Reservoir Geophysics Technologies and Workflows

Geophysics provides the only data set capable of imaging the entirety of the reservoir structure in a continuous and homogeneous manner. However geophysical data suffer from strong limitations related to propagation artifacts along with reduced spatial and temporal resolution and detection powers. They therefore require a detailed interpretation and calibration with well data. Historically seismic data have been used to provide reservoir structure maps, initially from 2D seismic lines, nowadays from 3D seismic cubes with a lot more accuracy. Seismic reservoir characterization has developed in the 1990s taking advantage of improved acquisition and processing capabilities provided by the industry. More recently Time Lapse seismic or 4D seismic has proved to show seismic effects related to the hydrocarbon production in the reservoir. Therefore we can say that Geophysics provides a significant contribution to the structure definition, to the characterization of reservoir properties and heterogeneities in combination with well data and geological models and finally to the understanding of the dynamic behavior of a field when integrated to reservoir models and history matching (HM). These multiple uses of seismic data assume a complete integration of the geophysical workflow into the global Geosciences and field operation workflows. The geophysical workflow includes 3D and 4D seismic feasibility studies, data acquisition, data processing (time and depth imaging), structural interpretation, stratigraphic inversion and reservoir characterization, along with the integration in the geo-model (gridded geological model). It is conditioned by the type of field (Green field or Brown field) and the type of reservoir (conventional, unconventional, tight/fractured reservoir, heavy oil etc.), but also by the drilling program, the geological and reservoir modeling phases, and the development and production operations. Therefore an integrated geophysical project planning must be set up with a long term view in order to ensure the full adequacy of the seismic data during the whole duration of the field development and production phases. Seismic acquisition and processing are the conditioning bases for the quality of seismic data. Technologies have enormously evolved in the past twenty years to provide adequate data not only in exploration contexts but also on the fields where seismic acquisition is made more difficult by surface obstructions (permanent installations such as platforms, FPSOs and pipe lines) and field operations (drilling, shipping, construction etc.). The selection of acquisition technique and the definition of survey parameters (3D survey design) are made during a 3D feasibility study that must also anticipate the needs and conditions for Time Lapse seismic. Seismic processing which is now much more sophisticated than in the past, is now capable of dealing with numerous aspects of noise reduction, multiple attenuation, signal deconvolution and phase control, velocity picking and inversion, time and depth imaging etc. Seismic quality can be monitored all along the processing sequence to make sure that the selected parameters will fulfill the quality requirements of AVO studies and pre-stack inversion. Regarding seismic reservoir characterization numerous tools and workflows have been developed throughout the years. Among them stratigraphic inversion plays an important role as it provides, in combination with Petro-elastic models or Rock Physics models, estimations of reservoir facies and petrophysical properties at seismic scale. Comprehensive workflows have been set up by TOTAL in its interpretation workstation SismageTM, to interpret inverted seismic data in terms of reservoir properties, calibrate these results with the well data and integrate them into a geo-model using state-of-the-art modeling rules. Finally a feedback loop helps verify the consistency between the inverted seismic data and the geo-model infilling. If necessary an additional iteration is performed to update the geo-model. The acquisition of 4D seismic data is nowadays envisaged on all the fields. For that purpose, 4D feasibility studies are performed using Petro-elastic models or Rock Physics models and seismic modeling in order to quantify the 4D effects and provide specifications for the acquisition and processing phases. 4D warping allows the time alignment of base and monitor(s) seismic traces before subtraction, and provides the inversion of the relative variations of the compressional velocity (?Vp/Vp) in the reservoir. 4D Pre-stack inversion may provide the estimation of other elastic parameters variations and then improve the quantification of the 4D effects in terms of fluid saturation and/or pressure variations. Uncertainties must be monitored all along the geophysical workflow. Specific tools have been developed to quantify the structural uncertainties. Reservoir characterization and 4D uncertainty tools are still under development to complete the geophysical uncertainty workflow. Reservoir Geophysics has become an important part of the field development studies. Specific technologies and workflows have been developed to optimize the seismic data quality and their integration in field studies. Nevertheless many research topics still remain in order to improve the interpretation, quantification and integration of seismic information into the geo-models and reservoir models.