un-triggered Triggered Anaerobic Stability of TX15834 Polymer A major risk to successful deployment of a polymer flood is the stability of the polymer in the reservoir. The reservoir is a reducing environment where a polymer could spend a number of years, particularly in an off-shore environment where well spacing is larger. The long term stability of the TX15834 Polymer was studied under reservoir conditions for a period of 15 months, by storing 1 ppm solutions in SSW at 58 C in an anaerobic environment (<5ppb O 2 ) using a Jacomex glove box. The viscosity of the samples was recorded at various time intervals to monitor degradation (Figure 5). The viscosity recorded was the lower Newtonian plateau measured in a shear rate-viscosity sweep on an Anton Paar low shear rheometer (MCR32).

8 1.1 Normalised Newt. Viscosity (58 C, cp) TX15834 SSW 1ppm Figure 5 Anaerobic Stability of TX15834 The TX15834 Polymer demonstrates only ~1% overall loss in viscosity over 15 months at reservoir conditions. This equates to a viscosity decrease of ~.4 cp from the viscosity recorded at seven days. The fluctuation observed over the first few days corresponds to the triggering of the Polymer. This level of viscosity loss is typical for HPAM polymers. It is evident from the modest viscosity loss that an insufficient proportion of the acrylamide groups on the Polymer backbone have been hydrolysed to result in precipitation by coordination of acrylate functional groups and multi-valent cations. Injectivity Test for TX15834 Injectivity test of the TX15834 Polymer in un-triggered an triggered states were carried out using Castlegate core plugs. Core plugs with the properties shown in Table 5 were used. Cores were initially saturated with SSW and characterized prior to use. Table 5 Typical Core Properties Length (cm) Diameter (cm) Cross-sectional Area (cm 2 ) PV (cm 3 ) Porosity Permeability (md) In these experiments, ~1 pore volumes (PV) of polymer solution at 1 ppm was injected into the cores followed by a brine flush for ~1 PV. The following procedure was followed: 1. Synthetic brine was prepared and filtered (SSW) 2. Dimensions and dry weight of the core plug were measured 3. Core was saturated with SSW 4. Saturated core weight was recorded for pore volume and porosity calculations 5. Core was placed in a Hassler-type core holder and flushed with several PV of SSW while applying back pressure (~1 psi) 6. Back pressure was released and differential pressures (DP) over the length of the core at several injection flow rates (ex. Q =.1,.2,.5 and 1 ml/min) were obtained

9 7. Permeability of the core was calculated from the gradient of the linear plot of differential pressure vs. Q 8. Polymer mother solution was prepared in filtered SSW and its viscosity measured 9. Mother solution was diluted to 1 ppm with filtered SSW and its viscosity measured (solution was triggered by aging at temperature for triggered solution injectivity test) 1. The Polymer solution was transferred to a transfer cylinder and 1 PV were injected into the core at.2 ml/min and the pressure drop recorded. The Resistance Factor (RF) value was calculated from the stabilized pressure drop 11. Polymer injection was ceased and the core plug was flooded with several PV of filtered SSW (at least 1 PV) at.2 ml/min and the pressure drop recorded. The Residual Resistance Factor (RRF) value was calculated from the stabilized pressure drop. 12. In all cases the RF of the initial brine injection is 1. RF is defined as the differential pressure of the injected fluid divided by the differential pressure of the initially injected brine. As the RF of the initial brine injection is always 1, it has been omitted from the graphs below. An un-triggered 1ppm solution of TX15834 Polymer not only shows near-water viscosity, but exhibits very low RF (~1.5) and RRF (~1.3) values in the core flood tests relative to an equivalent HPAM polymer (Figure 6). Even after injecting 5 PV of un-triggered solution at 1 ppm, a RF value of ~ 1.6 was obtained. Brine flush also resulted in a RRF value of ~1.35. Triggered 1 ppm solutions of TX15834 show good injectivity and stable pressure drops during both polymer solution injection and the subsequent brine flush in the core flood tests (RF ~12, RRF ~2. See Figure 7). This is consistent with traditional HPAM polymers of equivalent molecular weight, hydrolysis levels and concentration (e.g. TX1567, RF ~11, RRF ~2. See Figure 8). 2 Un-triggered 1 ppm TX15834 Injectivity Test 1.5 RF = 1.5 RRF = 1.3 RF / RRF 1 Polymer Injection Brine Flush.5 i) Total PV Injected

