Conditioning Polymer Solutions for Injection into Tight Reservoir Rocks

Abstract

Water soluble polymers have been used for decades as mobility control agents for tertiary recovery processes. Viscosity is conferred by the large hydrated size of the individual high molecular weight polymer molecules; their single-molecule hydrated size is so large that it can rival the diameters of the pore throats conducting the fluid, and it is widely understood that there are permeability limits below which solutions of such polymers cannot transport well. Delineating exactly where these limits are remains challenging, and operators are left to use whatever anecdotal evidence is available to decide whether to inject polymer, and, if so, what type and molecular weight to use. A rule of thumb is that when the permeability of a rock falls below 100 millidarcys, transport can be problematic. We have developed processing techniques for laboratory tests to condition polymer solutions for injection into reservoir carbonate cores with permeabilities below 10 millidarcys and median pore radii below one micron. Shearing and tight filtration were used to reduce the maximum size of polymers in solution while retaining as much viscosity as possible. Subsequent filtration was used to quantitatively assess the plugging behavior of the product solution across a range of pore sizes smaller than those which conduct in the rock sample. Coreflood injectivity tests revealed the onset of face plugging as a function of average polymer size. Co-solvent was shown to dramatically improve the transport of sulfonated polyacrylamides when face plugging did not occur, and those improvements were mirrored in benchtop filtration data. This improvement came despite equal-or-better viscosity in the polymer solution, demonstrating that the co-solvent did not reduce the polymer's hydrated size and therefore most likely weakens inter-molecular associations in solution. In sum, the data indicate that permeability loss occurred by two mechanisms: simple mechanical plugging and progressive adsorption, likely mediated by inter-molecular entanglements. These two permeability reduction mechanisms should be rectified by different means.

Flow Enhancement of Water-Soluble Polymers through Porous Media by Preshearing

“…decrease in viscosity in shear flow fields) has also been reported in several studies. [50][51][52][53][54]  Further studies are needed for determining the precise relationship between the shear and extensional flows as a result of pre-shearing. Moreover, direct measurements of MWD can better illustrate the concept of flow enhancement of water-soluble polymers in porous media.…”

Updated Perceptions on Polymer-Based Enhanced Oil Recovery toward High-Temperature High-Salinity Tolerance for Successful Field Applications in Carbonate Reservoirs

“…When particularly harsh conditions are present, however, NVP is essential in the case of SAV333, as previously noted. It is also worth noting that increased sulfonation levels cause more pore-plugging issues [ 173 ].…”

“…There are a few noteworthy recommendations that will aid operators in the appropriate design and planning of polymer-based EOR projects in carbonate reservoirs under harsh conditions, such as high temperature, high salinity, and low permeability. Several recent screening studies and core-flooding tests have led to these recommendations [ 42 , 62 , 65 , 67 , 70 , 71 , 88 , 119 , 120 , 128 , 129 , 150 , 155 , 162 , 163 , 164 , 169 , 170 , 171 , 172 , 173 , 175 , 176 ], along with the simulation runs and field pilots [ 42 , 44 , 78 , 88 , 104 , 169 , 176 , 202 , 203 , 204 , 205 , 206 , 207 , 212 ] in carbonate rocks. These following recommendations will aid in improved designs for field implementation and are intended to possibly resolve the challenges of high temperature, high salinity/hardness, and injectivity issues in low-permeability carbonate formations.…”

“…Although rather expensive, co/ter-polymers containing ATBS and NVP have high thermal stability of up to 140 C in saline solutions and easily last one year at this temperature (Gaillard et al, 2015; Dupuis et al, 2017). However, higher sulfonation levels can lead to increased pore plugging issues [ 173 ]. Furthermore, there is found to be great potential in thermo-viscosifying polymers (TVPs) and these would be considerably cheaper if applied, but more tests are required to ascertain the full capacities of these polymers, since these are only partially developed [ 71 , 163 , 164 ].…”

“…Pre-shearing polymer solutions may prove to be an effective method, facilitating the solution to retain its apparent viscosity during injection as well as abetting adequate injectivity. The procedure of performing pre-shearing on sulfonated polymers is detailed by Driver et al [ 173 ], which aims to achieve injectivity in very narrow/tight (5-10 mD) carbonate formations. It was demonstrated by them that the conditioning of polymer solutions using the pre-shearing method, tight filtration, and using co-solvents in the case of highly sulfonated polymers would result in very good injectivity [ 173 ].…”