Resistivity is an electrical property of rocks that indicates the presence of Hydrocarbons. This is because most rocks are saturated with low-resistivity water, while oil and gas are very high resistivity. It makes sense when Geosteering in a reservoir to be following the “sweet-spot” of maximum hydrocarbon saturation, rather than just following a given rock type. You can also avoid drilling water-wet sections of reservoir rock (not apparent from the gamma log). All this, and more, is exactly what the GRT makes possible.
Most MWD Resistivity tools are of the high-frequency “Wave-Propagation” type, and can accurately measure the average rock-formation resistivity. However, this type of tool may be difficult to use while steering a horizontal well because it requires complex interpretation of phase and attenuation logs to “see” an approaching bed-boundary. It is easy to mistake curve separations due to other effects such as Anisotropy or Invasion, resulting in drilling through a bed-boundary, and then having to backtrack. Who wants to learn complex log analysis just to steer a well? In addition, most wave-propagation tools are not directionally-sensitive, so course corrections are often based on guesses.
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CBG Corporation has invented and patented (US patent #7,657,377 , US patent #6,064,210 , Canada Patent #2,321,984, China Patent ZL200880018008.3, 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 GRT does not employ any third-party proprietary technology so there are no licensing fees to pay.
The addition of directional information to the resistivity reading provides a new enabling technology for Geosteering, at a price that is affordable for Directional-Drilling companies. Click to download a PDF Demo.
The GRT is an advanced Focused Laterolog device, incorporating a transmitting electrode and four independent receiving electrodes placed in quadrants around the collar to create an azimuthally-sensitive 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. With the advance warning, Geosteering is made easier!
In the diagram at left (from a computer model), the transmitter emits an electric current in all directions. The electric current density is higher in the upper conductive rock region (e.g. a shale), so the upper receiving electrode receives a bigger current than the lower electrode. By computing the ratio of the currents the tool can calculate the distance and direction to the bed boundary.
Unlike competing tools, it is not necessary to rotate the GRT to determine the direction to an adjacent bed boundary, because directional sensors are used to mathematically “reference” the result relative to gravity!
Directional resistivity data from the GRT can be graphically presented on a patented surface display in an enhanced version of the familiar compass rose drillers display typically employed in directional drilling.
A continuously-updated borehole image showing the distance and direction to conductive/resistive anomalies enables the operator to intelligently direct the drilling process, without having to analyze resistivity logs.
State-of-the-art techniques were applied in creating the GRT. For example, FEMLAB 3D Finite-Element-Analysis 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 self-calibrating 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, probe-based construction makes disassembly and reassembly quick and easy.
Of course, in addition to its application in directional drilling, the GRT performs exceptionally well as a conventional LWD resistivity tool. In the graph at left, 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 high-speed link to a PC when the GRT is retrieved from the well. Click to see actual log of a GTI Catoosa test well. It is not necessary to remove the tool from the BHA to download data. Furthermore, the GRT can include an internal Gamma-Ray tool (standard or directional), providing directional capability that is unique in the industry.
The GRT can be provided as a stand-alone tool, or part of a complete LWD resistivity system, including a surface display, display software, and data communications/control electronics. Click to download a Tool String Drawing.
The GRT is available compatible with Tensor-type systems, and can be readily integrated into other industry-standard tool strings. A built-in 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 laterolog-type tool, it will give less-accurate results in oil-based 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 high-resistivity formations, where it may be necessary to distinguish between oil and fresh water. An example is the heavy-oil Orinoco Basin of Venezuela, where the Wave-Propagation tools have been found unusable. Another example is in fresh-water injection well situations, such as in Saudi Arabia. Wave-Propagation tools are not accurate in resistivities above about 200 ohm.meters due to unpredictable dielectric effects.
The GRT has another advantage over other directional types of resistivity tools. As the graph below 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.
Unlike the “Wave-Propagation” types, the GRT can see both resistive and conductive boundaries equally well, so if you drill out of the reservoir (or pass through a fault) you know how to get back!