10 Un-triggered 1 ppm TX15834 Extended Injectivity Test 4 3 RF or RRG RRF 2 1 ii) Total PV Injected Figure 6. i) Un-Triggered 1 ppm TX15834 Solution Injectivity Test (Castlegate Core, k=52 md, SSW) ii) Extended Injection (Castlegate Core, k=48 md, SSW) Triggered 1 ppm TX15834 Injectivity Test RF or RRF Polymer Injection RF = RRF = 2 Brine Flush Total PV Injected Figure 7. Triggered 1 ppm TX15834 Solution Injectivity Test (Castlegate Core, k=64 md, SSW)

11 ppm Injectivity Test TX1567 RF = 11 1 RF or RRF 8 6 Polymer Injection 4 2 RRF = 2 Brine Flush Total PV Injected Figure 8. 1 ppm TX1567 Traditional HPAM Injectivity Test (Castlegate Core, K=71 md, SSW) To test the effect of in-situ triggering of the TX15834 Polymer, a core was saturated at 25 C with a 1 ppm solution (RF ~1.5) before being shut-in overnight at 65 C. Figure 9 details the effect of the triggering upon re-starting flow through the core. Upon recommencing flow, a brine flush was performed. The result was a transient RF value (referred to as RFMax) of ~14 and a RRF value of ~2, consistent with flow of HPAM through a core. 1 ppm In-situ Triggering 1ppm TX15834 Injectivity Test Overnight Shut-in and Triggering RF Max = 14 RF or RRF Ovrnight Shut-in and Triggering 4 2 RF = 1.5 Brine Flush RRF = Total PV Injected Figure 9. In-Situ Triggering of 1 ppm TX15834 Solution (Castlegate Core, k=58 md, SSW)

12 Polymer Adsorption Polymer dynamic adsorption experiments were performed using the same equipment and procedure as outlined above for the Injectivity Tests. Effluent analysis allowed the calculation of the level of dynamic adsorption of the un-triggered and triggered Polymer using material balance. Un-triggered TX15834 solution at 1 ppm showed near-zero adsorption density (.4 mg/g). Triggered 1 ppm solution exhibited a dynamic adsorption value of.26 mg/g. Table 6 below summarizes the results compared with reported data in the literature (Willhite, G., and Green, D. 1998). Table 6. Summary of Adsorption Data Polymer Adsorption (mg/g) Un-triggered TX Triggered TX Typical HPAM.1-.1 In-situ Triggering Under Porous Flow Conditions In order to probe the behaviour of the TX15834 Polymer under porous flow conditions a 4-ft sand pack experiment was conducted, the idea being that the residence time of the polymer in the pack would be sufficient to observe development of full viscosity while flowing. Each 5-ft section of Incoloy 825 tubing was individually heat-wrapped to carefully control the triggering point while ensuring that the solution had developed full viscosity by the time it exited the pack (Figure 1). i) Figure 1 i) Schematic of the 4ft Sand Pack; ii) 4ft Sand Pack ii)

13 A 5 ppm solution of the Polymer in SSW was injected into the 1 md sand pack at a rate of.5 ml/min. A 5-bar back pressure regulator was located on end of the pack. Pressure transducers recorded the differential pressure across each section during flow in order to detect progression of the triggering Polymer (Figure 11) Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Section 7 Section 8 Differential Pressure (psi) Pore Volume Injected Figure 11. In-Situ Dynamic Triggering Results from a 4ft Sand Pack (k=1 md) The progression of the Polymer through the pack is observed as the differential pressure rises sequentially through each section. The drop in maximum differential pressure through Sections 1 to 3 indicates that there is no triggering at these temperatures on the time-scale of this experiment and that the heating of the un-triggered solution is causing it to become less viscous. In Section 4 (55 C), triggering is starting, thereby resulting in a higher differential pressure than Section 3, despite an increase in temperature. The increase in the gradient of the differential pressure traces through Sections 4 to 6 indicates that an increasingly viscous solution is entering each section as triggering continues. The gradient of the differential pressure traces for Sections 7 and 8 are very similar, suggesting that triggering is complete and that full viscosity had been reached. This was confirmed by analysing the effluent for viscosity and polymer concentration using a modified Hyamine method (Figure 12). 1.2 Normalised Viscosity / Concentration Newtonian Viscosity (cp) Concentration (ppm) Pore Volumes 4 5 Figure 12. Viscosity and Concentration Analysis of the 4ft Sand Pack Effluent

