Shale producers around the world have learned in recent years that they cannot take wellbore integrity for granted.
During pressure-pumping treatments, the interplay of hydraulic fracturing and formation geomechanics is deforming and even shearing steel casing. In the worst of cases, this means that some operators have lost access to long sections of a wellbore before it produces a single barrel of oil.
This is largely not viewed as an inter-well challenge similar to frac hits. Rather, it is considered to be intrawell phenomena. The problem can take several forms and has no universal driver.
George King, an independent consultant and leading technical voice on hydraulic fracturing, points out that not every unconventional asset is affected—indicating that geology and understanding of the rock fabric matter. However, information he has collected from client operators shows that in certain US shale and tight oil fields, between 20%–30% of horizontal wells are impacted to some degree.
“Sometimes it is with the cement support, and sometimes it is with the casing itself,” said King, listing two of the problem’s general root causes. He added that the casing deformation is “an artifact of some damage that we are seeing right now in the realm of fracturing, and [during] the time period of fracturing.”
A commonly reported version of the problem emerges after just a few stages at the toe-end of a horizontal well have been fractured. As another plug is pumped downhole to continue the completion, it suddenly stops 1,000 ft to 5,000 ft short of its target. This could happen in the bend, or it could be somewhere deep in the lateral.
Often the deformation is described as ovalling. If a restriction consumes much more than a tenth of an inch of the pipe’s original internal diameter, there is the potential that unstimulated stages beyond the stopping point are lost to conventional plug-and-perf methods. This fate can sometimes be avoided by running slim-profile perforating guns, but then other isolation technologies are still needed to replace conventional plugs.
Unconventional operators in China, Argentina, Canada, and the US have all faced the issue. In response, the technical community has recently stepped up collaboration efforts at industry conferences to find answers.
In China, the issue appears to be more widespread on a per-well basis. National oil companies working the Sichuan Basin, the country’s most active unconventional area, have reported that more than 40% of gas wells have experienced casing deformation or failures. Like their US counterparts, Chinese operators typically realize the problem during a post-fracturing tool run.
Argentina’s Vaca Muerta is not immune either. A 2015 technical paper coauthored by researchers at Chevron and YPF (URTeC 178620) casts casing shear and restrictions as common and “extremely detrimental to the performance of the well.”
When explaining why the issue is cropping up today, more than a decade into the shale revolution, experts are quick to point out that a lot has changed since the first horizontally drilled and hydraulically fractured wells.
“Nowadays, the problem is that they are not making only four or five fractures,” as operators did during the dawn of unconventional exploration, explained Arash Dahi-Taleghani, an associate professor of petroleum engineering at Penn State University. “Sometimes, you have 150–200 fractures that are closely spaced together and the injections rates are high.” He added that new diverter technologies might even be playing role too since their job is to build up pressures close to the wellbore. “All of these things can put too much stress on the casing, as was not the case before.”
Dahi-Taleghani has studied both the US and Chinese sides of the problem. He said theories about casing deformation are still evolving but affirmed that there are multiple causes.
They may work independently, but it is far more likely that a combination of conditions are lining up to jeopardize wellbore integrity. The most widely acknowledged trigger mechanisms reflect that both well planning and nature have roles to play.
They include:
- Weak or low-grade casing and couplings
- Poor or unsupportive cement behind the casing
- Leaking fracture plugs that allow for severe sand erosion of the casing
- The drilling of highly undulated or contorted wellbores which create nonlinear casing loads
- High pressures involved in hydraulic fracturing
- Effects of contrasting fracturing fluid and reservoir temperatures
- The response of rock layers, bedding planes, and faults to being fractured and fluidized (i.e., tectonic reactivation or slippage)
A warning being broadcast to shale producers is that their economic models often neglect these issues based on the assumption that casing failure is a low-risk outcome.
“But the more that you try to skimp by on your pipe design, your cement, your couplings—that type of thing—the higher the risk that well is going to fail before you can get a return,” emphasized King. “That’s going to change your net present value calculations.”
More Industry Dialogue Needed
Last July, more than 200 industry professionals crowded into a room at the Unconventional Resources Technology Conference (URTeC) in Denver where a panel made up mostly of industry consultants offered insights on hydraulic fracturing-induced well integrity problems. Petroleum engineers used the opportunity to ask dozens of questions of the experts.
