Boosting well production to maximum sand-free rate

Calibrating flows with non-intrusive devices

Dag Aldal
Clampon

This ultrasonic intelligent sand sensor processes the signal at the site and sends the reading to a monitor. The unit is non-intrusive and is simply clamped onto the flowline.

Sand production from un consolidated formations in oil and gas wells has been a worldwide challenge for the petroleum industry for many decades. The challenge is not merely to avoid or stop sand production, but to be able to maintain commercial well productivity after efforts to control sand are implemented. At the same time, the control method selected must be justified by a reasonable payback time of the investment cost.

Produced sand is a major problem in many production situations since small amount of sand entrained in the produced fluid can result in significant erosion and erosion corrosion problems. Even in "sand free" or clean service situations where the sand production rate is only a few pounds per day, erosion damage could be very severe at high production velocities. Sand erosion can also cause localized erosion damages to protective corrosion scales on pipe walls and result in accelerated erosion/corrosion damage. In a high velocity gas well, sand erosion is a serious problem since it can erode holes in the pipework in a very short time period.

Reservoir problems

Produced sand can result in damage to the reservoir, where in some cases the reservoir collapses as a result of the sand production. Again, this scenario is a costly experience for the operator who has to overhaul and complete the producing reservoir zone.

The most commonly used practice for controlling sand erosion in a gas and oil producing well is simply to limit the production (or the flow velocity of the fluid). Quite often, we see that producers worry over the consequences of sand production such that oil and gas production is seriously limited. Guidelines for this practice are stated in American Petroleum Institute (API RP14E).

When sand is produced, it usually comes in batches - in other words, in large or small quantities. However, the time period for the batches can vary both for the individual well itself as from well the well. When sand has been produced constantly over a period of time, one should be aware of the conditions within the reservoir itself.

The above factors are the basic reasons for measuring sand production. By using a monitoring device with a high degree of repeatability and sensitivity, producers are able to avoid erosion, corrosion, and reservoir damage, but can also increase the oil and gas production rate. The increase in production can be dramatic. To be able to do this, you need repeatability, sensitivity, and real time measurement.

Measuring sand flow

There are several techniques available in order to measure or detect sand production. They can usually be divided into two groups - intrusive devices and non-intrusive devices. The intrusive devices can be divided into the following categories;

Intrusive sensors, where the gas intrudes when a hole has been eroded in the sensor and activates an alarm.
Tuning fork principle, which works on acoustics.
Erodible resistance probes.

Erodible resistance probes appear to be the most commonly used devices today. This type uses the "Wheatstone Bridge" as a principle for measurement, and a well-proven working principle. However, all the intrusive systems have some disadvantages due to their intrusiveness. Sand and particles in the production flow cause erosion of the probe and it has to be replaced periodically. In addition, the devices are not able to give the user a quick response to sand produced. It lacks real-time performance.

When installing these units into the pipe, installers have to be aware that the point of location is critical. If the probe is installed in a flow pattern or regime that is not repre sentative for the actual conditions, wrong readings will result.

During installation, welding and drilling of holes has to take place. When the unit has been installed, it cannot be relocated easily. When replacing such an existing probe, one has to remove pressure from the flow line or use a high pressure tool to replace the existing probe.

Non-intrusive devices

The clamp-on acoustic type monitors are installed following a bend in the flow. When the flow passes the bend, particles (chalk or sand) will be forced out of the flow and impact the inside of the pipe wall, generating an ultrasonic pulse. This ultrasonic signal is transmitted though the pipe wall and picked up by the acoustic sensor itself.

One type of clamp-on device in use today is the ClampOn 2000, manufactured by ClampOn. Installation of the device takes a short time. No welding or drilling of holes takes place. Several sensors can be connected to one computer or directly to the SCADA supervisory system. The ultrasonic head uses a small diameter area (5 mm) where the paint has been removed. Between the ultrasonic head and the pipe wall, silicon grease is used to improve the contact.

The signal from the field sensor can either be send directly to the main computer (SCADA or other) or to a ClampOn computer. The distance between the sensor and the computer, which is located in a safe area, can be as much as 2,000 meters. Twisted pair cables are used to transmit signals. With use of fiber cable, signals can be transmitted up to 5,000 meters. In the case of the ClampOn computer, the signal is transmitted from the field in ASCII form and the measurement results are presented to users in a Windows program.

Windows gives the user an easy and well known environment to work in. The report window gives the user quantitative information on sand produced in grams per second, minutes, or hours. The user has different ways of showing trends, such as maximum and minimum of produced sand over a period of time. Signal data can be stored up to a year. The results can be exported to Microsoft Excel and printed out by the user when needed.

Maintenance of devices

The most critical part for all ultrasonic systems is the field equipment. Through development of the system in cooperation with users, certification institutions and sub-suppliers, the sensor has become compact and extremely sensitive. For example, the sensor is able to take pipe wall surface temperatures as high as 180°C. All parts are 316 rated (or customers' specifications) to protect ultrasonic and electronic parts. No special maintenance is required. If failure occurs, the sensor should be disconnected and ship back to the supplier for replacement.

The three main problems for non-intrusive acoustic systems are sounds generated by other sources than sand particles from the well. These include electrical interference, mechanical structural noise, and noise from liquid/gas mixtures.

In development of the ClampOn system, mechanical and structural noise has been avoided by choosing a high frequency band where these problems do not exist. A simple test for a user is to hammer on the outside of the pipe wall. The system should not show pulsing on the monitor. This can be eliminated by the choice of a frequency band, the type of ultrasonic head used, and the fact that the processing takes place in the sensor itself.