14 Rheological analysis of the effluent shows that target viscosity is reached and that there is a good correlation between the concentration and viscosity profiles. Results indicate that the TX15834 Polymer triggers under porous flow conditions without issue. Triggering Kinetics The Polymer, as currently formulated, is designed to trigger within two days when made down in SSW and heated at 5 C. The triggering kinetics are influenced by the reservoir temperature, salinity, hardness and ph. The triggering progress curve for the Polymer, as monitored by following the viscosity of a 1 ppm Polymer solution in SSW over time at 5 C, is shown in Figure 13 (Formulation I). An identical solution incubated at 25 C does not develop any additional viscosity during the same period as there is insufficient energy to trigger the polymer. In addition, Figure 13 includes the triggering progress curve for an alternate formulation of the Polymer (Formulation II), specifically designed to trigger more slowly. The chemistry of the Polymer can therefore be tuned in order to control the triggering kinetics and tailor it to the specific conditions of each reservoir. Viscosity at 5 C, 1 s-1 (cp) Formulation I: 25 C Formulation I: 5 C Formulation II: 5 C Ageing Time (Hours) Figure 13. Triggering Progress Curves for the Triggering of 1 ppm Polymer Solutions in Synthetic Sea Water at 5 C PILOT-SCALE STUDIES Shear Degradation Pilot Tests Shear degradation of HPAM during injection has limited the deployment of polymer flooding globally. Flow through restrictions such as chokes, other flow control devices and manifolds is believed to be the greatest source of viscosity loss during a polymer flood. Polymer TX15834 has been demonstrated to be shear-resistant in laboratory tests (see above) but it was deemed necessary to test the stability of the Polymer at scale by injecting it at realistic pressure drops through a scaled subsea choke (1/3 th scale). Un-triggered solutions of 1 ppm and 2 ppm Polymer were prepared in 1m 3 totes. The solutions were injected through a scaled subsea choke over a range of pressure drops between 8 and 7 psi by controlling flow rate 15 l/hr 62 l/hr ( bbl/d) and choke opening (Figure 14). Samples were taken upstream and downstream of the restriction before being triggered and analysed for viscosity (Figure 15), Polymer concentration (via an adapted Hyamine method) and brine composition.

15 Fresh Polymer IBC Tote Upstream Sample Cylinder Downstream Sample Cylinder Used Polymer IBC Tote Scaled Choke Backpressure Valve Pumps Figure 14 Flow Loop Set-Up for the Choke Degradation Study Figure 15. Results of TX15834 Shear Degradation Test Through a Scaled Subsea Choke 1 ppm and 2 ppm solutions show no loss of viscosity over the full pressure drop range tested. All viscosities and concentrations were unchanged (within experimental error). In stark contrast, tests with high molecular weight HPAMs have seen viscosity losses of ~5-7% upon passing through similar restrictions. The shear-resistance of the TX15834 Polymer in its un-triggered form provides it with the ability to deliver the designed viscosity yield deep into the reservoir, beyond these identified regions of highest shear. Inversion Pilot Test Failure to completely invert an emulsion polymer will result in injection problems, either through blocking filters or plugging the sand face. The potential impact upon deployment warranted an indepth inversion study at scale to investigate the envelope of conditions under which the Polymer could be successfully inverted. A series of 24 latex polymer inversion experiments have been conducted with Polymer TX15834 and a conventional latex product, TX1567.

16 The inversion system was designed at 1/1th scale in order to mimic typical surface facility requirements and to enable the identification of the critical operating parameters required to deliver a consistent, homogenous, fully inverted and hydrated Polymer solution. It is common to make down an emulsion in one or two stages. The two-stage process consists of an initial inversion step where the water-in-oil emulsion product is inverted to an oil-in-water emulsion by mixing with the injection brine. This concentrated mother solution is then diluted to the target injection concentration with additional brine in a second step. The two-stage inversion process is typically more reliable than a single-stage inversion process straight to injection concentration, however, a single-stage inversion simplifies the logistics of deployment. In this study both methods were tested. Preparation of the mother solution was simulated at pilot scale using the design detailed in Figure 16, under the range of operating parameters outlined in Table 7. Figure 16 Schematic for the Inversion Pilot Test Make-Down System Table 7 Inversion Pilot Test Conditions Liquid Polyacrylamide Product TX15834 and TX1567 Brine Salinities 2.5wt% and 3.8 wt% Temperature 1 ºC and 6 ºC Flow Velocities 3.1 m/s (low), 9.4 m/s (med) and 12.6 m/s (high) Pressure Drop (across static mixer) 5-12 psi Make-Down Design Two-Stage and One-Stage Pressure drop, and therefore mixing energy, was varied by systematically adjusting flow rate, static mixer length and static mixer diameter. The synthetic brine was heated and pumped through a twoinch hose. The Polymer was injected into the brine upstream of a static mixer. The second stage of the make-down (i.e. dilution and hydration in the injection pipe) was simulated at laboratory scale by collecting samples at different residence times from the two-inch hose, and then mixing them under conditions designed to simulate the full-scale injection pipe. A filter ratio of <1.2 was considered a successful test.

