Various embodiments of electromagnetic, nuclear and acoustic measurements have been made for many years to determine geophysical properties of earth formations penetrated by a borehole. These measurements are usually displayed as a function of depth within the borehole at which they were measured forming a display known in the industry as a "log" of the borehole. The log of spontaneous potential of earth formations penetrated by a borehole was made in 1927 using a wireline device. In the following decades, borehole measurements using wireline devices were expanded to include nuclear and acoustic measurements, as well as more sophisticated electromagnetic measurements, to determine additional geophysical parameters of interest, and to also determine certain properties of the borehole.
In the late 1960's and early 1970's, two or more wireline sensor responses were combined to obtain parametric measurements which were more accurate than measurements from either single sensor, and to obtain measures of additional parameters not obtainable from the response of either single sensor. As an example, measures of the acoustic travel time of the formation was combined with a measure of formation porosity derived from a scattered gamma ray device to obtain not only an improved measure of formation porosity but an indication of the lithology of the formation not obtainable from the response of either individual measurement. U.S. Pat. No. 3,590,228 to Jack A. Burke teaches the combination of neutron porosity, formation density and acoustic travel time wireline logs to obtain an improved formation porosity measurements and fractional components of as many as three minerals. U.S. Pat. No. 3,638,484 to Maurice P. Tixier teaches the combination of neutron porosity, formation density, acoustic travel time, natural gamma radiation, spontaneous potential and resistivity wireline logs to obtain even more formation information including effective and shale fraction porosity. The first such combining "combination" logs were generated from individual logs, each being made with a given type of downhole sensor and an individual pass within the borehole. As the technology matured, multiple sensors of different types were combined within a single downhole instrument thereby allowing the input parameters of the combination log to be measured in a single pass within the borehole. This advancement provided advantages in that drilling rig time devoted to logging was reduced. In addition, some of the depth correlation problems encountered in combining multiple logs made at multiple borehole passes were eliminated. Serious depth correlation problems still exist even though all sensor measurements are made with a single pass of the logging instrument. This topic will be discussed in detail in later sections of this disclosure.
In the intervening years, increasing numbers and types of basic wireline sensors combined with increasingly complex response processing algorithms have been used to obtain new and improved determinations of geophysical parameters of formations penetrated by a borehole and the properties of the borehole. Most sensors used in current wireline technology are very data intensive. When such measurements are made simultaneously with a single pass of a multiple sensor wireline device, massive amounts of raw data are generated per depth interval of borehole traversed. These data are transmitted to the surface of the earth over the logging cable for depth correlation and other subsequent processing to obtain the combination logs of multiple parameters of interest. Current conventional wireline telemetry systems using seven conductor electrical logging cable can telemeter data to the surface at a rate of 500 kilobits to 1000 kilobits per second. Using emerging fiber optic logging cable and telemetry technology, these transmission rates are expected to increase by orders of magnitude. Because of the large telemetry bandwidths available in current and emerging wireline systems, data intensive multiple sensors can be employed and the raw data can be transmitted to the surface for processing. There are situations, however, in which wireline logging systems employ a single conductor cable, either for economic or operational reasons. These cables are often relatively small in diameter and are often required in the logging of wells under high pressure. A single conductor cable limits telemetry band width. Using this type of cable, raw sensor data generated by modern combination logging tools can exceed the telemetry capacity of the system.
The previous background discussion has been directed to wireline type measurements wherein the measurements are usually made after the borehole has been drilled. In some drilling operations, wireline logs are made intermittently during the drilling operation, but such logging requires that the drill string be removed from the borehole prior to logging. Logging after completion of the drilling operation often reveals that the target formation or formations have been missed by either drilling too shallow or too deep. In addition, unexpected zones, such as high pressure formations or salt zones, can be encountered during, and adversely affect, the drilling operation. Such encounters can be quite costly and can be fully analyzed with wireline logging only after the encounter. Intermittent logging is likewise costly in that the drilling operation must cease during logging operations. Possible damage to the borehole can occur during intermittent logging and costly drilling rig time and logging equipment time is wasted during stand-by periods for each operation.
