Rupture of Polyacrylamide Gel in a Tube in Response to Aqueous Pressure Gradients

Hydro‐gel and the evolution of its strength as the gel matures from the state of aqueous fluid to the state of viscoelastic solid provides opportunity to employ hydro‐gel in certain critical applications, such as sealing of fractures or high permeability streaks in a hydrocarbon reservoir. The mature gel, inside the channel, helps in diverting the injected water into an un‐swept oil bearing zone. For the gel treatment to be effective, the gel layer inside the channel has to resist failure under water pressure. This article describes evaluation of the strength of Cr(III)–partially hydrolyzed polyacrylamide gel, formed in transparent tubes. A pressure gradient was imposed on the gel by injecting water on one end of the tube, leaving the other end open to atmosphere. Pressure gradient that increased with time caused shear deformation, and bulging of gel at the outlet end of the tube. At a threshold pressure, the deformation culminated to a rupture at the center of the gel, and subsequent burst of water through the outlet. Rupture was visualized using colored water that fingered through the center of the gel, leaving the rest of the gel intact. Rupture pressure, similar to the ultimate strength of a material, is defined for the gel layer. The rupture pressure was found to increase with the length, and decrease with the width of aperture. A theoretical model in support of this trend is presented here. When the pressure was built up fast, the rupture took place at a much smaller pressure. In case of pressure build‐up over a long period of time, rupture pressure became independent of the duration of build‐up. The effect of the rate of pressure build‐up is analyzed using the concept of viscoelasticity. Also, the rupture model is extended to the situations where the porous media hinder the free movement of gel at the outlet, and allow partial movement of gel laterally through the surrounding wall.