SANDIA REPORTS IMPROVED DOWNHOLE SEISMIC RECEIVER

Sandia National Laboratories, Albuquerque, reports a successful field test of a downhole seismic receiver it says offers several improvements over previous models. The Sandia receiver has a broader frequency range and a new type of sensor for better resolution of seismic signals in cross-well imaging. The technique involves placing seismic sources and receivers in separate wells in an oil or gas field. The goal is to help obtain a better image of a field's structure.

July 15, 1991

4 min read

Sandia National Laboratories, Albuquerque, reports a successful field test of a downhole seismic receiver it says offers several improvements over previous models.

The Sandia receiver has a broader frequency range and a new type of sensor for better resolution of seismic signals in cross-well imaging.

The technique involves placing seismic sources and receivers in separate wells in an oil or gas field.

The goal is to help obtain a better image of a field's structure.

In addition, the Sandia unit has been designed to make it possible to string several stations in an array, thereby obtaining simultaneous readings at many levels in the well.

Gerard Sleefe, an engineer in Sandia's geoenergy technology department and leader of the downhole receiver project, called the array feature "another major advantage."

He said, "Currently, to get readings at several levels with a single receiver, you have to move the receiver for each source shot. This is time consuming and expensive because the well has to remain shut down during the entire test period."

THE MECHANICAL PACKAGE

The task of designing the mechanical package fell to Bruce Engler, who works with Sleefe at Sandia.

The resulting device is a 16 in. long steel cylinder weighing about 30 lb. The tool is 4 in. in diameter and is locked into the wellbore by a motor driven metal pad that pushes against the side of the well.

"The design challenges were threefold," Sleefe said.

"First, we had to solve the problem of mechanical packaging. Then we had to come up with an appropriate sensor. And finally we had to design the data telemetry."

To determine the appropriate mechanical package for the receiver, Engler called upon Sandia's capabilities in finite element modeling, a technique that uses computers to determine how a certain piece of equipment will perform under specific conditions.

Engler started with a design and refined it according to the modeling results.

The finite element modeling, done by Kenneth Gwinn of Sandia's fluid and thermal sciences department, predicted the natural resonant frequency that would be created by vibrations of the tool. This natural resonance could interfere with signals emitted by the seismic source if they are within the same range.

With finite element modeling, Sandia engineers were able to set the natural resonant frequency of the receiver at a level well beyond the seismic frequency range.

Sandia designers experimented with two types of sensors for the receiver. They first looked at the conventional geophone. But because it is a velocity sensor that uses a spring mounted moving coil, it lacks the sensitivity to measure higher frequencies and can distort signals due to spring deficiencies, Sandia said.

Engineers then established specifications for a more compact piezoelectric acceleration sensor, which was custom made by Wilcoxon Research and incorporated into the prototype receiver.

The Sandia unit uses a conventional wireline to relay information out of the hole. Information can be digitized in the hole, stored in a memory buffer, then transmitted to the surface over the seven standard wires.

FIBEROPTICS NEXT

The geoenergy technology department and researchers in other Sandia departments are working on fiberoptic packages for the sensor and data telemetry packages.

Members of Sandia's telemetry technology development division are working with Sleefe to design the telemetry package, and members of Sandia's optoelectronics components development division are designing a fiberoptic accelerometer to serve as a sensor.

Sleefe cited the advantages of fiberoptics, which would not require the electronics of traditional sensing and telemetry packages. Thus they would experience fewer problems at the high temperatures and pressures downhole. They also could be multiplexed so data could be transmitted out of the hole faster.

FIELD TEST

The receiver with the piezoelectric accelerometer was tested in a well in Humble oil field a few miles northeast of Houston Intercontinental Airport. Results showed the performance of the package was superior to other seismic receivers, including geophones cemented in a well bore, Sleefe said.

"The test showed the downhole receiver resulted in enhanced signals in every way-higher frequencies, better resolution, and no significant natural resonances," he said. The next stage of testing will be a field trial of an array of five receivers in a single well. That test is planned for early next fall.

OYO Geospace, manufacturer of the prototype receiver, is sharing the cost of the project. Exxon Production Research Co. provided the seismic source for the test, which was conducted at a Texaco USA well.

Sandia is a multiprogram research and development facility operated for the U.S. Department of Energy by AT&T. Sandia's main facilities are in Livermore, Calif., and Albuquerque.