Delayed Crosslink Polymer Flowing Gel System for Water Shutoff in Conventional and Unconventional Oil and Gas Reservoirs

Abstract

Abstract A new polymer based gel system has been developed to address the excessive water production problem in fractured unconventional gas wells. Currently available polymer based water shut-off agents are unsuitable for treating high temperature hydraulically-fractured tight gas and shale reservoirs, where some fractures connect to water rich zones. The new gel developed is a low-concentration, low-viscosity delayed-crosslink polymeric gel system and is a significant improvement over traditional flowing gels used for fracture water shutoff in conventional reservoirs. The gel uses high molecular weight hydrolyzed polyacrylamide (HPAM) at low concentrations with a delayed organic crosslinker that is more environmentally benign, provides much longer gelation time (up to several days at temperatures well above 100 °C) and stronger final gels than comparable polymer loadings with chromium carboxylate crosslinkers. Results indicate that gelant with a few tens of centipoise viscosity can have gelation delayed to 12 hours or longer at temperatures of 100 °C and higher. Gels prepared with 4000 to 7000 ppm of HPAM and Polyethylenimine (PEI) were significantly stronger than those prepared with the Chromium(III) Acetate crosslinker for the same HPAM concentrations. This new gel system allows low-pressure extrusion of gelant into narrow-aperture fractures. The system is especially promising for deeper, hotter formations where rapid pressure buildup or gel instability prevents the use of current flowing gel systems. The gelant can be pumped with low pressures due to low concentration of polymer and delayed gelation to effectively seal problem water zones thereby reducing operational costs and increasing recovery. By impeding water production, the gel system developed here can be used to delay water loading and subsequent premature abandonment (or installation of expensive equipment), thereby extending life and reserves of unconventional gas wells. Potential applications include the Barnett Shale, where 15 percent of wells produce more water than injected during drilling and stimulation, presumably due to hydraulic fracture growth into underlying water zones.