What is the process of well stimulation in offshore fields?

I. Objective Definition and Key KPIs

• I.1) Primary objectives: remove formation/completion damage (skin), bypass damage via fractures, dissolve scales/asphaltenes/paraffins, or shut off unwanted water/gas.
• I.2) KPIs:
  • I.2.1) Skin factor reduction (?s): target s ? 0 to -2 for matrix jobs; track via PBU analysis.
  • I.2.2) Productivity Index (PI, J) uplift: J_post/J_pre = 1.5–3.0, sustained = 90 days.
  • I.2.3) Rate gain: +20–100% oil/gas at stable drawdown; avoid water/gas coning.
  • I.2.4) Uptime/NPT: stimulation NPT = 5%; SIMOPS conflicts minimized.
  • I.2.5) Cost/OPEX: $/bbl-oil or $/Mscf incremental = design target.
  • I.2.6) HSE/emissions: zero recordables, controlled returns; minimize pumping fuel burn and flaring.
• I.3) Scope (offshore specifics): deck space, crane lifts, DP stimulation vessels, subsea access (tree kill/choke lines, riser/RLWI), well barriers, waste/effluent handling, hydrate management.

II. Critical Parameters and Target Ranges

III. Step-by-Step Procedure / Workflow

III.A Candidate Selection and Diagnostics

• III.A.1) Screen wells: rate decline vs offset, rising drawdown, high skin from pressure-buildup, PLT indicating near-wellbore damage, water or scale issues.
• III.A.2) Acquire data: recent PVT, water analysis (sulfate, Ba/Ca/Fe), solids, BHST/BHFP, completion schematic, integrity tests, perforation details, prior stim history.
• III.A.3) Tests (estimated): acid solubility/reactivity, compatibility, core/plug flow for fines, mini frac/DFIT if fracturing is considered.

III.B Treatment Selection

• III.B.1) Carbonate: matrix HCl or emulsified acid; acid fracturing if tight/heterogeneous.
• III.B.2) Sandstone: mud acid (HCl–HF) after preflush; low-rate matrix to avoid fines mobilization; proppant frac for tight zones.
• III.B.3) Remedial: solvent/xylene–aromatic for asphaltene; hot surfactant for paraffin; oxidizers/chelants for iron; scale squeeze for sulfate/carbonate scales; water shutoff (gel or RPM) if conformance is the primary issue.
• III.B.4) Placement method: bullhead vs CT spotted; WL straddle packer for zonal selectivity; diverters (ball sealers, particulates, VES) for profile control.

III.C Design and Calculations

• III.C.1) Pore volume and acid volume
  • III.C.1.a) Radial pore volume to radius r_t: \( V_p = \phi \,\pi h (r_t^2 - r_w^2) \)
  • III.C.1.b) Matrix acid volume: \( V_{acid} = N \times V_p \), with N = 0.5–3.0 PV (carbonates typically lower N due to wormholing; sandstones need careful HF exposure control).
• III.C.2) Hydraulics and horsepower
  • III.C.2.a) Friction pressure (pipe): \( \Delta P_f = f \frac{L}{D} \frac{\rho v^2}{2} \)
  • III.C.2.b) Hydraulic horsepower: \( \text{HHP} = \frac{P \,(\text{psi}) \times Q \,(\text{gpm})}{1{,}714} \)
  • III.C.2.c) Bottomhole pressure during pumping: \( P_{bh} = P_{surf} + \rho g H - \Delta P_f \)
• III.C.3) Fracture diagnostics
  • III.C.3.a) Net pressure: \( P_{net} = P_{frac} - \sigma_{hmin} \); ISIP from shut-in; closure from G-function/vt analysis.
  • III.C.3.b) Step-rate test to find \( P_{frac} \) and friction via slope change.
• III.C.4) Productivity and skin
  • III.C.4.a) Radial flow with skin: \( J = \frac{2\pi k h}{\mu B\,[\ln(r_e/r_w) + s]} \)
  • III.C.4.b) Expected uplift from \(\Delta s\): \( \frac{J_{post}}{J_{pre}} = \frac{\ln(r_e/r_w) + s_{pre}}{\ln(r_e/r_w) + s_{post}} \)
• III.C.5) Pump schedule (example template; volumes and rates are estimated):
  • III.C.5.a) Preflush: solvent/mutual solvent 0.2–0.5 PV; for sandstone, HCl 7.5–15% to remove carbonates before HF.
  • III.C.5.b) Main acid: carbonate—HCl 15–28% or emulsified acid at 5–25 bpm; sandstone—HCl–HF 3/1.5% at 1–6 bpm via CT if selectivity needed.
  • III.C.5.c) Diverter cycles: 1–3 stages; particulates or VES to redistribute into lower-k intervals.
  • III.C.5.d) Overflush: brine or diesel/brine 0.5–1.0 PV to push spent acid/reactants away from near-wellbore.
  • III.C.5.e) Optional squeeze: scale inhibitor post-acid, tailored for adsorption/return profile.

