Geomechanics and Rock physics

This study investigates whether thermally induced changes from steam assisted gravity drainage (SAGD) operations in a heavy oil sandstone reservoir are observable using seismic methods. Thermally induced changes can include stress changes, changes in characteristics arising from shear heating of heavy oils, and changes in fluid saturation and density driven by thermal heating. These variations are important to understand if seismic monitoring in "real time" is used for project optimization. The SAGD process injects steam from a "steam chamber" that grows vertically and horizontally in the reservoir and overburden. The heat from the steam reduces the viscosity of the heavy oil which enablesit to flow down into the producer well (where expectations of recovery can reach approximately 40% of oil in place). This study presents an analysis of the changes that are expected in seismically observed characteristics such as velocity (compressional and shear velocities) and seismic acoustic impedance. Variations of the rock and fluid properties from steam injection in a heavy oil sandstone reservoir were calibrated with ultrasonic laboratory testing using high temperatures (up to 150°C). A 2D synthetic seismogram using results from, logs, images (borehole and laboratory (PVT, thin section)) was evaluated for time-lapse modeling. The outputs from reservoir simulation (pressure, saturation and temperature) were used to calculate the variation in acoustic impedance and the fluid substitution was evaluated using Gassmann's equation. The results indicate that massive variation in fluid saturation, density, effective horizontal stresses and generation of shear dilation zones from thermally-induced volume changes affect seismic parameters. Variations in the horizontal stresses reached inside the steam chamber can be as great as 8%, and affect the seismic response by increasing acoustic impedance contrast by approximately 4.5%. Introduction Reserves of heavy oils have increased in importance because of the reduction of the reserves of light oil and rising oil prices. The world's largest reserves of heavy oil are in Canada and Venezuela. To produce these heavy oils, their viscosity must be reduced, and one method to reduce it is through heat transfer by steam injection. The steam injection not only alters the properties of heavy oils, but also the properties of the rock, and leads to the question of how thermal variations affect reservoir properties. The heavy oil reservoirs can suffer thermal impact during steam injection such as thermal expansion, softening and mechanical rock failure; previous investigations indicated (Samier, 2007) high values of effective normal stress and increased shear failure during stimulation. Therefore, a geomechanic model of the reservoir was created, which couples the fluid data set to the seismic (in order to understand the dimensions of the steam chamber and its steam front along the stimulation process). Because the conventional flow simulators do not take into account the geomechanic effects on the stimulation process, a better understanding on how the rock matrix affects the gravitational segregation, fluid phase changes and thermal expansion in the reservoir is necessary. The geomechanic model of the reservoir is constructed using petrographic, petrophysical and rock mechanical laboratory data. Time-lapse seismic interpretation is the process of linking differences between seismic surveys (base and monitor) that have endured changes in saturation, pressure and temperature in the reservoir. It is used for the propagation of mechanical and thermal properties of rocks and fluids and seismic simulation (time-lapse 4D), and to investigate whether seismic data is a useful technique for monitoring a SAGD process. The synthetic model was generated containing data from real seismic, velocity and density logs of The Faja del Orinoco Belt. The steam-assisted gravity drainage (SAGD) is a method to economically increase the recovery factor of the large accumulations of heavy oil from The Faja del Orinoco Belt. The Faja del Orinoco Belt comprises 55,314 km 2 with an area of 11,593 km 2 currently in production (includes Guárico, Anzoátegui and Monagas states). The Faja del Orinoco Belt comprises an estimated 186 billion barrels. 1,360 billion barrels