A device and method for suppressing acoustic tube wave noise in a borehole. The device comprises an absorptive material that is positioned in a borehole so that the device will encounter and absorb the tube waves before the waves are detected by receivers. The device can also contain a housing in which to place the absorptive material. The housing can also be surrounded by an absorptive material to help absorb energy reflecting off the borehole wall.
A well tool in accordance with the invention comprises an attenuation device which attenuates tube waves associated with an elastic wave emission and/or reception set (1, 12). The attenuation device comprises a bubble generator (2) which releases into the well a gas not readily soluble in the well fluid, slightly overpressured in relation to the hydrostatic pressure, in the form of calibrated bubbles which greatly attenuate the parasitic tube waves. The attenuation device is placed above or below the elastic wave emission and/or reception set having a plurality of pickups whose signals are acquired by a local electronic module (14) and/or a source of elastic waves such as a vibrator. A bubble trap (17) is preferably associated with the bubble generator (2) in order to limit the volume of gas to be generated in situ. The invention is applicable to VSP type seismic prospecting and acoustic logging.
An attenuator for use in attenuating tube waves in a borehole includes a body formed from a permeable material having a rigid matrix which can be saturated with fluid. In its simplest form, the attenuator is one or more cylindrical bodies as part of a borehole acoustic logging system. Alternatively, the shape, particularly at the end regions, can be modified to reduce the amount of reflection of tube waves in the borehole. Another attenuator includes a body for placement in a borehole formed from an attenuating material, the shape of the body being selected such that the change in impedance experienced by acoustic waves in the borehole is gradual rather than abrupt. One particularly preferred shape for the body comprises one in which the diameter is greatest towards the center of the body, for example the shape formed by two cones placed base to base, or a cylinder having conically tapered ends. A number of bodies can be used where appropriate.
A VSP or reversed VSP type seismic prospecting method according to the invention allows disturbances contained in seismic recordings produced by tube waves propagating along wells and refracted in zones close to the surface to be minimized. The method disperses a gas under pressure, having preferably a high compressibility, capable of absorbing these tube waves. The gas may be generated by an explosive chemical reaction set off in situ, at a certain depth of the well (which may constitute the seismic wave source). The method has an application for seismic prospecting or drilling noise monitoring.
In one embodiment the invention comprises a method of gathering borehole seismic data in which a borehole acoustic instrument is lowered into a borehole in the earth's surface along with a canister positioned about the instrument and a second open bottomed canister positioned below the instrument. The canisters are at least partially filled with gas after the canisters are lowered into borehole. In another embodiment the invention comprises an apparatus for reflecting tube waves in a fluid-filled borehole which includes a canister having a diameter adapted to enable the canister to be deployed within the fluid-filled borehole, and having an open lower end, and an enclosed upper end with an aperture through the upper end adapted for sealing engagement between said upper end and a conduit.
An acoustic system and method for investigating and monitoring a hydrocarbon or other mineral reservoir. Wellbore tube wave energy may be created by natural or ambient sources or tube waves may be excited intentionally. Wellbore tube wave energy is converted to body wave energy at minor borehole obstructions or irregularities that become point sources of seismic energy. A well bore may contain a plurality of minor obstructions or discontinuities. Each obstruction or discontinuity position along the borehole has an associated unique waveform source coda that may be measured for processing the body waves radiated into the earth formation surrounding the well bore. A plurality of sensors detects the radiated seismic energy after the seismic energy has transited intervening earth formations. The system may be employed for permanent monitoring of mineral resources and resource management. Measurements of reservoir characteristics may be acquired at many different times over the productive life of a reservoir. A time-lapse profile may be created that is representative of the temporal and areal changes in the petrophysical characteristics and mineral content or distribution of the intervening earth formation.
