Time-lapse (4D) seismic: monitoring reservoir changes

The 4D acquisition setup

A 4D project requires TWO (or more) seismic surveys acquired at DIFFERENT calendar dates over the SAME area:

• Baseline: the pre-production or early-production survey. This is the reference. Every monitor survey is compared to it.
• Monitor: subsequent surveys at 1-5 year intervals. Each is compared to the baseline to reveal cumulative changes.

The central challenge: the two surveys have to be REPEATABLE. Small changes in the reservoir produce small 4D signals — often 5-15% of the baseline amplitude. If the acquisition geometry is even slightly different, that alone creates differences of the same magnitude, drowning the real signal. This is why 4D projects invest enormously in ACQUISITION REPEATABILITY: same boat, same streamer geometry, same source, same time of year, same water temperature, same season-dependent pressure effects. For permanent-seabed surveys (ocean-bottom nodes, OBN), the sensors stay in place for years — the gold standard for repeatability.

The quantitative repeatability metric is the NRMS (Normalized RMS) difference in a NON-RESERVOIR zone (where nothing should change). NRMS < 10% is excellent (well-acquired 4D); 10-25% is typical field data; > 35% usually means the 4D signal can’t be trusted.

Exercise — run the time-lapse

• Open the widget in Baseline view with the Waterflood dynamic. Drag the Years slider to 0. You see the original reservoir: a blue oil-sand band (low Ip) between red shale (high Ip). This is the reference survey.
• Drag Years to 10. NOTHING visibly changes in the baseline view — because the baseline is the baseline, always year 0. Switch to Monitor: NOW the section evolves as you drag Years. At year 5, the left half of the reservoir has turned from blue (oil) toward greener (water-swept), while the right half is still oil.
• Switch to 4D amplitude difference. The overburden and non-reservoir zones are GRAY (no change). The left half of the reservoir is RED (Ip INCREASED — water replaced oil, raising acoustic impedance). The right half is still GRAY (not yet swept). The sharp boundary between red and gray IS the current water-flood front.
• Drag Years from 0 to 10. Watch the red water-swept zone EXPAND from left to right. This is exactly what a real waterflood-monitoring 4D project looks like — operators use this to identify unswept zones (future infill well targets) and to verify that injected water is actually reaching the producers.
• Switch to Gas depletion dynamic, Years=5. The 4D amp-difference becomes BLUE across the reservoir (Ip DECREASED — gas has come out of solution, reducing Ip). The effect is stronger at depth (higher temperature accelerates exsolution). This is a very different 4D signature from the waterflood case: area-wide decrease vs sharp front.
• Switch to Compaction dynamic. The 4D amplitude difference becomes RED across the whole reservoir (Ip rises uniformly as effective stress increases). Now switch to 4D time shift view: the overburden and reservoir show negative (blue) time shifts — reflectors BELOW the reservoir arrive EARLIER because seismic waves travel through the stiffened reservoir faster.
• Compaction is the CLASSIC case where time shifts are diagnostic. Amplitude alone wouldn’t tell you much (uniform area increase is ambiguous — could be water flood from above, too). Time shift confirms: compaction.

Rock-physics primer for 4D signatures

Each reservoir dynamic has a characteristic elastic fingerprint. Here’s the rock physics of the main ones:

• Water replacing oil: brine is denser and stiffer than oil. Gassmann fluid substitution predicts Ip RISES ∼5-10%, Vp/Vs rises slightly. 4D amp-difference is POSITIVE (redder, stiffer). Time shift roughly neutral.
• Oil replacing gas (rare, during repressurization): Ip DROPS slightly (gas goes back into solution and compressibility of the mix falls back toward oil). Inverse of depletion.
• Solution gas exsolution (pressure drops below bubble point): dramatic drop in Ip (∼10-20% drop) because gas bubbles in the pore space enormously increase fluid compressibility. 4D amp-difference is strongly NEGATIVE. Signature is characteristic of PRESSURE DEPLETION.
• Reservoir compaction: effective stress rises as Pp drops; rock matrix stiffens. Vp and Vs both rise ∼5-10%, density rises ∼2%. Ip rises ∼7-12%. Reflectors below the reservoir pull UP (earlier arrival) because the wave spends less time in the stiffened rock.
• CO2 injection: CO2 (supercritical state) has low bulk modulus like natural gas. Ip drops similarly to gas exsolution. The plume edge is the key 4D anomaly. See §8.6 for the full story.

