Researchers continue to overcome difficulties involved with cement bonds in multilateral wells and fluid contamination during spacer operations.
Both of these operations can dramatically affect the overall success of a producing well.
This conclusion of a two-part series provides information on cement techniques used to improve the bond at the junction between the pipestring and formation and ways to reclaim costly drilling fluid from a well.
Multilateral sealant
Logistic and economic considerations have recently contributed to an ever-growing interest in multilateral drilling for offshore exploration. In such operations, branches and laterals that extend from a parent wellbore are drilled and completed to broaden the exploration base of the hydrocarbon-producing formation beyond the limitations of the parent well.
Establishing a multilateral system often permits the operator to tap a deep, complex reservoir more economically with a single wellbore than with an array of independent wells harvesting multiple reserves from the same reservoir.
Thus, from a single drilling platform, a limited number of drilling slots allow various secondary zones to be drained. This strategic design can provide production viability to marginal fields and, at the same time, reduce the quantity of drill cuttings for disposal.
Three characteristics are vital to the success and acceptability of a multilateral system:
- Connectivity: Defines the mechanical union of the lateral wellbore to its parent. The lateral casing is only one size smaller than the main casing, allowing the introduction into the lateral drillstring of larger downhole tools.
- Isolation: Segregates the parent well from the lateral leg and is required because of pressure differentials that exist between the various zones.
Isolation of the laterals from each other and from the parent well requires cementing the lateral liner with a two-stage cement job. The initial phase uses conventional cement, and the second phase requires a special mix for improved impact resistance.
- Access requirements: Allow individual lateral reentry with jointed pipe, coiled tubing, or wireline. With ease of access, full-bore service tools and other oilfield equipment may be installed in any lateral to facilitate workover operations without an onsite rig.
In 1996, a multilateral system was developed that incorporated connectivity, isolation, and access in a single system. The comprehensive system, which has full lateral liner connectivity, a hydraulically isolated junction, and full-bore reentry access, was the first to provide full-bore access to the lateral without impairing the integrity of the main wellbore.
The deepest multilateral junction and longest lateral well of its class to date is in the North Sea. Early customer involvement in the development, planning, and application of the system contributed to its success.
Tangential lines from the main wellbore may vary in size and degree of deviation from vertical; some lines, in fact, may be oriented horizontally. Multilateral wells can range from simple, openhole completions to sophisticated cased-hole completions with selective access to all laterals in the system.
Unlike openhole wells and slotted-liner completions, cased-hole wells provide additional control over production as such completions permit reentry during perforation and logging operations.
Where the hydraulic seal between the casing interior and the formation is affected only by a sealant, however, zonal isolation becomes difficult to achieve. The sealant system must therefore withstand many destructive forces during completion and production.1
In the quest for a sealant that ensures zonal isolation, researchers used mathematical models to quantify the levels of stress exerted on the sealant during the life phases of a multilateral well.
Additional studies involve test procedures involving:
- API fluids.
- Linear expansion.
- Abrasive wear.
- Permeability.
- Triaxial testing.
- Resistance to both impact and chemical influence.
- A calculation of the Young's Modulus.
Researchers found that superior elasticity and strength, needed to endure these impacts as well as standard oilfield cementing properties, were imperative to maintain an effective seal.1
Workers tested two radically different slurries:
- The standard Class G cement containing a common fluid-loss additive and retardant that becomes strong, but brittle in its set state.
- The non-Portland elastomer, or latex-based fluid exhibited good elasticity that has inferior strength when set.
Strength and elasticity
Believing that the ideal formula lay somewhere between these two extremes, researchers settled on an elastomer with a high-butadiene ratio injected with stabilizers, thinners, retardants, and other additives. They also determined the ideal ratio of elastomer to cement.1
Altogether, four slurries were blended and examined, and the effectiveness of incorporating fibers into the mix was also studied. In the final analysis, the slurry containing a high concentration of latex by weight of cement (BWOC) was determined to be the optimal choice.
