Experimental Study of Oil/Water Flow With Paraffin Precipitation in Subsea Pipelines

Abstract

The deposition of paraffins was studied in a laboratory simulation of the subsea operational conditions, where the oil pipelines laying on the seabed are subjected to low temperatures. An experimental apparatus was built for a pipe-flow model submerged in a chilling bath of near frozen water. Streamwise pressure and temperature gradients were measured to evaluate the differences in the behavior of paraffin deposition between one-phase oil flow and two-phase oil-water flow. Most works on the behavior of paraffin deposition consider one liquid phase only, while two phases - oil and water - are considered here. Measurements in two-phase oil-water flows show a reduction of the head loss compared to one-phase oil flows, when both are subjected to paraffin precipitation, and photographs show that the layer of paraffin deposits on the pipe wall was significantly reduced in the presence of water. Introduction Petroleum is a mixture of hydrocarbon molecules, amongst which we find the alkanes, whose general formula is CnH2n+2, including linear, ramified and cyclical chains with simple covalent bonds, where the number o carbon atoms may range from a few to hundreds. Above a certain molecular weight the molecules will present a solid phase under standard conditions, known as paraffin, but while in their natural underground petroleum reservoir the paraffins may be in a stable liquid phase solution of oil. However, when the temperature of the oil falls below a certain value, determined from the thermodynamics of phase equilibrium, called the temperature of paraffin precipitation (TPP), the solubility of the paraffins in the oil decreases and the precipitation of solid particles occurs. The solid particles of paraffin may agglutinate, and also, through physical-chemical processes, they could deposit in the neighboring solid surfaces. The deposition of paraffins in submarine pipelines poses a serious problem for the offshore petroleum production. The warm oil from the petroleum reservoir, emerging from the well-head, is cooled down by the seawater, surrounding the pipeline at temperatures as low as 4 °C (around 40 °F), to the point where paraffin precipitation occurs somewhere along the pipe. After some time, the continuous precipitation leads to deposits in the internal walls of the pipe, clogging the lines and promoting an increase in the head loss of the flow. Consequently, there is an increase in the pressure gradient required to maintain the flow, and the flow rate is reduced. In time, a complete obstruction of the pipeline may occur. Significant financial losses result from the reduction of revenue and from the increase of maintenance costs. Figure 1 shows a section of a pipeline with paraffin incrustation (from an offshore field in the Campos basin in Brazil).

Liquid-Liquid Flow Pattern Prediction Using Relevant Dimensionless Parameter Groups

“…Annular flow, a flow pattern associated with the highly viscous oil-water flow system, is more pronounced in those studies where high viscosity oil is used [41][42][43]. Detailed studies of the annular flow pattern are necessary to reduce the head-loss due to high oil viscosity during transportation [40,44]. Charles et al [7] suggested that high and low viscosity oils behave differently due to their wall wetting effects.…”

Development, Testing, and Field Application of a Heavy-Oil Pipeline-Cleaning Chemical: A Cradle-to-Grave Case History

“…A review of generic oil degreaser and solid-removal technology yielded fi ve key parameters important to effi cient chemical cleaning (Buzelin and de Campos Lima 2008;Javora et al 2008;Bordalo and Oliveira 2007). These parameters collectively enable the hydrocarbon and inorganic scale to stabilize in the water or chemical phase.…”

SiO2-Based Nanofluids for the Inhibition of Wax Precipitation in Production Pipelines

“…Generally, wax molecules are dissolved in a balanced state in crude oil under reservoir conditions, − however, when the temperature of the crude oil falls below the wax appearance temperature (WAT) since WAT is approximating the thermodynamic solubility limit of the wax . Below this temperature, wax molecules tend to precipitate from crude oil matrix, forming wax crystals, and getting deposited onto pipe surfaces. − In this respect, wax precipitation produces deposits on pipe walls and progressively clogs the production causing restriction in the flow, abnormal pressure drops, fluid gelation, lower flowability, and higher pumping costs. − In addition to temperature, other factors that affect the wax solubility and precipitation degree are related to the crude oil composition, , the gas–oil ratio, − the water–oil ratio, − pressure, flow rate, and inner pipe-surface roughness ,, as well as the presence of asphaltene aggregates, which are known to provide sites for wax crystal buildup …”