How Do Expandables Work?

How Do Expandables Work?

Expandable tubulars are downhole pipes that are plastically enlarged in-situ with a conical swage (or hydraulic expansion) to restore or increase internal diameter, seal off problem zones, or create near-monobore well architectures without sacrificing drift.

I. High-Level Purpose and Value-Chain Placement

• I.1 Position in the value chain: Well construction and workover/intervention—bridging drilling and completion by preserving hole size, restoring casing integrity, and enabling zonal isolation when conventional casing programs are constrained.
• I.2 Core objectives:
  • Diameter preservation: Maintain drift when adding a string (e.g., contingency liners, monobore designs).
  • Integrity repair: Patch leaks, parted casing, or damaged connections in cased hole without pulling tubulars.
  • Zonal isolation: Seal thief zones, water/gas breakthrough intervals, or unstable formations.
  • Slot recovery and conductor repair: Re-enter and restore legacy wells with minimal removal of existing tubulars.
• I.3 Economic thesis: Defer sidetracks, reduce telescoping, cut rig time and materials, and expand operating envelope in narrow pore/fracture gradients.

II. Step-by-Step Process Flow

• II.1 Candidate selection and design
  • Define use case: openhole expandable liner, cased-hole patch, or near-monobore section.
  • Run caliper/tortuosity and review casing tally; confirm achievable expansion ratio and dogleg severity limits.
  • Engineer post-expansion ratings (burst, collapse, tension, compression, sealability) and drift.
  • Specify expansion method (hydraulic cone push, mechanical pull, or hybrid) and fluids (rheology, filtration).
• II.2 Make-up and pre-job QA
  • Assemble expandable string with pre-machined features (grooved wall, ductile section, special connections).
  • Make up anchors/hanger, seals, polished bore receptacle (if used), and running tool.
  • Pressure test and function test the expansion system; confirm cone clearances and stroke.
• II.3 Run-in-hole (RIH)
  • RIH at controlled speed; monitor drag and ECD; condition hole for debris/ledges.
  • Land at depth; set anchor/hanger (openhole or inside casing).
• II.4 Expansion
  • Displace to clean, filtered fluid; engage the swage/cone at the expandable’s shoe.
  • Apply hydraulic pressure and/or mechanical overpull to drive the cone through the tubular, inducing controlled plastic deformation.
  • Maintain rate/pressure envelope to avoid under- or over-expansion; track progress via stroke, pressure, and returns.
• II.5 Post-expansion operations
  • Pressure test the expanded string; drift/ID gauge run; verify top-of-expansion.
  • For cemented cases, pump tail slurry matched to reduced annular clearance; clean out.
  • Set top packer or tie-back if required; log for isolation confirmation.
• II.6 Contingencies
  • If cone stalls, work string (gentle reciprocation/rotation if allowed), adjust flow, or reverse circulate debris.
  • For under-expansion, perform re-pass (within design limits) or set remedial seal assembly.

III. Major Equipment/Components and Functions

IV. Key Physics and Governing Equations

IV.1 Expansion ratio and strains

• Expansion ratio: \( ER = \dfrac{D_{\text{exp}}}{D_{\text{run}}} \)
• Hoop (circumferential) engineering strain: \( \varepsilon_\theta = \dfrac{D_{\text{exp}} - D_{\text{run}}}{D_{\text{run}}} = ER - 1 \)
• Assuming near-constant volume through thickness (estimated), wall thickness after expansion: \( t_{\text{exp}} \approx \dfrac{t_{\text{run}}}{ER} \)

IV.2 Thin-wall pressure to reach yield during hydraulic expansion (estimated)

• At onset of plasticity: \( \sigma_\theta \approx \dfrac{P\,D_m}{2\,t} \Rightarrow P_y \approx \dfrac{2\,t\,\sigma_y}{D_m} \)
• Where \( D_m \) is mid-wall diameter, \( \sigma_y \) is flow/yield stress at downhole temperature.

IV.3 Cone force requirement (simplified)

• Axial force to drive cone: \( F \approx \dfrac{\pi\,D_m\,t\,\sigma_{\text{flow}}}{\sin\alpha}\,\bigl(1 + \mu\,\cot\alpha\bigr) \)
• Where \( \alpha \) is cone half-angle, \( \mu \) is friction coefficient, \( \sigma_{\text{flow}} \) is effective flow stress during plastic work.

IV.4 Post-expansion ratings (first-order estimates)

• Burst (thin-wall): \( P_{\text{burst}} \approx \dfrac{2\,t_{\text{exp}}\,\sigma_{\text{uts}}}{D_{\text{exp}}\,SF} \)
• Plastic collapse (long cylinder, simplified): \( P_{\text{coll}} \approx 2\,\sigma_y\,\dfrac{t_{\text{exp}}}{D_{\text{exp}}} \)
• Both must be de-rated for cold work, ovality, connections, temperature, and bending.

