Mulitiple spacing resistivity measurements with receiver arrays

Details Mulitiple spacing resistivity measurements with receiver arrays Mulitiple spacing resistivity measurements with receiver arrays

1999-02-22

2001-02-06

Strecker, Gerard (Department: 2862)

Electricity: measuring and testing

Of geophysical surface or subsurface in situ

With radiant energy or nonconductive-type transmitter

C324S339000

Reexamination Certificate

active

06184685

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
Wells, also known as wellbores or boreholes, are drilled to reach underground petroleum and other subterranean hydrocarbons. While or after a well is being drilled, a great quantity of information relating to parameters and conditions downhole is desirable. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods. In conventional oil well wireline logging, a probe or “sonde” is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed cylinder for housing the sensors, which hangs at the end of a long cable or “wireline.” The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
While wireline logging is useful in assimilating information relating to formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drillstring and bottomhole assembly must first be removed or tripped from the borehole, resulting in considerable cost and loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, drillers must at times make decisions (such as the direction to drill, etc.) possibly without sufficient information, or else incur the cost of tripping the drillstring to run a logging tool to gather more information relating to conditions downhole. In addition, because wireline logging occurs a relatively long period after the wellbore is drilled, the accuracy of the wireline measurement can be questionable. As one skilled in the art will understand, the wellbore conditions tend to degrade as drilling fluids invade the formation in the vicinity of the wellbore. In addition, the borehole shape may begin to degrade, reducing the accuracy of the measurements.
Because of these limitations associated with wireline logging, there was an emphasis on developing tools that could collect data during the drilling process itself. By collecting and processing data during the drilling process, without the necessity of tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections “real-time”, as necessary, to optimize drilling performance. With a steerable system the driller may change the direction in which the drill bit is headed. By detecting the adjacent bed boundaries, adjustments can be made to keep the drill bit in an oil rich layer or region. Moreover, the measurement of formation parameters during drilling, and hopefully before invasion of the formation, increases the usefulness of the measured data. Further, making formation and borehole measurements during drilling can save the additional rig time which otherwise would be required to run a wireline logging tool.
Designs for measuring conditions downhole and the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as “measurement-while-drilling” techniques, or “MWD.” Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, commonly have been referred to as “logging while drilling” techniques, or “LWD.” While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term MWD will be used with the understanding that the term encompasses both the collection of formation parameters and the collection of information relating to the position of the drilling assembly while the bottomhole assembly is in the well.
The measurement of formation properties during drilling of the well by MWD systems improves the timeliness of measurement data and, consequently, increases the efficiency of drilling operations. Typically, MWD measurements are used to provide information regarding the particular formation through which the borehole crosses. Currently, logging sensors or tools that commonly are used as part of either a wireline or an MWD system include resistivity tools. Resistivity tools are effective because for a formation to contain petroleum, and for the formation to permit the petroleum to flow through it, the rock comprising the formation must have certain well known physical characteristics. One characteristic is that the formation has a certain measurable resistivity (the inverse of conductivity), which can be determined by providing an electromagnetic wave of a particular frequency through the formation. As will be apparent to one skilled in the art, the propagating wave suffers both attenuation and phase shift as it travels through the formation. Analysis of this attenuation and phase shift provides the resistivity of the formation surrounding the resistivity tool.
Ordinarily, a well is drilled vertically for at least a portion of its final depth. The layers or strata that make up the earth's crust are generally substantially horizontal. Therefore, during vertical drilling, the well is substantially perpendicular to the geological formations through which it passes. In certain applications, however, such as when drilling from an off-shore platform, or when drilling through formations in which the reservoir boundaries extend horizontally, it is desirable to drill wells that are oriented more horizontally. When drilling horizontally, it is desirable to maintain the well bore in the pay zone (the formation which contains hydrocarbons) as much as possible so as to maximize the recovery. This can be difficult since formations may dip or divert. Thus, while attempting to drill and maintain the well bore within a particular formation, the drill bit may approach a bed boundary. Many in the industry have noted the desirability of an MWD system that could be especially used to detect bed boundaries and to provide real-time data to the driller to enable the driller to make directional corrections to stay in the pay zone. Alternatively, the MWD system could be used as part of a “Smart” system to automatically maintain the drill bit in the pay zone. See, e.g. commonly assigned U.S. Pat. No. 5,332,048, the teachings of which are incorporated by reference herein. The assignee has also developed a system that permits the measurement of MWD data at the drill bit to provide an earlier indication of bed boundaries and formation characteristics. See U.S. Pat. No. 5,160,925. The use of an MWD system with these other systems makes it possible to conduct at least certain portions of drilling automatically.
The various “beds” or layers in the earth have characteristic resistivities which can be used to identify their position. For example, in a so-called “shaley-sand” formation, the shale bed can have a low resistivity of about 1 ohm per meter. A bed of oil-saturated sandstone, on the other hand, is likely to have a higher resistivity of about 10 ohms per meter, or more. The su

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