Operational challenges in transportation of diverse hydrocarbon liquids: Commingled flow of Gas-to-Liquids (GTL) products, Conventional Alaska North Slope (ANS) crude and heavy crude through the Trans Alaska Pipeline System (TAPS)

TAPS was originally designed and built for transporting 29°API Prudhoe Bay type crude oil from ANS to the marine terminal in Valdez. Combined oil production from ANS steadily declined since the peak output of 2.03 million barrels per day in 1988. The dwindling ANS oil production is, however, exerting increased burden on the economic operation of TAPS, raising an important question related to its sustained economic operation in the future. The proven and recoverable reserves of conventional natural gas in the developed and undeveloped fields on ANS are estimated to be 38 trillion standard cubic feet (TCF). Additionally, estimates of unconventional gas resources in the form of hydrates and coalbed methane are also significant. This in fact creates an interesting scenario; in that these vast gas resources are stranded because the domestic gas market is far way from ANS. The in-place resources of heavy oil on ANS are also huge, ranging between 20-25 billion barrels. However, heavy oil resources on ANS have received little attention, mainly because these oils are viscous and are difficult to transport. A feasible solution, in fact, lies in the combination of the foregoing problems and scenarios. The vast natural gas resources on the ANS can be chemically converted to produce GTL products on ANS which will provide additional liquid capacity to fill up TAPS. Additionally, ANS' heavy oil resources also have the potential to become future TAPS throughput. In this manner all three issues can be addressed, i.e., increase the economic life of TAPS; monetize the huge gas resources on the ANS by bringing it to market in the form of GTL and also bringing Alaska's heavy oil to market. Although, transportation of commingled GTL products, conventional and unconventional (heavy) oils is theoretically feasible; it is this diverse throughput and Alaska's arctic environment that poses several operational challenges such as pumpability, gel strength (cold restart problem), effect on density and viscosity, and solid organic deposition. Therefore, these challenges ought to be addressed before the proposed diverse throughput becomes a reality in the future. This is precisely the purpose of the study presented in this paper. The ternary blends of varying compositions of GTL, conventional ANS crude and synthetically created heavy crude were prepared and evaluated for their densities, rheology, solid organic deposition and gel strength. The results indicate that certain blends consist of characteristics akin to existing ANS crude oil conditions favorable for flow through TAPS.