Objective
This project is based on the utilisation of the drill bit as an acoustic source. Seismic signals emitted during the abrasion of the rock by the drill bit are recorded via seismic pick-ups placed in a special manner on the surface or in nearby wells.
After these signals have been processed and interpreted in real time, it is possible to obtain data comparable to that provided presently off-line by seismic measurements taken in wells, for example the equivalent of a "speed log" in relation to the depth, the seismic profile of a well, or a 3D image of formations crossed in the vicinity of the well. Thus, it is a matter of studying and finalizing appropriated methods and equipment, thus providing access to a better instant acknowledgement of the crossed formations, and allowing to improve drilling procedures.
Studied carried out during phase 3, that is the combination of drill string measurements and surface measurements, have led to the following conclusions :
1. Seismic profiles for transposed wells can be obtained from the sole recording of drill bit noises, but the quality of the results remain poor, and depends partly on the hardness of the formation and on the drilling mode (ordinary rotation or percussion rotation).
2. The signal to noise ratio remaining low, it would be useful to improve it by using a seismic source on the drill string near the drill bit.
3. The spectrum of seismic signals, emitted at the bit during drilling and transmitted in the soil is relatively high frequency and may reach 300 to 400 Hz.
4. In most cases, seismic signals recorded at the top of the drill string are not faithfully representative of the signals transmitted at the bit.
For a better correlation with recordings on devices spread at the surface, it is advised, during cross-correlations of traces,to use the "pseudo-signatures" recorded near the bit. However, this raises the problem of their transmission up to the surface, along the drill string.
5. Obtaining an evaluation of the mean speeds of the formation sections between the bit and the surface, and in a correlative manner, of the speeds of seismic waves in vertical sections (section speed) is possible through a statistical study of the curvatures on seismic profiles without the use of plots at the top of the drill string. However, the results obtained like the previous ones are closely linked to the quality of the signal to noise ratio (soft terrains providing hardly any positive result). Their improvement implies the elaboration of a seismic source.
6. Different drilling equipment have been studied both in Europe and in the U.S.A inherent to the study of a seismic source while drilling. There are very few studies on this subject and because of the present context, there is no real incentive for manufacturers to pursue research in this domain. Consequently studies have been interrupted at this stage of the project.
To achieve this goal, it is necessary to use both, the seismic signals received at the surface on a seismic type apparatus, or on special sensors placed in nearby wells, and the signals transmitted by the drill string and received at its head, as the latter signals can be representive of a pseudo-signature of the seismic signals transmitted by the drill bit.
This project has been divided in five major phases :
Phase 1 - Measures at the drill string
Phase 2 - Measures at the surface geophone
Phase 3 - Combination of both types of measures
Phase 4 - Well seismic source
Phase 5 - Special data acquisition and processing device on the site.
Phase 1, 2 and 3 are closely related, as phase 3 can only be executed once Phases 1 and 2 have been carried out on the same site and at the same time.
In this regard, measures were made at three different sites in France :
LE MAYET DE MONTAGNE, in July 1984, while drilling in granite for the purpose of geothermals studies. Measures were taken at a depth between 200 and 800 m, and recorded with a 47-trace surface device over a length of about 700 m and placed radially to the well, and on a set of sensors placed in bores near the well. An accelerometer was fitted at the end of the drill string. Drilling was executed out in the open with a bottomhole drill hammer.
LACQ SUPERIOR, in May 1985, during a vertical drilling operation, very near a deviated well. This particularity has allowed us to record with the help of well geophones, the signals transmitted by the bit drilling at depths of about 400 m, and measured at horizontal distances inferior to 50 m. The purpose was to compare the signals recorded at the upper end of the drill string with those recorded at the drill bit.
SOUDRON 116, in October 1985, when drilling a production well on the Soudron field, south of Chalons sur Marne in the Paris Basin. During this field data acquisition, measures were taken between 1000 and 2100 m deep (one level of measures for every string adjunction) on seismic devices comprising sensors placed, both, at the surface and in bores at a depth of about 40 m. Phase 3 consisted mainly of the acquisition via numerical processing of seismic profiles of the transposed and multi-offset wells, or of data on the vertical seismic speeds by using the records provided by surface devices and drill strings, and the repeated cross-correlations.
Phase 4 consisted in a feasibility study and in the definition of specifications in view of designing a prototype seismic source while drilling in case the feasibility results of phases 1 to 3 proved insufficient at the level of the signal to noise ratio transmitted only by the drill bit.
Phase 5 of the project depended mainly on the positive results obtained during the previous phases. The purpose of phase 5 was to study and make an in-situ acquisition and processing prototype unit, capable of providing in real time the data required to run the drilling operation and to pursue exploration.
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Coordinator
92500 Rueil Malmaison
France
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