All frontiers invariably inspire expeditions. This has been true for the exploration and development of hydrocarbon as it was for the discovery and exploration of Mt. Everest. And it has been true for the frontier inside the bore wall, behind the casing. Advanced materials, methods, and wellbore designs are needed in this environment for safe and economical resource production.
While ultra-deepwater represents an epic frontier for our industry, there is a global network of scientists, chemists, rheologists, mechanists, and engineers who are pioneering a realm of the microscopic: the molecular frontier behind the casing. Their job is to interrogate molecular bonds, fluid flow dynamics, and the formation of hydraulic seals in spaces as narrow as three quarters of an inch.
|The low yield point of resin enables it to flow freely into micron-sized leaks without prior acid clean up.|
Drilling has always been the route to hidden hydrocarbon. It is generally agreed that the first oil well was drilled in 1859 in Pennsylvania with a method known as cable-tool drilling, the lifting and dropping of a drill bit. When the rotary drill punched onto the scene in the 1880s, it efficiency opened a route through the overburden to the hydrocarbon-bearing strata.
Those drills were put on wheels, and these wheels created opportunity, but also limitation. Wells could only be drilled where the drilling rigs could roll. This included the first offshore wells. The rigs rolled to the end of wooden piers built off the Santa Barbara coast of California in 1896. Then came the concept of floating, not rolling, a rig to where the oil was. A barge sailed beyond view of the shoreline in the Gulf of Mexico in 1947; at the end of 2012, there were 569 offshore rigs actively pursuing oil and gas, 44% of which were specifically designed and built for deepwater.
The frontier emerging behind the casing is smaller than ever. Fracture gradient windows are narrowing, flow paths are tight and tortuous, and mechanical demands are increasing. In this annular world there is one constant: the need for confining, isolating, and protecting multitudes of strata from the upward migration of gas and fluids.
In 1919, when Halliburton ran the first cementing job in Oklahoma, the goal was not to transform oil and gas wellbore architecture, but to help reduce risk and solve a problem. Nearly 30 years later, the company was part of the offshore frontier, cementing the first offshore well. As early as 1929, the company was investigating the properties of cement and the phenomena of cement hydration. These investigations expanded to the study of variables that could lead to incomplete cement placement or a loss of zonal isolation: the effects from mud contamination on cement thickening time and tensile strength; increases in drag or incomplete mud removal due to casing standoff and interface geometries; and increases in displacement pressures due to narrow pathways for bypassing fluid, to name a few.
|Conventional resins (left) react exothermically with water. WellLock resin (right) is compatible with water.|
These investigations have resulted in hundreds of cement additives designed to optimize the chemical, rheological, and mechanical properties of cement blends, and to adjust performance to meet the challenges of each individual well. The goal in this world of chemistry, fluid dynamics, and zonal isolation always has been to help operators balance the cost versus ultimate recovery equation while contributing to environmental safety.
Obtaining a 360° annular seal for the life of the well is never easy. It is a realm defined by equivalent circulating density, in situ corrosive elements, flow dynamics, thermal gradients, pressure and temperature fluctuations, regulatory statutes, and economics.
Cement remains the primary way to establish a barrier against the upward migration of fluids via the annulus, simply because cement provides the balance between cost management and pump-to-placement before setup. However, those two attributes have been challenged by ultra-deepwater, extreme downhole conditions, and unconventional source rock.
A cement alternative
Laboratory-modified resins for commercial use can be traced back to the 1860s, but the use of resins in the oil field begins some 80 years later. Once introduced, it was clear what resins had to offer: superior adhesion, resistance to many caustic and corrosive chemicals, excellent mechanical properties such as low yield point and low viscosity in the unset state, and flexibility and toughness after setting.
|Throughout the transition from a liquid to a solid state, the specially formulated resin continually transmits hydrostatic pressure to the formation until a gas impermeable barrier forms.|
Despite these promises of performance, practical application of resin as an annular barrier has remained just out of reach. Getting a resin downhole requires easy mixing and pumping without hardening before placement. Moreover, exothermic reactions triggered by water could cause damage downhole or to the surface equipment used for mixing and pumping.
In the past year, however, WellLock resin has overcome some challenges and has been applied in situations where conventional cement cannot effectively go. This resin lends itself to specific adjustments of rheology, density, and curing time to allow reliable placement. The resin can withstand impurities in the wellbore without significant degradation in performance and is compatible with water. The resin can be pumped in a fluid train ahead of or behind aqueous fluids without issues, and can be mixed and pumped through conventional equipment that may have aqueous residue.
As a result of this resin's compatibility with water, it has been used successfully to P&A an offshore well in the Gulf of Mexico that represented a closed system because the platform sheared away during a hurricane. The well was unable to accept fluid, yet regulations required decommissioning. A technical team determined that a weighted, low-viscosity fluid could be placed with minimal injectivity, yet provide high-compressive strength after setting. The resin ultimately displaced the seawater in the well without generating heat downhole. The system remained in cohesive association – in other words, the resin formed a polymer network based on covalent bonds to develop a single molecule, the size of which is determined by the volume of material. This single molecule retains optimum shear bond values and tolerates up to 30% contamination without compromised mechanical properties, while resisting cracking.
P&A operations are nearly as active as new installations. Yet, due to the increasingly complex installations and challenging environments, many operators are faced with the fact that conventional solutions may not address the challenges of a particular well. Care must be taken to avoid contamination, or channeling of wellbore fluids into the cement plug.
The resin system was used in 2012 in another Gulf of Mexico well that could not be decommissioned by conventional methods. After an initial cut on the casing, bubbles were observed coming from the annulus. The probability that this bubble stream would channel through cement was high because pressure could not be held on the cement plug. In this case, the resin was used in a squeeze application, stopping the annular leak. Subsequently, a 50-ft (15-m) resin plug was set. The resin has a low yield point, and due to its ability to be formulated free of solids, it can penetrate small casing leaks, micro-annuli, or gravel pack pores without the risk of particle bridging. Unlike cement, there is no prior clean-up required, eliminating the time and cost of an acid treatment. In this case, permanent abandonment was established.
Sally Charpiot is manager, marketing & business analysis, cementing, at Halliburton. Paul Jones is the company's principle scientist, cementing.
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