Optimization of production chemical delivery is possible with a monitoring and control system.

A key component of recent advances in production optimization has been improvement in the ability to monitor and control production systems in real time, particularly at remote locations. The focus has been on enhancing an engineer's ability to monitor the flowing performance of wells and the operating performance of processing equipment through sensors, and to make precise adjustments to the system at the most appropriate time, from a distance. The addition of chemicals to the production and process stream, a task carried out by legions of sturdy little injection pumps in oil fields all over the world, has more than likely been at the bottom of the list of activities where real-time monitoring and control is implemented. This is no longer the case, for two important reasons.

First, the impact of a process shutdown on the high volume, remote, often unmanned, production systems in place today is much greater, operationally and economically, than it was on yesterday's oilfield systems. Second, there is increased reliance on the use of tailored chemical additives in maintaining flow and corrosion control during the production process. A larger number of chemicals are playing an increasingly important role in optimizing production. So it should come as no surprise that operators are looking much more closely at real-time monitoring and control of chemical delivery in production systems. One approach is Baker Petrolite's SentryNet system introduced 2 years ago and currently in use at a number of remote and unmanned facilities in the Gulf of Mexico and elsewhere.

The system consists of a pump control module, a high-precision flowmeter, and a Modbus communication package for communication to a central processing location. There are two versions of the pump control module. The SentryNet version provides electronic actuation of electric pumps via a 4-20mA signal. The same signal can be used in conjunction with a DCS controller or variable speed drive to control motor speed and hence injection rate. The Sentry MP unit is designed for control of pneumatic pumps through a multi-configurable pulse output that controls air or gas flow to the pump via a solenoid valve. The controller maintains a precise chemical injection rate under changing process conditions and reports injection data to a central location.

Central to the operation of the system is a high-precision flowmeter. This meter reads the actual pumped chemical rate and is accurate at rates as low as two quarts per day. A variety of onboard alarm systems indicate low flow, no flow and leaking solenoid situations, ensuring that the specified amount of chemical is injected with each pulse.

The control modules communicate over an RS-485 network bus. In addition to accepting injection rate set-point signals, the units can also report back on performance variables such as actual rate vs. set point, batch and cumulative injection totals and other operating parameters. As many as 56 units can be handled with one RS-485 cable, which is connected to a modem/telemetry unit. This device transmits the information from the remote site to a similar modem on a central processing location. Commonly, a multi-point broadcast radio frequency modem is used as the communication device on remote platforms. The central location modem is configured to transmit and receive signals from many remote modems at once, in each case addressing only the intended individual control module and its associated pump. Where there is no existing DCS system, an Internet interface may be used.

Benefits of fingertip control

The architecture of the software utilized for monitoring and controlling the chemical pumps is structured so that a single central location has control over the entire injection pump network. The software provides an operator with real-time data on all control units as well as any other instrumentation installed on the network (e.g., corrosion probes). The managing software overlies a customized database that eliminates day-to-day manual tasks by continuously logging chemical pump performance, injection rate changes and alarm conditions. The database can be remotely accessed via telephone modem, for near-real-time viewing of data.

Some of the benefits of fingertip control of chemical injection are obvious: more efficient operation of processing equipment, reduced labor and transportation costs, etc. However, according to Mitch Means, manager of Baker Petrolite's Chemical Automation Group, some of the most important impacts are not immediately obvious. "If a chemical pump failure on a remote platform goes undetected for a significant period of time, several miles of pipeline volume can quickly become out of spec before a back-up pump is started," said Means. "The latest version of the SentryNet controller unit can be configured to operate as both a monitor and backup system for the installed chemical injection system. If the chemical pump fails, the SentryNet controller will automatically activate a backup pump and maintain a pre-determined injection rate. The cost of lost production associated with facility upsets caused by chemical injection pump failures can be significant, particularly in the case of deepwater facilities. These situations can be minimized with real-time monitoring of the chemical system."

For multi-point injection of the same chemical, most current systems employ a design with one pump per injection point, each independently and manually controlled. Another alternative is to configure a single large chemical pump supplying a number of SentryNet-controlled valves that are monitored and adjusted independently. This design eliminates the constant maintenance issues associated with multiple pumps and provides more precise process control.

Sensor-driven chemical delivery

When chemical dosage can be precisely tuned to other measured parameters (e.g., corrosion indicators) the overall use of a chemical can be optimized, resulting in lower chemical usage over time and improved chemical performance. For example, a producing company found that it was experiencing a base corrosion rate of 10 mils per year (mpy) in a major flow line where the maximum allowable rate had been set at only 1 mpy. In order to meet the specification, a corrosion inhibitor was continuously injected at a concentration of 25 parts per million (ppm), based on available data and conventional corrosion control procedures. Subsequently, a SentryNet system was installed and data from a corrosion probe was collected in real time. The data indicated that in fact, the 1 mpy target could be achieved with a chemical dosage of only 10 ppm corrosion inhibitor. The reduction saved the producer more than US $2 million per year.

The system can accept inputs from any type of standard commercially available process monitoring device (e.g., level sensors, flowmeters, corrosion probes, H2S monitors, etc.). Baker Petrolite has developed several proprietary devices for monitoring specific characteristics relevant to chemical remediation. One such device is a patented probe that employs a spectroscopic technique to directly measure dissolved asphaltenes in crude oil. The probe output compares with traditional methods of asphaltene characterization including IP-143 extraction gravimetric analysis and ADT spectroscopic methods. Another device also employs spectroscopic detection of oil mist in a gas stream, providing an immediate indication of process changes such as foaming or carryover. Devices such as these can be coupled with a SentryNet system to control the precise and timely addition of asphaltene inhibitor/dispersant or defoamer chemicals.

A third probe developed by Baker Petrolite senses the onset of scale formation in a mixed fluid system and can be used to optimize the injection rate of scale inhibitor. Other sensor systems under development include a corrosion probe for gas systems and a biofilm monitoring device.

"Even the most modern, reliable chemical pump will experience failures during its lifetime" Means said. "The SentryNet approach recognizes this fact and protects critical process operations through vigilance and immediate action."