Monitoring Water Disposal Wells with WirelessHART Transmitters
By Josh Hernandez, Emerson Automation Solutions
A large oil and gas company operates a series of wells and a steam flood field, where steam is injected to provide pressure to force oil and gas out of the well. The water used in this process is treated and injected into an underground reservoir via water disposal wells.
These wells need to be cleaned periodically using a process called well-bore cleanout, but it’s difficult to determine precisely when to do a cleanout. Installing wireless pressure and totalizing transmitters on 15 water disposal wells helped the company monitor productivity, eliminate unnecessary cleanouts and save $150,000 the first year, with similar savings expected going forward. Significant savings were also realized because they were able to determine that fewer new water disposal wells were required.
Monitoring Water Disposal Well Operation
A separate underground reservoir is used to store treated wastewater from nearby production wells. Steam is used for injection into production wells to create a pressure that will push the gas and oil to the surface. Water comes to the surface with the oil and gas, is treated, then injected back into the underground reservoir via water disposal wells (Figure 2).When water is needed for steam injection, it is taken from the reservoir.
Water injection rates and totals are carefully monitored to meet EPA requirements, as well as to optimize water disposal well performance. Surface or injection pressure is also monitored to meet EPA and safety requirements, and to provide further insights.
This field has over 20 water disposal wells, each about 500 feet apart. The job is to maximize the productivity of these existing wells by monitoring their performance, determining when they need to be cleaned out, and keeping them running to minimize the number of new wells that need to be drilled.
Not all wells are created equally as they vary in depth, total length of perforations and other parameters. Well performance is indicated by the volume of water injected per foot of perforations, a value the company wishes to maximize because it’s closely related to the total injected volume of water, which indicates productivity of that well. Before the wireless installation, manual rounds were made daily to read local chart recorders indicating water flow into the well, and to read local gauges measuring surface pressure.
However, these rounds were not sufficient to optimize cleaning or to determine when new water disposal wells needed to be drilled. Daily measurements fed into the efficiency model indicated that 10 new wells were required. But when a more rigorous study was done, using many more manual measurements daily, it was determined that only five new wells were required, saving nearly a million dollars.
It quickly became apparent that daily rounds were not supplying enough information for accurate analysis of water disposal well conditions. Well operation changes from hour to hour, so a single measurement at 8a.m. can be quite different than a measurement made at 1p.m. on the same day. A single daily measurement, therefore, does not necessarily represent what is happening in the field that day, and can lead to erroneous conclusions, such as the need to drill too many new wells.
A study showed they needed on-line monitoring and trending of key parameters. This would improve well modeling to optimize cleaning, measure well productivity more accurately, and determine the need for more wells. The study was quite expensive, and it isn’t practical to conduct such studies on a regular basis. So, they analyzed its results very carefully, and concluded on-line monitoring was the best solution.
Because the water disposal wells are in remote areas, installing wired instrumentation was not economically feasible. It would cost too much to install power supplies, wiring and safety equipment at each site for wired transmitters, and then install cabling or a remote terminal unit to bring the information back to the central control system. Instead, they installed batterypowered Rosemount transmitters.
Wireless to the Wells
The company installed 15 Rosemount 3051S wireless pressure transmitters on existing manifolds (Figure 3) to monitor the surface pressure – or injection pressure – of each water disposal well. These were installed in two hours each. They were factory configured for the application, including the Network ID and Join Key, so they were ready to go out of the box once the battery was connected at each transmitter.
One Emerson WirelessHART gateway was installed to receive the signals. Because of the distances involved for some of the wells, three high gain antennas were also installed (Figure 4). The field gateway communicates to the base station gateway, and the base station gateway is integrated to the Rockwell Automation control system via EtherNet/IP. The 3051S wireless transmitters began transmitting useful data to the control system within minutes after the transmitters and gateway were installed.
Fifteen Rosemount 705 Wireless Totalizing Transmitters were installed shortly thereafter on existing turbine meters to measure injection rates and totalized values of injection water. These were installed on a Saturday morning in about four hours. There was a hard deadline, but the two maintenance technicians were able to meet it because of the simple installation requirements of wireless totalizing transmitters.
Emerson was on site to assist with the installation. Lead wires from the turbine meter were attached to the Rosemount 705 terminal block. They attached the Rosemount 705 direct mount threads (Figure 5) to the threads of the turbine meter and powered the device.
Within five minutes the Rockwell Automation control system was communicating with the wireless totalizer. Emerson assisted with the next 14 wells, and the total time for installation and startup was 10 minutes per well.
The wireless transmitters provide continuous data to the well performance model, which runs in a PC at the company’s well field control. The data is trended and used to calculate the Injectivity Index, a key parameter for productivity.
The index is a function of surface or injection pressure, injection rate, and the fixed number of perforations per foot. The wireless measurements give richer, more reliable data to optimize the model.
The biggest benefit was optimizing the wellbore cleanout process. They can now trend the surface or injection pressure at each water disposal well to detect stoppage. This is much better than single daily measurements.
Well-bore cleanout is expensive and time consuming, and they save $10,000 each time they avoid an unnecessary cleanout. This has saved at least one cleanout a year for each of the 15 wells, totaling $150,000 in savings.
Trended data gives much more insight into the condition of each water disposal well, and avoids erroneous conclusions that were too prevalent with single daily measurements. Equally important, they can optimize future development and avoid unnecessary drilling of new water disposal wells, all without the need for expensive and time consuming studies.
The success has led to four more water disposal wells being monitored since the initial installation, with more planned.
Wireless measurements eliminate the need to send personnel into the field on a daily basis, and provide a much better picture of what is going on at each water disposal well. This has enabled them to improve compliance reporting, improve operator productivity, optimize wellbore cleaning, improve well efficiency, and optimize future development.
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