Abstract Radioactive Marker Technique (RMT) is a systems to evaluate layer compaction and land subsidence due to underground fluid production, based on the periodic monitoring of the distance between markers inserted into the formation, or onto casing. RMT is designed for the in situ measurement of the uniaxial compressibility coefficient (Cm) of layers under depletion, which is crucial to implement mathematical models to forecast compaction and its effects on the earth surface. After a brief description of measurement principles, the paper describes some aspects of RMT interpretation. A review of the main features of modern tools is given, weak points of measurement procedures and interpretation techniques are underlined and statistical methods utilized to reduce errors of Cm computation are presented. The paper addresses also a new subject of discussion, i.e., the effect of casing on the deformation of the surrounding layers, that are to be quantified in order to obtain more realistic measurements. Researches in this direction must be carried out, in the attempt to clarify some still vague aspects of interpretation techniques, and to assure reliable measurements and clear criteria of RMT applications. In general, RMT-derived Cm's match with sufficient precision with the ones derived from surface subsidence observed at surface, but not with the ones measured in laboratory. Finally, an application in Northern Adriatic Sea is presented, where RMT helped to improve the quality of lab-measured Cm's, allowing for a more precise environmental impact assessment of a possible hydrocarbon development project in a very sensitive area. At last, the well-studied case of Groningen gas field, and other offshore North Sea and Gulf of Mexico applications are reported.
Introduction Land subsidence is a key priority in the environmental management of hydrocarbon reservoirs. This is particularly true in environmentally sensitive areas and in offshore operations, characterized by very high development costs, where it is essential to understand the long-term behavior of producing reservoirs. Subsidence prediction is an essential step in any sound technical procedure aimed to control this phenomenon. Rock compressibility is a key factor driving formation compaction, subsidence of earth surface, and operative problems to production facilities. As a general rule, from an applicative standpoint, rock compressibility and reservoir compaction raise at least three major issues.
The first one concerns land or seafloor subsidence, its impact on the natural environment (sensu lato), and the possible impairments both on land settlements and on production structure and their design. Recent and well-known examples of these critical interactions are the cases of Groningen, The Netherlands, Northern Adriatic Sea Italian gas fields and, as far as offshore is concerned, Ekofisk oil field, North Sea. The second issue is casing deformation in front of compacting layers. In general, compaction imposes severe stresses on casing, which might compromise its integrity. Casing failure resulting from compaction has occurred in many fields throughout the world. Moreover, casing deformation is a critical issue in the evaluation of in-situ estimation of reservoir compaction, as examined later in the paper. The third issue is the possible influence of rock compressibility on reservoir productivity. In fact, on one hand compaction can effectively aid the production as a major driving force, squeezing the saturating fluids into the well. On the other hand, it is well known the effect of compressibility in impairing reservoir rock permeability, eventually reducing production and final recovery.