DELINEATING THE FAILURE MECHANISMS OF PARAFFIN GELS

摘要

The deposition of paraffin-oil gels in sub-sea crude oil pipelines causes flow restriction and results in tremendous production losses. Quiescent gelation of paraffinic crude oils in pipelines during a temporary production shutdown results in the 'restart problem' inasmuch as the gel has to be broken down before flow can be restarted. Any mechanical method of remediating either the deposition problem or the restart problem, such as pigging or applying pressure, has to overcome the cohesive/adhesive forces holding the paraffin gel together. Thus, knowledge of the gel strength is very important in designing remediation techniques. The cohesive strength of the paraffin gel is manifested in the form of the yield stress and the adhesive strength is determined by the van der Waals attractive forces between the gel and the pipe wall. While using a mechanical remediation technique, the breakdown of the paraffin gel could occur either due to cohesive failure or adhesive failure. In order to study the mode of failure under the given operating conditions, it is necessary to understand the behavior of the yield strength and the adhesion strength of paraffin-oil gels. Rheometric studies on these properties show some very interesting results that provide valuable pointers on the mode of pipeline operation. The yield stress of the paraffin-oil gel is a complex function of the shear and thermal histories, apart from the dependence on the composition of the gel. The yield stress of a model paraffin-oil system showed completely different dependencies on the thermal history, based on whether the sample was cooled under static or flowing conditions. Under static conditions (that resemble the restart problem), the yield stress decreased with an increasing cooling rate. Additionally, the yield stress of the wax-oil gel was observed to reach a maximum at a particular flow stress. The dependence of the adhesion strength of the paraffin-oil gels (to the deposition surface) on the thermal history under static conditions follows a trend that is exactly the opposite to that of the yield stress. Such behavior of the yield strength and adhesion strength is explained based on the size and shape of the crystals formed under various conditions. The result of the opposing trends leads to an optimum condition that has direct bearing on field operation.