Method and Apparatus for Conducting Seismic Surveys From a Single Well Having Both Seismic Sources and Receivers Deployed Therein

The present invention relates to a method and apparatus for conducting seismic surveys from within a single well borehole and in particular for having both a seismic energy source and plural seismic receivers deployed in an array suspended in the wellbore by and inside coiled steel tubing.

It is important to map the termination of oil bearing formations as they near a salt dome. Such mapping is very difficult to accomplish using traditional surface seismic techniques. This is primarily because surface generated seismic energy reaching the vertical or near vertical surfaces of an active or passive salt dome or diapir can be reflected or scattered in directions other than back to the surface seismic detectors. Vertical Seismic Profiling surveys, or salt proximity surveys, in which seismic detectors are deployed in a wellbore and in which a seismic source is located on the surface can provide additional information, but not an adequate amount of information because of similar geometrical problems.

It is beneficial to deploy both a seismic energy source and one or more spaced apart seismic detectors in a single wellbore simultaneously. The seismic source/receiver system can be placed in a wellbore drilled near to the vertical or near vertical flank, but outside of, a salt dome. The seismic energy generated by the borehole deployed seismic source can travel along nearly horizontal paths and can be reflected or scattered back from the side or flank of the salt dome or diapir and from the steeply dipping earth formation beds near the flank of such a salt dome. The reflected seismic signals can then be detected from the spaced apart seismic detectors and recorded as a function of time.

The present invention concerns method and apparatus in which both a seismic source and seismic receivers or detectors, such as hydrophones, three component geophones, or three component accelerometers, are suspended in a single wellbore. To investigate a salt dome or like configuration seismic signals are sent from the suspended source, such as an airgun, and received by the receivers to define a more accurate map of the salt dome flank configuration than possible with surface located seismic sources and detectors. In the present invention an airgun seismic source is suspended at the end of a roll of coiled steel tubing run into the borehole from the surface. A surface compressed air source is connected to the airgun by a smaller diameter steel tubing, run internally to the outer coil steel tubing. An electrical cable is also run internally to the coiled tubing down to the airgun seismic source to effect control of the source. Seismic detectors, such as hydrophones, are connected to the same internally carried electrical cable above the seismic source at a spaced distance therefrom as desired. If preferred, a separate electric cable could be run to the detectors from that used to control the seismic source. If necessary, coolant water can be pumped from the surface down the coiled tubing annulus between the inner tubing and the electrical cable to cool the airgun. The coolant water is allowed to exit the airgun into the wellbore where it circulates back to the surface.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: FIG. 1 is a schematic vertical section through a well penetrating geological formations adjacent a salt dome and showing equipment according to the invention deployed by coiled steel tubing in the wellbore, and FIG. 2 and FIG. 3 are schematics of a seismic section recorded by equipment similar to that deployed in FIG. 1 and illustrates the attenuation of tube waves above the airgun source.

Referring initially to FIG. 1, a well 10 penetrates a plurality of earth formations adjacent a salt dome or diapir 12 which has an oil bearing formation 14 adjacent thereto. A plurality of seismic detectors 16 are suspended inside coiled steel tubing in the well in a spaced array with a seismic airgun source 18 suspended at the end of the coiled tubing 20 further on down the wellbore from the seismic detectors 16. In operation the seismic airgun energy source 18 generates seismic energy waves which are reflected, refracted and scattered by the surrounding acoustic impedance discontinuities (including the salt dome) and reflected, refracted and scattered back to the seismic detectors 16. Signals from the detectors 16 are sent to a surface computer system for recording. Detectors 16 contemplated for use herein can comprise pressure sensitive hydrophones or geophones or accelerometers attached to an armored electrical cable carried interiorly to the coiled tubing 20. If 3 axis geophones or accelerator meters are used, then some method of determining the orientation of the detectors spatially will be needed. One such method could be the inclusion at the end of the geophone package of an inertial guidance system such as a gyroscope.

