"Near" real-time scale deposition measurement making advances

May 1, 1999
Fundamental Visco-Elastic Response of TSMR. [5,065 bytes] The correction yields a more accurate response versus the non-linear, conventional QCM frequency measurement. [5,951 bytes] Rapid change of frequency with time is indicative of a fluid at high risk of scaling. [4,789 bytes]

Spot sampling yielding to continuous, automated system

Kevin McMillin
Engineering Editor
The natural percolation of formation fluids through geological rock formations yields soluble solids in the formation water phase. These soluble solids are produced with the recoverable hydrocarbons. Solubility, temperature, pressure, flow rates and an over abundance of certain ions interact to create a scale-forming environment.

The solubility of some scales vary proportionally, and others inversely proportional to these parameters, making scaling chemistry very complex. Production engineers have always wrestled with the problem of scale formation and the associated economics of treatment, remedial work, or abandonment.

The unavailability of continuous monitoring technology requires frequent "spot" sampling and testing methods. Wells producing with inconsistent flow rates, fluid phases, and different gas-oil-water ratios render this method of monitoring questionable and subject to error. A more finite, predictive method is required for better monitoring and control of scale formation.

Some operators have indicated their willingness to contract out all operations involving scale control, giving service companies overall management responsibilities of the operator's production scale problems. This creates an opportunity for service companies to develop measurement technologies and project management strategies to address this service need.

A new measurement system has been developed giving a "near real-time" evaluation of scale buildup tendency. Two different methods of measurement are currently available to the industry. A portable wellsite suit case system for "grab samples" and a side-stream unit for on-site testing during normal production operations.

Remedial work

Current practice for control of downhole scale deposition is to forcibly inject (squeeze) chemical inhibitor into the formation. Previous analysis of the produced water provides an insight into what the minimum concentration of the inhibitor should be to effect scale control. The inhibitor is then produced with the natural formation fluids, reacting with and neutralizing the scale forming carbonates and/or sulfates of the produced fluid.

Available testing methods are not performance-based measurements and only related to the amount of inhibitor present and not its effectiveness. Additionally, available testing methods do not allow for quick on-site testing and evaluation. Some results can take days, or weeks, to obtain.

Well production parameters can change significantly in this period of time, rendering the analysis invalid. The alternative is to take another sample and repeat the process. This results in a constant "playing catch up" mode of operations, limiting the ability of an operator to have pro-active control of operations.

The ability to obtain more frequent data points and continuously monitor produced fluid stability, and not inhibitor concentrations, will optimize the scale treatment process. More current information will yield an "evaluate, then treat" scenario, versus the current "treat, then evaluate" scenario. The significance of this statement is realized in the cost of treatment alternatives.

Squeeze jobs are big expense items. Eliminating just one of these operations can change well economics drastically. More current information can also help optimize a required squeeze operation, resulting in less "insurance" chemical(s) required at the wellsite.

Measurement fundamentals

The measurement is acquired with the aid of an acoustic device called a thickness-shear mode resonator (TSMR). The TSMR operates on the fundamental working principal of a standard Quartz Crystal Microbalance (QCM). The application of an electric potential across a piezoelectric wafer results in its shear deformation at a resonant frequency. Mass adherence to the wafer surface changes its deformation characteristics and the resonant frequency, resulting in a quantitative mass measurement. Mass deposition is directly proportional to the change in frequency. Formula 1 [17,814 bytes]

The TSMR electronics developed by Sandia National Laboratories monitors two separate outputs, resonator frequency and damping voltage. Enhanced Lever oscillator electronics in the TSMR give it more accurate and robust measurement capability over the conventional QCM. As illustrated in Formula 2, the frequency response of a resonator can be significantly affected by visco-elastic fluid effects, rendering frequency-based mass measurements meaningless or confusing.Formula 2 [16,868 bytes]

The TSMR can better differentiate between mass deposition and visco-elastic interactions. The damping voltage output is directly proportional to visco-elastic effects and is often more responsive to these interactions than conventional QCM frequency output. This responsiveness can be used to qualify the frequency output, monitor changes in fluid properties, and linearly correct for visco-elastic effects.Formula 3 [16,220 bytes]

The well site grab sample test is performed in minutes with changes in frequency being induced by "stressing" the test sample with known quantities of scaling ions. Depending on the chemical makeup of the produced fluid sample, a deviation in the frequency curve may be observed. The severity of scaling tendency can be interpreted from the slope of the frequency-time curve. Rapid change in frequency indicates a system extremely close to the scaling point, and a high risk fluid. Close monitoring will most likely result to ascertain when the point will be breached, and/or how well resulting treatment is adjusting the system.

This graphical response can be extrapolated to a long-term monitoring situation with the system "stresses" resulting from changes in the produced fluid phase, temperature, pressure, flow rate, etc. The ideal response would be a flat line with no change in frequency. Continuous monitoring capability, automated or not, will allow operators to control fluid scaling tendencies with optimal, more timely treatment corrections.

Future development

Applications of this technology to the offshore environment are feasible with currently available hardware. Both the portable grab sampling and the side-stream equipment systems have onshore and offshore capability. Both systems demand a small footprint, making them very "rig friendly."

An obvious question from the offshore operators is, "when will there be a system capable of reliable downhole operation?" Best estimates are early 2000. The robustness of the Lever oscillator electronics make the transition from a surface to a downhole environment possible. Technical issues to be dealt with include ruggedizing electronics for a downhole environment, mechanical conveyance, reliability, and deployment concerns.

Nalco/Exxon has exclusive development rights from Sandia National Laboratories to further advance the TSMR for petroleum and petrochemical use.

Acknowledgements: The following provided information for this article: P. Kraus, PhD.; R. McClain, PhD.; K. Minyard, all of Nalco/Exxon (Sugarland, Texas).

Copyright 1999 Oil & Gas Journal. All Rights Reserved.