Geosteering Resistivity Tool

Transcription

1 L o g g i n g t o o l s f o r o i l e x p l o r a t i o n Geosteering Resistivity Tool Page 1 of 6

2 Geosteering Resistivity Tool Enhanced Resistivity Measurement Most MWD Resistivity tools are of the Wave Propagation type, and can accurately measure the average rockformation resistivity. However, this type of tool may be difficult to use in horizontal drilling because it requires complex interpretation of phase and attenuation logs to see an adjacent bedboundary. It is easy to mistake curve separations due to other effects such as anisotropy or invasion, resulting in drilling through a bedboundary, and then having to backtrack. CBG Corporation has invented and patented (US patent #6,064,210) and UK patent 2,353,596, a method for measuring resistivity as a vector quantity. For example, up to one meter before mistakenly drilling out of a reservoir, advance indication of the distance and direction to a nearby shale bed is available so that corrective action can be taken. The addition of directional information to the resistivity reading provides a new enabling technology for geosteering, at a price that is affordable for directionaldrilling companies The GRT is an advanced imaging laterolog device, incorporating four independent receiving electrodes to create an azimuthallysensitive resistivity measurement. This capability allows the GRT to detect a nearby bed, determine whether it is conductive or resistive, and ascertain its direction and distance. In the diagram at top right (from a computer model), the electric current density is higher in the upper conductive rock region (e.g. a shale), so the upper electrode receives a bigger current. Unlike competing tools, it is not necessary to rotate the GRT to obtain directionallysensitive resistivity measurements. Modeled GRT current density Directional resistivity data from the GRT can be graphically presented on a surface display (patents pending) in an enhanced version of the familiar compass rose drillers display typically employed in directional drilling. A continuouslyupdated borehole image showing the distance and direction to conductive/resistive anomalies enables the operator to intelligently direct the drilling process, without having to study resistivity logs. Sample resistivity display on compass rose Page 2 of 6

3 Advanced CostEffective Design Stateoftheart techniques were applied in creating the GRT. For example, FEMLAB 3D FiniteElementAnalysis modeling was used to optimize mechanical and electrical parameters. A graph (at right) from the model of the GRT illustrates how the ratio of current received by a pair of electrodes can be used to distinguish a parallel conductive bed from a resistive bed, and determine the distance to the boundary. Benefiting from its innovative electronic and mechanical design, the GRT provides accurate and reliable operation, combined with low cost and serviceability. Sophisticated selfcalibrating measurement circuitry ensures that stability and precision are maintained across the full range of operating conditions. Should it be necessary to service or repair the GRT, its rugged, probebased construction makes disassembly and reassembly quick and easy. The GRT does not employ any thirdparty proprietary technology so there are no licensing fees required. GRT receiver electrodes Page 3 of 6

4 Geosteering Resistivity Tool Logging While Drilling Of course, in addition to its application in directional drilling, the GRT performs exceptionally as a conventional LWD resistivity tool. In the graph at right, the 4.75 inch diameter tool is shown capable of very accurate measurements in a 6.5 inch diameter hole whatever the mud resistivity. Even in severe washouts up to 12 inches in diameter, the tool retains acceptable accuracy. A complete log is saved in the GRT internal memory and can be subsequently downloaded through a highspeed link to a PC when the GRT is retrieved from the well. It is not necessary to remove the tool from the BHA to download data. Furthermore, the GRT can include an internal GammaRay tool (standard or directional), providing directional capability that is unique in the industry. The GRT can be provided as a standalone tool, or part of a complete LWD resistivity system, including a surface display, display software, and data communications/control electronics. A mud Pulser can be mounted above or below due to the full 10wire bus passing through the tool. The GRT is available compatible with Tensortype systems, and can be readily integrated into other industrystandard tool strings. A builtin Muleshoe helix at the top end of the tool ensures that the Directional Sensor package is always correctly oriented with the GRT sensors. Since the GRT is a laterologtype tool, it will not give accurate results in oilbased muds if there is a significant amount of emulsion breakdown. However, in high resistivity formations operation at reduced accuracy is still possible. Laterolog tools excel in very highresistivity formations, where it may be necessary to distinguish between oil and fresh water. An example is the heavyoil Orinoco Basin of Venezuela, where the wavepropagation tools have been found unusable. Another example is in freshwater injection well situations, such as in Saudi Arabia. Wavepropagation tools are not accurate in resistivities above about 200 ohm.meters. formation resistivity GRT Maximum Detection Distance measurement accuracy +/ 2.5% The GRT has another advantage over other directional types of resistivity tools. As the graph (at left) indicates, the detection distance for an adjacent contrasting rock formation is dependant only on resistivity contrast, and not on the actual resistivities. Detection distance is constant for a wide range of resistivities, particularly for the most important high values typical of oil reservoirs bed resistivity distance (inches) Page 4 of 6

5 Geosteering Resistivity Tool GRT Calibrator/Verifier Product Description The Calibrator/Verifier is a dual purpose accessory for CBG s Geosteering Resistivity Tool. It can be used to test the GRT over its full measurement range and confirm that it is operating properly. It is also capable of downloading the entire resistivity log from the internal memory of the GRT, and can subsequently transfer the log to a PC for analysis. Since the GRT electrodes are utilized for data communications, the log dump does not require timeconsuming disassembly of the tool. A 3hour log can be transferred from the GRT to the Calibrator/Verifier in roughly one minute, and over 400 hours of log data can be stored. The Calibrator/Verifier is batteryoperated and completely portable, and is housed in a rugged weatherresistant enclosure for use in the field. The Calibrator/Verifier has a simple menubased user interface with an LCD display and 3button keypad. It is quickly and easily attached to and removed from the GRT without the use of special tools or hardware. Calibrator/Verifier specifications: Battery Complement = six rechargeable cells Operating Time = 6 hours Recharge Time = 1 hour Comm. link to GRT = proprietary serial interface, 57.6Kbaud Comm. link to PC = RS232 serial link, 57.6Kbaud (USB soon to be available) GRT field test log Page 5 of 6

6 G R T S p e c i f i c a t i o n s Operational Specifications Power Source Battery Life Types of Measurement Average Resistivity Range/Accuracy Vertical ResolutionThinBed Resolution External Lithium Battery Pack (14.4V to 28.8V nominal) 600 hours with one 28.8 volt Battery Pack (10% duty cycle) Realtime Resistivity Vector (magnitude + angle), LWD Average Resistivity Log 0.2 to 20,000 Ωm., +/10% (worstcase), +/1.5% (if borehole is ongauge) 17 cm / 6.75 inches Maximum Operating Temperature 150 C / 300 F Maximum Pressure Detection Distance for Parallel Rock Formation Depth of Investigation (Invasion Tolerance) Internal Memory 1400 bar / 20,000 PSI > 1 meter / 40 inches with >10:1 resistivity contrast 25 inches 50% point on PGF curve 16 MBytes, up to 1600 hours data log (1 sample / 10 seconds) with highspeed port, 400 hours (1 sample / 4 seconds) Mechanical Specifications Tool Length Nominal O.D. / Wear Band O.D. Tool Connections Maximum Mud Flow Volume Maximum Dogleg Severity Electrical Connections Toolface / Directional Package Orientation 3.5 meters / 11.5 feet 4.75 inches / 5 inches A.P.I. 3.5 inch IF, box upper/pin lower feet/minute flow rate Rotating 15 /100 feet, Sliding 20 /100 feet 10pin Rotary, male upper/female lower Muleshoe Helix built into upper box connection 2601 McHale Court Suite 145 Austin, Texas Tel: Fax: w w w. c b g c o r p. c o m Page 6 of 6