The criteria that singled out this multilateral junction sealant (MJS) included solid placement properties, mechanical-strength profile (indicating sufficient strength to withstand more than 100 stress cycles with little initial deformation), and impact resistance capable of sustaining more than twenty 50-Nm blows.1
Multilateral junction sealant has logged a successful application record for numerous offshore wells in the North Sea. In one such operation, a multilateral well was drilled after an old well was abandoned and sidetracked to a new parent well from the same slot.
Because of a low TD fracture gradient and a hole diameter of 8.5 in., a stage tool was run in the 7-in. liner 70 m below the junction to ensure the appropriate placement of uncompromised sealant across the junction.
The first stage of the operation placed the top of cement 500 ft (152 m) below the stage collar to eliminate the possibility of cement preventing the collar's opening. The second cementing phase customized the multilateral junction sealant to the conditions of the well.1
After the field crew ran a 150-bbl (23.8 cu m) spacer train of alternating viscous and thin fluids to ensure good mud displacement, the multilateral junction sealant was pumped into the wellbore at 7 bbl/min (1,113 l./min). Once this slurry had been placed, the drill pipe was removed from the hole and the well was circulated.
Subsequent production has subjected the sealant to drawdown pressures of up to 2,700 psi; yet the multilateral junction sealant continued to isolate the junction.1
Variable-viscosity spacer
A Gulf of Mexico operator recently used a variable-viscosity, optimized rheology (VVOR) spacer in combination with a precise job design to remove and reclaim costly drilling fluid from a well during cementing.
The use of this single-operation procedure, along with the superior performance of the spacer, saved 1 day in rig time along with associated expenses. These savings, along with the value of the salvaged drilling mud, amounted to about $150,000. Moreover, the cost of a remedial job was avoided.
In this case, the primary objectives included returning the mud to the surface without contamination from either the cement or spacer fluid, achieving optimal cement placement, and eliminating subsequent squeeze operations.
The relatively stable water-based spacer rheology, formulated throughout an extended shear range, ensured a more consistent flow velocity and provided an ideal interface with the synthetic drilling fluid.
The VVOR not only removed the mud removed from the annulus efficiently and in an unadulterated condition, but the integrity of the cement bond also remained competent.
Wellbore cleansing
The principal purposes for primary cement operations include:
- The creation of a strong bond between the pipestring and formation.
- Zonal isolation from formation fluids by sealing the annulus.
- Prevention of casing deterioration.2
If these goals are to be met, the wellbore must be free of residual materials deposited by the drilling mud at the time the cement is pumped into it. These materials are often left in the drillstring and annulus in the form of partially dehydrated, gelled (PDG) mud; moderately gelled (MG) drilling fluid; and dried filter cake.
These elements can cause cement channeling and bonding irregularities, which can lead to improper displacement and cementing that renders the annulus and formation susceptible to lost circulation, downhole stress, and other harmful effects. The gel strength of the MG and PDG must be broken, and as much of the mudcake as possible must be expunged to prepare the wellbore for the introduction of the cement.2
Interfacial isolation
Another major consideration of cementing is the mutual isolation of the drilling mud and cement during the displacement process. If the slurry and mud are permitted to commingle at the interface, incompatibilities in their components can seriously damage the chemical and physical integrity of the cement and ultimately lead to well failure.
Variations in available cements, their additives, and drilling fluids can be quite extensive. Incompatibilities are common, primarily due to the effects of "bentonite-yielding" in water-based drilling fluids or the wetting properties of oil-based muds.
"Bleeding" the two fluids into each other during interfacial contact generally produces a thick mass that can be deposited on the annular face for an extended period of time.
When materials in the drilling mud contaminate the cement, such critical properties as curing time and compressive-strength development are adversely affected. Other consequences include:
- A rise in circulation pressures and the concomitant loss of fluid to the formation.
- Impaired mud replacement.
- A loss of zonal isolation.
- Corrosion or collapse of the pipes.
- The eventual need for an expensive squeeze job.
Spacers, preflushes
To solve displacement and contamination problems, the oil industry has developed spacers and preflushes (Fig. 1). These fluids, which are pumped downhole in the interval between the other two fluids, are similar because they effectively displace the mud with the cement, purge the wellbore of drilling-fluid residue or other impurities, and help ensure that the composition of both the slurry and mud remain intact during displacement.
