Understanding the Plugging Performance of HPAM-Cr(III) Polymer Gel for CO2 Conformance Control

With the demand for conformance control in CO₂ flooding fields, the traditional HPAM-Cr(III) polymer gel has been applied to reduce excess CO₂ production. However, the field application results are mixed. In order to understand the HPAM-Cr(III) polymer gel plugging performance in reservoirs, a numerical reservoir analysis is used to interpret the laboratory results and analyze how the laboratory results are connected to the field application results.

The HPAM-Cr(III) polymer gel plugging performance was evaluated in the laboratory with sandstone core plugs. CO₂ and water effective permeability was measured. Gelant was injected into the cores until the injection pressure stabilized. Breakthrough pressure was tested and followed by two WAG cycles injection. The injection pressure was monitored. Different permeability cores (100- 1200 md) were used to mimic different reservoir conditions. Residual resistance factor was calculated using the stabilized water/CO₂ injection pressure before and after gel treatment.

HPAM-Cr(III) polymer gel has higher breakthrough pressure in the low permeability cores. The polymer gel behavior is discussed using two scenarios: no breakthrough and with breakthrough. If no breakthrough, the numerical reservoir analysis reveals that the polymer gel can successfully divert the CO₂ and water flow to the low permeability layers before breakthrough. The damage to polymer gel is caused by CO₂ diffusion from the low permeability layers. The gel damage rate depends on the diffusion rate. This is the reason that polymer gel treatment can last for a longer time in some fields, In another scenario, CO₂ breakthrough the gel treated formation, CO₂ will penetrate to the depth at where the reservoir pressure gradient in low permeability layer is higher than injection pressure gradient. After this point, CO₂ will be diverted to low permeability layers. In this case, polymer gel has a larger contact area with CO₂ and the gel damage rate will be faster.