Fine-tuned fracturing

Even after a year of falling prices early 2016 has seen some of the biggest drops in oil prices since the 2008 financial crisis. January saw the price of oil dip below US$30/bbl for the first time in 12 years. With conditions unlikely to improve significantly in the short term, the industry is faced with the challenge of maximising recovery rates and efficiency to keep production high but costs low.

Unconventional plays are acutely sensitive to these pressures and the challenge of responding to them is particularly tough as it is a market that has relied heavily on scale. With the current market conditions that strategy is simply much less viable. As a result operators are looking at ways to substantially reduce the break-even point on any given well for them to remain commercially viable.

It has been estimated that fracking a new well can cost up to US$8 million in addition to the drilling costs. An existing well can be refracked for the same or substantially less. Operators are having to reconsider the viability of continuing to open new wells or whether to recomplete and extract more from current wells.

Refracturing provides the way to access hydrocarbons left in place after the first completion and to increase the optimal recovery of the well. It has been estimated that only 8% of a reservoir’s oil from shale wells is recovered initially. Other data has suggested that many wells do not produce from every stage and that recovery rates can be as low as 5% in some unconventional plays.

With oil at US$30 /bbl, 5 - 10% recovery rates mean that many wells are simply not economic. As a result, operators are forced to leave huge numbers of wells inactive and it is extremely difficult to contemplate new completions. In a climate where every barrel counts, improving production efficiency for unconventionals and maximising recovery rates from existing assets is paramount.

The need for greater downhole intelligence

The key challenge for operators trying to improve efficiency is acquiring the right data during the frack process. Well engineers have limited visibility during this activity, so overcoming this challenge relies on gaining more real time intelligence. Operations now include the adoption and deployment of the latest sensing technologies to plan and monitor completion and production activities.

Hydraulic fracturing is inherently difficult to monitor and there remains significant uncertainty regarding various subsurface parameters including: effectiveness of chemical diverters, cluster contribution, and fracture propagation geometry among others.

One of the key sensing technologies is the distributed acoustic sensor (DAS). DAS is offering new levels of insight, changing the production economics in unconventionals when used in combination with the traditional distributed temperature sensors (DTS). These technologies working together give operators a multi-dimensional and dynamic profile of well conditions. Ultimately, the additional information DAS delivers is vital in removing the ‘guesswork’ from downhole activities.

By providing accurate and actionable intelligence to well operators, DAS enables real time adjustments in each stage. It also provides insights over a longer series of operations to maximise efficiency and safety. As this article will show, DAS can successfully enhance operations in a number of key activities such as:

Fundamentally DAS will minimise the resource requirements for each frack, thereby reducing the production cost for each well and enabling more wells to remain economically viable, even in a depressed market.

Moreover, deployment of this technology will turn refracking into an economically viable prospect, rather than a hit and miss operation. DAS provides the data needed to make it a much more controlled process throughout the operation, enabling engineers to see where the refrack is being directed and to make adjustments in real time to get maximum benefit.

What is DAS?

The DAS interrogator unit uses a laser to send thousands of pulses of light every second into an optical fibre deployed in or near an asset. A small amount of that light returns to the DAS interrogator through the process of backscatter, which DAS continually monitors. When sound and vibrations disturb the fibre, the characteristics of that backscatter are changed. The DAS interrogator analyses those changes to identify and locate the disturbance.

In downhole applications the optical fibre cable is run along the entire length of the well, either permanently installed outside the casing in cement or deployed by an intervention method, such as wireline, slickline, or coiled tubing. Here the acoustic disturbance at each point along the fibre allows engineers to ‘visualise’ and record what is going on downhole in real time. Acoustic energy can be generated by different events in the well bore, such as fluid flow and leakage, movement of proppant and rock or cement fracturing.

Without DAS, operators are reliant on surface data such as pressure and flow rate. This information does not allow operators to monitor a hydraulic fracture operation in real time from within the well or determine what is actually occurring downhole. In contrast, the enhanced visibility that DAS delivers allows engineers to gain a far greater short and long-term understanding of completion, production performance and well integrity. This not only helps to focus time and effort on value-adding activity, but also increases the success rate of operations and improves the efficiency and safety of well completion.

DAS systems are capable of being deployed on new wells or retrofitted to old. If in-well fibre optics is already in place, then DAS only requires the deployment of new surface equipment.

