Field of the Invention
1. The present invention relates to a method of automatic classification, according to selection criteria, of records of seismic or microseismic events picked up by seismic receivers coupled with an underground formation under development.
2. Description of the Prior Art
Locating points of an underground zone, whether in a reservoir zone or in a cavity, in which occur microseismic events related to an activity which has the effect of modifying the stress field, is of great significance for the development of the zone, whether production of fluids extracted from a reservoir through one or more wells or injection of fluids into the zone.
It is for example possible to monitor the evolution of a hydrocarbon reservoir under development or of geothermal sites. In the case of enhanced recovery notably, it is desired to sweep the oil out of the rock by injecting fluids at pressures and temperatures that may be very different from those of the medium. The resulting stress variations may lead to a fracturation of the medium, which modifies the circulation of the fluids inside the reservoir and which it is important to locate properly.
It is also well-known to use underground reservoirs for fluid storage. The latter may be, for example, liquid or gas phase storage reservoirs wherein a certain microseismic activity induced by significant variations in the flow rate of the liquid or the gas taken from or injected into these reservoirs can be observed.
The reservoirs used may also be reservoir zones or cavities used for waste storage, which have to be monitored so as to respect the environment and to comply with the increasingly restricting pollution control regulations. An underground reservoir can be used to inject drilling fluids containing large amounts of solid particles since dumping these fluids after use on drilling sites is forbidden by regulations.
The temperature of the fluids injected is generally very different from the temperature of the medium at the depth where they are injected, which, in case of massive injection, has the effect of producing thermal stresses which generate fractures and therefore lead to a certain seismic activity. The injection pressure of these muds can also create stresses and lead to great changes in the medium.
The seismic activity induced by the effects of the pressure or of the temperature can reveal, for example, the formation of fractures or give rise to stresses in already existing fractures. They contribute to modifying the flow paths of the fluids in the medium, or they create escape paths for fluids out of the reservoir, containment breaks with possible pollution of the surrounding zones, notably of an aquifer used for drinkable water supply, which it is important to detect.
It is also of very great significance to monitor reservoirs used for storage of nuclear waste so as to prevent injection operations or local temperature rises due to storage from causing fractures in the layers providing containment.
With microseismic monitoring, also known as passive seismic, the developer's aim is, in the end, to interpret very quickly the data in connection with the conventional working data (pressure, flow rate, temperature, etc.) so as to be able to account for the mechanical response of the site in the development protocol in order to preserve the productivity of the well(s) or of the site. The microseismic activity observed can be combined with a mechanical degradation of the medium which can generate solids encroachment, with the opening of fractures likely to communicate the reservoir with an aquifer, or with other phenomena of thermo-poro-mechanical origin whose consequences may lead to a performance degradation of the wells or even to well damage.
U.S. Pat. Nos. 4,775,009 and 5,303,773 and EP 546,892 notably describe various techniques for monitoring the evolution over time of underground reservoirs, which comprise using seismic or other pickups permanently installed in one or more wells (embedded in the cement coupling the casing with the formation or externally combined with a production tubing and pressed against the inner face of the casing) without disturbing the various operations carried out (production, injection, various servicing operations performed by means of these wells). Permanent installation of pickups in wells allows seismic monitoring of a reservoir in order to detect different phenomena related to the development thereof.
French Patents 2,703,457 and 2,703,470 and U.S. Pat. No. 5,724,311 describe methods for long-term repetitive active monitoring of a reservoir through application of elastic waves to a formation and acquisition of the response signals sent back by the formation, by means of the permanent installation of emission and reception means in wells or close to the ground surface. Differential processing operations are carried out on acquisitions performed under identical conditions.
French Patent 2,688,896 and U.S. Pat. No. 5,481,502 notably describe electronic acquisition and transmission systems specially designed to collect signals from permanent pickups installed in wells outside casings or production tubings, and to transmit them to a surface recording and control equipment during long-term repetitive monitoring or seismic surveys.
Patent application EP-A-1,074,858 and U.S. Pat. No. 6,113,388 also describe a method for automatic analysis of signals acquired by one or more elastic or acoustic wave pickups so as to automatically locate thereon, with high accuracy, at least one significant time such as the time of first arrival on each pickup and/or the end of these signals.
U.S. Pat. No. 6,049,508 describes a method for automatic discrimination, from among all the events recorded, of those of type E which are of interest for characterization of the site, while taking into account the events induced at completion level, which are referred to as type C events. These events are generated by injection stops and resumptions, opening or closing of one of the completion elements (valve, packer, etc.) at any depth in the well or even at the surface (at the wellhead), including at surface installation level (pipes, various devices). Some of these actions, such as communication of the underground zone (reservoir) with the surface network, may lead, as a result of pressure variations notably, to type E events, often deferred, which it is desired to record and interpret. Type C events, which can be in very great number within a relatively short time interval (more than 3400 events in one week, by way of practical example), are detrimental to real-time monitoring of the geomechanical phenomena induced through the acquisition of type E events which are often in relatively small number for the same period (some ten events for example).
Besides the seismic receivers coupled with the formation, one or more reference pickups are used, which provide direct acoustic coupling with elements of the technical zone development equipment for detection of the elastic waves directly linked with the development. By means of a comparative analysis of the signals coming from these receivers and from each reference pickup, the records are sorted into different families according to whether the events in the underground zone are independent of the events detected by each reference pickup or depend on them directly or indirectly.
Systematic listening to microseismic events likely to occur in an underground formation generates, as mentioned above, a large amount of seismic records that have to be carefully classified so as to simplify later discrimination and analysis work.