Measurement of Waxy Crude Properties Using Novel Laboratory Techniques

Kunal Karan, John Ratulowski, Philip German
2000 SPE Annual Technical Conference and Exhibition   unpublished
TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract Transportation and production of waxy crude oils poses a variety of challenges including wax deposition, increased viscosity and pressure drop at temperatures below the cloud point, and high re-start pressure for gelled oil in the pipeline at temperatures below the pour point. The driving force for this study is an investigation of live oil transportation through a 600 km long sub-sea pipeline where ambient temperatures range from 0 to 5°C
more » ... ge from 0 to 5°C and pressures up to 3000 psia. The design and operation of such a system warrants unique set of input data. Measurement of such data involves the development of novel experimental techniques. In this paper, newly developed experimental techniques and apparatus used for measuring key waxy crude properties is presented. A novel experimental apparatus was used to visually determine live oil pour points at pressures up to 3000 psi. The gel strength measurements for pipeline restart pressures for both dead and live oils were conducted using a model pipeline test (MPT) apparatus. Measurements of the live oil wax appearance temperature (WAT) were performed using a state-of-the-art High Pressure Cross Polar Microscope (HP-CPM) and a laser-based solids detection system. 2 KUNAL KARAN, JOHN RATULOWSKI AND PHIL GERMAN SPE 62945 − a region defined by temperatures below the PP where the fluid exhibits highly non-Newtonian behavior and where oil may gel under quiescent conditions;. − a region of mildly non-Newtonian behavior defined by the temperatures between the WAT and the PP. This study focuses primarily on the laboratory techniques for the measurement of waxy crude properties, namely, WAT, PP and gel strength. Further, it is demonstrated why tests under realistic conditions, i.e. for live oils, need to be conducted for improved estimates. Wax Appearance Temperature When a waxy crude is cooled, at the WAT the wax begins to separate out as solid crystals when the solubility limit is exceeded. A distinction must be made between the thermodynamic WAT and the experimentally measured WAT. The thermodynamic WAT defines the true solid-liquid phase boundary temperature, i.e. the maximum temperature at which the solid and liquid phases exist in equilibrium at a fixed pressure. The experimental WAT represents the temperature at which the first crystals are "detected" and, consequently, depends on the sensitivity of the measurement technique. Normally, the experimental WAT would be well within the thermodynamic Solid-Liquid phase envelope 21 , i.e. at a fixed pressure the (WAT) expt < (WAT) true . On Measurement Techniques Several techniques have been employed for experimental determination of WAT. These range from simple cold finger, visual ASTM D2500-66 method, filter plugging (FP) and viscometry to more sophisticated techniques such as cross polar microscopy (CPM), light transmission, Fourier transform infrared (FTIR) light scattering, differential scanning calorimetery (DSC) ultrasonics etc. A comparison of WAT for crude oils measured by different techniques have been reported in the literature 20,22 . Monger-McClure et al. 20 evaluated a number of laboratory techniques to find out the most suitable one for flow assurance studies. They recommended CPM and DSC as methods of choice when the sample volumes were limited, and, FP and FTIR as methods of choice for live oil measurements. In their study on seventeen North Sea oils, Ronningsen et al. 22 found CPM to invariably provide the highest WAT amongst those measured by DSC, CPM, and, viscometry. More recently, Cazaux et al. 23 have measured WAT using CPM, viscometry and X-Ray diffraction and found CPM to give the highest WAT. The relevancy of the experimental WAT to the field observation has been addressed by Ronningsen et al. 22 and Monger-McClure et al. 20 and correlated by Hamammi and Raines 24 , Erickson et al. 12 and Monger-McClure et al 20 . The WAT of Stock Tank Oil (STO) was found surprisingly to be in agreement with the live oil deposition temperature. Now, the WAT for live crude oils are generally lower than the respective STO WAT. Therefore, a conclusive explanation of why the STO WATs matches with the field deposition temperature was not provided. One possible explanation is that
doi:10.2118/62945-ms fatcat:e46ld2dxprfbjjszyg46ov7mwa