The DAS system also allows for a number of other applications throughout the life of a well. This includes the ability to monitor warm-back, flow back and production, as well as providing continued well integrity for the whole lifecycle of the well. It also enables borehole seismic vertical seismic profiling (VSP), which can be used to further optimise well performance and guide the placement of future wells.

DAS and frack optimisation

By providing a real time log of the operation, DAS provides a clear indication of fracture performance in the exposed formation. DAS data is acquired and interpreted to help build an understanding of the entire operation, giving operators a multi-dimensional and dynamic profile of well conditions. This enables real time adjustments to be made in each stage, as well as providing insights over a longer series of operations, maximising efficiency and safety.

During a standard first time frack operation DAS can track the wireline logging equipment that goes into the well in the initial phases. DAS can provide assurance that the perforating guns, or the devices to set the packers, are on depth, that the packer is set properly and that the guns fire. In a traditional cement and perf operation, the DAS also enables engineers to see that the fracture balls seat on the right packer.

Conversely, in a refrack operation the DAS allows operators to understand the current state of production in a well and which zones are producing, inactive or depleted. This information means it is possible to design a refrack specifically suited to that well. It is then also possible to confirm the placement of the chemical diverters before monitoring the actual frack pumping.

During the operation DAS provides far more sophisticated insights to help engineers fully optimise the frack. With an accuracy of 1 - 2 m, DAS not only assures engineers that the process is being carried out successfully, but confirms if there are any downhole events that need to be addressed.

For example, engineers will see where integrity failures are causing cross stage communication issues or opening up previously fracked stages to frack fluid. Both scenarios can potentially waste significant amounts of resource. Although the operational impact depends on the severity of the integrity issue, DAS highlights problems as soon as they occur. This alerts engineers to cease activity and adapt to re-establish an effective and efficient operation. Well engineers also face significant challenges in deciding how pressure is introduced in the frack and how to optimise pumping application. Pumping too hard, too soon risks destroying the cement rather than penetrating the formation as intended. Furthermore, errors in the pumping application risk driving the fractures out of the formation of interest altogether – again leading to the wasting of water, proppant and time. By enhancing the fracture process design to optimise how pressure is introduced, DAS helps operators ‘cooperate’ with the demands of the frac, minimising cement damage and keeping the frack within stage. This enables jobs to be concluded far more efficiently in terms of time and resources, rather than ‘fighting’ the frack.

Other data sources may also be introduced to further enhance the monitoring solution – such as micro-seismic, geophysical, petrophysics and rock mechanics. Fotech’s oilfield logging service, for example, deploys some novel techniques to view and correlate these combined data sources. Correlation of these inputs allows engineers to understand the relationships between them and to provide a more complete understanding of what is happening below the surface.

Post-operation analysis

As well as accurate and reliable real time monitoring, operators also want to ensure that production at each well site is being maximised, both immediately for each fracture operation and over a series of operations.

For this post-process analysis, Fotech’s Helios DAS system provides an in-depth data and visualisation report within 30 minutes of completing a frack stage. This data helps engineers to rapidly analyse and significantly enhance performance of each stage in a well incrementally. For an entire well bore Helios DAS quickly highlights the stages that have taken the most sand and fluid.

By analysing the DAS data from a larger sample of operations, engineers determine operational inefficiencies and assess what may need to be changed in future. Furthermore, correlating both distributed temperature and acoustic data over time under flowing conditions also allows for new insight into flow profiles and reservoir response to surface operations. Divergence from an idealised temperature and/or acoustic profile can quickly identify and precisely locate and diagnose operational problems.

Conclusion

As this article has demonstrated DAS has a fundamental role to play in terms of enhancing production efficiency. The combination of real time monitoring and post-operation processes means that DAS presents a powerful new well surveillance tool to enhance production and rapidly diagnose operational issues.

This is crucial to understand. DAS and other optical sensors represent a value-add rather than a cost in a downturned market. Fibre sensing systems such as DAS are the only technologies that can deliver the spatial information and insights from within the well during the frack process that operators need to improve efficiency and recovery.

Distributed fibre optic sensing is also likely the key to making refracturing and the recompletion of wells not just a consistently economically viable process, but also preferable to drilling and completing new wells. The technology has the potential to enable old wells to achieve the same or better production rates than the original completion.

When margins are squeezed and production efficiency is more important than ever before, operators need to capitalise on the tools available to them to bring down the overall break-even point for wells. It is only by embracing these technologies that operators can ensure a brighter future for the oil and gas industry.

Read the article online at: https://www.oilfieldtechnology.com/special-reports/14102016/fine-tuned-fracturing/