Apr-2010

Detecting and dealing with hydrate formation

A review of cause, detection and treatment of hydrate plug formation in gas pipelines. Gas hydrates are solid crystalline compounds, which have a structure wherein guest molecules are entrapped in a cage-like framework of the host molecules without forming a chemical bond.

Bahubali Chandragupthan PL Engineering
Girish Babu Nounchi L&T - Gulf

Article Summary

It is a result of the hydrogen bond that water can form hydrates. The hydrogen bond causes water molecules to align in regular orientations. The presence of certain compounds causes the aligned molecules to stabilise, and a solid mixture precipitates. The water molecules are referred to as the host molecules, and the other compounds, which stabilise the crystal, are called the guest molecules. The hydrate crystals have complex, three-dimensional structures in which the water molecules form a cage, and the guest molecules are entrapped in the cages.

The stabilisation resulting from the guest molecule is postulated to be caused by Vander Waal’s forces, an attraction between molecules that is not a result of electrostatic attraction. The hydrogen bond is different from the vander Waal’s force because it is due to strong electrostatic attraction, although some classify the hydrogen bond as a Vander Waal’s force. Another interesting thing about gas hydrates is that no bonding exits between the guest and host molecules. The guest molecules are free to rotate inside the cages built up from the host molecules. This rotation has been measured by spectroscopic means. No hydrate without guest molecules has been found in nature. Thus, clathrates are stabilised by the weak attractive interactions between guest and water molecules. However, the guest species has some restrictions on its size. This arises from the fact that there are a limited number of cage types that encapsulate guest molecules without deviation of the hydrogen bond lengths and angle from ideal ones. All of the cages are not necessarily dependent on the temperature and the pressure of the guest compound in equilibrium with clathrate hydrate.

The formation of a hydrate requires three conditions: low temperature and high pressure; the presence of hydrate formers such as CH4, C2H4, CO2 and H2S; and sufficient quantities of water and formation time.

Early warning signals
There is no single indicator that gives the best warning of hydrate formation, but pressure drop is the most common indicator. Hydrates in a well are announced most often by abrupt flow blockages accompanied by a high pressure drop. In normal operation, however, the well’s temperature is high enough to prevent hydrate formation:
• Pigging returns should be examined carefully for evidence of hydrate particles. Hydrate masses are stable even at atmospheric pressure (metastable equilibrium) in a pig receiver discharges.

• If water arrival decreases appreciably at the separator, hydrates may be forming inline. Several field trials have indicated that the earliest sign for hydrate formation in a pipeline is a decline or stoppage of the production of water. An increase of water hold-up in the pipeline is due to the formation of non-transportable hydrates. Unfortunately, the water production rate is not accurately and continuously monitored enough, and therefore this early warning sign is often not noticed.

Hydrates denude H2S from natural gases owing to the near optimal fit of H2S in hydrate cavities. The same is not true of other acid gases and carbon dioxide.   

The pressure drop (∆P) increases and the flow rate decreases if the pipe diameter is decreased by hydrate formation at the wall in a gas line. Since ∆P in pipes is proportional to the square of turbulent flow rates, the change in flow with hydrates present can be substantial; however, a large restriction may be necessary over a long length before a substantial pressure drop occurs.

• In some cases, the pressure drop over the line increases gradually because of a build-up of hydrate layer at the pipeline wall. This behaviour has been observed in gas/condensate lines that operate in the annular flow regime at a relatively low water cut. Usually the increase in pressure drop is accompanied by a decreasing gas production rate.

Austvik suggests that while a gradual pressure increase in hydrate formation occurs for gas systems, a gradual pressure increase is not typical for gas/oil/condensate systems. The flow regime of a system may change due to the formation of viscous hydrate slurries. The flow in a system that normally operates in the annular flow regime may change to slug flow because of hydrate formation. This may result in a severely fluctuating pressure drop over the line after an initial period of gradually increasing pressure drop.

• If a pipeline operates in the stratified or slug flow regime, hydrate formation may cause large fluctuations in the pressure drop. This behaviour is due to the formation of an increasingly viscous slurry of hydrates that intermingles with water, gas and oil. The continuous formation and subsequent dispersions of slugs of this viscous slurry initially cause variable and temporary restrictions in the pipeline (although the bare line pressure drop often increases continuously). Once these fluctuations are observed, a sudden jamming of the flow line by a slug of hydrate slush may be imminent. In gas/oil/condensate systems, Statoil’s experiences are that, without advance warning, the line pressure drop shows sharp spikes just before blockages occur.

• Dead ends, tees and narrow tubing are easily blocked by hydrate. Pressure transducers connected to a pipeline via narrow tubing may give erratic readings due to hydrate blockage in the instrument lines before notable hydrate restrictions are formed in the pipelines itself.

• Patches of hydrate may be heard to move through flow line or production facilities. If these hydrates dissociate in heat-traced or low-pressure facilities (for instance, heated separators) the temperature may notably decrease because of heat consumption by dissociating hydrates.

Detection of hydrate blockages
Two methods are suitable for detection of hydrates in onshore systems: thermal imaging and gamma ray detection. A thermal imaging camera is a handheld device that measures infrared spectral transmission as an indicator of system temperature. It is applicable only to onshore or offshore topsides.