Recommendations are offered for liquid level measurement at the various stages of oil production, as well as a drilling and a pipeline scenario. 

Donald Koeneman, Ametek Drexelbrook

A number of technologies have been used to tackle the challenges of measuring liquid levels in oil production facilities. Some have fallen by the wayside due to performance or maintenance problems. Others have evolved along with the application challenges and have proven effective in the field. Oil production applications range from fairly simple level measurements to those that involve harsh operating conditions and include high temperatures and pressures. Because of the variety of applications, there is no single technology that will be appropriate for all applications.

This article presents a basic user’s guide to the most appropriate level measurement technologies for various oil production applications. Its purpose is to briefly explain the technology choices and recommend the most practical solutions for each situation.

POINT LEVEL VS. CONTINUOUS

There are two major types of level measurement instruments: those that take point-level measurements and those that take continuous-level measurements.

Point-level (on/off) measurement devices indicate the presence or absence of liquid at a given point or location in a process vessel or storage tank, or a phase change such as an oil/water interface in a separator. Point-level measurement can be used for either alarm or control of a process.

Continuous-level (proportional) measurement devices measure the liquid level over the full span of a process. Continuous-level measurement can be used to indicate inventory or control a process.

TECHNOLOGY CHOICES

The technologies used to measure high level, low level, total level and phase separation (interface) are affected differently depending on the process environment. Some of the most common non-mechanical technologies used are listed below.

RF admittance employs a radio frequency signal to a sensor. A change in RF admittance indicates either the presence or the absence of liquid, or how much liquid is in contact with the sensor. This makes RF a popular, versatile and robust technology for a wide range of conditions and process materials for both point-level and continuous-level measurements. Additionally, RF admittance has the inherent ability to ignore coating deposits that may develop on the sensor.

Radar utilizes Frequency Modulated Continuous Wave (FMCW) or Pulsed Time Of Flight (PTOF) through-air transmission, which allows for an accurate non-contact reading of the reflected electromagnetic signals under some adverse measurement environments.

Continuous-level ultrasonic measurement uses a piezoelectric crystal transducer to generate a high-frequency ultrasonic pulse, and measures the transit time for the pulse to be reflected from the material surface and returned to the transducer to determine level or distance.

Point-level ultrasonic measurement electronically resonates a crystal at a fixed frequency to generate sound waves that travel across an air gap to a second crystal. As the gap between the two crystals fills with a liquid medium, the second crystal begins to resonate with the first. Due to the construction of the devices, these are frequently referred to as gap switches.

Tuning forks utilize piezoelectric crystals that vibrate the fork at a specific frequency. When the fork is covered by product, the system detects a change in frequency, which causes a change in the output state of the switch.

Magnetostrictive measurement utilizes a float with embedded magnets that rides on a rigid or flexible tube containing a magnetostrictive wire, i.e., a wire that changes its shape in the presence of a magnetic field. The wire is pulsed with a low-voltage signal that detects the position of the magnetic field within the float. This results in a transit time measurement that often exceeds the accuracy of radar. Magnetostrictive systems may also contain temperature sensors to provide several important measurements in one: vessel penetration, total level, interface level and several temperature points.

Time Domain Reflectivity (TDR) takes a highly focused electromagnetic wave, guided by a metallic rod or flexible cable, to the surface of the process liquid and reflects it back along the wave guide to determine the level.

APPLICATIONS

Below, we discuss level measurement recommendations for typical applications at the various stages of oil production, as well as a drilling and a pipeline scenario.

Separation phase. Fluid coming directly from an oil well is a varying mixture of oil and water (from 30% to 90% water). Automatic Well Test (AWT) systems are used to periodically determine a well’s operating behavior. Well test systems are small vessels typically 2–4 ft in diameter and about 10 ft long, designed to measure the production from one well at a time on a regular basis.

The well tester measures the total output, watercut and net oil. A typical AWT system will control the oil/water interface with an RF admittance probe and use various flow devices all controlled by a programmable logic controller with computer interface. The computer “automatically” selects the wells, opens and closes valves at a manifold, and records the data from a number of wells, one at a time.

The well fluid is then pumped to a Free Water KnockOut (FWKO), or a series of them. These vessels are gravity separators, typically 12 ft in diameter by 60 ft long; they can also be vertical cylinder vessels. The water that is not bonded to the crude oil is allowed to separate over time. Well fluid entering the vessel with 60% water may exit, after free water separation, with 30% water. Level controls are used to monitor the oil/water interface level as separation occurs and to dump the excess free water.

