Almost all oil and gas is found within the tiny spaces in sedimentary rocks, mainly sandstone and coarse-grained limestone. Bedrock limestone and sandstone, even though hard, also contain lots of pores that can contain water, oil, gas and other elements.

Along with the gas and oil is a mixture of sand, silt and mud. This sludgy silt mud is rich in heavier metal elements including uranium and mercury. Even the heavy uranium can be suspended in the product as it is drawn from the well. However because mercury exists naturally in vapour form it is readily drawn along with the gas product.

Where these contaminants end up in the plant depends on many factors. Mercury as suspended vapour in the product will act in the same way as many other oil and gas products. During fractional distillation temperature and pressure changes allow vapours to condense into selected by-products.

In the same way, certain vessels on the rig or in the refinery may lend the right conditions for vapourised mercury to condense back into liquid. Here it will form as either small globules or in small puddles when collected in the base of pipes and vessels.

In the case of the heavier elements suspended in the product, over time these form scale that can be eggshell-like, brown or grey. They can even form whitish slate like layers or layered variations like tree rings. These in turn can develop into heavier deposits such as in the pipe cross section shown in Fig.1. Here deep layers of tree ring type deposits are clearly seen. These deposits are formed of various elements that may contain heavy metals including uranium ores and mercury.

Breaking containment

As wells and their products get older, it is expected that increasing amounts of heavier elements such as uranium and mercury will also be drawn up from the sludge. In fact, when breaking containment in vessels on offshore installations, engineers are already finding deposits and even small puddles of liquid mercury.

Mercury evaporates and produces harmful vapours at relatively low temperatures. When a pressurised vessel that contains mercury droplets is depressurised, these vapours will start to be released. So when breaking confinement, even though a thorough gas test has proved there is no flammable gas present, the vessel could still present a hazardous health risk from invisible mercury vapours.

As a result, interest is growing in portable analysers that can determine the level of mercury vapour at any given location.

One company leading the way here is Able. The Reading, UK-based instrumentation and control specialist has been in business for almost a quarter of a century now and portable mercury analysers are among the newest additions to its product line.

For example, the Jerome431-XE mercury vapour analyser uses a patented gold film sensor for accurate detection and measurement of toxic mercury vapour in the air. According to Able, this portable hand-held unit can easily be carried to locations with mercury concerns for applications such as industrial hygiene monitoring, mercury spill clean up and mercury exclusion testing. Simple, push-button operation allows users to measure mercury levels from 0.003 to 0.999mg/m

The gold film sensor is inherently stable and selective to mercury, eliminating interferences common to ultraviolet analysers, such as water vapour and hydrocarbons. When the sample cycle is activated, the internal pump in the 431-XE draws a precise volume of air over the sensor. Mercury in the sample is adsorbed and integrated by the sensor, registering it as proportional change in electrical resistance. The instrument computes the concentration of mercury in milligrams per cubic metre or nanograms, and displays the final result in the LCD readout.

An improved film regeneration circuit in the 431-XE makes the sensor last even longer than in earlier models.

For extra versatility and to meet specific applications, additional accessories are available to customise the Jerome431-XE.

For example, an optional communications configuration allows data logging and computer interface capabilities. For data acquisition during portable surveys, a Jerome data logger plugs into the 431-XE. Using the Jerome Communications Software (JCS), the analyser and data logger download recorded data to the computer for analysis, printout, and permanent record storage. The JCS software can also program the instrument for stand-alone monitoring. If the sensor becomes saturated while the 431-XE is attached to the data logger or computer, the analyser automatically regenerates the sensor and then resumes sampling. They can also be used as collection devices for applications such as gas stream analysis.

An internal option board allows auto-zeroing, dc power operation, timed regeneration, and timed sampling during prolonged unattended sampling periods. The option board also allows external fresh air solenoid support and 4–20mA or 0–2V analogue output. A moulded hard carrying case or soft field case give added versatility and organised storage for the instrument and its accessories.

Other suppliers are also focusing on simplifying their monitoring technologies. One such company is OhioLumex. Based in Ohio, US, the company both manufactures and distributes analytical and scientific instrumentation.

Its RA-915+ Mercury Analyser is a portable spectrometer designed for interference free analysis/monitoring of mercury content in ambient air, water, soil, natural and stack gases, chlorine alkali manufacturing, spill response, hazardous waste, foodstuff, and biological materials.

This US Environmental Protection Agency-validated high sensitivity and selectivity instrument does not require gold amalgam pre-concentration and subsequent regeneration steps. This enables the user to conduct real time monitoring.

Being combined with an RP-91 Cold Vapour accessory for water and RP-91C pyrolysis attachment for solid sample testing, the instrument is designed to determine mercury content in ambient air, water, soil, natural and stack gases. Its detection limits are as low as 0.5ng/m

Meanwhile another UK company, Shawcity, has sold over 1000 of its portable mercury vapour instruments (MVI) worldwide.

Marketed as the world’s only mercury vapour surveyor, the MVI gives continuous readings. Shawcity says that the MVI’s high-performance pump means that users know immediately when tiny levels of mercury vapour are present. Added to this, says the company, are highly accurate measurement and an almost instantaneous reaction to mercury. 3 in seconds.3 in air,0.5ng/l in water and 5µg in soil.