Cementing is an integral part of the drilling process. Operators appreciate the importance of good cementing designs and its repercussions on overall well costs and drilling schedules.
BP's cementing guru, Ashley Hibbert, believes that as more oil and gas wells are drilled in deeper waters, they demand increasingly efficient execution. The water temperature gradient from surface to seabed is nonlinear, and the properties of shallow formations vary from location to location. These factors add further challenges to the already difficult issues in deepwater drilling and impact the design of cementing operations more than is generally recognized.
Hibbert said better modeling of deepwater temperature gradients from surface to seabed is required to build a temperature history of the cement slurry during placement, and in lab testing to reduce rig time.
Conventional procedures used to design cement slurries are inadequate for deepwater applications because they do not thermally match the laboratory tests with the field behavior.
Workers at Halliburton and BP have developed a procedure to estimate realistic temperatures during slurry placement and waiting on cement (WOC). The temperature profile mainly depends on water depth
and temperature.
Hibbert said operators and contractors need to develop a better understanding of the mechanical behavior of cement in the annulus, adding this could allow cement formulations to be designed to optimize well life.
The US Department of the Interior's Minerals Management Service and the American Petroleum Institute have agreed to prepare a document on well cementing practices to mitigate annular flows.
Slurry designs
Another exciting area of interest is the introduction of very light and very heavy slurry designs, including Schlumberger's LiteCrete and DenseCrete technologies.
These new cement systems are designed to provide successful isolation where conventional cement densities or placement techniques would not work. This includes cementing weak formations that otherwise could not be completely cemented, the replacement of two-stage cementing operations with single-slurry cementing, longer cement columns in a single casing string and production liner cementing using lightweight cements that minimize hydrostatic weight.
The US contractor believes the elimination of two-stage cementing saves time, money and potential well problems. Full string replacement by liners cemented with low-density slurries saves rig time and casing costs and improves future value in terms of increased well diameters for completion mechanics.
LiteCrete technology is being pioneered in the Middle East. The Simsima and Umm El Radhuma are loss circulation carbonate formations. During drilling of the intermediate zone, lost circulation can occur with 8.6 lb/gal mud. Using the Crete technology, one Middle East operator was able to circulate 120 bbl of cement to surface successfully. The set LiteCrete provided 1,400psi in 24 hours at 337°F (170°C).
Other new products such as Flexstone offer long-term isolation in wells exposed to post-cementing temperature or pressure variations over time.
Schlumberger believes that with its Flexstone cementing technology, downhole stresses can be accounted for by adjusting set cement properties as easily as cement slurry properties are modified.
Flexstone uses nonmineral-based particles in proven engineered particle blends to create cements with an adjustable Young's Modulus and linear expansion after full cement hydration. Due to the constant solids content, the compressive strength and tensile strength remain higher than systems with comparable flexibility.
These systems are designed to replace conventional cements in wells expecting large changes in temperature or pressure during their lifetime or suffering from long-term cement failure. This would apply to large changes in wellbore fluid weight, production from deep water, tectonically charged areas, steam flood wells and gas wells developing annular pressure over time.
Flexstone is the first system in the oil
field designed specifically with long-term properties for zonal isolation. Using proven particle distribution technology, the system modifies the properties of the cement after
it has set and remediates long-term
cement failures.
Schlumberger claims the concept can help operators reduce well maintenance costs, ensure isolation for stimulation treatments and reduce a gas well's annular pressure during its producing life, and in cases of steam flood or tectonically charged areas.
In the gas fields of south Texas, wells can be lost to tectonic stresses. A flexible wellbore was created for the first new well in this project by placing concentric casing strings inside an annulus cemented with Flexstone. Schlumberger reported that after more than 1 year, this well has
shown no signs of tectonic damage and
is still producing.
Alternative
The third new product in Schlumberger's portfolio is Durastone, a cement alternative engineered to provide superior wellbore integrity during rugged drilling conditions.
The group claims this new technology offers the advantages of steel-reinforced concrete and provides higher compressive strengths, lower permeability, and flexural and impact strengths far superior to conventional Portland cement blends.
Durastone uses patented particle-size distribution coupled with a new stainless-steel microribbon reinforcing to create a Portland cement alternative with higher matrix strength and lower permeability than conventional cements. This new product is designed to increase cement durability for difficult well conditions. Primary applications would be multilateral junctions, windows, whipstock plugs, incompetent casing points, shoe tracks drilled with bicentered bits and zones intended for high-density perforating.
Durastone has been demonstrated in a test well in Oklahoma. Wells are pending for level 6 multilateral isolation in Venezuela and for improving cement sheath integrity in Saudi Arabia.
Foamed slurries
Hundreds of case histories have proven foamed cement is the preferred approach
to cementing casing across shallowwater flow zones.
According to workers at Halliburton, the benefits of foamed cement include vastly improved displacement efficiency and flow control due to expansive fluids in place during the phase transition from liquid slurry to gel.
However, specialized blends often are required so that the cement (foamed or nonfoamed) will function as intended at the low temperatures encountered across
these zones.
Foamed cement offers the option of changing slurry density on the job without starting over in the lab and retesting thickening times, fluid loss, free water and settling control. The capability to make last-minute density changes is especially important given that the tops of the water- flow formations and associated gradients vary greatly from well to well.
More importantly, the compressible nature of a foamed fluid allows it to expand once in place, maintaining a positive pressure against a formation.
A recently developed system greatly enhances the ability to make last-minute changes. Using all liquid additives, the system imparts water-flow resistance to class A or H cement. The performance properties of this system have proven acceptable for cementing across shallowwater flow zones.
This allows neat cement to be loaded on the rig, and the operator then can customize the slurry for various degrees of flow control as conditions develop.
In addition to all the slurry attributes of foamed cement, recent research suggested foamed cement is a superior annular sealant due to its ducal nature.
Experience in deep water has shown it is vital to maintain hydrostatic control to prevent severe washouts during a shallowwater flow. Foamed cements are well suited to prevent fluid influx and subsequent water flows during the cementing operation. Case histories have shown the combination of new riserless drilling fluids and new-generation foamed cements are effective in dealing with shallowwater flows.