17 The TX15834 Polymer is successfully inverted under a wider range of conditions than the HPAM formulation tested (Table 8). Table 8. Inversion Study Results Brine (x1 3 ppm TDS) Temperature (ºC) Flow Rate HPAM TX High N/A Pass 25 6 Med Pass Pass 25 6 Low Fail Fail Standard Static Mixer 38 6 Med Pass N/A 38 6 High Pass Pass 38 1 Low Fail Pass 38 1 Med Fail Pass 38 1 High N/A Pass No Static Mixer 25 6 Med Pass Pass Half Diameter Static Mixer 25 6 Low N/A Pass Half Length Static Mixer 38 1 Low N/A Fail Single Dilution to 1ppm 25 6 Med Fail Pass 38 6 High Fail Pass The results show that a minimum pressure drop of about 2 psi across the static mixer was adequate to ensure complete inversion and hydration of both latex polymers under most conditions. The TX15834 Polymer outperformed the TX1567 polymer, passing the tests under low temperatures and lower flow rates where the TX1567 failed. This is thought to be a combination of factors including the lower energy requirement to mix a less viscous solution and the use of an effective surfactant package. The TX15834 Polymer was the only product that could be made down directly to a 1 ppm solution, requiring a pressure drop of 5 psi or greater. A simple direct injection of the product, instead of the two-stage make down and dilution, is a more desirable way to apply the product from a design and operational perspective. Conclusions A comprehensive experimental program including laboratory and pilot-scale tests has successfully demonstrated the shear-resistance of the TX15834 Polymer. The mechanisim of shear resistance has the added benefit of lowering the viscosity of injection solutions to near water levels. The ability to inject the Polymer is therefore improved relative to an equivalent HPAM solution. Rheological analysis confirms thermal triggering of the Polymer to achieve the same viscosity as an HPAM polymer of equivalent molecular weight and level of hydrolysis. Sand pack and coreflood tests have shown that the Polymer can be easily injected into, propagated through and triggered within a porous medium, with adsorption levels consistent with HPAM. Proven shear-resistance through a scaled subsea choke and reliable inversion at scale, combined with water-like viscosity in the un-triggered state and confidence in the Polymer s behaviour in porous media means that TX15834 is now the subject of an ongoing inter-well onshore pilot test. Acknowledgments We would like to thank many of our colleagues at TIORCO, Nalco Champion, and Nalco Water for their collaboration and contribution to this project. Also, a special thank you to BP for their input and contributions to validating and de-risking this polymer technology.

18 References API Recommended Practice 63 [199] Recommended Practices for evaluation of polymers used in enhanced oil recovery operations, First edition, June 1, 199, American Petroleum Institute, 122 L Street, Washington DC 25. Caulfield, M.J., Qiao, G.G., and Solomon, D.H. [22] Some Aspects of the Properties and Degradation of Polyacrylamides. Chem. Rev. 12 (9): Gogarty, W. B. [1967] Mobility Control with Polymer Solutions. SPE J. 23 (3): Maerker, J.M. [1975] Shear Degradation of Partially Hydrolyzed Polyacrylamide Solutions. SPE J. 15 (4): SPE-511-PA. Sorbie, K.S. [1991] Mechanical stability of polymers. In Polymer-Improved Oil Recovery, ed. Chap. 4.4, Glasgow, Scotland: Blackie &Sons/CRC Press. Seright, R.S., Adamski, R.P., Roffall, J.C., and Liauh, W.W. [1983] Rheology and mechanical degradation of EOR polymers. Presented at the SPEI British Society of Rheology Conference on Rheology in Crude Oil Production, Imperial College, London, April. Seright, R.S. [1983] The Effects of Mechanical Degradation and Viscoelastic Behavior on Injectivity of Polyacrylamide Solutions. SPE J. 23 (3): Willhite, G., and Green, D. [1998]. Enhanced Oil Recovery. Society of Petroleum Engineers.