Using 4D Seismic Surveys and History Matching to Estimate Critical and Maximum Gas Saturation

Abstract

An approach is explored for estimating critical and maximum gas saturation using 4D seismic data from multiple surveys shot during gas exsolution and dissolution in a producing hydrocarbon reservoir. To guide this process, hydrocarbon gas properties and behaviour are studied, and their relation to the fluid-flow physics is understood using numerical simulation and seismic modelling. This understanding is then used to interpret observed seismic data, which has surveys repeated every 12 to 24 months, from a turbidite field in the United Kingdom Continental Shelf (UKCS). Furthermore, the field reservoir simulation model is then history matched to the production data and the gas saturation effects observed on the 4D seismic data. The 4D seismic response is a function of pressure changes, fluid (oil/water/gas) changes and noise. The effects of the gas mechanism are extracted from the seismic data based on its unique relationship to the seismic amplitudes. It is found that these changes can be represented by a binary model (presence or absence of gas) which enables the use of a logical objective function to compute the misfit between the observed data and simulated data, and thus guide the parameterisation process of the history matching exercise. This approach circumvents full physics modelling in a joint history matching workflow that includes conditioning to both production data and multiple time lapse seismic data. It is concluded that for seismic surveys repeated at intervals of six months or more, the gas saturation distribution during either liberation or dissolution exists at two fixed saturations defined by the critical and the maximum gas saturation. From analysing only the 4D seismic data, we find a low critical gas saturation and a maximum gas saturation that is relatively unconstrained. The history matching exercise also gives us similar low values for the critical gas saturation, and highlights the importance of the vertical permeability in getting an extensively corroborated model. This paper explores a direct link between 4D seismic and the fluid flow parameters, a link between the gas saturation distribution and seismic response, as well as a quantitative analysis using multiple 4D seismic surveys for history matching.