Spacers and preflushes differ principally in their yield-point design. The yield point allows lost-circulation agents and weighting materials to be added to the spacer formulation and allows the spacer to bring solids procured in the wellbore to the surface.
The weighting of the spacer helps prevent gas cutting of the cement by controlling formation stress.
Versatility factors
VVOR can displace both water and oil-based drilling fluids, and its formula can be tailored to the wellbore geometry stipulations and drilling-fluid properties of any operation. Additionally, it can be adapted to a broad range of BHCTs from 60 to 325° F. while its yield point can be changed to satisfy needed criteria.
Through an adjustment of the base VVOR mixture concentration in the mix water, spacer viscosity can also be adapted to meet the demands of the drilling mud.2 The components of VVOR have an intrinsic chemical congruity with both conventional cements and drilling-fluid systems.
VVOR is compatible with most water-based muds, cement mixes, and brines. The use of VVOR with oil-based muds, however, requires adding suitable surfactants that create water-wet surfaces to enhance the cement's bonding properties. Surfactants may also be added to help purge the wellbore and prepare it for the cement.2
The inherent compatibility of the VVOR is further expanded by the use of dispersant in the spacer formula that allows adjustment of the rheological profile. The spacer's rheology curtails the possibility of bypassing mud in the narrow side of an eccentric casing-formation annulus-a particularly important function, because all annu* have eccentricities.
The stability of this rheology permits greater versatility in the fine-tuning of the interposed shear adjustment between variations in slurry and mud rheologies without disturbing the rheology balance of the spacer itself.2
VVOR's base mix consists of a single-sack powder that forms a viscous fluid when blended with fresh water, seawater, NaCl-saturated water, or KCl brines. It can be batch-mixed or it can be mixed on-the-fly if all components are preblended in bulk form.
If the materials are prepared in bulk-cement equipment, an iron-control agent should be dissolved in the mix water beforehand to help ensure the proper development of the spacer's rheology.2
References
- Xenakis, H., Vijn, P., and Covington, R., "A New Sealant System for Multilateral Junction Geometries," SPE paper 38495 presented at the Offshore European Conference, Aberdeen, Sept. 9-12, 1997.
- Crook, R.J., et al., "Spacer Tech Saves," Hart's Oil and Gas World, April 1998, pp. 37-42.
Spacer-fluid research
Displacement mechanics were first scrutinized in 1940 by P.H. Jones and D. Berdine. During this time, investigation into spacer fluids had no place in early research, which instead focused on basic displacement factors.
In the mid-1960s, R.H. McLean, C.W. Manry, and W.W. Whitaker promoted water preflushes. They thought that heavy drilling fluid would release itself from casing walls and fall into a less viscous, flowing liquid if the displacing fluid could not support it. Although workers studied the effectiveness of various spacers, no field testing took place.
Within a decade, L.L. Carney performed wellsite research in his quest for a universal spacer. Carney, who questioned the adequacy of invert-emulsion muds, lower-viscosity oil-based drilling fluids, and oil-based spacers, also advocated an alternative to costly multiple-spacer systems.
He tried to develop an agent that would prohibit the intermingling of the mud and cement slurry but would, simultaneously, be compatible with both. Aspiring to design a comprehensive spacer capable of being weighted at a prescribed level, Carney surmised that the separating fluid should be slightly heavier than the mud and somewhat lighter than the cement but contended that this balance was difficult to achieve with some types of spacers. Carney's emulsion spacer produced favorable results in four diverse-design, onsite applications.
By simulating field conditions through the use of a large-scale apparatus, researchers in the early 1980s examined low-viscosity, lightweight spacers. R.C. Haut and R.J. Crook discovered that such fluids appeared to remove mud completely through the erosion of filter cake and an increase in mud mobility.
Their research was founded on the premise that a drilling mud's mobility indicates its varying fluid properties downhole and that this mobility, along with flow velocity, governs displacement. Therefore, the spacers and slurries used in Haut and Crook's study were pumped at maximum speeds.
Crook, et al., also investigated the importance of filtrate fluid loss, the use of water as the spacer fluid, results derived from increases in velocity and mud mobility, and the infusion of sodium chloride in a water-based spacer.2
