Polymer Retention Determination in Porous Media for Polymer Flooding in Unconsolidated Reservoir

Abstract

Polymer flooding is a well-established technique aimed at improved recovery factors from oilfields. Among the important parameters affecting the feasibility of a large deployment, polymer retention is one of the most critical since it directly impacts the oil bank delay and therefore the final economics of the project. This paper describes the work performed for the East-Messoyakhskoe oilfield located in Northern Siberia (Russia). A literature review was first performed to select the most appropriate methodology to assess polymer retention in unconsolidated cores at residual oil saturation. 4 polyacrylamide polymers were selected with molecular weights between 7 and 18 M Da and sulfonated monomer (ATBS) content between 0 and 5% molar. An improved 2-fronts dynamic retention method along with total organic carbon-total nitrogen analyzers were used for concentration measurement. Retention values vary between 93 and 444 The sentence could be rephrased μg/g, with the lowest given by the polymers containing ATBS, corroborating other publications on the topic. This paper also summarizes the main learnings gathered during the adaptation of laboratory procedures and paves the way for a faster and more efficient retention estimation for unconsolidated reservoirs.

Keywords: adsorption; non-absorbable tracers; polymer dynamic retention; polymer flooding; unconsolidated core; viscous oil.

Figure 1

Examples of calculations of oil bank delay vs. retention and polymer concentrations values.

Figure 2

Example of retention calculation using the 2-fronts method. The first figure shows the overall principle of the method and how to calculate inaccessible pore volume. Once the polymer curves are normalized (C/C₀ which is the ratio of measured concentration and initial concentration), the retention is calculated by considering the area between both curves (first & second fronts). The value is taken at the 0.5 cutoff for the ratio C_eff/C_i, being concentration in the effluents and initial concentration respectively.

Figure 3

Single front method. Polymer and tracer are injected together and then flushed with water. The concentrations are analyzed during the process to determine the delay between tracer and polymer.

Figure 4

Hydraulic diagram of the Tech-ViP unit used for testing polymer compositions.

Figure 5

Electrochemical detector signal vs polymer A concentration, graduation dependence.

Figure 6

Pressure drops and polymer concentration changes during experiment No. 6 (core 1229 md, 0.01 Pa·s at 7.3 s⁻¹ shear rate) as a function of pore volume injected (PV). A bump is noticed for pressure during the first polymer front (orange curve with the cross symbols or Grad center) which corresponds to a little volume of oil displaced by polymer, considered for the rest of the study.

Figure 7

Hall plot for the entire core model at equal viscosities of the polymer solution (10 cP at 7.3 s⁻¹) during experiments No. 2 and 6, upper and lower curves respectively. P stands for pressure while dt stands for time increment.

Figure 8

Normalized concentration (C_eff/C_i) of tracers and polymer in successive samples of the 6th filtration experiment. C_eff represents the measured concentration in the effluents at each sampling interval (each dot on the curves) and C_i represents the concentration of the fresh solution before injection.