Several inquiries centered around how to remediate casing leaks. Should a liner patch be used, or a resin squeeze? If a leak is detected while fracturing (indicated by an unexpected pressure drop), should the pumping stop? Or is it better to keep pumping and hope that the sand will bridge the hole?
How strong should the cement be? Would foamed cement better absorb some of the shear forces? Could corrosive bacteria living in the reservoir be exacerbating casing collapse? Can predictions improve by using integrated models that account for geomechanics, cement quality, and casing loads?
One person asked perhaps the most important question: Is it possible to eliminate casing failures? The answer to that one is no—at least not in every scenario. The answer to many of the other questions is “it depends.”
The industry remains in a position where describing the problem is easier than describing what to do in response. This is partly due to a lack of publicly available information. And not everyone shares the same experiences. Some companies may be seeing the problem in one well a year, others may see it in more than a dozen.
Outside of case studies from China, shale producers have produced few relevant papers on the subject in recent years. Most information about casing ovalling and deformation comes from the offshore arena where reservoir compaction due to production and injection has been a known issue for decades.
But the absence of knowledge sharing within the unconventional sector has left it without a consensus on best practices, or a strong set of field examples that demonstrate effective mitigation strategies. However, more information will soon become known.
At next month’s SPE Hydraulic Fracturing Technology Conference (HFTC) in The Woodlands, Texas, an all-operator panel will present case studies on the effects that hydraulic fracturing has on well integrity in various basins. The oil and gas companies set to present their findings include Shell, BP, ConocoPhillips, Encana (soon to be renamed Ovintiv), and XTO.
Terry Palisch, the global engineering advisor at Carbo Ceramics and the SPE technical director for completions, helped organize both the URTeC and upcoming HFTC panel. For him, the overarching goal has been to convince operators that now is the time to begin sharing information.
“We’re cognizant of the sensitivities,” he said, “but this is an industry issue that we need to solve, and we’re only going to solve it if we work together.”
Palisch added that as has been the case with frac hits, the shale sector tends to accelerate fruitful collaboration around the time that remediation efforts start coming into view. The panel at HFTC will provide clarity on whether the sector is reaching this point with respect to casing failures.
Why Now, Why During Fracturing?
Current stimulation strategies and their effect on the reservoir are by all accounts the biggest drivers behind casing deformation in shale plays.
Today’s completion prescriptions call for the application of high downhole pressures—a range that spans 8,000 psi to 14,000 psi. Inside the reservoir, these pressures are not always applied evenly.
Models have long showed operators that their stimulations result in so-called bi-wing fractures that are roughly symmetric. But thanks to diagnostics such as microseismic surveys, it is well understood that bi-wing fractures are a rarity. The norm is asymmetric fracturing, especially in infill scenarios.
In extreme cases, Dahi-Taleghani said some wells in China have seen 80%–90% of the stimulation energy impact on just one side of the reservoir rock. The issue was the subject of a paper he recently presented at the SPE Annual Technical Conference and Exhibition (ATCE) in Calgary (SPE 195944).
“When you are pushing to one side, you will develop some shear stresses,” he explained, adding that, “casing is very strong in terms of tensile stress or compressive stress, but not so much when it comes to shear stresses.”
A shearing event does not always cut a well apart, as the word implies, but it can deform the pipe enough to essentially close the wellbore. In the Chinese example, researchers concluded that when such failure mechanisms were activated, the fracturing of subsequent stages exacerbated the casing deformation.
Their proposal was to use new workflows and a 3D Earth model to better understand how the reservoir would react to asymmetric fracturing. Additionally, the study argues that better modeling enables “rational spacing design” and that coupling it with real-time microseismic monitoring will help operators actively mitigate the casing failures.
Pressuring up to extreme levels can also cause the pipe to balloon outward by a 100th of an inch. Such an expansion might seem small, but it is significant.
“As you continue doing the perforating and the fracturing, you get that pressure back again, so you’re cycling pressure down [the casing for] the duration of the frac job—and every time you do that, you’re expanding that pipe,” said King. And when pipe expands and contracts over and over, the door to fatigue issues opens.
Then there is the issue of temperature and its effect on steel casing. Bottomhole temperatures in a shale well may be in excess of 200°F, whereas fracturing fluids are usually pumped at surface temperatures that on a warm day might be 70°–80°F.
