Well or borehole logging is the practice of making an accurate record, known as a well log, of the geologic formations through which a borehole creates a path or conduit. Well logging activities are performed during all phases of an oil and gas well's development; drilling and evaluation, completion, production and abandonment.
The oil and gas industry logs rock and fluid properties to find hydrocarbon-bearing strata in the formations intersected by the borehole. The logging procedure consists of lowering a tool on the end of a wireline into the well to measure the properties of the formation. An interpretation of these measurements is then made to locate and quantify potential zones containing hydrocarbons and at which depths these zones exist.
Logging is usually performed as the logging tools are pulled out of the hole. This data is recorded in real-time via a data connection to the surface logging unit or using a memory unit aboard the tool to create either a printed record or electronic presentation called a well log which is then provided to the client. Well logging is performed at various intervals during the drilling of the well and when the total depth is drilled.
Density logging is the practice of using a specific well logging tool to determine the bulk density of the formation along the length of a wellbore. The bulk density is the overall density of a rock including the density of the minerals forming the rock and the fluid enclosed in the pores within the rock.
A radioactive isotope-based source, usually Cesium 137 (137Cs), applied to the wall of the borehole emits gamma rays into the formation so these gamma rays may be thought of as high velocity particles which collide with the electrons of the atoms that compose the formation. At each collision the gamma rays lose some of their energy -to the electrons, and then continues with diminished energy. This type of interaction is known as Compton scattering. A proportion of the scattered gamma rays reach detectors located at fixed distances from the source, and is counted as an indication of formation density.
In oilfield operations isotopes can be lost into the well as a result of the breakage of the logging tool at the risk of being irretrievable. Such events can lead to the closure of the well or measures taken to ensure that radioactive material cannot circulate or permeate out of the well. Indeed, direct contamination and the risk to oilfield workers of dangerous levels of exposure are not uncommon. Although comprehensive control measures are in place, the risk associated with the use of highly radioactive isotopes during oilfield operations will always be present.
As is the nature of radioactive materials, the half-life of the material also determines its useful lifetime. Although density logging tools are calibrated to take into account the reduction in activity of an isotope, the useful life of the isotope is somewhat short-lived. A 137Cs source will produce only one-half of its initial gamma-ray output after a period of 30 years. Consequently, isotope-based sources require periodic replacement, and the older isotopes disposed of. Disposal requirements include precautions similar to that of normal nuclear waste, such as that produced as a waste product at nuclear power stations.
The typical regulatory limit for the amount of 137Cs which may be used during a logging operation is a maximum of 1.3 Curie. During density logging operations, a certain number of photons per second are required to enter into the detectors to ensure a high enough statistic for the purposes of data quality consistency and interpretation. As a result, density logging operations are normally performed such that the tool is moved at a rate of 1,800 feet per hour to ensure sufficient photons enter the detectors at any particular depth to offer a data resolution acceptable to the client (typically a repeatability to 0.01 g/cc density). In a 15,000 ft long well, this can translate to just over 8 hours of logging time, bottom to surface (or at least 2 hours in the zone of the reservoir).
Operations cannot currently be performed faster as the speed of logging relates to an acquisition speed proportional to the output of the gamma-source. For safety reasons, the amount of 137Cs used is capped, with a resultant cap in the minimum amount of time required to perform a log.
Various means have been published purporting to mitigate this issue by using x-ray sources as a substitute for gamma-ray sources. By changing the source from a chemical source, with its known output rate, for an x-ray tube, the output of which is dependent upon a number of factors, e.g., input set voltage, input frequency, number of multiplier stages, the designed peak-peak voltage for each multiplier stage, the capacitance of the ladder, the temperature of the generator, etc., introduces variations into the output of the generator that imperil the ability to achieve the statistical ability required to achieve a repeatability of 0.01 g/cc density.
In some cases, prior art discloses methods to monitor the output radiation of x-ray tubes, and to use the changes in resultant radiation output levels as input modify the input voltage of the generator. However, modification of x-ray output after it has already been produced would not help correct the log data that would already have been detrimentally affected by the changes in x-ray energy/output.
A resistor-based feedback loop may be implemented such that the output potential of the multiplier can be monitored. As the capacitance of the capacitors in the multiplier reduces with ambient temperature, the natural hail ionic frequency of the multiplier will increase as a function of temperature. If the input frequency of the ladder is fixed (which is typical), then the result will be a reduction in the efficiency of the multiplier with increase in ambient temperature, and the output voltage of the generator will reduce. With the exception of NP0, which exhibits a low capacitance per unit volume (and therefore very large in comparison to equivalent value capacitor materials), the majority of capacitor materials that are useful within a downhole tool tend to exhibit a variable capacitance with respect to environmental temperature. Within high-temperature environments the capacitance can degrade to such an extent that efficiency of the high-voltage ladder will degrade such that thermal-run-away and failure can be expected.
