Controlling pressure
In the surface interval of offshore wells, shallow-water flows from overpressured shallow sediments can be a challenge during casing and cementing operations. In deepwater, this challenge can be exacerbated because of the proximity of the overpressurized zones to the mudline, the relative water depth and deepwater drilling techniques. “When we go to cement across the section, we have to design a slurry that can keep the shallow-water flow in place and not let it penetrate or flow,” Simon Turton, Halliburton’s Strategic Business Manager for Cementing, said.
To combat the shallow-water flows, Halliburton has developed Deep FX-L, a liquid cement additive that enables the cement to reach critical gel strength in an average of 11 minutes and stem the shallow-water flows. It allows gel strength to build by virtue of the calcium-silicate-hydrate setting. Without the additive, Halliburton says, this would take hours. Critical gel strength is defined as 500 lb/sq ft.
“It’s assumed that when a slurry reaches that number, then it is impenetrable by gas, oil, water or by any other substance that might flow,” Mr Turton said. The additive has no known slurry limitations and can be used in both heavy and lightweight cements, according to the company.
The Deep FX-L additive has undergone field trials in the deepwater Gulf of Mexico, and Halliburton is currently commercializing this product.
The company is also developing additives to address challenges that occur further downhole. “We quite often find that the deeper we go, the lighter cement that we may need to use,” Mr Turton said. Lightweight cements with densities under 15.8 ppg are often used to achieve the lower ECD required to stay within the narrow pore pressure and fracture gradients common in deepwater wells.
There are a few ways to make a lightweight cement. The first is to add more water to the cement, but this can compromise the final properties of the cement, Mr Turton said. Adding a gaseous phase – using nitrogen, for example – can make a lightweight foam cement. However, there is often not enough space on the rig floor to accommodate the equipment required to add gas to cement. Such equipment includes tanks and potentially a nitrogen converter. The final option for creating a lightweight cement is to use a lightweight particulate additive.
Halliburton is preparing to launch a new lightweight particulate additive, Liquilite, later this year. It consists of hollow glass spheres suspended in a liquid phase and is added to the cement as it is going downhole. Existing light-weight cements use hollow glass spheres blended in with dry cement before being mixed and pumped downhole. This creates an opportunity for the glass spheres to be crushed while blending the cement and transferring the dry cement, especially as pressure is used for both of these two processes. Service companies then have to use additional spheres to compensate for glass spheres lost to crushing. With Liquilite, this crushing effect is avoided, allowing Halliburton to do away with extra glass spheres and, in turn, reducing the cost of the additive.
Both the Liquilite and Deep FX-L additives are added as the cement is pumped downhole, not pre-blended with the cement. This allows the operator to increase or reduce the amount of additive used on the fly, if needed. “If they don’t use it on the job, there’s no waste cost or disposal associated,” Mr Turton said. “It’s just the neat cement that’s left in the tanks and it can be topped off and then used on the next hole section.”
Halliburton’s Advantage-1 deepwater cementing unit, which was designed and built in 2015, can mix liquid additives into the cement as it’s pumped into the annulus. In addition to a 25-bbl, three-component mixing system, the unit features a six-pump liquid additive system with an automated metering and pumping unit. The PLC-based system can be run from a cabin that can be placed anywhere on the rig.
To ensure the system’s reliability, Halliburton built health diagnostics into the unit that perform predictive maintenance to alert the crew to any potential failures or issues before they occur. Diagnostic software monitors more than 300 parameters related to the system’s critical components and deliver alerts on any maintenance needs. “Reliability is a very important feature,” Mr Turton said. “The deepwater environment is one in which we need to operate perfectly every time we do a job.”
Measuring success
When it comes to evaluating the quality of a cement job, the density of the cement and the thickness of the casing can present challenges for traditional acoustic cement evaluation tools. These tools often either fail to properly detect the presence of lightweight or foam cements or perceive these cements as being poorly bonded to the formation or casing.
“Logging lightweight cement slurries has been a challenge in the industry for a long time,” said Rajdeep Das, Product Champion for the Integrity eXplorer cement evaluation service at Baker Hughes. On the flip side, these technologies can be confounded by heavy fluid, which interferes with the representative evaluation of the cement. Operators rely on accurate cement evaluation data to determine if cement has correctly bonded to the casing and formation and provided the needed zonal isolation. “It’s important for the operator to know if he has the zonal isolation to drill further,” Mr Das said.
In mid-2015, the company launched the Integrity eXplorer cement evaluation service, which uses electromagnetic acoustic transducers. These sensors generate and propagate shear and lamb acoustic waves along the casing itself, rather than through the borehole fluid, to provide acoustic measurements of the cement bond to the casing. “This is the only type of transducer that can produce shear horizontal waves on the casing today,” Mr Das said. “These waves are the only true indicator of solids behind the casing.”
Operators commonly run into a problem when using traditional cement evaluation services to evaluate lightweight cement – which is often used in deepwater to minimize hydrostatic pressure in weak and depleted formations and in narrow pressure windows – or when cement becomes contaminated with borehole mud. Lightweight or contaminated cements have low acoustic impedance and, thus, can be misinterpreted as a partial or nonexistent bond by traditional cement evaluation services.
“Logging lightweight cement slurries has been a challenge in the industry for a long time,” said Rajdeep Das, Product Champion for the Integrity eXplorer cement evaluation service at Baker Hughes. On the flip side, these technologies can be confounded by heavy fluid, which interferes with the representative evaluation of the cement. Operators rely on accurate cement evaluation data to determine if cement has correctly bonded to the casing and formation and provided the needed zonal isolation. “It’s important for the operator to know if he has the zonal isolation to drill further,” Mr Das said.
In mid-2015, the company launched the Integrity eXplorer cement evaluation service, which uses electromagnetic acoustic transducers. These sensors generate and propagate shear and lamb acoustic waves along the casing itself, rather than through the borehole fluid, to provide acoustic measurements of the cement bond to the casing. “This is the only type of transducer that can produce shear horizontal waves on the casing today,” Mr Das said. “These waves are the only true indicator of solids behind the casing.”
Operators commonly run into a problem when using traditional cement evaluation services to evaluate lightweight cement – which is often used in deepwater to minimize hydrostatic pressure in weak and depleted formations and in narrow pressure windows – or when cement becomes contaminated with borehole mud. Lightweight or contaminated cements have low acoustic impedance and, thus, can be misinterpreted as a partial or nonexistent bond by traditional cement evaluation services.
In addition, the transducers also generate lamb waves, which are able to detect the presence of a microannulus, a small micron-level gap between the cement and the casing. Most cement evaluation services would classify a microannulus as debonding in a regular run. “Correctly detecting it as a microannulus is very important, so that the operator can determine the need for any necessary remediation,” Mr Das said. Because traditional cement evaluation services would not be able to properly identify a microannulus, he said, operators would have to pressurize the casing and do an additional run to detect a microannulus. However, because the Integrity eXplorer’s transducers generate lamb waves, it can detect a microannulus in the same run as the cement evaluation.
Electromagnetic acoustic sensor technology also allows for quality cement evaluation in the presence of thick casing. “Deepwater is one of the prime markets where you use a lot of thick casing because of the high downhole pressure,” Mr Das said. “In terms of deepwater cement evaluation, I think that evaluating cement through thick casing is one of the biggest challenges that exists.” Thicker casing has a higher impedance to acoustic waves, which can be misinterpreted as the presence of greater amounts of cement. “The thicker the casing gets, the more critical it is to obtain accurate measurements of the annular material. We believe this technology has overcome that hurdle and can provide the accurate measurements needed to be successful,” Mr Das said.
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