Further Insights into the Mechanism of Disproportionate Permeability Reduction

Abstract

Disproportionate permeability reduction (DPR) is a phenomenon whereby many water-soluble polymer solution and polymer gels reduce the permeability to water flow more than to oil or gas flow. DPR is important for some gels that are applied to production wells in water-shutoff treatments. The mechanisms of DPR need more investigation and clarification. In this study, experiments on two different scales, core- and micro-model-scale, were conducted to study the mechanism of DPR in flow through permeable medium treated with Cr(III)-acetate-HPAM gels. The procedures for the two experiments can be similarly divided into five phases: gelant injection and gelation, oil flooding, gel rehydration with no oil phase pressure difference, oil re-flooding, and water flooding. In the core-scale experiments, we used nuclear magnetic resonance (NMR) to monitor the T2 signal change during fluorine-tagged oil and water flooding after the gel treatment. NMR can detect the signal change of trapped and free water inside the gel in different pore sizes in porous medium. In the micro-model-scale experiments, a microscopic glass-etched model is subject to red oil and blue water flooding alternately after the gel treatment. Oil extrusion under an oil phase pressure gradient and the evolution of oil flow channel are recorded by a video sensor. The subsequent water flooding is also investigated. Results show that a gel-displacement mechanism is a primary reason for the development of the oil flow path initially. As the displacement proceeds, the gel dehydration occurs induced by the oil phase pressure and therefore the flow channel continues forming, but no gel is produced during this phase. During gel rehydration, the flow channel is blocked, which can be inferred from the T2 spectrum and visual microscope image. However, the rehydrated gel can only partially reduce permeability and oil pathways re-establish easily with the subsequent oil flooding. In water flooding, water permeability decreases abruptly. The mechanisms for the disproportionate permeability reduction involve channel segregation, gel rehydration, residual oil effects, and the low permeability of gel relative to water.

A Multiscale Investigation of Cross-Linked Polymer Gel Injection in Sandstone Gas Reservoirs: Implications for Water Shutoff Treatment

“…Zhang et al (2016) suggest that as gas saturation decreases, the potential of the gel to limit water relative permeability sharply decreases. 77 Moreover, recent investigations 33,35,36,78,79 state that significant trapping of residual nonwetting saturation due to RPM treatment results in a greater restriction to water (wetting) flow and eventually significantly increases Frr w Considering all above, we conclude that the reduction in Frr w with increasing water flow rates is due to both the deformation of the gel layer and decreasing S gr .…”

“…These observations are considerably different from those reported previously, where the gel formed only in the edges and crevices of grains. 23,31,35 Most likely, in these previous studies, gel layer formation was not observed as the layer was too thin and totally transparent (but still existed, see above).…”

“…The rheological behavior of the P­(AAM- co -AA)­Na–chromium cross-linked solution was examined. It has been reported that high-salinity brine and high temperature may affect the behavior of such gels. , However, Karimi et al (2014) and Zhu et al (2017) report that chromium cross-linked polyacrylamide gels can be utilized in the reservoirs with high temperature (≈127 °C) and salinity. , In addition, to make such polymer gel systems more tolerant to thermal and salinity effects, it is suggested to be combined with nanoparticle technology or some additives. , It can also improve the gels’ rheological properties and lubricity for different applications, making the gels more environmentally friendly by reducing its toxicity. ,, Moreover, cost-effective polyacrylamide and chemical additives such as chromium­(III) acetate have been successfully applied in lab-scale and reservoir-scale sandstone oil/gas-field operations. ,,,,− ,− …”

“…Furthermore, it was previously shown that the efficiency of a water shutoff treatment strongly depends on the gel quality (i.e., the gel’s rigidity and stability). − Moreover, while there have been recent insights into the mechanism of gel deformation and the associated disproportionate permeability reduction (DPR), ,− further scientific understanding of gel behavior in relation to injection rates is required to guarantee successful gel placement in gas wells. The previous literature works mostly focused on the use of gels to mitigate water production in oil wells.…”

Rock/Fluid/Polymer Interaction Mechanisms: Implications for Water Shut-off Treatment

“…24,41,44,152,153,157 Similarly, these roles would reverse if the pore surface was oil/ gas wet. 24,44 Elsewhere in the literature, this mechanism is referred to as segregated pathways, 24,28,29,44,46,152,153,157,162,163 fl u i d p a r t i t i o n i n g , 4 1 , 1 6 0 a n d fl u i d d i s t r i b ution. 22,23,110,111,119,151,164 For consistency, the term "fluid segregated pathways" is used in this review.…”