The interpretation of amplitude changes in 4D seismic data arising from gas exsolution and dissolution

Abstract

This study examines the four-dimensional (4D) seismic signatures from multiple seismic surveys shot during gas exsolution and dissolution in a producing hydrocarbon reservoir, and focuses in particular on what reservoir information may be extracted from their analysis. To aid in this process, hydrocarbon gas properties and behaviour are studied, and their relationship to the fluid-flow physics is understood using numerical simulation. This knowledge is then applied to interpret the seismic response of a turbidite field in the UK Continental Shelf (UKCS). It is concluded that for a repeat seismic survey shot 6 months or more after a pressure change above or below bubble point (as in our field case), the gas-saturation distribution during either exsolution or dissolution exists in two fixed saturation conditions defined by the critical and the maximum possible gas saturation. Awareness of this condition facilitates an interpretation of the data from our field example, which has surveys repeated at intervals of 12–24 months, to obtain an estimate of the critical gas saturation of between 0.6 and 4.0%. These low values are consistent with a range of measurements from laboratory and numerical studies in the open literature. Our critical gas-saturation estimate is also in qualitative agreement with the solution gas–oil ratios estimated in a material balance exercise using our data. It is not found possible to quantify the maximum gas saturation using the 4D seismic data alone, despite the advantage of having multiple surveys, owing to the insensitivity of the seismic amplitudes to the magnitude of this gas saturation. Assessment of the residual gas saturation left behind after secondary gas-cap contraction during the dissolution phase suggests that small values of less than a few per cent may be appropriate. The results are masked to some extent by an underlying water flood. It is believed that the methodology and approach used in this study may be readily generalized to other moderate- to high-permeability oil reservoirs, and used as input in simulation model updating.

Review Paper: Post‐stack 4D seismic time‐shifts: Interpretation and evaluation

“…b, also for example the Emeraude, Baobab or Schiehallion fields) is observed to also exhibit a more moderate range of slowdowns of up to 10 ms, as the gas tends to occupy most of the reservoir volume at the critical gas saturation (Falahat et al . ). By comparison, saturation‐related speedups (hardening) due to water replacing oil (or gas), water flooding or an upward progression of the aquifer water are typically of up to 2 ms (e.g. Fig.…”

“…This leads to a clearly visible amplitude brightening and distinct time‐shift slowdown due to the small amount of gas at critical saturation but also the trapped gas or gas caps (Falahat et al . ). The pressure drop will produce a speedup, but this is usually very small in comparison to the gas effects.…”

“…Laboratory, pore‐scale and simulation studies suggest critical gas saturation values as low as 0.05 and as high as 0.38 (Falahat et al . ). Low values are more common in moderate porosity reservoirs, although there is no definite trend.…”

Seismic Assisted History Matching Using Binary Image Matching

“…In this field, the reservoir fluid is black oil with an API gravity ranging from 22°to 28°at a temperature of 120°F (48.89°C). Initial reservoir pressure is approximately 2900 psi (19.99 MPa) (at depth 1940m TVDSS) whilst bubble point is 2850 psi (19.65 MPa) at the top reservoir level, and the solution gas-oil ratio (GOR) is 354 scf/bbl (62.99 sm 3 /m 3 ) (Falahat et al, 2014). In this particular field, there is known to be gas exsolution, gas mobilisation, and then re-pressurisation with subsequent dissolution.…”

“…These data have been cross-equalised by the operator for 4D seismic data interpretation purposes, and have a non-repeatability NRMS noise metric (Kragh and Christie, 2001) of approximately 31% (Falahat et al, 2014). The data have been transformed into relative impedance traces by coloured inversion (Lancaster and Whitcombe, 2000).…”