III.D Offshore Execution (Platform or Vessel)

• III.D.1) Permits/SIMOPS: integrated plan with marine ops, crane lifts, exclusion zones, flare management.
• III.D.2) Barriers and testing: verify dual barriers; pressure test tree/lines/PCE to 1.1× MAWP; function-test SCSSV; leak-off testing if needed.
• III.D.3) Rig-up: connect stimulation iron to kill/choke or riser; install check valves; set WL/CT BOP; test to job pressure.
• III.D.4) Displacements: calculate tubing/lines capacity; pre-cool/warm lines per BHST; hydrate inhibition plan (methanol/MEG) if gas service.
• III.D.5) Pumping:
  • III.D.5.a) Start with step-rate/step-down to confirm friction and matrix/frac threshold.
  • III.D.5.b) Execute preflush–main–diverter–overflush stages; hold rates to design; avoid exceeding P_frac for matrix jobs.
  • III.D.5.c) Monitor WHP, rate, density, pH, returns; calculate real-time \(P_{bh}\), \( \Delta P_f \), ISIP/closure (if fracturing).
• III.D.6) Soak and cleanup: soak 0.5–2 hours (matrix); controlled flowback to avoid fines surge; gradually open choke; capture samples for iron/solids.
• III.D.7) Selective placement (subsea/horizontal): WL tractor with straddle packer or CT jetting for zonal isolation; repeat stages with diverters as needed.

III.E Post-Job

• III.E.1) Return well to production at controlled drawdown; increase stepwise to design drawdown.
• III.E.2) Conduct pressure-buildup test within 24–72 hours for skin/PI; PLT if zonal distribution is critical.
• III.E.3) If a squeeze was placed, track inhibitor returns/tracer to estimate squeeze life.
• III.E.4) Document job: plots, volumes, pressures, deviations; lessons learned.

IV. Risks & Mitigations (HSE, Reliability, Integrity)

• IV.1) Well control/overpressure: risk of unintentional fracturing or barrier failure. Mitigation: verified barriers, MASP compliance, real-time step-rate, auto-shutdown on high WHP.
• IV.2) Corrosion/HF exposure: acid and HF risks to metallurgy and personnel. Mitigation: proper alloy compatibility, corrosion inhibitor design at BHST, calcium fluoride precipitation control, full HF PPE and exclusion zones.
• IV.3) Fines/sanding/screen plugging: especially in sandstones. Mitigation: low-rate HF, clay stabilizers, diverters, graded return-to-flow, filtration of fluids (2–10 µm).
• IV.4) Scale/precipitation (Fe, CaSO4, BaSO4): use iron control, chelants; brine compatibility checks; post-acid inhibitor squeeze.
• IV.5) Hydrates (gas wells/subsea lines): depressurization and cold fluids. Mitigation: MEG/methanol pre-injection, insulation, warm-back procedures.
• IV.6) Marine/SIMOPS: DP excursion, crane lifts, spills. Mitigation: DP trials, lift plans, drip trays, closed-drain systems, spill kits, ROV standby.
• IV.7) Waste/returns handling: spent-acid neutralization and solids. Mitigation: closed-loop tanks, neutralization plan, certified disposal onshore.
• IV.8) Equipment reliability: pump/iron failures. Mitigation: N+1 pumping redundancy, pressure-tested iron, spare hoses/valves, predictive maintenance on vessels.

V. Optimization Levers (Performance, Cost, Emissions)

• V.1) Data-driven design: use DFIT/mini-frac and step-down data to calibrate friction and closure; adapt schedule in real-time.
• V.2) Zonal selectivity: WL straddles, CT jetting, mechanical diversion to reduce fluid overuse and improve stage placement efficiency.
• V.3) Diverter optimization: alternate particulate/VES diverters; monitor pressure response for diversion effectiveness.
• V.4) Chemistry tailoring: emulsified acids for high T; solvent preflush for asphaltene; non-damaging breakers for gels; low-emission chemistries where feasible.
• V.5) Logistics: batch-stim multiple wells per mobilization; pre-rig pumps/iron; maximize vessel utilization; minimize fuel burn by right-sizing HHP.
• V.6) Completion-aware designs: ICD/AICD, screens, and gravel packs require low-solids fluids and controlled rates; avoid screen blinding.
• V.7) Flowback strategy: choke management to avoid fines surge; use surface sand traps and high-efficiency filtration.

VI. Verification & Monitoring Plan

• VI.1) Before job: baseline well test (J, s by PBU), fluid samples, scale/solids baseline, integrity test records.
• VI.2) During job: record Q, WHP, rate steps, density, pH, temperature, returns; compute \( P_{bh} \), ISIP, \( \Delta P_f \) in real-time; HHP utilization.
• VI.3) After job (Day 0–7): stabilized well test; PBU for skin; PLT if zonal conformance critical; solids monitoring on separators; chemical returns (if squeeze).
• VI.4) 30–90 days: KPI review—sustained rate/PI, water/gas trends, scale return decay; compare to forecast; plan re-stim/squeeze intervals.
• VI.5) Reporting: standardized plots (pressure-rate-time), treatment charts, variances vs design, learnings for next well.

Summary Notes (Operational)

• S.1) Offshore stimulation is a closed-loop, barrier-managed operation using matrix or fracture mechanisms depending on rock and damage type.
• S.2) Success hinges on accurate pressure control (stay below/above frac pressure by design), selective placement, compatible chemistry, and disciplined cleanup.
• S.3) Validate with PBU/PLT and sustain with inhibitor squeezes and proactive surveillance.