How Do Solid Expandable Tubulars Work in Frac Applications?

At-a-Glance: Solid expandable tubulars (SET) are run smaller and plastically expanded in-situ with a swage to restore or create a near-monobore inside existing casing or open hole. In fracturing, they enable stage isolation, casing reinforcement, or refrac liners while preserving drift ID and pressure rating for plug-and-perf.

I. Objective Definition and Key KPIs

• I.1 Objective: Use SET to maintain or restore internal diameter and pressure integrity so multi-stage hydraulic fracturing can proceed safely and efficiently (new well, remediation, or refrac) with minimal ID loss and high differential-pressure capacity.
• I.2 Typical frac uses:
  • I.2.1 Preemptive reinforcement across weak/depleted intervals to prevent casing ovalization/deformation during high treating pressures.
  • I.2.2 Zonal isolation of thief/water/gas zones pre-frac to achieve target frac gradients.
  • I.2.3 Refrac liner with minimal ID loss to enable new stage architecture inside legacy casing.
  • I.2.4 Rigless repair of leaks or restrictions to regain drift for plug-and-perf.
• I.3 KPIs:
  • I.3.1 Retained drift ID vs plan (inches) and clearance to largest planned plugging tool (=0.25 in radial estimated).
  • I.3.2 Pressure integrity: pressure test at liner-top and across expanded section (e.g., = treating pressure + 10–20% margin).
  • I.3.3 Leak-tightness: zero sustained annulus pressure; seal acceptance (e.g., ISO14310 V0/V1 equivalent, if applicable).
  • I.3.4 Treating envelope: burst/collapse margin =10–30% over maximum expected surface treating pressure (estimated).
  • I.3.5 Operational performance: run/expand success >98%, NPT hours, expansion length per hour, and debris-free wellbore post-expansion.
  • I.3.6 Stage enablement: number of stages regained/added vs plan; frac-rate capability (bpm) within friction limits.
  • I.3.7 Post-frac: no deformation alarms; plugs pass/mill as planned; sustained rate/pressure performance.

II. Critical Parameters and Target Ranges

Key Mechanics (formulas)

• II.1 Hoop plastic strain during expansion:

  $$\varepsilon_\theta \approx \ln\!\left(\frac{D_f}{D_i}\right)$$

• II.2 Thin-wall hoop stress during frac:

  $$\sigma_\theta \approx \frac{p\,r_m}{t} \quad\Rightarrow\quad p_{\text{allow, burst}} \approx \frac{2\,t\,\sigma_{\text{allow}}}{D_m}$$

• II.3 Approximate expansion pressure (hydraulic swage, includes hardening/friction factor K):

  $$p_{\text{exp}} \approx \frac{2\,t\,\sigma_{\text{flow}}\,K}{D_m}$$

• II.4 Frictional pressure loss at frac rate (Darcy–Weisbach):

  $$\Delta p_f = f\,\frac{L}{D}\,\frac{\rho v^2}{2}$$

• II.5 Treating margin:

  $$M_{\text{burst}} = p_{\text{burst,min}} - \left(p_{\text{treat,max}} + p_{\text{surge}} + p_{\text{temp}}\right)$$

III. Step-by-Step Procedure / Workflow

III.1 Design and Pre-Job Engineering

• III.1.1 Define completion objective: reinforcement, isolation, patch, or refrac liner; set target drift ID and treating rates/pressures.
• III.1.2 Collect inputs (estimated if unknown): casing tally/IDs, connection types, curvature, BHST/BHBP, fluids, H2S/CO2, frac program (rate, proppant, plug/gun OD).
• III.1.3 Size the expandable string: choose Di for run-in clearance and Df to meet drift ID and pressure envelope; verify expansion ratio and material grade.
• III.1.4 Model burst/collapse and thermal cycles pre-/post-expansion; confirm margin equations above; validate seal stack ratings.
• III.1.5 Plan expansion method: hydraulic push with pressure and swage progress; define max p_exp, rate of advance, and contingency pull-off criteria.
• III.1.6 QA/QC: shop expand representative joints; pressure test seals; drift test with worst-case plug/gun assembly.

III.2 Well Preparation

• III.2.1 Cleanout to below target interval; circulate to clear solids; condition mud/brine to low solids and lubricity.
• III.2.2 Caliper/ultrasonic log (if available) to confirm restriction length/ID; verify DLS and set depth window.
• III.2.3 Function-test surface pumps to required p_exp with 10–20% headroom.
• III.2.4 Drift the well with a drift representative of planned frac tools.

III.3 Run and Expand

• III.3.1 Run expandable assembly on workstring; maintain low overpull, circulate conditioned brine; centralize across expansion zone.
• III.3.2 Space-out and position top of liner/patch relative to planned perforations or isolation interval; verify depth with correlation.
• III.3.3 Initiate expansion: apply hydraulic pressure to drive swage; control advancement rate; monitor pressure/weight signature for uniform plastic flow.
• III.3.4 Set anchors and seal stacks at liner top/bottom; confirm set by pressure signature and displacement volume.
• III.3.5 If in open hole: pump preflush; expand; cement as per design (if cemented expandable liner), then pressure test.
• III.3.6 Pressure test: hold test above maximum expected treating pressure (plus margin) for defined duration; record leak-off.

