Summary
Water production during oil and gas recovery is a longstanding problem that is becoming critical with maturing fields worldwide. Lifting, processing, treating, and reinjection 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 U.S. $40 billion per year.
Polymer gels have been widely used as blocking agents for excessive water production. In this study, two different polymers were crosslinked with polyethyleneimine (PEI). The first is a copolymer of polyacrylamide tert-butyl acrylate (PAtBA), and the second is a polyacrylamide (PAM). The PAtBA/PEI system was previously shown to be stable at temperatures up to 160°C, 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 using C-13 nuclear magnetic resonance (NMR) spectroscopy. Understanding these changes is a first step toward the identification of the crosslinking mechanisms of PAtBA and PAM with PEI. This will have a strong impact on the design of water shutoff 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 tens of billions of dollars to deal with excessive water production (Bailey et al. 2000).
Hydrophilic polymer gels have been widely used to reduce (Zaitoun and Kohler 1988) or completely block (Hutchins et al. 1996.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 (Prud'homme et al. 1983; Lockhart 1994; te Nijenhuis et al. 2003).
Organic crosslinkers were introduced to obtain gels that are stable over a wider temperature range (Moradi-Araghi 1991; Albonico et al. 1994; Hardy et al. 1999). This is possible because in this case, the crosslinking is done by a covalent bonding, which is much more stable than ionic 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 121 DEGREE C for 13.3 years (Moradi-Araghi 2000, 1993). However, its toxicity has limited its broad use in the field. Chemical alternatives for the phenol/formaldehyde system were also reported (Moradi-Araghi 1994; Dovan et al. 1997).
Overview of Water Shutoff Operations in Oil and Gas Wells; Chemical and Mechanical Solutions