Preconditioning Concepts in Polymer Flooding in High-Salinity Reservoirs: Laboratory Investigations and Case Histories
- Wolfgang O. Sohn (Edeleanu GmbH) | Balram K. Maitin (RWE-DEA A.G.) | Volz Hartwig (RWE-DEA A.G.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1990
- Document Type
- Journal Paper
- 503 - 507
- 1990. Society of Petroleum Engineers
- 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.4.1 Waterflooding, 5.2 Reservoir Fluid Dynamics, 2.4.3 Sand/Solids Control, 5.6.5 Tracers, 5.3.2 Multiphase Flow, 6.5.2 Water use, produced water discharge and disposal, 5.7.2 Recovery Factors, 5.5 Reservoir Simulation
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In polymer-flood field projects with partially hydrolized polyacrylamide (PH PAA) solutions, Deutsche Texaco A.G. (DTA) has applied two methods of preconditioning: a preflush with fresh water and the use of a relatively small slug of a less-salt-sensitive polymer. Results of laboratory work that led to an improved preconditioning concept with polymer are described. Case histories of two projects with two different preconditioning processes are presented and discussed in detail.
For a certain set of reservoir conditions, polymer flooding is one of the most promising EOR methods for sandstone reservoirs and has frequently been applied in onshore operations. In this process, water-soluble polymers increase the viscosity of the flood water and thus improve the water/oil mobility ratio. The incremental oil production is a result of improved vertical and areal sweep efficiency. The production profile is favorable because of accelerated recovery.
Since 1975, DTA has implemented a number of commercial polymer-flood projects applying polyacrylamides. These projects are located in the Gifhorn trough, one of the major oil-producing areas in West Germany.1-3 The sandstone reservoirs involved have permeabilities of 600 to 4,000 md, temperatures of about 140°F [60°C], and oil viscosities of 6 to 17 cp [6 to 17 mPa·s]. At the beginning of polymer-flood activities, the reservoir brine salinities were 40,000 to 170,000 ppm, depending on the type of water injected over the last 25 to 30 years of production history. The six operational projects with polyacrylamides have produced about 4.4 million bbl [700 000 m3] of enhanced oil. Polymer floods have raised oil recovery by 15% of the original oil in place (OOIP).
Polymer Selection and Flood Concept
The polymers for flooding are basically polyacrylamides (frequently partially hydrolized), biopolysaccharides (xanthan and scleroglucan), and hydroxyethylcellulose (HEC).
The chemical costs, based on market prices in Europe, of polymer solutions under standardized test conditions are listed in Table 1,4 which shows the polymer-solution costs for the concentration required to obtain an apparent viscosity of 10 cp [10 mPa·s] at 140°F [60°C] and a shear rate of 1 second-1. The costs, lowest for chemical solutions of PH PAA in fresh water, are between $0.20 and $O.30/bbl [$1.26 and $1.89/m3]. On the basis of these economic considerations, DTA has decided to use polyacrylamide for all polymer projects performed so far. This meant, however, that the salt-sensitive polymer had to be not only dissolved in fresh water, but also protected from high-salinity reservoir brine.
The standard procedure of preconditioning the reservoir is to flush it with a sufficiently large quantity offresh water. This process entails the following problems.
1. The low-viscosity fresh water follows paths of highest permeability and leaves parts of the reservoir uncontacted. Most of the residual oil may be contacted later by the high-viscosity polymer slug.
2. Preconditioning takes a long injection time with increasing operational costs, which makes the project less attractive economically.
3. A late start of polymer injection adversely affects ultimate oil recovery because, with higher water saturations, the injected polymer is increasingly consumed for water displacement.
For these reasons, a preconditioning process was developed in which the freshwater preflush is replaced by a small slug of salt-insensitive or less-salt-sensitive polymer.5 This procedure has the following advantages.
1. The small, salt-insensitive polymer preflush is part of the total EOR polymer slug.
2. Protection of the salt-sensitive PH PAA can be achieved with a preflush of just 10 to 15% of the total slug.
3. Both preconditioning time and total project life are shortened considerably.
4. The inner surface of the near-wellbore area will be saturated with adsorbed polymer of the preflush phase, so the main polymer slug does not lose viscosity by further adsorption.
To compare the effects of preflushing with fresh water and with a polymer solution in a laboratory experiment, a plexiglass model (37.8 in. [96 cm] long×0.75 in. [1.9 cm] thick×3.15 in. [8 cm] high) filled with synthetic sand (k=4,000 md) was flushed with Dogger brine [175,000 ppm total dissolved. solids (TDS)]. The Dogger brine was displaced by fresh water that contained a dye material as tracer, and the flood front was measured optically. As Fig. 1 shows, there was a strong overriding of the low-density fresh water over the high-density brine. After injection of 1 PV, a significant portion of the brine remained undisplaced.
Conversely, when the same experiment was made with fresh water thickened with 750 ppm of a xanthan-type polysaccharide, a defined vertical flood front and a much more piston-like displacement of the brine was observed. A polysaccharide solution slug size much smaller than that of the fresh water is needed to precondition a reservoir effectively.
Because nonplugging xanthans were not commercially available in Europe in 1979, the field projects were performed with a low-hydrolysis polyacrylamide (LH PAA) (10 mol% of acrylate) as preflush polymer. It is well-known that polyacrylamides with a low degree of hydrolysis are less susceptible to flocculation by divalent ions.6,7 Fig. 2 shows the viscosity yield of the LH PAA as a function of salinity compared with polyacrylamide (PH PAA) with a higher degree of hydrolysis (30 mol % of acrylate). Both polymers are "emulsion polymers" with an active matter content of about 30% and a polymer molecular weight above 6 million g/mol. The apparent viscosity of LH PAA is also influenced to a certain degree by the solution salinity, but under the conditions of 140°F [60°C] and 10 seconds-1, the apparent viscosity is still about 3 cp [3 mPa·s] - i.e., five to six times the brine viscosity. For solutions in fresh water, on the other hand, a higher concentration of LH PAA is needed to obtain viscosities comparable with PH PAA solutions.
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