Wells – Processes – Cementing – plugging or consolidating
Reexamination Certificate
2000-08-23
2003-09-30
Bagnell, David (Department: 3672)
Wells
Processes
Cementing, plugging or consolidating
C166S285000
Reexamination Certificate
active
06626243
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to methods and compositions for cementing, and, more specifically to methods and compositions for cementing in cold environments. In particular, this invention relates to methods and compositions for well cementing in permafrost environments utilizing low heat of hydration mixtures of hydraulic cement, aluminum silicate and accelerators.
DESCRIPTION OF RELATED ART
Cementing is a common technique employed during many phases of wellbore operations. For example, cement may be employed to cement or secure various casing strings and/or liners in a well. In other cases, cementing may be used in remedial operations to repair casing and/or to achieve formation isolation. In still other cases, cementing may be employed during well abandonment. Cement operations performed in wellbores having relatively cold temperatures, i.e., bottomhole circulating temperatures typically less than about 50° F., may present particular problems, among other things, in obtaining good wellbore isolation. These problems may be exacerbated in those cases where wellbore and/or formation conditions promote fluid intrusion during or after cement curing, including intrusion of water, gas, or other fluids. Furthermore, relatively cold temperatures may lead to excessive thickening times, resulting in costly delays while waiting on cement to cure (“WOC”).
Deepwater well operations typically include operations performed on offshore wells drilled in water depths over about 1,000 feet (especially in Northern latitudes), and more typically, greater than about 2,000 feet deep. Under deepwater conditions, relatively cool temperatures promoted by seawater, in some cases coupled with poorly consolidated formations often make the prevention of fluid intrusion during cementing a challenge. In such cases, relatively cool temperatures (typically less than about 60° F., and more typically, less than about 50° F.) may slow cement curing or hydration, extending the transition time of a cement slurry. Transition time may be defined as the time required for a cement slurry to develop gel strength, or quantitatively as the time for a cement slurry gel strength to go from 100 lb/100 ft
2
to 500 lb/100 ft
2
.
Because longer transition times means that the gel strength of a cement increases relatively slowly, there is more opportunity for intrusion of water or other fluids, such as oil or gas, to migrate through or displace a cement slurry. When such fluid migration occurs, channels, pockets or other cavities may form in the setting cement. Such cavities or channels may create a permanent flow passage or otherwise compromise the integrity of a cement sheath, such as exists between a pipe string and a formation. Furthermore, intrusion of a fluid such as water may dilute a cement slurry and thus prevent it from developing sufficient compressive strength. Fluid migration into a cement is typically more extensive when cement transition times are lengthened because although the cement column in a wellbore has typically built enough gel strength to support itself and to thereby reduce hydrostatic pressure on the surrounding formation, it has not developed sufficient gel strength to prevent fluid intrusion or migration. Although reduced gel strength, extended transition times, and fluid intrusion during cement curing are problems commonly encountered in deepwater completions, such problems may also be encountered in any wellbore having relatively cool formation temperatures, such as in wellbores drilled in cool or cold climates.
In those cases where formation sands are overpressured by fluids such as gas and/or water, fluid intrusion into the setting cement during the cement transition time may be a particular problem. In this regard, shallow formations in deepwater wells typically are unconsolidated, making them weak, prone to fracture, and prone to producing relatively high flows of water. Such a problem may be further exacerbated in those situations in which a relatively lightweight cement slurry is required. Such situations include those in which formations are susceptible to fracture, such as naturally weak or unconsolidated formations, or those with reduced bottom-hole pressures. Lightweight cements typically have longer transition times at relatively cool formation temperatures. Such cements are often referred to as “water extended cement slurries.” Due to the relatively long transition times of water extended or lightweight cement slurries, there is increased opportunity for fluid intrusion and cement contamination. Such contamination may result in the loss of formation isolation and/or in casing damage. Resulting cement job failures may result in many undesirable consequences, such as the need for expensive remedial work, increased rig time, loss of production, and/or loss of the wellbore itself.
In cold weather regions, such as the Arctic, the temperature of shallow formations may not exceed 32° F. for several hundred feet of depth. Such formations are typically referred to as “permafrost” which denotes a permanently frozen subsurface formation. Depending on the location, a permafrost or frozen section may extend from a few feet to depths greater than about 1500 feet. In such situations, even where fluid intrusion is not a problem, a cement slurry may not have the opportunity to set and provide needed strength before it freezes. Conventional methods for downhole cementing in permafrost formations have traditionally employed gypsum/Portland cement blends. As compared to conventional Portland cements, these gypsum/Portland cement blends offer reduced BTU output when hydrated, and therefore reduced degree of permafrost melting during and after cement placement. Gypsum/Portland cement blends are also noted for an ability to set under freezing conditions. The density of conventional gypsum/Portland cement blends typically ranges from about 12.0 pound per gallon (“ppg” or “PPG”) to about 15 ppg. These cement blends typically contain from about 20% by weight of dry blend (“BWOB”) to about 40% BWOB Portland cement, and typically suffer from low compressive strength and high cost.
In some wellbores, gas intrusion may be a particular problem during and after cementing. Such wellbores include, for example, those where a wellbore penetrates a gas formation having a pressure corresponding to a first pressure gradient and a relatively underbalanced permeable zone having a pressure corresponding to a second pressure gradient that is lower than the first pressure gradient. In such cases, hydrostatic pressure exerted by the cement slurry may keep gas intrusion from occurring while the cement is still fluid. However, due to chemical hydration of the slurry and/or dehydration of the slurry across the permeable zone, the pore pressure of the slurry may decrease below the gas pressure in the reservoir allowing the gas to enter the cement. This underbalanced pressure may result, for example, in gas channeling to the surface or to another lower pressure permeable zone.
SUMMARY OF THE INVENTION
Disclosed are cement compositions and methods which, in one embodiment, may be formulated with aluminum silicate and metal sulfate, such as aluminum sulfate, to achieve improved gel and/or compressive strength characteristics in relatively low temperature environments and/or in relatively short periods of time as compared to conventional well cements. Such cement systems may be characterized by the ability to form cement slurries having relatively short transition times, a characteristic which may be particularly advantageous in cold environments and/or in wellbores having relatively weak formations and fracture gradients, both of which are typically found in deepwater offshore wells. Further, the disclosed cement compositions may be formulated to have reduced heat of hydration as compared to conventional cements, making them well suited for cementing in permafrost environments, or in other cold environments such as those where the soil surface temperature does not exceed 32° F. and/or those environments wher
Bagnell David
BJ Services Company
Dougherty Jennifer R.
O'Keefe, Egan & Peterman
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