Time-lapse Seismology to Determine Foamy Oil and Wormhole Footprints in a Heavy Oil Cold Production Reservoir
Qiaozhi (Sandy) Chen
Simultaneous extraction of oil and sand during cold production of heavy oil generates high-porosity channels termed "wormholes". The development of wormholes causes reservoir pressure to fall below the bubble point, resulting in dissolved-gas coming out of solution to form foamy oil. Amplitude anomalies in the vicinity of the borehole observed in time-lapse seismic surveys are possibly a result of the presence of foamy oil and wormholes. These results can be used to image the drainage pattern, which reflects the foamy oil and wormhole footprints, for infill drilling designs. The use of time-lapse reflection seismology for detecting the presence of foamy oil and wormholes is discussed here. This thesis presents one of the first comprehensive seismic modeling studies of these cold production phenomena.
The presence of small amounts of gas within the foamy oil can dramatically decrease the fluid bulk modulus, reducing the P-wave velocity of saturated sands. Elastic moduli and velocities, under gas influence, have been modeled to an in-situ reservoir in a Western Canadian heavy oil field. 2D numerical seismic responses of foamy oil zones show amplitude anomalies and traveltime delays when pre- and postproduction results are compared.
On the other hand, wormholes, with porosities greater than the critical porosity, can strongly change the rigidity and frame bulk modulus of the reservoir sands, thus resulting in the decrease of both P-wave and S-wave velocities of the saturated sands. The macroscopic effects of wormholes on P-wave and S-wave velocities of the reservoir rock can be observed, when large amounts of sands are produced. 2D numerical models designed to examine cumulative effects of wormholes using both PP and PS seismic data indicate that PS data are better at imaging the wormhole footprints due to its significant impact on S-wave velocity. Also, the increase in amplitude anomalies is proportional to wormhole density.
The combined effects of both foamy oil and wormholes are also examined to show the amplitude anomalies and traveltime delays within the drainage regions. Considering that most of the cold production reservoirs are thin, seismic resolution of anomalous areas is highly dependent on frequency bandwidths.
In this thesis, the Vp/Vs values from four different reservoir states have been studied. When compared to the initial reservoir state, Vp/Vs of the post-production reservoir with both foamy oil and wormhole effects can be higher or lower. If the wormhole effect is dominant, Vp/Vs will increase. If the foamy oil effect is dominant, Vp/Vs will decrease. This makes it complicated to analyze the AVO effects under the influence of the foamy oil and wormholes in heavy oil cold production.