The economic, technical, operational and safety advantages of measuring geophysical parameters as well as drilling management parameters, during the actual drilling of the borehole, was recognized in the early 1950's. Commercial measurements-while-drilling (MWD) became available in the late 1970's and early 1980's. These measurements included directional information and a limited number of formation evaluation type services. Additional sensors and services have been added as MWD technology matures. U.S. Pat. No. 5,250,806 to Erik Rhein-Knudsen et al discloses a MWD apparatus for measuring formation characteristics and is more specifically directed to the simultaneous measurement of neutron porosity and formation density with suitable borehole corrections for each measurement. Methods for downhole depth correlation and resolution matching are not addressed. In many respects, the sophistication of MWD sensors are comparable to their wireline counterparts in spite of the harsh environment experienced in using such sensors in the drilling environment. It is feasible, at least in principle, to utilize multiple sensor, data intensive, combination logging methods developed for wireline tools to obtain new and improved parametric measurements while drilling. Furthermore, it is feasible, in principle, to utilize additional sensors responding to drilling related parameters simultaneously with formation evaluation type sensors. In practice, however, the combination of multiple sensor response techniques, comparable in sophistication to corresponding wireline applications, is limited by current MWD telemetry rates and downhole storage capacities. The simultaneous transmission of drilling dynamics sensor information such as directional information, weight on the drill bit, and other non-formation evaluation type measurements further overloads current MWD telemetry transmission rates which are of the order of 2 to 60 bits per second. Furthermore, it is not feasible to store copious amounts of raw sensor data downhole for subsequent retrieval and processing due to relatively limited storage capacity of current MWD systems. MWD means for making formation evaluation combination logs comparable to current wireline logs require the computation of the desired parameters downhole, and the transfer of the computed parameters of interest to the surface. By using downhole computational means and methods, the transmission requirements are reduced by orders of magnitude in that only "answers" are telemetered, or alternately stored, rather than raw data. This type of downhole computation is also applicable to other types of non formation evaluation type measurements such as signals indicative of the operational characteristics of the downhole equipment as well as measurements indicative of drilling direction and efficiency. Furthermore, downhole processing is applicable to wireline systems with limited telemetry capacity.
Attention is again directed to depth correlation of multiple sensors conveyed along the borehole within a single downhole subassembly. It is virtually impossible to position all sensors in the same plane intersecting the downhole subassembly perpendicular to the axis of the borehole. The varying dimensions of different types of sensors as well as associated power and control circuitry require that multiple sensors be positioned axially along the major axis of the downhole subassembly which is parallel with the axis of the borehole. With the downhole subassembly positioned at a given depth within the borehole, each sensor is responding to a different depth interval of formation penetrated by the borehole. Using terminology common in the industry, each sensor has a different "measure point". Furthermore, the physics of many types and classes of sensors introduce different "effective" measure points. Nuclear detectors such as thermal neutron devices and scatter gamma ray devices used to obtain porosity and density measurements comprise a nuclear source and one or more radiation detector spaced axially from the source. The sensors respond primarily to formation between the source and the one or more detectors. The effective measure point is, therefore between the source and detectors. Likewise, electromagnetic and acoustic sensors often comprise multiple, axially spaced, transmitters and receivers with the effective measure point lying within the axial within the axial array. Even electromagnetic sensors comprising a single transmitter and a single receiver with the transmitter operating at multiple frequencies can exhibit different effective measure points within a given formation. The effective measure point is not usually at the mid point of the axial array and, in fact, can vary with the type of intervening formation and/or the environmental conditions of the borehole such as the mud weight.
In combining the responses of multiple sensors to obtain one or more parameters of interest, variations in the vertical resolutions of the various sensors must be also addressed as well as variations in effective measure points. Some sensors respond rapidly as a formation bed boundary is crossed while others respond more slowly. Vertical resolution is governed by many factors including the physical arrangement of the sensor, the basic physics of the sensor and the physical properties of the formation and borehole environment being traversed. As an example, a measure of formation photoelectric factor will exhibit better defined or "sharper" vertical resolution than a formation bulk density measurement even though the same gamma ray source and gamma ray detector are used to make both measurements. Unless vertical resolutions of all sensors are matched or otherwise processed, parameters computed from the combined responses of multiple sensors will be erroneous, especially in intervals of rapidly changing geology such as laminated sand-shale sequences.
The above discussion of sensor measure points, effective measure points and vertical resolution is applicable to both wireline and MWD devices. U.S Pat. No. 5,282,133 to Charles C. Watson teaches adaptive filtering and deconvolution of primarily gamma ray spectra in obtaining optimum measures of bulk density and formation photoelectric factor; however these teachings are directed toward wireline measurements. Effective measure points of all sensors responses from a downhole subassembly must first be shifted or correlated to a common reference point along the axis of the downhole subassembly before meaningful combination logs can be computed. The common reference point is preferably selected to be near the midpoint of the multiple sensor array. The depth reference point is defined as the depth within the borehole at which the common reference point of the subassembly is positioned. Vertical resolutions of all sensors are also normalized to a common value as will be detailed in subsequent sections of this disclosure. If the effective measure points and vertical resolutions of the sensors are not properly correlated to a common reference, erroneous combination logs will result especially in the regions of bed boundaries where one or more sensors may be responding to one formation and the remaining sensors may be responding to different formations.
