Riding the acoustic wave


Mark Tolley, Acoustic Data, UK, examines the benefits of a real time acoustic monitoring system that measures downhole pressure and temperature.

If a method exists where wellbore pressure and temperature data can be brought to surface more cost-effectively then it should be utilised.

This is however the nub of why, until now, acoustic communication systems including wireless, EM and fluid pulse systems have remained accessible only to those operators with high value and high production rate wells. It is a comfortable fit for these high priced systems and operators accustomed to high budgets; after all, no one ever got fired for buying IBM. As for the rest of the wells in the world, their data is obtained from wireline deployed memory gauges and cabled gauge systems; all priced to what the market can stand.


Figure 1: Hourly bottomhole pressure chart.

Causing disruption

Acoustic Data set out to disrupt the status quo in wellbore pressure and temperature acquisition. Over a 10 year period the company produced the AD250 real time acoustic communication monitoring system that is designed to break all of the industry’s pre-conceptions on how downhole pressure monitoring should be.

The company designed a system in which its downhole gauge measures the pressure and temperature, writes this to onboard memory, and then at pre-programmed intervals anywhere from 45 seconds to several days, takes a sample data pair, converts it into an encoded acoustic sound burst and transmits it to surface via the steel tubing wall.

At surface the signal is picked up by an intrinsically safe matchbox-sized sensor magnetically attached to the tubing head, completely without wellhead penetrations. From there the signal is routed to a shoe box-sized receiver/decoder and the data is simultaneously displayed in an OLED screen, saved to an SD card and sent by Modbus 485 communication protocol to any SCADA system.

The company’s greatest achievement has been in developing tremendous signal generation energy efficiency. A single gauge powered by two C sized batteries can transmit data to surface from a depth of up to 2500 m and can do this over a period of years depending on data rate.

Cost savings

Economically viable and ‘green’ acoustic wellbore communication technology is now available to almost every well in the world, facilitated by the most important break with tradition, a dramatic reduction in cost. At £20 000 to £50 000 all in, depending on depth for five years of daily real time P&T data, operators Capex and Opex expectations can be slashed, and considering the current oil price this is timely.

The next important break with tradition is how compact the AD250 system is. It can be hand carried it to site in a Pelicase. The savings in logistical costs and carbon footprint associated with mobilising and demobilising your wireline pressure surveys and cabled gauge installations, particularly offshore are dramatic.

Simplicity of installation and operation brings further savings by a reduction in personnel count and associated mobilisation costs. One of the company’s engineers or a technician attends the first installation with a new customer, trains their operators how to run the system and then leaves it for them to manage. If requested, site visits can be arranged to change batteries and handle the gauge data when the gauges are pulled and re-run, but how many experts does it really need at site. Desirably none and that is the goal.

To underline the point, it takes approximately 5 minutes to mount each gauge on tubing, and this is done on the catwalk well before the gauges are required to be run in hole. As the completion progresses the tubing with gauges are picked up and run straight in hole so there is no incremental well completion time as with cabled gauges for example.

Life of well

Another significant break with tradition is that the AD250 system was designed as a life of well instrument. Designed by an ex-operator for operators the downhole gauges are serviceable at well site, do not require redressing and do not require re-calibration. The piezo resistive pressure sensor is accurate to <0.05% FSD with a resolution of 0.01 psi and pressure measurement stability actually improves with time.

Acoustic Data has never considered servicing HPHT wells or high data rate drilling applications because the major service companies already do this very well. The gauge’s operating pressure and temperature limits are 700 atm and 125°C. Instead, it critically analysed what made economic sense to operators running the thousands of brownfield wells that need wellbore data to optimally produce them to the economic limit.

This means wells such as slugging, water-producing low rate gas wells and the struggling rod-lifted oil wells that usually cannot afford to be instrumented so that their real IPR potential is never known. Traditional pressure survey costs just bring their shut-in forward. In response to this need, we provide the operator with low cost, daily, real time data over a sufficient duration to permit them to manage their wells to extended economic limits. These gauges can then be pulled and moved to a different well, re-run and the process continued at no further cost.

A conversation that the company regularly has with new customers is about data rate, and it generally takes some discussion to move reservoir engineers away from 1-second data expectation to 1 hr or greater. The proof is in the ability to analyse the data recovered. It is clear from these snapshots that 1 hourly data is a vast improvement over occasional fluid level shots and that pump-off control can be effective with 1-hourly data. Our experience is that even less frequent data generally produces data of sufficient resolution for analysis of pressure buildup and derivative plots and certainly for IPR trends.


Figure 2:AD250 gauge with three repeaters.

Ageing assets

Ageing oilfield assets, particularly offshore assets like unmanned platforms require low cost wellbore data for the reasons stated above. In the context of acoustic communication in wellbores, Statoil and BP for example explain in recently published articles how they are experimenting with intervention-based ‘cableless’ alternatives to retrofitting a cabled or permanent downhole gauge where these had failed or were not available. The economics for acoustics are compelling.

To provide a solution for these sorts of applications, the AD250 was made deployable in any circumstance; on wireline installed hangers as a retrofit in completions where workovers are undesirable or uneconomical, on clamps on the outside of tubing for packer-less completions and on welded internally ported mounts for packer-based completions.

There has been high interest for the wireline run hanger retrofit solution, which ran up against an unexpected problem. The gauge hanger rental companies charged so much for hanging the gauges that the gauge system became uneconomic relative to conventional pressure survey methods. Acoustic Data therefore designed its own low cost, compact and simple gauge hanger system as an ‘enabler’ for deployment of the acoustic gauge system.The gauge hanger will hang gauges produced by other companies and will be released to the market in general on a sale and lease basis later in the year.

Other applications

There are other perhaps unexpected applications for this cost-effective acoustic technology.


Figure 3: AD250 downhole gauge and surface retriever.


Figure 4:Four minute installation on the rig floor.


Figure 5: AD250 gauge being run in hole.


DAS

Some operators cannot measure wellbore pressure or temperature with their distributed acoustic sensing (DAS) system and there are few solutions available to remedy the issue after the cable is cemented in the hole. By placing the acoustic gauge near the DAS cable in the wellbore the pressure and temperature data could be measured and transmitted to surface over the DAS without any repeaters, a simple, low cost and high value win for an operator.

CO2

In the high CO2 well scenario where tubing needs to be pulled every year or so operators definitely do not want to be running cabled PDHG systems, its enough trouble if the tubing parts by itself without birds-nesting cable in the hole as well. Running the standard super-duplex gauge provides a good solution and is well suited for stripping out of the hole without killing the well.

H2S

There are some operators producing high H2S content oil and gas wells. A routine annual pressure survey can transform into a nightmare when the safety and anti-venting measures are accounted for in the operations plan, particularly in populated areas. The company’s Inconel version of the AD250 is designed to stay in the wellbore for the expected time between workovers allowing the operator to avoid the HSE issues associated with wireline pressure survey operations.

Conclusion

The wireless smart well has long been a holy grail for operators and thanks to recent advances in acoustic communication technology, manufacturing efficiency and operator friendly pricing this is now a reality.


Adapted by Louise Mulhall

Published on 26/08/2015


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