Hydrate Inhibition in Headers With No Production Flow
- B. Herrmann (Independent Consultant) | C. Bargas (BP) | S.J. Svedeman (Southwest Petroleum Inst.) | J.C. Buckingham (Southwest Petroleum Inst.)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 26-29 September, Houston, Texas
- Publication Date
- Document Type
- Conference Paper
- 2004. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 4.3.1 Hydrates, 4.1.2 Separation and Treating, 4.2 Pipelines, Flowlines and Risers, 5.4.2 Gas Injection Methods, 5.1.1 Exploration, Development, Structural Geology, 4.1.5 Processing Equipment, 3.4.1 Inhibition and Remediation of Hydrates, Scale, Paraffin / Wax and Asphaltene
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When deep-water subsea production is shut in for long periods, measures are normally taken to prevent the formation of hydrate blockages in the flowline network. One common inhibition method is to inject methanol into the jumpers, manifold headers, and well bores. This paper presents the result of experiments that were conducted to investigate the distribution of inhibitor injected into a static (no production flow) header or bore that contains oil, water, and gas. The test results were used to help develop a more effective header design and also to shape operating guidelines for inhibiting after shut-in. A test facility was constructed to test various header configurations and production conditions. Tests were conducted to investigate the effect of varying the water cut, gas volume fraction, header inclination angle from horizontal, and the oil density. Experiments also examined the distribution of methanol injected into a vertical well bore.
Results are presented that show the dispersal of oil, water, and gas in the header after shut-in and after injecting inhibitor into the header. In all test cases, uninhibited water remained in the header after methanol injection and no protection was provided against hydrate formation. Either the methanol never came into contact with the water or the diffusion in the static system was so small that the methanol and the water remained segregated.
Test results also show that the maximum amount of inhibitor present in the header after methanol injection depends upon the oil density, header geometry, and the amount of gas trapped in the header. These results can be used to determine the maximum amount of inhibitor that needs to be pumped to inhibit a shut-in header section.
Given the apparent failure of methanol to disperse in such a way as to inhibit water, further tests were run to examine why this common practice of methanol inhibition is successful. These experiments gave some explanation for the success of this procedure and yielded insights to the timing of chemical injection.
Upon shutdown, gravity causes the fluids in a subsea header to separate. Free gas migrates to the top portion of the header, water migrates to the lower section, and both are separated by a layer of oil. If the shutdown is for a prolonged period, then normal practice is to pump a couple of volumes of methanol into the header to mix with the water. The assumption is that the alcohol mixes with water and this prevents methane from bonding with the water to form hydrates.
How the methanol mixes with water is not obvious. In a static system, gravity distributes not only the oil, gas, and water, but also the methanol. If the methanol is lighter than the oil then it floats along the top of the oil and pools at the oil-gas interface. If the methanol is heavier than oil, then it floats along the top of the water and tends to pool at the oil-water interface. Sometimes the headers are sloped to prevent water from accumulating in the dead end. In this case, when methanol is lighter than oil, the injected fluid will pool at the top of the oil and displace the heavier fluids out of the header until enough methanol has accumulated that it can itself flow out of the header. See Figure 1.
However, if the methanol is heaver than oil, it will pool at the bottom of the oil layer and displace oil out of the system again until enough methanol has amassed that it flows out of the header into the next section. See Figure 2.
Gravity dominates the distribution. The problem is that there are other factors affecting the relative density of the methanol and its dispersal. Firstly, the pressure inside the header determines the amount of gas in solution with the oil and the density of the oil. The oil can be either lighter or heavier than methanol. For a typical GOM oil with a 650 GOR, the breakover point is 1,800 psi. At pressures less than 1,800 psi, the methanol is lighter than the oil. At pressures greater than 1,800 psi, the methanol is heaver than the oil. See Figure 3.
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