Mitigating Explosion Risks in High Pressure Air Injection Compressors

Abstract

Abstract This paper describes research undertaken by Encore Acquisition Company and the University of Calgary regarding the flammability safety of synthetic lubricants used in Encore's compressors in their high pressure air injection (HPAI) projects in Southeastern Montana. With over 2,270 e3m3(ST)/d (80 MMscfd) of installed air compression capacity discharging at pressures of 31.0 to 34.5 MPa (4,500 to 5,000 psi), a critical aspect of this project is the safe and uninterrupted operation of the compressors. Experience gained by Encore and other HPAI operators shows that the reaction of high pressure air with compressor lubricants in high temperature interstage and discharge regions of the compressors can be a source of trouble (destructive overpressures), even when synthetic diester-based lubricants are employed. Using an Accelerating Rate Calorimeter (ARC), samples of fresh and used synthetic lubricants were heated in the presence of air at initial pressures up to 34.5 MPa (5,000 psi). Self-heating rates and pressure responses were measured. The results highlighted the significant effect of pressure on auto-ignition temperature. Most significantly, the auto-ignition temperature of the diester-based lubricant dropped from the manufacturer's reported level of 410 °C (770 °F) at atmospheric pressure to 180 °C (365 °F) at pressures in the range of 17.2 to 34.5 MPa (2,500 to 5,000 psi). Also, the auto-ignition temperature of used (oxidized) synthetic lubricant was further reduced to values close to the operating temperature levels of the compressors. Finally, it was noted that the auto-ignition temperatures for different brands of diester-based lubricants were all very similar. The significance of this study is not only in the temperature data, but also in the discussion of several significant changes that Encore made to the design and operation of their high pressure air compressors as a result of this study. This information will assist future HPAI operators in designing safe and reliable air compression systems. Introduction Improved oil recovery of conventional light oils by high pressure air injection (HPAI) is becoming a well known process. With increasing oil demand and dwindling primary and secondary production-based reserves, more producers are showing increasing interest in HPAI. Typical examples of such increasing interest include Encore's eight injector 17 e3m3(ST)/d (6 MMscfd) HPAI project initiated in 2002 in their Pennel Unit that has since expanded to 850 e3m3(ST)/d (30 MMscfd) with an ultimate design of 1,700 e3m3(ST)/d (60 MMscfd). Air has been continually injected at 34.5 MPa (5,000 psi) for four years in the original portion of the flood. In addition, a new 566 e3m3(ST)/d (20 MMscfd) HPAI project was initiated in the Cedar Creek-Little Beaver East Field located on the Montana/North Dakota border in 2004. This project has 28 injection wells and is operated at 31.0 MPa (4,500 psi). Figure 1 is a map indicating Encore's presence in the Cedar Creek Anticline [over 160 km (100 miles) long] showing the HPAI and waterflood projects sites. Safe and uninterrupted compressor operation is the most important factor in the successful implementation of every HPAI project. To improve operational safety and reliability, ester-based synthetic lubricating oils are widely used in HPAI compressors.