IV.5 Hydraulics during expansion

• Pressure losses: \( \Delta P \approx f\,\dfrac{L}{D}\,\dfrac{\rho v^2}{2} \)
• Equivalent circulating density: \( \text{ECD} \approx \text{MW} + \dfrac{\Delta P}{0.052\,\text{TVD}} \) [ppg]

IV.6 Worked example (estimated)

• Assumptions: 7.000 in run OD, \( t_{\text{run}} = 0.317 \) in, \( ER = 1.10 \), \( \sigma_y = 80{,}000 \) psi, \( \sigma_{\text{uts}} = 95{,}000 \) psi, \( SF = 1.1 \).
• Mid-wall diameter pre-yield \( D_m \approx 6.683 \) in; required pressure to yield:

  \( P_y \approx \dfrac{2\cdot 0.317 \cdot 80{,}000}{6.683} \approx 7{,}590 \) psi (add 20–40% for friction/dynamics ? ~9{,}100–10{,}600 psi, estimated).

• Post-expansion thickness: \( t_{\text{exp}} \approx 0.317/1.10 \approx 0.288 \) in; expanded OD: \( D_{\text{exp}} = 7.700 \) in.
• Estimated burst: \( P_{\text{burst}} \approx \dfrac{2\cdot 0.288 \cdot 95{,}000}{7.700 \cdot 1.1} \approx 6{,}480 \) psi (not including connection de-rates).
• Estimated plastic collapse: \( P_{\text{coll}} \approx 2 \cdot 80{,}000 \cdot \dfrac{0.288}{7.700} \approx 5{,}980 \) psi.

V. Key Performance Drivers

• V.1 Expansion quality control
  • Stable cone travel (pressure/overpull control) to avoid local thinning or wrinkling.
  • Clean, filtered fluid to minimize scoring and stick–slip of the cone.
  • Wellbore tortuosity within spec to limit ovalization and dogleg binding.
• V.2 Efficiency and cost
  • Higher expansion rate with minimal re-passes; optimized swage angle and friction reducers.
  • Reduced trips and cement volumes compared with additional conventional strings.
  • Rig time savings vs. sidetracks or full casing retrieval.
• V.3 Integrity and reliability
  • Post-expansion ratings that meet worst-case loads (pressure, temperature, bending).
  • Seal system design for differential pressures and thermal cycles.
• V.4 Safety and emissions
  • Well control readiness during high-rate pumping; verified barriers before/after expansion.
  • Material savings and fewer trips reduce embedded carbon and fuel burn.

VI. Typical Challenges and Mitigations

• VI.1 Cone sticking or erratic advancement
  • Cause: Ledges/debris, high friction, ovality, solids.
  • Mitigation: Pre-job conditioning, high-quality filtration, lubricious pills, controlled WOB/overpull, short reciprocation (if permitted).
• VI.2 Under- or over-expansion
  • Under: Reduced ID and sealability—perform re-pass within limits; validate with drift/ID log.
  • Over: Excess thinning—tighten pressure envelope, limit surge/pack-off, monitor wall strain trends.
• VI.3 Post-expansion rating reduction
  • Issue: Cold work and thickness loss de-rate burst/collapse; connections are critical points.
  • Mitigation: Conservative load cases, de-rate factors, temperature-dependent properties, and quality control on connections.
• VI.4 Cementing in tight annuli
  • Issue: Narrow annulus raises ECD and channeling risk.
  • Mitigation: Low-ECD spacer trains, right-angle-set fluids, fine systems for laminar gaps, centralization planning.
• VI.5 Thermal and cyclic loads
  • Issue: Thermal expansion/contraction can challenge seals and anchors.
  • Mitigation: Use compliant seal stacks, PBRs, and validate via thermal–mechanical cycling analysis.
• VI.6 Corrosion and sour service
  • Issue: Cold work may affect SSC resistance.
  • Mitigation: Appropriate metallurgy, inhibitors, and de-rated sour service envelopes.

VII. Why This Matters Economically and Operationally

• VII.1 Access and reach: Preserve hole size to TD through troublesome intervals without adding multiple telescoping strings.
• VII.2 Cost avoidance: Avoid sidetracks, fishing, or pulling damaged casing; reduce cement and steel consumption.
• VII.3 Production assurance: Rapid zonal isolation and water/gas shutoff restore or protect production.
• VII.4 Flexibility: Provides a contingency option in narrow windows or unexpected losses without abandoning the well plan.

Quick Recap

• Mechanism: A cone plastically enlarges a specially engineered tubular in place, recovering ID and establishing seals.
• Success factors: Clean fluids, controlled pressure/overpull, suitable metallurgy, and load de-rates.
• Value: Time and cost savings, reduced risk, and enabling well objectives that conventional casing alone cannot achieve.