Before discussing the depth correlation of multiple sensor measurements, a brief background of techniques used to measure the depth of the downhole subassembly as well as the movement of the subassembly within the borehole will be provided. In wireline measurements, the depth of the down hole subassembly or logging "sonde" is determined by passing the logging cable over a calibrated measure wheel at the surface of known circumference. Often, the stretch of the cable is compensated for by the use of a microprocessor which uses as an input the length of cable in the borehole, the weight of the cable in the borehole, the weight of the sonde and even the history of the cable whose stretch characteristics can change with usage. Logging speed is also determined by the rate of rotation of the calibrated measure wheel. In MWD operations, the depth and rate of penetration of the downhole subassembly is determined from a calibrated measure wheel which contacts the drill string at the surface. If logging sondes and downhole MWD subassemblies were conveyed smoothly along the borehole, multiple sensor responses could be correlated, at least to a first order of approximation, by simply shifting all sensor responses measured as a function of depth to a common reference point using the known physical spacings of the sensors and an assumed or computed effective measure point of each sensor. In practice, however, logging sondes and downhole MWD assemblies are not conveyed smoothly along the borehole. In wireline logging, which is usually performed with the sonde being conveyed up hole, the sonde often sticks within the borehole and subsequently releases or "jumps" as the cable is retrieved. Although the surface measure wheel indicates a constant velocity, the logging sonde and sensors therein are actually moving sporadically up the borehole. In MWD operations, the drill bit and nearby sensor subassemblies often "bounce" as the borehole is being drilled. Even though the surface measurement indicates a constant rate of penetration, the sensors are likewise moving sporadically down the borehole. First order sensor depth correlations mentioned above can be erroneous since each sensor measurement is recorded as a function of depth using depth measurements made at the surface. As an example, assume that two axially spaced sensors are sensing two different formations. Further assume that the lower sensor is in a thin formation bed and the upper sensor is in a relatively thick formation bed. If the sensors sporadically drops within the borehole, the upper sensor could move past the thin bed at an abnormally high velocity and obtain an abnormally small number of measurements within the this bed. The upper sensor could conceivably drop through the thin bed and obtain no measurements within the thin bed. If the surface measurement indicates a constant sensor array velocity, the log produced by the second sensor will indicate an abnormally thin formation bed or, conceivably indicate no bed if the second scenario is encountered. Any combination log computed from the combination of the two sensors responses would obviously be erroneous in the vicinity of the thin bed unless depth correlation methods are designed to handle such situations.
Resolution matching was originally accomplished in the prior art by "smearing" vertical resolutions to the sensor exhibiting the poorest vertical resolution. Although simple to execute, the obvious disadvantage of this technique is that logs of computed parameters exhibit vertical resolution no better than the poorest resolving sensor in the combination array. Deconvolution techniques based upon model sensor responses are now employed to enhance resolution of the poorer resolving sensors thereby yielding computed log response with significantly improved vertical resolution. Deconvolution algorithms are a function, to some extent, of the borehole and formation environment in which the sensors are operating. In order to obtain maximum vertical resolution enhancement through deconvolution, a continuous update of deconvolution parameters based upon current environmental conditions is desirable.
The correlations of depth and vertical resolution of multiple wireline sensors are performed at the surface in the prior art. Raw response data from each sensor, which exhibits the maximum vertical resolution from that sensor, is telemetered to the surface and first recorded as a function of depth as measured by the calibrated measure wheel. Assuming that the downhole geology is varying, each raw sensor response log will exhibit characteristic excursions or "signatures" as a function of depth. Various correlation techniques are used to correlate the response signatures as will be discussed in a following section. Should the sensors be moving along the borehole at a velocity as indicated by the calibrated measure wheel, depth correlation will comprise simple depth shifts to align the effective measure points to a common reference point. Should the sensors be actually moving sporadically along the borehole, correlation will be required to "stretch" or "shrink" as well as depth shift the individual raw sensor responses logs for reasons cited and illustrated previously by example. Once the raw sensor response logs have been properly depth correlated and aligned with respect to resolution, combination logs of parameters of interest are computed and exhibited as a function of measured depth which is usually the depth reference point as previously defined. The above discussion assumes, of course, that the wireline telemetry bandwidth is sufficient to handle the transmission of raw data logs from all sensors to the surface.
In principle, the methods of wireline sensor correlation can be applied to the response of multiple MWD sensors. The application of these wireline techniques to MWD measurements has been prevented, however, by limited telemetry and downhole data storage capacity of current MWD systems, Using current MWD telemetry and storage capacity, raw sensor measurements exhibiting suitable vertical resolution can not be telemetered to the surface for correlation or, alternately, stored downhole for subsequent retrieval and processing. This disclosure is directed toward overcoming these problems so that MWD multiple sensor measurements can be properly correlated downhole n order to provide combination logs of maximum accuracy, precision and vertical resolution.