Chris Mills, senior consultant engineer at TUV SUD National Engineering Laboratory, highlights techniques that would help the oil and gas industry to efficiently tackle flow management issues within mature fields
Flow measurement is a vital aspect of hydrocarbon production. Understanding, monitoring and controlling flow rate, as part of key processes, are essential elements to the viable operation of production systems across the world. It provides the means for well testing, process monitoring and production optimisation. Flow metres are also used as the basis for fiscal and custody transfer measurement of hydrocarbons. In this sense, they can be thought of as the ‘cash registers’ of the industry. Accuracy in such applications is paramount and even small measurement errors over the course of a year can amount to significant loss in revenue.
Flow metres, like most types of process equipment have fundamental performance limitations making them susceptible to changing field conditions that occur as a reservoir matures. Over time, reservoirs that once produced high volumes of almost exclusively hydrocarbon can gradually transition to a state where they produce low volumes of almost exclusively water. These large shifts in production rates and composition impose a heavy burden on production equipment which typically have a limited operating envelope and are increasingly pushed to perform under conditions they were not originally designed for. This impacts flow measurement in a number of ways. One of the most significant impacts comes from the diminished hydrocarbon production rate forcing metres to operate below their operating or calibrated range. The range over which a flow metre can effectively operate, known as ‘Turndown’, is the ratio of the maximum to minimum flow rate over which the flow metre can perform. This is sometimes also referred to as ‘flow range’ or ‘rangeability’. Different types of flow metres typically have different ‘Turndown’ ranges depending upon their fundamental principle of operation.
Considering the turbine flow metre as an example, these metres if implemented correctly, can be extremely accurate and repeatable, and are frequently used in fiscal applications where accuracy is vital. However, like most flow metres, they have a‘Turndown’ and care must be taken in selecting the correct metre or number of metres for a given application. Since the flow metre returns a pulse signal which is proportional to the flow rate, the metre is calibrated in terms of the number of pulses per unit volume (K-Factor) of fluid which flows through the metre.
Intuitively, one might expect that the relationship between KFactor and flow rate should be linear, however in reality that is not the case. The mechanics of the metre means that there is a limited range of which this linear relationship is true. Outside these ranges, the behaviour of the metre becomes significantly non-linear and more complex, generally resulting in lower accuracy. Flow metres are typically calibrated over this linear and ‘predictable’ flow range.
All metres exhibit this limitation in one way or another. However, depending upon the principle of operation, some metres have wider turndown than others. Differential pressure metres for instance, such as orifice plates or Venturis, have historically had a small turndown of around 4:1 (although now improved with newer differential pressure transmitter technology), whereas some modern types such as ultrasonic metres can operate at turndown ranges of 200:1 or higher. As reservoirs mature, dropping hydrocarbon rates typically force metres to operate at the bottom of their operating range, if not below, whereas on the opposite side, produced or wastewater flow metres tend to operate at the high end of their operating range. In some cases, it is possible to characterise the behaviour of a device below its defined operating range and thereby extend its capability - so long as the behaviour of the device does not become significantly non-linear. In other cases, the only solution will be to either replace the flow metre or redesign the flow measurement system.
To provide confidence that the measurement taken by the device remains accurate, the device should demonstrate traceability to a higher-level standard. This occurs throughflow calibration at a traceable and accredited flow laboratory or onsite prover system if space, weight and financial constraints allow. Specific industry standards or agreements normally dictate the calibration frequency. For mostapplications however, it is the user who must define the calibration methodology and flow conditions. If the field conditions have changed, then the calibration temperature, pressure, fluid viscosity and flowrange should be amended to match the service conditions.
In addition to the operating range of the metre, another significant factor is phase contamination. As separation systems are pushed to operate at the extremes of their performance envelope, phase contamination becomes a serious risk, resulting in a mismeasurement of the hydrocarbon produced.
Electromagnetic water metres for instance are affected by the presence of oil since oil is not conductive, and transit-time ultrasonic metres are seriously affected by the presence of a secondary phase. Great care must be taken in understanding the performance limitations of the metre and the conditions in which it operates. Moreover, there are several other problems that can arise from changing field conditions. These include material erosion caused by increased sand loading, distorted, swirling or asymmetric flow profiles caused by upstream process equipment or pipework resulting from system modifications, or changes in fluid physical properties to name just a few. All of these factors must be monitored as they can have a detrimental effect on metre performance.
Since every application is unique, the solution to this problem is not always straightforward. All factors should be considered before selecting a course of action. Regardless of the measurement technology used, a key consideration is to understand the measurement uncertainty and calibration requirements for the application. In addition, performance, calibration frequency, recognition of these points in the quality system and accuracy requirements should be considered along with reviewing other relevant technical factors such as fluid properties, ideal turndown, contamination, and operating conditions. Economic and human factors should further be considered. Finally, manufacturers and regulators should be consulted, and appropriate standards observed.