After the FWKO, fluid is typically sent to heater-treater gravity separation vessels. This stage uses heat and sometimes chemical additives to reduce the crude oil’s viscosity and density. The drier oil is skimmed off and sent to the stock tank, and the water is removed through a dump valve and sent to the skim tank.

Oil/water interface control. In these vessels, continuous RF admittance measurement of the interface level can be used to control the water dump valve to prevent the water phase from becoming high enough to reach the oil output piping from the vessel. In lieu of continuous measurement, a point-level RF admittance system mounted horizontally in the vessel below the oil outlet piping may control the water dump valve.

Alternatively, a TDR device may be used to measure the interface level, depending on the thickness of the oil pad and characteristics of any emulsion layer.

Total level alarm. Point-level RF admittance should be used for a low-level alarm (i.e., high in the vessel) to indicate a loss of total oil level in the separator.

Water content. RF admittance should be used as a water-in-oil analyzer (i.e., cut monitor) to indicate the percentage of water entrained in the oil outlet piping. Cut monitor ranges will depend on the separation stage.

Stock tanks. After the dry oil has been removed from the separation phase, it is sent to the stock tank as a storage point before it goes on to the pipeline. Since the dry oil in the stock tank will still have some small amount of entrained water that will separate over its residence time, an interface control for water dump is used, along with a high-level alarm to prevent overfill.

Oil/water interface control. Continuous RF admittance or TDR may be used to measure the interface level in much the same way recommended for separation vessels. Point-level RF admittance is not recommended.

Water dump valve control. A point-level RF admittance system can be mounted horizontally, low in the vessel, as a backup to the continuous control to prevent any oil from being dumped along with the excess collected water.

High-level alarm can be accomplished by point-level RF admittance.

Water content. RF admittance is recommended for use as a cut monitor in the oil outlet piping. Watercut is typically in the 0–5% range; when it is high, the out-of-spec oil is diverted from entering the pipeline.

LACT units. A Lease Automatic Custody Transfer (LACT) unit is found wherever the produced oil is fed into a pipeline. It is designed for measuring, sampling and testing crude oil. Watercut is measured, and if the oil does not meet pipeline specifications, it is diverted for reprocessing.

RF admittance can be used for water-in-oil analysis to indicate the percentage of water in the oil outlet to the pipeline, typically in the 0–3% range.

Skim tank. Separated water goes to the water plant and is collected in the skim tank, where it is processed for further use. It is typical for this water to still have some emulsified oil in it. That oil can be recovered and the water further processed before it is either used or disposed.

Oil/water interface control. Continuous RF admittance level measurement of the oil/water interface can be used to control the water dump valve, to prevent the water phase from getting too low and reducing water storage capacity. Alternately, a TDR may be used to measure the interface level, depending on the thickness of the oil pad and the characteristics of any emulsion layer.

Water dump valve control can be achieved using a point-level RF admittance system mounted horizontally, low in the vessel, as a backup to the continuous control, in order to prevent any oil from being dumped along with the excess collected water.

High-level alarm. Point-level RF admittance is recommended to prevent an overfill condition.

Flotation cells. Flotation cells are often used to remove the last traces of oil and solids before disposal of the produced water. A water level is maintained at an exact location on a trough overflow. As air and a detergent are introduced at the bottom of the trough, bottom sediment solids float to the top with the foam that is created. The foam and its contaminants are forced over the trough and collected. Precise control of the water level is necessary to ensure that the foam will float over the top of the trough.

Continuous-level RF admittance is recommended for water level control. It ignores the foam and controls the exact water level below the foam in relation to the top of the trough overflow.

Alternatively, ultrasonic technologies can be used with the aid of a target pedestal that floats on the water/foam interface. The pedestal is extended to a point that is well above the foam as a target surface for the ultrasonic reflection.

Drilling mud. On both onshore and offshore drilling rigs, the mud is processed to remove solids in a series of rectangular steel tanks that are partitioned to hold about 200 bbl each. These tanks are set in series for the mud circulation system, which pumps the mud down through the drilling pipe to where it cools and lubricates the drillbit. The mud then flows back to the surface carrying away the formation cuttings.

Ultrasonic technology is used to provide an accurate level measurement in the dirty environment of the mud pit. These measurement systems provide the fast response time that is required to ensure that the mud circulation system is not overfilled or underfilled.

Pipeline slug detection. To signal a change in the source and ownership of oil being transported through pipelines, a slug of water may be placed between the differently sourced oil quantities.

RF admittance is recommended for use as a cut monitor in this context, to indicate the precise watercut and, thus, the change of oil source. This technology has the inherent advantage that a process upset, due to the presence of a water slug, will not cause the system to short out or hang up in its readings.