Measurements of electrical properties of an earth formation penetrated by a borehole have been used for decades in hydrocarbon exploration and production operations. The resistivity of hydrocarbon is greater than saline water. Therefore, a measure of formation resistivity can be used to delineate hydrocarbon-bearing formations from saline water-bearing formations.
Electrical borehole measurements are also used to determine a wide range of geophysical parameters of interest, including the location of bed boundaries, the dip of formations intersecting by the borehole, and anisotropy of material intersected by the borehole. Electrical measurements can also be used to “steer” the drilling of the borehole.
Borehole instruments or “tools” used to obtain electrical measurements typically have one or more antennas or transmitting coils and have one or more receivers or groups of electrodes. The one or more antennas are energized by an alternating electrical current, and resulting EM energy interacts with the surrounding formation and borehole environs by propagation or by induction of currents within the borehole environs. In turn, the one or more receivers on the tool respond to this EM energy or current. In general, a single coil or antenna can serve as both a transmitter and a receiver on the tool. Other types of tools used to evaluate electrical properties of the formations contain an array of electrodes that inject currents into the formation and read voltages and/or currents.
The electrical measurements may be telemetered to the surface of the earth via a conveyance, such as wireline or drill string equipped with a borehole telemetry system. Alternately, the electrical measurements can be stored within the borehole tool for subsequent retrieval at the surface. Based on the measurements, various parameters of interest, such as those listed above, can be determined using calculations and formulation well known in the art.
One borehole tool used to obtain electrical measurements is a resistivity imager, which can be conveyed downhole on a drill string or a wireline. When disposed in a borehole, the imager can image a borehole's resistivity (conductivity) and characterize faults, fractures, folds, unconformities, reefs, salt domes, sedimentary bodies, dip direction, beds, and the like.
When used in a borehole having conductive mud (i.e., water-based mud (WBM)), the resistivity imager can use a measuring electrode that emits or discharges current to a return electrode through the formation. When the current and voltage drop between the electrodes is measured, calculations based on Ohm's Law can determine the resistivity of the formation adjacent the measuring electrode. The imager's resolution is inversely proportional to the size of measuring electrode used. Accordingly, a larger measuring electrode produces a lower resolution and vice versa.
A resistivity imager used in non-conductive mud (i.e., oil-based mud (OBM)) typically has four-terminals and includes an injection electrode, a return electrode, and at least two sensor electrodes. The injection and return electrodes are used for injecting current into a formation, while the measuring electrodes measure a potential gradient between the injection and return electrodes. In these imagers, the image resolution is inversely proportional to the space between the measuring electrodes, and the measured signal strength is proportional to the distance between the measuring electrodes. Accordingly, a large distance between the measuring electrodes produces a lower resolution and vice versa.
Existing resistivity imagers use a sinusoidal signal to image the borehole, and conventional electronics produce this sinusoidal signal continuously during operation. As a result, existing imagers can use a considerable amount of energy to image the borehole, and power supplies downhole can be limited.
Nevertheless, the continuous sinusoidal signal allows the imager to use standard correlation techniques to improve the signal-to-noise ratio of the resulting resistivity measurements. As is known, a standard correlation filter takes running averages of successive waveforms and correlates these at different points in the signal. This essentially reduces any noise present in the signal to improve the signal-to-noise ratio of the measurements for better analysis. Understandably, averaging for the correlation process requires time to obtain successive waveforms. In a downhole MWD/LWD environment, the rotation of the imager about the borehole when obtaining measurements can limit how much improvement in the signal-to-noise ratio can be achieved. In this situation, time becomes space, and the azimuthal resolution of the measurements becomes degraded when too much time is needed to obtain successive waveforms for correlation and noise reduction.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.