Hydrate Inhibition in Headers With No Production Flow
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 90127, "Hydrate Inhibition in Headers With No Production Flow," by B. Herrmann, SPE, Independent Consultant; C. Bargas, BP; and S.J. Svedeman and J.C. Buckingham, SPE, Southwest Research Inst., prepared for the 2004 SPE Annual Technical Conference and Exhibition, Houston, 26-29 September.

When deepwater subsea production is shut in for long periods, measures normally are taken to prevent the formation of hydrate blockages in the flowline net-work. One common inhibition method is to inject methanol into the jumpers, manifold headers, and wellbores. Experiments were conducted to investigate the distribution of inhibitor injected into a static (no production flow) header or bore that contained oil, water, and gas. The test results were used to help develop a more effective header design and to develop operating guidelines for inhibiting after shut-in.

Introduction

Upon shutdown, gravity causes fluid in a subsea header to separate. Free gas migrates to the top portion of the header and water to the lower portion, separated by a layer of oil. If the shutdown is for a prolonged period, normal practice is to pump two volumes of methanol into the header to mix with the water. The assumption is that the alcohol mixes with water, preventing methane from bonding with the water to form hydrates.

How the methanol mixes with water is not obvious. In a static system, gravity distributes the oil, gas, and water as well as the methanol. If the methanol is lighter than the oil, it floats on top of the oil and pools at the oil/gas interface. If the methanol is heavier than the oil, 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 flow out of the header itself. However, if the methanol is heavier than oil, it will pool at the bottom of the oil layer and displace oil out of the system until enough methanol has amassed that it flows out of the header into the next section.

A high water cut will influence the relative density of the methanol mixture and complicate the dispersal. Some of the methanol mixes with the water, and this “heavy” mixture pools at the oil/water interface. However, after a period of time, pure methanol collects at the top of the mixture and moves to the oil/gas interface.

The only mixing of methanol and water occurs when the methanol comes in contact with the water. The methanol-injection rates are small compared to the volume of the header, so the methanol flow is laminar. Thus, diffusion of methanol into the water is very slow. Moreover, it can occur only where a sufficiently large percentage of water exists that the injection point is inside the water volume or the methanol is heavier than the oil and it pools on top of the water.