Thermally Stable Gels for Near-Wellbore Permeability Contrast Corrections

Abstract

The application of metallic or organic crosslinkers for gelation of polyacrylamides or polysaccharides is limited to relatively low temperatures and hardness levels. These polymers will initially produce good gels under hostile environment conditions (Hardness > 200 ppm, Temperature>167°F (75°C)), but the acrylamide gels undergo substantial syneresis due to the hydrolysis of the amide groups followed by additional crosslinking with the divalent cations present in the brine. Due to chain scission of the inherently weak ether linkages in the polymer structure, polysaccharide gels do not survive extended aging under hostile environment conditions. Copolymerization of acrylamide with other monomers can provide protection against excessive thermal hydrolysis. These acrylamide containing polymers can be crosslinked with mixtures of phenolic compounds and formaldehyde to produce gels which are tolerant to the harsh conditions common to most deeper reservoirs. The gels produced by the reaction of these polymers with phenol and formaldehyde in a brine such as seawater containing 3.4% TDS and 1700 ppm hardness have survived over 2 1/2 years of aging at 235°F (113°C). A similar gel had survived over 9 months of aging at 300°F (149°C) in seawater when the test was terminated. Metallic crosslinkers such as Cr(III) produced in-situ, when used in conjunction with a different thermally stable polymer, resulted in the formation of a stable gel in a brine containing 2.45% TDS and 1, 555 ppm Ca(II). This gel had survived 2 years of aging at 220°F (104°C) before the test was terminated. A similar gel made with polyacrylamide began to synerese within a month and shrank more than 80% in less than three months of aging at 200°F (93°C). A producing oil well with a bottomhole temperature of 230°F (110°C), treated with organic crosslinking of a thermally stable polymer, has produced over 33, 000 bbls of incremental oil in 54 months since the treatment.