Cam Matthews, a research fellow at the Edmonton-based engineering firm C-FER Technologies, noted that the differential on a cold day might be as much as 200°F (i.e., 50°F injection water and a 250°F reservoir temperature).
“Every time you inject cold fluid at high rates for any reasonable period, you are transmitting that surface temperature down to the bottom of the wellbore,” he explained. “This affects the tubular from a mechanical loading standpoint, especially when you add it on to all the other stresses.”
The temperature variance also puts the pipe into tension, which if not for cement, would retract the pipe upwards by as much as 20–30 ft. While cemented casing is not likely to move to such a large degree, damaging axial forces may still build up. King explained that this causes some “back and forth” movement of the casing, at which point “you get into some dynamic loading and point loading of that pipe.”
Mitigation Begins with Well Planning
Though they are half a world apart, the Sichuan Basin and the Vaca Muerta share an important feature: they lie in the shadow of two of the world’s largest mountain ranges. In China, it is the Himalayas, while in Argentina it is the Andes.
Both ranges pull up the geologic fabric of the two shale basins like a tent, creating severe tectonic forces and fault zones. When unaccounted for, this could spell doom for multimillion-dollar wellbores.
If a fault near a wellbore is activated and slips, then steel casing is at the mercy of the Earth’s movements. This issue has been reported mostly in China.
Dahi-Taleghani said that, after years of trouble, Chinese operators now avoid fault-related shearing by spending more time detecting hazardous zones. Their next course of action is to not place fracturing clusters within 200–250 ft on either side of a fault. Aside from seismic data, Dahi-Taleghani said faults can be “fingerprinted” during drilling since they correlate strongly to lost circulation events.
Bigger sources of concern in the US and Canada center around the well-construction process, which for one thing means that the shape of a well is a factor. If a horizontal well features too severe a dogleg curve, or too many sharp undulations thereafter, then the casing is bound to suffer from a stressor called point loading, or nonlinear loading.
In simple terms, the sharper the dogleg, the higher the potential for casing fatigue. To understand point loading in this context, it helps to imagine how a paper clip weakens and eventually buckles or breaks as it is repeatedly bent backwards and forwards.
Reported dogleg severity is usually on the order of 7°–10° per hundred feet. However, this measurement can deceive since it includes only the points between the start of a stand of pipe and the end. When measurements are taken continuously inside the well, doglegs of over 30° per hundred feet have been recorded.
Long pipe intervals are not likely to move past such sharp angles, and while short intervals can, they are subject to point or nonlinear loading as they move downhole. Drilling gentler trajectories and smoothing out sections of undulations are among the recommendations being discussed to lower the number of stress points along the casing.
Point loading also becomes less of a concern when a quality cement job has been achieved. However, the definition of “quality” has been the subject of debate in the shale patch since the inception of horizontal well cementing.
King said that from a zonal isolation standpoint, most cement jobs do their job. Less certain within the shale sector is whether most cement jobs support the pipe from nonuniform loading imposed by the reservoir.
And while stronger casing may seem an obvious remedy, that is not what people studying the issue are calling for. Thicker walls and using higher-quality steel will not always solve these problems, and are often impractical options from an economic point of view.
“Casing is around 20% to 30% of the total well cost—that’s a huge amount of money, and because of constant budgeting issues, operators are choosing to pay the minimum cost of designing any well,” Christine Noshi, a petroleum engineer who researched casing integrity in unconventional resources as part of her graduate program at Texas A&M University.
Noshi, now an intern with Halliburton’s Landmark software division, presented her findings on casing failure prediction and prevention last year at the International Petroleum Technology Conference in Beijing (IPTC 19311). Among her conclusions is that the shale sector lacks the computational tools needed to scale up casing failure prediction.
“Everything has to be input by the engineer,” she explained. “There is no software today that automates the process—even the simplest workflow for designing a casing string—or tells you that there is a higher risk of a failure in this string or that will give you alerts about potential hazards.”
Matthews is also arguing that the traditional design methods are simply overwhelmed by the number of complexities involved in unconventional environments. For the most part, the calculations used today focus squarely on stress.
“You determine your allowable stresses, you figure out what the loads will be, then you put a safety factor on it, and life is good,” he said, adding that the only catch is that this simple workflow only truly applies to the conventional world. “The severe pressure-temperature cycling and potential formation movement-induced loads (strains) definitely represent different loading conditions and design considerations in these wells that are not traditional.”
