Clara Andreoletti

Seismic exploration on ice: the flexural wave noise challenge.

In the context of an increasing world demand for oil & gas the untapped reserves of the arctic are becoming nowadays of great interest. Since the late 70’s many attempts have been made to carry out seismic exploration in arctic environments. Although many successes have been achieved both onshore and offshore during these years, offshore exploration is still effectively restricted to the very short arctic summer. An acquisition test was conducted in winter 2007 in Alaska by Eni, Shell and Repsol with CGGVeritas as contractor to prove the feasibility of acquiring offshore seismic surveys on floating ice. Different source receiver combinations were tested in order to find the one able to solve the existing tradeoff between data quality and operational constrains. A subsequent phase of processing tests have been started in order to address specific problems related to seismic acquisition in this environment. In particular the most relevant of these is the Flexural Ice Wave (FIW) noise: a novel approach has been developed in order to address it. FIW noise suppression preliminary results indicate that acquisitions on floating ice are feasible in terms of time, cost, HSE and data quality.

Seismic interferometry application to improve seismic reflection signals affected by ice-plate flexural noise

Seismic interferometry based on cross-correlation of traces recorded at different positions makes it possible to recover new Greens functions and to obtain signals as if sources were placed at the receiver positions. This case history describes results obtained using sea-bottom hydrophone signals recorded in shallow water below the ice plate in an Alaska seismic survey. The aim was to simulate new shots at the sea-bottom receiver positions and to redatum the surface-generated seismic signals below the ice layer. The application was performed and the method investigated to attenuate the strong noise due to dispersive wave modes (mainly flexural waves) which propagate in the ice layer.

Some practical issues related to migration in the angle domain

Summary The benefits of recasting pre-stack depth migration in the angle domain are such to suggest its application in industrial Kirchhoff PSDM software. Here we examine some practical aspects of the implementation in the angle domain, namely those that mostly impact performances and effectiveness: memory requirements, poor illumination and errors in velocity estimation.

Seismic Velocity Uncertainty Analysis In An Underexplored Basin

In areas characterized by complex geology, Depth Velocity Analysis is the mandatory instrument to estimate the sharp velocity variations that can be expected; seismic data and processing tools are necessary but not sufficient ingredients for successful depth imaging results, so sound a priori information must be retrieved from well information and from the geological knowledge of the area of study, as well as a close integration between the geophysical and geological contexts. A proper estimation of seismic velocities is not only fundamental to obtain good imaging quality and correct depth positioning, but it can also be a key input for detecting the presence of overpressures: usually, overpressured areas are associated with seismic velocity inversions. Moreover, in areas where anisotropy is unknown because of lack of well information, seismic velocities are affected by uncertainty: this must be taken into account in order to quantify the consequent uncertainty both in depth positioning of the target and in pore pressure prediction. In this paper a case history from Offshore Ghana is presented, dealing with the above mentioned issues: the intimate correlation that exists between anisotropy and velocity analysis is investigated, and the impacts of the uncertainty that affects them is considered.

Integration between Pore Pressure Prediction and Petroleum System Modelling Methodologies

The necessity to satisfy the world needs of oil & gas presses Oil Companies to drill in conditions that are getting harder and harder in terms of geopressure environment. In exploration, pore pressure prediction is critical for the evaluation of vertical and lateral sealing, the estimation of maximum possible column of hydrocarbons in place, and consequently to rank the prospects and evaluate the economics. Moreover, the overpressures have created serious problems during drilling operations in the past and also at the present time; for all these reasons, the geopressures prediction is important to define the best well design in order to reduce NPT, costs and reservoir damages. In order to build the appropriate model and to face this complex problem in the right way, the necessary information have to be collect, interpreted, elaborated and evaluated by several disciplines and for this kind of item, geology, geophysics and engineering have to be strongly integrated to give the best. We know from several years that information about geopressure can be derived from seismic velocities, and several relationships exist and applied with considerable success, but generally, in complex and deep areas, conventional velocity fields derived from seismic time processing are often not accurate enough to make a correct pore pressure prediction. For this reason, several methods have been developed trying to obtain more appropriate velocity fields.