Investigation of a High Temperature Organic Water Shutoff Gel: Reaction Mechanisms

Abstract

Abstract Water production during oil and gas recovery is a longstanding problem that is becoming critical with maturing fields worldwide. Lifting, processing, treating and re-injection of the unwanted water add to the overall oil production costs. Also, water disposal may pose environmental problems. Recent statistical studies indicate that processing unwanted water costs the oil industry nearly 40 billion dollars per year. Polymer gels have been widely used as blocking agents for excessive water production. In this study, a copolymer of polyacrylamide tert-butyl acrylate (PAtBA) and polyacrylamide (PAM) crosslinked with polyethyleneimine (PEI) have been investigated. This PAtBA/PEI system was previously shown to be stable at temperatures up to 160ºC, typical of those encountered in deep oil and gas reservoirs. However, the crosslinking mechanisms of this system at high temperatures have not been well defined. This study examined the structural changes of PAtBA and PAM using C-13 Nuclear Magnetic Resonance (NMR) spectroscopy. Understanding these changes is a first step towards the identification of the crosslinking mechanisms of PAtBA and PAM with PEI. This will have a strong impact on the design of water shut-off treatments utilizing these systems. Introduction As oil and gas fields mature, larger volumes of water are produced. Separating, treating and disposing this water add extra costs to the petroleum production. It has been reported that the petroleum industry spends several tenths of billions of dollars to deal with excessive water production.[1] Hydrophilic polymer gels have been widely used to reduce[2] or completely block[3] water from its producing zones. Polyacrylamides have been the most commonly used base polymers crosslinked with either inorganic or organic crosslinkers. Inorganic crosslinkers include Cr+3, Al+3 and Zr+4 and have been mostly utilized to crosslink partially hydrolyzed polyacrylamide (HPAM). Inorganically crosslinked gels result from the ionic bonding between the negatively charged carboxylate groups and the multivalent cation.[4–6] However, these ionic bonds break at temperatures higher than 75ºC and therefore ionic gels are thermally unstable.[7] Organic crosslinkers were introduced to obtain gels that are stable over a wider temperature range.[8–10] This is possible because in this case the crosslinking is done via a covalent bonding, which is much more stable than ionic type of bonds. The covalent bonds often involve the amide groups on the polymer backbone. A typical example of an organically crosslinked gel is the polyacrylamide-phenol/formaldehyde system, which has been reported to be stable at 121ºC for 13.3 years.[11] However, its toxicity has limited its broad use in the field. Chemical alternatives for the phenol/formaldehyde system were also reported.[12,13] A gel system which is stable at temperatures up to 160ºC was introduced.[14,15] This gel is based on the crosslinking of polyacrylamide/tert-butyl acrylate (PAtBA) copolymer with polyethyleneimine (PEI). It was first applied in a carbonate reservoir at nearly 130ºC and in sandstone reservoirs at 75 and 82ºC.[10,16] Rheological studies concerning the gelation kinetics,[10,17] viscoelastic properties of the final gel18 and gel strength in porous media[19] of the above system have also been reported. The performance of this system in porous media was also examined under field conditions.[15,20–22] The gelation mechanism proposed by Hardy et al.[10] involves the formation of covalent bonds between the carbonyl carbon at the ester group and imine nitrogen from PEI (Fig. 1). Reddy et al.[23] proposed a second mechanism (Fig. 2), where the PEI nitrogens form covalent bonds with the carbonyl carbon at the amide group of PAtBA through a transamidation reaction. In both cases the hydrolysis of the polymer is believed to play a key role in the gelation process, but detailed studies identifying either mechanism are lacking.