1 May 2002

Density profiles crude

By Nicholas Sheble

Ray Lees, a technical manager for Synetix/ICI, writes from Aberdeen, Scotland, about his organization's approach to the separation of oil/well fluids. It has foregone the single point transmitters and is producing more product and using fewer chemicals.

In oil production, well fluids route to production separators. These vessels are long horizontal cylinders, typically 3 meters in diameter and up to 25 meters in length, with a weir positioned close to one end. Well fluids enter the separator as a mixture of oil, water, gas, and sand.

As the fluids flow along the vessel, they start to separate, under gravity, into their constituent parts. In order to have within specification oil flowing over the weir and uncontaminated water at the water outlet, it is necessary to control the flow rate. The only way to control this effectively is to accurately measure the levels of the various phases at the weir. Often in a separator there is not a distinct interface but rather a continuous graduation from one phase to another.

Traditionally, individual single-point instrument transmitters for the oil/water interface and bulk oil level determine the production separator level. There are numerous types of level instruments available, the most common being displacement systems, differential pressure, capacitance probes, microwave radar, ultrasonic, thermal conductivity, and radioactive. Of these types, probably only displacement and capacitance work for interface measurement.

Capacitance probes or similar admittance probes operate by measuring electrical characteristics that will be dependent on the levels of fluids in contact with the probe. As an interface level rises on the probe, the measured admittance or capacitance changes.

The problem with this method is that if the electrical characteristics of either fluid change significantly from the expected values, as will happen when fluids from different wells are introduced to the vessel or emulsions are formed, then the admittance or capacitance measured will change, incorrectly signifying level change as well.

Displacement instruments consist of a submerged body (displacer) suspended in the fluid. The weight of the displacer balances with the upward force (buoyancy) exerted by a particular level of fluid.

These instruments are suitable only where a well-defined step change in densities is evident, such as a step change from 100% oil to 100% water.

In oil production separators, the lack of accurate measurement of the levels of oil and water, as well as any sand, emulsion, or foam, means facilities are often operated at less than their design capability.

Incorrect separation information can cause a high concentration of oil to carry through the water outlet. It can allow a high concentration of water to carry over with the oil phase and cause downstream knock-on separation problems. Foam carryover through the gas outlet can cause rapid buildup in downstream flare liquid knock-out vessels. Sand accumulation reduces the working volume of the vessel and therefore its efficiency. All of these problems can result in unnecessary trips or production and environmental problems.

Coast-to-coast landscape

Density profiling instruments define all the fluids inside the vessel to a greater degree than conventional instruments. In place of the traditional two single points, oil/water interface and bulk oil level, operators can see all the phases and the quality of the interfaces. This enables within-specification oil throughput to increase while limiting chemical usage.

The density profiler measures the density and the extent of different phases within a vessel. It maps different densities of materials such as gases, liquids, and interface levels between phases within a vessel. These materials separate into different user-variable density bands or phases. One can then calculate the interface of the various phases with respect to the vessel height. This information controls the interface levels and determines the effect of chemicals injected.

A density profiler resides in dip pipes-sealed pockets similar to thermowells-installed within the separator through a single 6-inch flange. A narrow dip pipe holds an array of Americium-241 sources, a low-energy gamma emitter. The other dip pipes hold radiation detectors made up of a vertical array of up to 96 Geiger Muller (GM) tubes, each one 28 millimeters in height. Each tube matches to the radiation source on the same plane.

Three dip pipes project into the vessel, one collimator, and two probe tubes. The narrower of these tubes holds a source-arming rod that contains a series of sources along its length. A tube, a collimator, surrounds the rod. The collimator has small holes drilled in it at each source level. These holes direct a narrow beam of radiation toward a selected GM tube in the probes, which reside in separate dip pipes.

The material between the two dip pipes will attenuate the radiation; the intensity of radiation seen by a GM tube relates to the density of the intervening material. Each sensor produces a train of voltage pulses as its output signal.

The rate at which these pulses occur is directly proportional to the intensity of the radiation incident on the sensor. The density of the process fluid at the sensor level determines how much radiation passes through the process fluid.

These counts transmit for analysis via a fiber-optic link and an RS-232 converter to a programmable logic controller that collects the information and calculates the density of the material for each individual GM tube. This is the density profile. Use it to control loop process variables. IT

Read the full text of Raymond Paul Lees' ISA Emerging Technologies Conference paper, or write him at ray_lees@ici.com.