Offshore Chemical Processing Of Associated Natural Gas: Acase Study Of Methanol Production At The Offshore Well

ABSTRACT Offshore gas sources too small to support a pipeline to shore represent a serious gas management problem. This is particularly true of associated gas from a producing oil field which for environmental or political reasons can no longer be flared. One possible solution to this problem is to convert the gas to some product which can be transported with the oil or in tankers. One such product is methanol. Using methanol as an example, a procedure is presented illustrating how an analysis of the technical and economic feasibility of an offshore processing plant can be conducted. INTRODUCTION The relatively small amounts of natural gas which often accompany oil production have become increasingly difficult to manage because of environmental, economic, and political considerations. A typical gas-oil ratio of an offshore well might be 1000 SCF of gas per barrel of crude. Using this ratio, a 35,000 BOPD field would produce about 35 MMSCF of gas per day. Unless the field was near shore or close to an existing gas pipeline, the flow rate would be insufficient for commercial use and the gas would be flared or re-injected. A possible alternative might be to chemically process the gas at the well site to produce a marketable product which can be handled more easilythan gas. One such product is methanol. This paper examines the concept of converting gas to methanol at the offshore well using a mobile offshore process plant which could be employed at several well sites if necessary over the expected process plant life. The major elements of the mobile offshore methanol facility consist of the gas source, the methanol plant and platform, a single point mooring buoy with a permanently moored barge, product transportation facilities, and shore facilities. Gas coming from an oil production platform is piped from the jack up through a single point mooring buoy to a storage barge. The transit tankers receive the methanol from the moored storage barge and transportation atmospheric pressure and ambient temperature to the shore facilities where it is stored prior to input into the distribution system. The well site is arbitrarily selected as being in 100 feet of water in the South China Sea where the weather and sea conditions are normally moderate, and yet the facility must be designed to withstand an occasional typhoon force storm. The major traditional use of methanol is as a feedstock to produce resins for particle board and plywood. For this reason the methanol market is concentrated in industrialized areas of the United States, Western Europe, and Japan. Yokohama, Japan was selected as a reasonable market site for use herein. METHANOL PROCESS DESCRIPTION As shown in the simplified process flow sheet, Figure 1, production of methanol from natural gas involves four steps:Feed gas purificationSynthesis gas preparationMethanol synthesisMethanol purification