In addition to imaging salt domes, this system can also be useful in imaging any high angle faults or any other steeply dipping (from horizontal) feature. It is also possible to use such a seismic source-detector assembly which is run into a wellbore on coiled steel tubing in a horizontal well to image horizontally aligned parallel beds above and below the horizontal wellbore.

The concept of doing seismic energy transmission and seismic reception in a single wellbore by instruments inserted therein and carried interiorly to coiled tubing is new. In addition to the mechanical problems associated with building tools to contain both a powerful seismic source and sensitive receivers that can be easily deployed i.e. rolled up on hoist drum, there are other formidable complications as well.

For example, most borehole seismic sources generate strong tube waves that travel in the borehole fluid for the full length of the well with very little attenuation. These tube waves are also reflected from the top and the bottom of the fluid column and therefore reverberations or echoes of them can persist for several seconds. Such tube waves are usually very high in amplitude and can completely mask the reception of any desired horizontally scattered seismic signals. Unsuccessful prior art attempts to perform single well seismic surveying have involved electrically operated seismic sources. Cross coupling of high amplitude electrical signals from the high power electrical drive cables by electromagnetic induction into the seismic detectors can also swamp or mask the low level desired seismic signals from the detectors.

The present invention solves the problems described above by preferably using a borehole airgun which is deployed from the end of a coiled steel tubing. The coiled tubing deployed airgun has been developed for high temperature borehole operation. Cooling water is pumped down through the tubing in the annulus between the electrical cable and the smaller diameter compressed air supply steel tubing run internally therein. The coolant water passes over the airgun and back up the wellbore annulus outside the coiled tubing to the surface. A multi-conductor armored well logging wire line runs inside the coiled tubing providing electrical control signals to the airgun. A second multi-conductor armored wire line also run inside the coiled tubing provides electrical connection to the detectors. Compressed air is provided by a smaller diameter (such as 1/4") steel tubing which is also run through the typically 11/2" inside diameter coiled steel tubing. An array of seismic detectors (preferably hydrophones) are connected to the cable at spaced intervals inside the 11/2" diameter coiled steel tubing above the airgun.

Air released from the airgun upon its activation into the borehole fluid is a very effective tube wave attenuator. Tube waves travelling along the borehole are highly attenuated after travelling only a few tens of feet in the array of released air bubbles. This is clearly illustrated from field data of FIGS. 2 and 3. In FIG. 2 time in milliseconds from source firing is on the horizontal axis and depth is on the vertical axis for 3 different detectors. These data were gathered using a 3 component geophone detector at a depth of 1200 ft in a first well and moving an airgun source in a second parallel wellbore to different shot depths from 600 foot depth to 1800 foot depth firing at 5 ft. intervals. The first of the geophone components is oriented vertically, and is shown in FIG. 2A. The second and third geophone components are oriented horizontally, and at 90.degree. with respect to each other. These are shown in FIG. 2B and FIG. 2C. The strong linear events are caused by downgoing tube waves in the source well. The up-going tube waves in the source well are highly attenuated by the air bubbles above the source. In all 3 illustrations the attenuation of direct arriving tube waves is dramatic as the source (airgun) moves below the detector. FIGS. 3A, B,C shows seismic data recorded with the borehole fluid treated both above and below the source. Both up-going and downgoing tube waves are eliminated allowing the weaker but desirable seismic signals to be visible.

Electrical cross-coupling from the airgun control signals is not difficult to control. The only high level electrical signal required in the system of the invention is a 30 millisecond duration pulse to operate the airgun by firing a control solenoid. This signal is carried by a twisted pair of wires inside the armor shielded wireline and does not present any major interference problem especially since it precedes by a significant interval the time window of interest for recording back scattered signals at the spaced detectors.

Vibration isolation between the seismic source at the bottom of the coiled steel tubing and the seismic detectors located higher up inside the coiled steel tubing can be provided if desired.

The present invention may be subject to many modifications and changes without departing from the spirit or actual characteristics thereof. For example, other types of seismic sources could be used if desired. The present embodiment should therefore be considered in all respects as illustrative and not restrictive of the scope of the invention as defined by the appended claims.