None of the prior art teaches of practical methods that can be employed to ensure that the output stability of the x-ray tube remains good enough to perform the critical measurement, while compensating for variations in temperature typical in a tool that is being logged in a borehole. In addition, none of the prior art deals with the effects of temperature on the generator electronics as a separable error input to the control scheme, thereby permitting temperature effects to be controlled independently of other issues affecting the output stability of the generator.
For example, U.S. Pat. No. 3,327,199 to Gardner et al. discloses a DC to DC high voltage power supply system including a transistor chopper DC to AC inverter type circuit and a high voltage output transformer that includes an extra secondary winding, hereinafter referred to as a tertiary winding, which generates a feedback voltage which is proportional to the high voltage output. A passing element, for example, a transistor, is located between the DC input voltage and the DC to AC inverter circuit and is controlled by the voltage generated by the tertiary winding to provide a highly regulated DC input voltage to said inverter circuit. The DC voltage appearing across a tertiary winding rectifier circuit is coupled to a filter circuit comprising capacitors in combination with a temperature compensating resistance.
U.S. Pat. No. 5,023,768 to Collier discloses a high voltage, high power DC power supply that includes a single turn primary winding driven through a resonating capacitor by an AC source having a frequency in excess of about 100 kHz. The primary winding includes a pair of concentric cylindrical metal walls having opposite ends electrically connected to each other. A volume between the walls includes plural secondary winding assemblies, having different axial positions along the walls. Each of the assemblies includes an annular magnetic core surrounding the interior wall, a winding on the core and a voltage-doubling rectifier. DC voltages developed across each secondary winding assembly by the rectifier are added together to provide the high voltage, high power output.
U.S. Pat. No. 5,400,385 to Blake et al. discloses a supply for a high bias voltage in an X-ray imaging system has an inverter and a voltage multiplier that produce an alternating output voltage in response to control signals. A voltage sensor produces a signal indicating a magnitude of the output voltage. A circuit determines a difference between the sensor signal and a reference signal that specifies a desired magnitude for the output voltage and that difference is integrated to produce an error signal. The error signal preferably is summed with a precondition signal that is an approximation of a nominal value for the signal sum and the summation producing a resultant signal. Another summation device arithmetically combines the resultant signal and the sensor signal with a signal corresponding to a one-hundred percent duty cycle of the inverter operation in order to produce a duty cycle command. An inverter driver generates the inverter control signals-that have frequencies defined by the resultant signal and have duty cycles defined by the duty cycle command. A unique state machine is described which generates those control signals.
U.S. Pat. No. 4,641,330 to Herwig et al. discloses of a high voltage supply circuit for an x-ray tube includes a high voltage transformer having a primary side driven by voltage pulses generated by a drive circuit. The drive circuit includes sub-circuits for controlling the pulse repetition frequency, which is selected as equal to a parallel resonant frequency of a high voltage generator connected to the secondary side of the transformer, for the purpose of saving energy. The drive circuit also includes a sub-circuit for controlling the pulse duration, with the filament voltage in the x-ray tube being regulated by this pulse duration.
U.S. Pat. No. 7,564,948 to Wraight et al. discloses of an x-ray source being used as a replacement for a chemical source during density logging along with various means of arranging the apparatus and associated power-supply, also teaches of the means of filtering the primary beam from the x-ray source such that a filtered dual-peak spectrum can be detected by a reference detector which is then used to directly control (feedback) the x-ray tube voltage and current for stability purposes. However, the patent discloses only a compact x-ray device (bipolar) with a grid, a power supply in the form of a Cockcroft-Walton rolled up into a cylinder between two Teflon cylinders in order to save space, and the aforementioned filtered reference detector method.
Finally, U.S. Pat. No. 8,481,919 to Teague teaches a means of creating and controlling the electrical power necessary, by serially stepping up the DC reference and creating high potential field control surfaces, to control either a bipolar or unipolar x-ray tube for the purposes of replacing chemical sources in reservoir logging. The reference also teaches one or more moveable/manipulatable beam hardening filters and rotating light-house collimation on the source, the use of gaseous insulators including SF6 as an electrical insulator in a downhole x-ray generator. However, the reference does not disclose a method of using the increased output of the x-ray device to enable longer offset detectors to enable analysis of the non-invaded zone of the formation. It also fails to teach of a method to increase the permissible count rate within a detector volume by doubling the number of PMTs for a given detector volume.
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