This invention relates generally to apparatus and methods for making induced polarization and resistivity measurements in a borehole.
In the study of geological formations it is known that induced polarization and resistivity measurements can be made by means of surface-located equipment. See, for example, U.S. Pat. No. 4,295,056. Through the use of such surface equipment, induced polarization measurements which may include phase angle anomalies can be obtained. The phase angle anomalies can be created by a shallow subsurface polarizable material which is of significance to the oil and gas industry if its composition can be geochemically tested and found to contain hydrocarbons which have seeped from a deeper reservoir. However, before the material can be tested, its location must be pinpointed. Therefore, there is the need for an apparatus and a method which utilize electrodes in a borehole to make downhole induced polarization and resistivity measurements in the vicinity of the formations which are creating the anomalies so their locations can be accurately determined. Once located, the formations can be geochemically tested to determine if hydrocarbons are present (see, for example, U.S. Pat. No. 4,360,359). From such information one can tell whether hydrocarbons are seeping from a reservoir, for example. The need for an apparatus and a method for making such downhole measurements has been recognized. See, for example, Complex Formation Resistivity--The Forgotten Half of the Resistivity Log by D. D. Snyder, R. H. Merkel, and J. T. Williams and Induced Polarization Measurements In and Around Boreholes by D. D. Snyder and R. H. Merkel.
It has been observed that some of these anomalies arise from shallow or thin formations, thereby making it desirable to utilize an apparatus which can make measurements of sufficient resolution for such thin formations. This desirability indicates that a short array spacing of the electrodes be used, which short array spacing corresponds to the size of the thin layer; however, for such a short array spacing of the type needed to make the induced polarization and resistivity measurements, the measurements made thereby would be adversely affected by the electrical properties of the drilling mud or other fluid surrounding the electrodes at their measurement locations in the borehole. That is, a short array spacing would cause the apparatus to read the electrical properties of the drilling mud or other fluid rather than the electrical properties of the desired formation.
The borehole fluid can have a substantial effect on the phase readings made by such an apparatus because the borehole fluid may be a drilling mud typically containing highly polarizable montmorillonite clay, for example. Additionally, it is difficult, if not virtually impossible, to determine ahead of time exactly what the effect of the borehole fluid will be. Still further, a drilling mud tends to be electrochemically unstable so that it is difficult to determine at any given time what the electrical properties of the mud are. Furthermore, the depth of invasion of the mud or other borehole fluid into the rock formation is highly variable. Therefore, there is the need for the apparatus and method to be constructed and performed so that several suitable measurements with variable length arrays can be made and compared to derive a reasonable estimate of the electrical properties of thin formations. This reasonable estimate can be further refined by suitable modeling of the formation surrounding the borehole.
It is also desirable that the apparatus and method which meet the foregoing needs be usable with commercially available interconnection equipment presently found in the oil and gas industry. This commercially available equipment includes electrical logging cable comprising parallel electrical conductors. Such commercially available cable is susceptible to significant electromagnetic coupling between the various parallel conductors. This electromagnetic coupling adversely affects the induced polarization measurements.
One way to avoid the electromagnetic coupling is to use separately twisted pairs of wires; however, such twisted pairs of wires are not standard equipment in the oil and gas industry and thus their use would require special orders with possibly long lead times and high costs. Additionally, separately twisted wires would make for a thick cable which would reduce the amount of cable which could be retained on available winches.
Such a problem of electromagnetic coupling could also be overcome by placing certain equipment in the implement which is to be used in the borehole and telemetering the readings to the surface. A shortcoming of placing additional equipment in the borehole is that it requires packaging the equipment for use in the extreme pressures and temperatures found in the borehole. Furthermore, having the equipment downhole would limit the ability to control it, which ability would be present if the equipment were to be located at the surface. Telemetering signals to the surface would limit the dynamic range and accuracy of the measurements made in the borehole. Therefore, there is the need for the apparatus and method to be constructed and performed so that such adverse electromagnetic coupling is eliminated without preventing the apparatus and method from being used with or implemented by commercially available equipment.
It is known that such induced polarization and resistivity measurements as are referred to hereinabove are made by introducing a current into the formation and then detecting a voltage produced in the formation by the current. It has been discovered that it is important to control the density of the current because if the current density is too large, the induced polarization response will be non-linear with respect to the current and thus will not be representative of the response that a surface induced polarization survey would obtain. Therefore, the apparatus and method which meet the aforementioned needs should also meet the needs of maintaining an accurately controlled current output and utilizing a low-noise receiver which can accommodate very small transmitted currents.
To insure that accurate measurements are made, the apparatus should be constructed so that electrical leakage in the portion of the apparatus which is to be used in the borehole is reduced. It has been found that the phase measurement which is indicative of the induced polarization response is much more sensitive to minor amounts of electrical leakage than are conventional magnitude-only resistivity measurements. To prevent such electrical leakage, there is the need for the apparatus to be constructed so that there is substantially a zero pressure differential between the interior of the downhole implement and the borehole pressure to thereby preclude the flow of borehole fluid into the downhole implement.
To further insure the accuracy, the desired method is to be performed discretely whereby discrete, rather than continuous, measurements of the induced polarization and resistivity are made. Discrete measurements preclude the introduction of noise and instability which can occur by means of the movement of the downhole implement through the borehole fluid as measurements are taken. Additionally, the preferred current used to obtain such induced polarization and resistivity measurements is at a very low frequency so that if a continuous logging run were made, it could cover a substantial distance before one cycle of the current were transmitted.
More generally, there is the need for an apparatus and a method for obtaining measurements which can be used not only to qualitatively look for anomalies, but also to quantitatively understand surface-obtained induced polarization data, other surface geological and geophysical data and downhole electrical properties.