Space Radar Monitoring Identifies Earth Surface Subsidence Caused by Oil and Gas Field Development

December 5, 2012

Even when these observations are taken regularly, however, generating a reliable continuous map of subsidence is impossible since reference marks only provide a discrete measurement net with general interpolation between its nodal points. Furthermore, model calculation of earth surface deformation requires significant assumptions (particularly, estimation of the compression modulus of the rock skeleton from the reservoir bed to the earth surface).

Differential Radar Interferometry Shows Displacement

Differential radar interferometry (DRI) directly and efficiently maps earth surface displacement and structural deformation (Fig. 1-4). The main advantage of DRI over other methods of vertical and horizontal deformation monitoring is direct measurement of the changes in relief that occur between surveys.
The picture of displacement obtained with interferometric processing usually gives an integral view of displacement, including both natural and industrial components.

Naturally occurring soil deformation results from the movement of crustal blocks due to the lunar tidal cycle, seasonal thawing of soil in permafrost areas, subsidence in areas of high thermokarst activity, and modern geodynamic processes.

Production induced displacements mostly occur due to a significant (by several times) pressure drop in reservoir beds accompanying hydrocarbon production, depletion of water-bearing horizons, and mineral recovery with a mining method that produces large cavities in the skeleton of the sedimentary mantle. Underground gas storage facilities (UGS) are associated with an approximately semi-annual cycle of alternating displacements of the earth surface. Radar interferometry enables the tracing of deformation of main pipelines caused by industrial impact on the soil.

Space radar monitoring of subsidence from hydrocarbon field development has been practiced abroad since the mid-1990s. No systematic study of applying radar interferometry to the gas industry had been done in Russia until now.
In 2006, VNIIGAZ specialists began to analyze foreign experience with radar interferometry in monitoring earth surface displacements.

Analysis of the results of NASA, ESA, etc., enabled the following conclusions:
1. Two-, three- and four-pass chains of radar photos are used for differential interferometric processing.
2. Minimum displacements recorded in the course of interferometric processing of non-simultaneous radar photos taken in X- and С-bands of the radio-wave electromagnetic spectrum are several millimeters.
3. The rate of the earth surface subsidence in hydrocarbon production areas can exceed 1 mm/day.
4. As production induced vertical displacements are due to the recovery of hydrocarbons from productive horizons, the outline of the subsidence area can correspond to the gas-water contact, the outline of actual reserves, or can show the location of these outlines (as in various standard cases).
5. Best results are achieved by combining results from radar survey interferometric processing and data from ground reference observations, and by setting radio signal corner reflectors.

Gazprom Refines DRI Technology

For practical refinement of differential radar interferometry technology, Gazprom is planning pilot projects at its fields. No comprehensive testing of this technology has been done in Russia. In 2006, VNIIGAZ started planning an interferometry radar survey of a complicated natural environment, specifically northern areas in Western Siberia. Initial results are presented in this article.

The researchers planned to analyze a five-pass