A more serious problem is that other existing systems on the market are pulsed by noise made by liquid/oil mixture or gas mixtures. A good sand monitoring system should, when sand is not present in the flow line, show a very calm and steady line on the computer screen. Too often, systems show sand-free oil and gas production when there actually is sand. The consequence for the operator is that the production throughput must be reduced unnecessarily. ClampOn's solution to this is to have the brain of the system in the field together with a sensor working in the correct frequency range. A good signal-to-noise ratio helps eliminate this problem.

Calibration

Accuracy and repeatability of sand monitoring systems are two key elements. Such sand-monitoring words as calibration, accuracy, and repeat ability can sometimes be confusing. For some oil producers, accuracy is not of great importance; their main concern is repeatability. They need equipment that provides the best possible repeatability (ClampOn's is better than 1%). A sand injector can be used on site to verify performance.

Other oil production systems need a high degree of accuracy. The reason is that the volume of sand produced is a concern. In such cases, producers are concerned about how the sand affects the piping (erosion); how the sand affects the reservoir itself; when do they have to clean out the separator. The sand volume is also important because it has to be removed.

For reference, the ClampOn 2000 monitor, when calibrated in-house, has a deviation in the range of 25-50%, with a repeatability of plus or minus 1%. When the meter is installed at the well site and adjusted to read for the actual flow conditions, then it should be in the range of 15-25%, with a repeatability of plus or minus 1%.

When field calibrating by using a sand injector, the meter is installed downstream of the sand injector. Sand is then injected into the flowline to adjust/verify the ClampOn 2000 particle monitor. The accuracy should be in the range of plus or minus 5-15%. In other words, a repeat ability of plus or minus 1% should be expected and not dependent on the method of calibration.

After the monitor is installed on the well, the well should be operated for some time in order to determine the zero level for the monitor. This procedure allows for adjustment to background noise. Usually, this procedure requires around 30-60 minutes.

If the well is already in production and producing sand, it should be detected and shown immediately on the computer screen. The repeatability is still plus or minus 1%, but the the accuracy will suffer. Our experience is that sand is never produced continuously, therefore we measure the trend until a sand free period comes up in order to establish the zero level.

Obtaining sand-free rate

Too often, operators are not not fully aware of the rate of sand production, so the production rate is reduced unnecessarily. A cutback in production in the range of 20-75% is fairly common in oil and gas wells. Bearing in mind the values such production limitations represent, it is well worth evaluating sand monitoring systems as a way of increasing production without high investment costs.Also, it is important to have a system that responds rapidly and accurately, to improve the sand detection. Usually, the operator chokes back production immediately when sand is present (or the operator believes sand is present).

An accompanying graph shows a producing well in which sand production is declining. The curve represents what we call a "good" pattern. As the figure shows, sand is being produced due to the increase in production (opening the choke valve). However, by using a reliable sand-monitoring system, the operator can monitor the development of the sand production. The figure shows how sand production is being reduced over time, due to consolidation of the producing reservoir. When this pattern develops, the operator knows that the production of oil and gas can continue at this level since a sand-free well will soon result.

When the well is finally flowing, with no sand production for some time, the operator can once again open up the choke and increase production. This will again probably result in sand production as shown in the figure. However, the operator should let the production continue in order to observe the trend in sand production.The operator is searching for a "good" pattern such as that shown in the graph. When this curve appears after a time, the operator has a consolidated reservoir.

This method of increasing production requires time (usually a couple of days) until the operator sees the opposite - a "bad" pattern - meaning that sand production is increasing.

When the "bad" pattern appears on the screen, the operator restricts production by returning to the previous setting of the choke valve. Then the operator knows the maximum sand-free level of the well, and reduces oil and gas production to the previous sand-free level. Then the well is produced over a period of at least 24 hours to ensure that the formation is consolidated and stable.

Production optimization

In the following, we take a closer look at well production optimization workover for Conoco in the UK sector. The curves are marked and we are going to examine those that indicate choke position versus sand production. The accompanying graphs show how much sand is being produced in grams per second, the choke opening in percent, pressure in bars, and temperature in degrees Celsius.

The early peaks on the sand curve, at 11:00 to 12:00, were caused by testing the system. The test involved scratching very fine sandpaper on the pipe wall. At 12:00, the choke opens up to 15% and runs for one hour, after which it is opened 20-30%. At this choke position, the sand begins to flow immediately. This shows that sand is loosened from the pipe wall when the choke opening is increased.

At 17:00, the choke is once again opened up 35%. The well starts to bring up more sand from the wellbore. This is sand from the well itself, and in approximately half an hour, the sand rate begins declining. Production continues at this level and begins bottoming out at 18:26. Production continues at this level throughout the night in order ensure that no sand is being produced from the reservoir. By 11:00, the well is clean and is producing virtually no sand.

At 11:35, the choke is opened to 40% and once again, the well begins to produce sand. Opening the the choke gives the well a "kick," and this can be seen clearly in the sand production curve. Sand production continues to increase and probably comes from the reservoir itself. This sandstorm lasts from around 14:30 to 15:20. After 15:00, there is a clear reduction in the quantity of sand coming to the surface.

At about 16:40, we can see that the pressure is increasing. This means that the reservoir is consolidating. There is a reduction in sand produced and at some point, the well will be free of sand. However, in this case, the platform was not able to handle the sand being produced, so the choke setting from reduced from 40% to 35%. Sand production is kept to a minimum and continues to decline. Production stabilized at gradually higher positions, and eventually, the choke position was moved up to the 60% mark.

Examples of well optimization programs run in the North Sea with the use of ClampOn equipment include the following: BP's gas production on Cleeton was raised from 115 MMcf/d to 180 MMcf/d; BHP's production on Ravenspurn was increased by 300%; Conoco's production on Caister was raised by 35%; and Statoil's production on Statfjord was increased by a value by US $540,000 per day.