Cold Finger Method Effective at Testing Paraffin Inhibitors
The deposition of paraffin with a high molecular weight in crude oil is difficult and often expensive to treat using chemical methods. As owners and operators look to reduce flow assurance costs, the development of inhibitors of paraffin deposition has become a greater priority, an expert said.
Speaking at the 2016 Gulf of Mexico Deepwater Technical Symposium in New Orleans, Gee Williams, a manager of business development at Halliburton, discussed a strategy the company used to test the effectiveness of paraffin inhibitors, as well as the results of the company’s test on a sample of condensate from the Eagle Ford shale formation.
A typical paraffin field test involves three stages: a test for wax solubility, a test for wax deposition, and the analysis of the wax deposits. Williams said cross-polarized microscopy is the best testing method in deep water for wax solubility in crude oil.
Flow loop testing and cold finger testing are two common methodologies for evaluating wax deposition. In a flow loop test, an oil sample containing paraffin inhibitors circulates through a loop via a recirculation pump. A cooling bath gradually lowers the sample’s temperature until it reaches the wax appearance temperature (WAT). In a cold finger test, heated oil stirs around a cooled metal finger that simulates a pipeline’s inner wall, leaving wax deposits after the finger’s temperature falls below the wax appearance temperature.
Williams said that while a flow loop test is good for simulating field conditions, it can be a cumbersome process because of a bulky, expensive setup that requires significant amounts of oil. Because of that, he said companies typically run cold finger tests despite their inability to adequately depict real-world field conditions.
“[Cold finger] has its limitations. We’re not really able to simulate real-world field conditions, but it does allow us to run through various iterations very quickly,” Williams said.
Halliburton used a cold finger methodology to test the efficiency of six paraffin inhibitors on a sample of condensate from the Eagle Ford. In the test, four inhibitors showed a reduction in efficiency on paraffins with an increased molecular weight, while two inhibitors showed more than 50% inhibition.
Williams said the type of wax deposits obtained during a cold finger test can be altered by manipulating the differential between the wax appearance temperature and the cooled metal finger. A smaller temperature differential will result in a wax chain with a higher molecular weight, and as the differential increases, the molecular weight of the wax chain decreases. Because it wanted to test paraffin inhibitors against the high-molecular-weight deposits, Williams said Halliburton sought to maintain a low temperature differential in the cold finger test.
“Our objective is really to target more of these chains. Maybe we get a little bit of the midrange, but we want to focus on this high-molecular-weight paraffin because that is the problematic paraffin,” Williams said.
A low temperature differential in a cold finger test often leads to small amounts of wax deposited on the cold finger apparatus, so one of the challenges in running a test under such conditions is obtaining a sufficient amount of paraffin to run a gas chromatograph analysis. This can be done be altering the stirring speed of the cold finger. Williams said that while a test cannot replicate the shear one might see in a pipeline system, it can be manipulated enough to increase the paraffin deposition number.
For Further Reading
SPE 174817 Study on Inhibition of High-Molecular-Weight Paraffins for South Eagle Ford Condensate by K. Gawas, P. Krishnamurthy, F. Wei et al., Halliburton.
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07 May 2018