Sensitivity of compaction-induced multicomponent seismic time shifts to variations in reservoir properties

Abstract

Pore-pressure variations inside producing reservoirs result in excess stress and strain that influence the arrival times of reflected waves. Inversion of seismic data for pressure changes requires better understanding of the dependence of compactioninduced time shifts on reservoir pressure reduction. Using geomechanical and full-waveform seismic modeling, we investigate pressure-dependent behavior of P-, S-, and PS-wave time shifts from reflectors located above and below a rectangular reservoir embedded in a homogeneous half-space. Our geomechanical modeling algorithm generates the excess stress/strain field and the stress-induced stiffness tensor as linear functions of reservoir pressure. Analysis of time shifts obtained from full-waveform synthetic data shows that they vary almost linearly with pressure for reflectors above the reservoir, but become nonlinear for reflections from the reservoir or deeper interfaces. Time-shift misfit curves computed with respect to noise-contaminated data from a reference reservoir for a wide range of pressure reductions display well-defined global minima corresponding to the actual pressure. In addition, we evaluate the influence of the reservoir width on time shifts and the possibility of constraining the width using time-lapse data. We also discuss the impact of moderate perturbations in the strain-sensitivity coefficients (i.e., third-order stiffnesses) on time shifts and on the accuracy of pressure inversion. Our feasibility analysis indicates that the most stable pressure estimation from noisy data is provided by multicomponent time shifts from reflectors below the reservoir. For multicompartment reservoirs, time shifts can be accurately modeled by linear superposition of the excess stress/strains computed for the individual compartments.

Inversion of multicomponent seismic time shifts for reservoir pressure and length: a feasibility study

“…Multicompartment reservoirs exhibit time‐shift distributions similar to those in Fig. but with strain and time‐shift perturbations caused by the intercompartment pressure differences or variations in shape (Smith and Tsvankin , ). The results for a single‐compartment reservoir presented here highlight the general properties of time shifts of different wave types for a wide range of reflector depths.…”

“…The spatial patterns of compaction‐induced time shifts remain consistent over a range of pressure drops, and reflectors A, B, and C are positioned to sample these distributions for inversion purposes. Compaction‐induced strains in the reservoir volume are one‐to‐two orders of magnitude larger than those in the surrounding medium (Smith and Tsvankin ). The resulting velocity increase inside the reservoir causes time shifts to change from lags in the overburden to leads beneath the reservoir.…”

“…Typical two‐way time‐shift distributions for (a) P‐waves, (b) S‐waves, and (c) PS‐waves measured using 22 reflectors around the reservoir (white box) from Fig. (Smith and Tsvankin ). The time shifts correspond to hypothetical specular reflection points at each ( X , Z ) location in the subsurface; source location is indicated by the white asterisk at the top.…”

“…Large compaction‐induced strains inside the reservoir generate S‐wave time shifts for deep reflectors that are two to three times larger than those for P‐waves. A sensitivity study by Smith and Tsvankin () demonstrates that S‐wave time shifts for reflectors beneath the reservoir become nonlinear in pressure for relatively large stiffness changes corresponding to pressure reductions exceeding approximately 10%. Their results suggest that time shifts of shear‐wave reflections may be large enough to invert for reservoir pressure when the smaller P‐wave shifts are obfuscated by noise.…”

Double-difference waveform inversion: Feasibility and robustness study with pressure data

“…In particular, overburden structures may change between surveys. For example, compaction within the reservoir can change the stress field and velocity above and below it (Smith and Tsvankin, 2013). Water velocity also varies seasonally.…”