Matthews said cementing could be improved by rotating the casing, a normal practice in vertical wells. However, rotating pipe is rarely done in the unconventional sector due in part to concerns over the inherent bending-fatigue and torsional-loading impacts it has on pipe connections.
This tradeoff also highlights that pipe connections are considered to be yet another common source of casing failure, especially in the build sections of horizontal shale wells. C-FER is one of several companies represented on an API committee that has been established to review industry standards for casing connections. The recently formed committee will help decide if new connection qualification protocols need to be drafted for their application in such horizontal well applications (SPE 194369).
Problems with Collars and Plugs
By their very nature, casing failures are often difficult to diagnose. Sometimes this is because the culprit stage is inaccessible by the time the problem is detected. But new technologies have made it easier to see the problems. Two of the companies leading this charge are EV and DarkVision.
In 2018, the visual-diagnostic specialist EV teamed up with Anadarko Petroleum (acquired last year by Occidental Petroleum) to deploy a downhole camera system that was integrated with a multifinger caliper tool. The project, described as a first-of-its-kind and detailed in a paper shared at last year’s HFTC (SPE 194252), demonstrated how the combination of visual data and caliper measurements offers certainty as to the source of casing deformations.
Anadarko elected to run the system into one of its west Texas wells after an obstruction was discovered while pumping down a plug and a perforating gun.
The system found dozens of casing collars that had lost some portion of their original internal diameter. Of those, four collars were found to have suffered severe ovalling. Normal collars in the well showed an average diameter of 3.92 in. while the deformed collars averaged 3.71 in. The collar closest to the hung up depth was just 3.46 in. diameter and another located a couple of hundred feet away was even tighter at 3.18 in.
The dual data set was used by Anadarko to conclude that, in this case, the driver behind the deformations was over-torqueing. This was supported by the fact that ovalization was also seen 1–2 ft above each of the anomalous collars.
Instead of cameras, DarkVision uses acoustic-based scanning technology to capture images of the entire wellbore as its tool moves through one. Operators have come to the Vancouver-based technology startup to analyze perforation erosion. In the course of that work, the company has also found unexpected well integrity issues—including the first well that the tool was ever run inside.
“There was just a big hole there, and we weren’t sure that there was a perforation although it didn’t line up with where the perforations were supposed to be,” recalled Stephen Robinson, the chief executive of DarkVision. “It turned out that the data lined up very well to where the plugs were set.”
In that first deployment, five casing holes were found; enough to persuade the operator to consider a different type of plug going forward. Not every plug failed in this well, or in subsequent wells, but DarkVision has only recorded such defects where plugs were set. “For the same reason that the perforations erode, there is proppant getting by and that creates a bigger and bigger leak path,” explained Robinson.
Such casing holes can lead to big pressure drops during hydraulic fracturing, which reduces the stimulation energy that is delivered into the shale matrix.
DarkVision has also seen several wells where plug erosion severed fiber optic cables attached to the outside of the casing. The high-definition diagnostic costs about $1 million per well and is among the most delicate technologies used downhole. A cut in the fiber is easily confirmed by matching the point at which a hole was discovered to where the fiber data stops streaming. “You don’t know where it’s going to start, or where it’s going to end, or what stages it’s going to affect. But every time you set a plug, you have a chance of breaching your fiber,” said Robinson.
For Additional Reading
SPE 184868 A Causation Investigation for Observed Casing Failures Occurring During Fracturing Operations by Neal Adams, Neal Adams Services, et al.
IPTC 19331 Data Mining Approaches for Casing Failure Prediction and Prevention by Christine Noshi, Texas A&M University, et al.
SPE 194369 Developing an Evaluation Method for Casing Connections Used in Hydraulically Fractured Wells by Kirk Hamilton, C-FER Technologies, et al.
SPE 195944 Impact of Asymmetric Stimulated Rock Volume on Casing Deformation in Multi-Stage Fracturing; A Case Study by Hao Yu, Southwest Petroleum University, et al.
URTeC 178620 Interference Behavior Analysis in Vaca Muerta Shale Oil Development, Loma Campana Field, Argentina by Milena Rimedio, YPF, et al.
SPE 194252 Long Lateral Restriction Diagnosed by Camera-Caliper Combo on E-Line Tractor in One Run by Allison Lay, Anadarko, et al.
