P. Cheung and A. Hayman - Riboug Research Centre, Schlumberger, France

Drilling an oil well requires a good knowledge of the subsoil. Electrical imaging provides a fuzzy picture of underground structures. Experts at Schlumberger carry out simulations with COMSOL to sharpen the image and analyze the crisp snapshots that result.

Electrical imaging is an essential tool used in oil prospecting and geological work. A measurement of the subsoil's microresistance limits the need for classic core drilling, which provides only a partial impression of the oil field, generating additional costs and delays.

Together with laboratory and field tests, P. Cheung and A. Hayman from the Riboug Research Centre (Schlumberger-France) simulate the behavior of the electric imaging system, both to improve its design and also to predict the response according to the geological characteristics that are found.

Logarithm of electrical potential. The electrodes simulation the injected current are in red.

Optimize the geometry and the position of the electrodes

The measuring device is made of several electrodes that are attached to the material scientists wish to sample. They set up an alternating current between two of the electrodes while measuring the difference in electrical potential from the others. From this information they can apply Ohm?s Law to calculate the resistance of the material being tested.

Using COMSOL to optimize the design of the electrode arrays involves predicting how the electrical current moves for each electrode geometry and the layout of the arrays. Another area of intervention involves the mechanical casing of the device. 2D and then 3D models allow investigators to simulate various possibilities.

Schlumberger engineers conduct the multiparameter studies from the MATLAB platform using models they have developed through the COMSOL graphical interface. One of these researchers, Mr. Hayman, deems working with COMSOL and MATLAB in combination as being particularly user-friendly.

In the field

Once the measurement has been taken, an important step consists of predicting the signal measured according to the characteristics of the subsoil: varying levels of conductivity or the presence of geological objects. A sudden change in resistance distorts the signal, both in terms of its range and space. On a practical level it is impossible to correct such variations on an ongoing basis. In fact, the quality of the measurement is monitored in order to detect presimulated variations. This does not obstruct any automatic measurements that are taken.

The use of digital simulation with COMSOL is not limited exclusively to the design of the measuring device, but it is also applied in the field. On one hand, checks carried out using more classic measurements are pushed ever further. On the other hand, new operating conditions require simulations to be restarted. Finally, as A. Hayman emphasizes, "using COMSOL speeds up the development of our measuring systems."