Each signature is a TELL. Skilled 4D interpreters can often distinguish water flood, depletion, and compaction from the amplitude + time-shift patterns alone — without needing additional logs. But quantitative predictions require FORWARD MODELING (taking your rock-physics template, applying the hypothesized dynamic, computing the expected 4D signature, and comparing to observed).

Quantitative 4D: pressure-saturation decomposition

Sometimes two dynamics happen simultaneously — waterflood AND compaction, or injection AND production. The 4D signal is a MIXTURE. Quantitative 4D (Q-4D) aims to DECOMPOSE the observed signal into pressure and saturation contributions:

Δ(Amplitude) = f(ΔP) + g(ΔS) + ε

Using pre-stack 4D data (angle-dependent 4D differences), you can separately solve for ΔP and ΔS per voxel. The math is essentially a 4D version of the pre-stack elastic inversion from §7.3 — instead of inverting for (Ip, Is, ρ) at a fixed time, you invert for (ΔP, ΔS) between two time instants.

Outputs: pressure-change cube + saturation-change cube, volume by volume. Directly feeds reservoir-engineering models. The high-value deliverable of a mature 4D program.

Repeatability and common pitfalls

• Misaligned geometry. The baseline boat was 25 m off-line from the monitor boat. Result: 4D signal dominated by geometry-induced differences in fold, azimuth, offset coverage. Mitigation: cross-equalize with careful match-filter processing; use PRE-STACK methods that are less sensitive to geometry.
• Seasonal water column. Baseline acquired in January (cold water, fast sound speed), monitor in July (warm, slow). Travel times differ BEFORE anything reservoir-related. Correct via: time-of-year matching, or water-velocity time-lapse correction using measured profiles.
• Processing differences. The two surveys were processed on different software versions with slightly different demultiple parameters. Fixed: REPROCESS baseline and monitor TOGETHER through the same flow.
• Overburden changes. Regional fluid withdrawal in a field above your reservoir changes seismic velocities in the overburden. 4D signal in the reservoir can be shifted and scaled by overburden effects. Correct with: volumetric time-shift inversion, or explicit overburden geomechanical model.
• False 4D from NOISE. Low NRMS in the reservoir zone but HIGH NRMS in nearby shale. The noise floor could be producing false 4D "anomalies." Always compare reservoir 4D against shallow-overburden NRMS as a sanity check.
• Acquisition gaps. Missing line or infill zone with no monitor coverage. Clearly flag these areas; don’t over-interpret edge effects.

Applications in practice

• Norwegian Sea (Ekofisk, Valhall): world-class 4D programs in compaction-dominated chalk reservoirs. Time shifts up to 30 ms documented over decades. Drives repressurization + waterflood programs.
• Troll Field (North Sea): waterflood 4D revealed that gas was bypassing water in certain zones — changed development strategy dramatically.
• Sleipner (Norway): flagship CO2 storage 4D project. Annual repeat surveys since 1996 clearly show the CO2 plume growth within the Utsira aquifer. See §8.6 for the case study.
• Gulf of Mexico deep-water: 4D on permanent ocean-bottom-node surveys has become the standard for high-value subsea developments. Near-daily updates in some cases.
• Unconventional (shale): 4D in completion-induced fracture networks, microseismic 4D, and DAS (Distributed Acoustic Sensing) are rapidly maturing.