Measuring Seismic Velocity Sensitivity To Production-Induced Strain at the Ekofisk Field

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIn chalk reservoirs such as the Ekofisk Field, fluid flow and geomechanical effects combine to change both the location and properties of the reservoir and overburden. Pore pressure and fluid saturation changes cause reservoir compaction and perturb the elastic properties of the reservoir rocks. The overburden responds to the compaction with piston-like seafloor subsidence and length changes (strains). These overburden strains change the seismic velocity. The resulting velocity changes are observed on time-lapse seismic data as time-shifts that accumulate though the overburden.These processes are being monitored by GPS surveys of the production facilities, repeat logging of radioactive marker bullets, repeat bathymetry surveys, core sample analysis, and time lapse seismic data. The goal of this paper is to combine these measurements to better understand the relationship between overburden strains, changes in overburden velocity, and resulting time-lapse time-shifts.

Review Paper: A semi‐empirical model of strain sensitivity for 4D seismic interpretation

“…The exception to this is a synthetic sandstone, for which R is measured under unloading as 75, whilst loading gives 15, but unloading after loading is 55. Similarly, Janssen et al (2007) observed values of 10-30 for Ekofisk chalk in the laboratory (as opposed to 2-6 in the 4D seismic data). Bathija, Batzle and Prasad (2009) measured three sandstones and one shale sample under hydrostatic and uniaxial loading, and found Rfactors of between 6 and 91 for the sandstones and 6-11 for the shales.…”

“…Similarly, Janssen et al . () observed values of 10–30 for Ekofisk chalk in the laboratory (as opposed to 2–6 in the 4D seismic data). Bathija, Batzle and Prasad () measured three sandstones and one shale sample under hydrostatic and uniaxial loading, and found R‐ factors of between 6 and 91 for the sandstones and 6–11 for the shales.…”

“…), and also within the chalk reservoir of Ekofisk (Janssen et al . ). Anomalously high values in the overburden can signal that the rocks have failed significantly and may be mechanically unstable and highly permeable.…”

“…noticeable feature of the time-shift estimates is the prominence of slowdown (velocity decrease) due to extension in the overburden as a consequence of reservoir depletion and also due to reservoir inflation (pressuring up by fluid injection). For these effects, the values tend to be large, ranging from 2 ms in deep (>3 km) reservoirs over short production/injection time periods of several months -see, for example Curlew-D (Fehmers et al 2007) and Skua (Staples et al 2007), to more dramatic effects (>20 ms) for compacting chalks such as in the Valhall field or Ekofisk field (Janssen, Smith and Byerley 2007) over a period of several years. Indeed, some of the largest reported signals of overburden extension are for the high porosity compacting chalk reservoirs of the North Sea, although many unconsolidated clastics such as in the Genesis field, Gulf of Mexico also exhibit well-developed overburden time-shifts .…”

“…), to more dramatic effects (>20 ms) for compacting chalks such as in the Valhall field (Barkved et al . ) or Ekofisk field (Janssen, Smith and Byerley ) over a period of several years. Indeed, some of the largest reported signals of overburden extension are for the high porosity compacting chalk reservoirs of the North Sea, although many unconsolidated clastics such as in the Genesis field, Gulf of Mexico also exhibit well‐developed overburden time‐shifts (Hodgson et al .…”