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Colombo, D., G.R. Foulger, L. De Luca and R. Grossi, Integrated analysis of passive and active seismic data in a producing oil field, 68th European Association of Geoscientists & Engineers (EAGE) Conference, Vienna, Austria, 12-15th June, 2006.

Integrated analysis of passive and active seismic data in a producing oil field

D. Colombo1, G.R. Foulger2, L. De Luca1 and R. Grossi1

1Geosystem s.r.l., via Clericetti 42/A – 20133 Milano, Italy
2Dept. of Earth Sciences, Univeristy of Durham, Durham, U. K.

Recently, seismic exploration (e.g. near/global offset active seismic data) has been enhanced by new seismological techniques in reservoir monitoring and exploration. Enhanced oil recovery procedures, such as water, steam and CO2 injection, generate microearthquakes due stress changes induced in the reservoir. Monitoring of induced microearthquakes during production and/or well testing is becoming common practice in the oil industry, in order to optimize oil recovery procedures and for exploration purposes. The passive seismic technique is thus becoming an important tool for exploration purposes, especially in particularly tectonically complex areas. Changes in Vp and Vs as a result of fluid substitution (e.g. steam substituting oil) can be detected via Vp, Vs or Vp/Vs 3D tomography, where microearthquakes are used as seismic sources.

We present the results of an integrated analysis of passive (microearthquakes) and active seismic data in a producing oil field in Papua New Guinea. We performed a test on synthetic data to evaluate the effect of using joint tomographic inversion of microearthquakes and shallow seismic shots.

Vp, Vs and Vp/Vs 3D tomography was conducted by inverting the arrival times of seismic waves from microearthquakes recorded by a seismological network deployed in an area about 40 km by 40 km wide. Shots from 2D seismic lines in the area were also used. The microearthquakes, which occur deep beneath the surface, can illuminate the reservoir directly from beneath and constrain velocities in the deeper parts, whilst shallow seismic shots contribute to optimizing the shallower part of the model. Microearthquakes are more efficient generators of shear waves and this enables the study of variations in the Vp/Vs ratio in reservoir rocks which cannot be achieved with shots alone. The velocity model from wide-offset pre-stack depth migration, seismic-line interpretation, sonic logs and a digital elevation model were used to construct a starting model for the high-resolution 3D tomographic inversion.

To study processes such as opening cracks, shear faulting and cavity collapse resulting from fluid removal, the dynamics of fracture growth was studied by calculating moment tensors for the microearthquakes. This analysis also revealed the local and regional stress.

last update 22nd February, 2006