Production Engineering - Multistage Stabilization of Crude (T. P. 1085, with discussion)

A process that has come to be known locally as "multistage stabilization" has been developed in the Haft Kel field of the Anglo-Iranian Oil Co. in southwest Iran, for the recovery of casinghead gasoline. It consists in releasing the dissolved gases from the crude by stages, a practice common in many fields, but which, so far as is known, has nowhere been carried to the extent to which it is employed in Iran. It has been in use there for nearly nine years, during which time some 200 million barrels of crude have been handled by the process. This process of gasoline retention consists in preventing the greater part of the casinghead gasoline hydrocarbons (the butanes and pentanes) from ever leaving the crude, instead of allowing them to be vaporized and then recovered from the lighter gases by the usual compression or absorption processes. Its efficiency is fully equal to that obtained by the absorption process; its disadvantage is that it increases the quantities of ethane and propane in the crude sent to the refinery, but with the growing demand for propane this drawback is vanishing. When casinghead gasoline is recovered as a separate liquid in the Iranian fields, it is blended with the crude, as the only outlet is through the refinery, and the crude-oil pipe line offers the cheapest means of transportation. It is under these conditions that the retention of the butanes and pentanes in the crude by multistage stabilization is a most attractive process, possessing as it does the following advantages: (1) extreme simplicity, (2) low first cost, (3) easy installation! (4) practically 100 per cent operating-time efficiency, (5) no operators required, (6) no water or other services required, (7) negligible maintenance, (8) once established, very little routine testing is necessary. The one essential for reasonably efficient multistage stabilization is that the flowing pressures of the wells should be relatively high; i.e., they should not be less than about one-third the saturation pressure of the crude in the reservoir. This is a very general statement, of course, and much depends on the type of crude and its temperature.