III.4 Resume Frac Operations

• III.4.1 Drift through expanded section with largest planned frac plug/gun assembly.
• III.4.2 Execute plug-and-perf or sleeve-based frac program; ensure rate/pressure within verified envelope; track friction vs model.
• III.4.3 Post-frac, remove temporary tools as planned (mill/drill plugs) ensuring debris control across expanded section.

IV. Risk & Mitigation

• IV.1 Stuck during expansion:
  • IV.1.1 Mitigate with pre-job caliper, solids-free fluid, centralization, and controlled swage rate; pre-expand shop joints.
  • IV.1.2 Contingency: pressure step-down, reciprocation limits, reverse-circulate; defined abort criteria to protect seals.
• IV.2 Insufficient pressure envelope:
  • IV.2.1 Verify burst/collapse margins per Section II; consider shorter patch, higher-grade material, or lower frac rate (reduce friction).
  • IV.2.2 Install additional anchors to distribute load and reduce local hoop stress.
• IV.3 Seal failure/leak:
  • IV.3.1 Ensure clean sealing surfaces; avoid expansion across heavy pitting or large ID steps unless designed for it.
  • IV.3.2 Pressure test in stages; if leak detected, consider secondary expansion or setting an upper patch.
• IV.4 Thermal/mechanical cycling during multi-stage frac:
  • IV.4.1 Use appropriate elastomer/metal stack; manage stage sequencing to minimize rapid temperature transients.
• IV.5 Sour service (H2S):
  • IV.5.1 Select SSC-resistant metallurgy and sour-rated seals; control pH and avoid acid exposure on seals unless validated.
• IV.6 Debris and erosion during frac:
  • IV.6.1 Keep proppant away from unprotected seal bores; use flow diverters, erosion sleeves, and rate caps as needed.
• IV.7 FDI (frac-driven interactions) and deformation risk:
  • IV.7.1 Preemptively reinforce high-risk intervals; monitor offset pressure/strain if available; adjust stage spacing and pump schedule accordingly.
• IV.8 HSE at high pressure:
  • IV.8.1 Pressure barriers audited; red-zone control; hydrotest certificates verified; emergency bleed-down procedures rehearsed.

V. Optimization Levers

• V.1 ID Retention Strategy: Engineer Df to pass the largest plug/gun OD with friction constraints; minimize telescoping vs conventional casing to maximize stage count and frac rate.
• V.2 Expansion Execution: Use filtered, lubricious fluid and steady swage advance; real-time pressure/volume matching to detect off-nominal strain hardening early.
• V.3 Seal Placement: Land seals/anchors on round, uniform IDs; avoid coupling locations; if crossing collars, use dedicated collar-crossing elements.
• V.4 Rate/Friction Management: Model ?p_f with worst-case fluids; if margins tight, select lower-roughness ID coatings or adjust stage rate without compromising proppant transport.
• V.5 Data Analytics: Compare measured p_exp vs model to back-calculate K (hardening/friction). Calibrate treating pressure model and adjust stage designs in real time.
• V.6 Refrac Design: Short, targeted expandable patches can isolate damaged sections while preserving most of the original casing ID; combine with limited-entry or diversion to re-cluster inflow.
• V.7 Preemptive Installation: In basins prone to casing shear/ovalization, install SET across the risk interval before stimulation to avoid NPT and retain full tool access.
• V.8 Reliability: Tight QA/QC on heat treatment, wall thickness, and swage geometry; drift and hydrotest every joint pre-job to reduce run risk.

VI. Verification & Monitoring Plan

• VI.1 Pre-Expansion:
  • VI.1.1 Baseline caliper/USIT of target interval; friction/torque model; pump capability test to p_exp + margin.
• VI.2 During Expansion:
  • VI.2.1 Record pressure, rate, swage position, displacement; acceptance = smooth signature without spikes; correlate volume to theoretical plastic work.
• VI.3 Post-Expansion Integrity:
  • VI.3.1 Pressure test liner top and expanded interval (e.g., 30–60 min hold); no pressure decay beyond criterion.
  • VI.3.2 Drift test with gauge ring matching largest frac tool OD.
  • VI.3.3 Optional: multifinger caliper/UT for ID confirmation and seal placement verification.
• VI.4 During Frac:
  • VI.4.1 Monitor treating pressure vs modeled friction; alarm if exceeding M_burst threshold or seeing abnormal step changes (seal compromise or deformation).
  • VI.4.2 Track plug set/mill data; any overpull trends across expanded interval trigger inspection.
• VI.5 Post-Frac:
  • VI.5.1 Pressure test to production envelope; verify no sustained annulus pressure.
  • VI.5.2 If refrac, evaluate stage coverage and inflow via tracers/production logging to validate isolation effectiveness.

How SET “Work” in Frac—Mechanism Summary

• Plastic expansion: A hardened swage is pushed/pulled through the tubular using hydraulic pressure. The pipe yields plastically, growing OD/ID to near the host casing/open-hole diameter, creating a tight interference fit.
• Sealing/anchoring: Metal and elastomer elements at the liner ends are expanded to create a high-load metal-to-metal seal backed by elastomer and slips, achieving differential-pressure integrity.
• ID preservation: Because the string is run smaller and expanded in-situ, the final ID is larger than an equivalent non-expandable patch, preserving frac tool passage and pump-rate capability.
• Pressure envelope: Proper grade/thickness and expansion control retain sufficient burst/collapse margins for high-rate, high-pressure plug-and-perf operations.