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.
This paper presents a history matching scheme that has been applied to production data and time lapse seismic data. The production data objective function is calculated using the conventional least squares method between the historical production data and simulation predictions, while the seismic objective function uses the Hamming distance between two binary images of the gas distribution (presence of gas (1) or absence of gas (0)) sequenced over the different acquisition times. The technique is applied to a UKCS (United Kingdom Continental Shelf) field that has deep-water tertiary turbidite sands and multiple stacked reservoirs defining some degree of compartmentalisation. Thirty five parameters are perturbed in this history match, they can be classified as volumetric parameters (net-to-gross, pore volume), transmissibility parameters (permeability, transmissibility), and end points of the relative permeability curves (critical saturation points). An initial ensemble of fluid flow simulation models is created where the full range of uncertain parameters are acknowledged using experimental design methods, and an evolutionary algorithm is used for optimization in the history matching process. It is found that permeability and critical gas saturation are key parameters for achieving a good history match, and that the volumetric parameters are not significant for this match in this particular reservoir. We also observe that matching only to production data marginally improves the seismic match, whilst matching to only seismic data improves the fit to production data. Combining both sets of data delivers an improvement for the production data and seismic data, as well as an overall reduction in the uncertainties. A unique feature of this technique is the use of the Hamming distance metric for seismic data history matching analysis, as this circumvents the use of the uncertain petroelastic model. This approach is easy to implement, and also helps achieve an effective global history match.
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.
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