The use of measurement and logging while drilling has matured a great deal in the last 10 years. The use of these tools that have been developed for the oil and gas industry for use in primarily sedimentary depositional environments must be investigated in light of the goals set for EGS systems. Let us first define what is meant in this section by the terms, realizing that the line between these two areas continue to blur.
1) Measurement While Drilling (MWD): Tools that measure downhole parameters of the bit interaction with the rock are MWD tool. These measurements typically include vibration and shock, mudflow rate, direction and angle of the bit, weight on bit, torque on bit, and downhole pressure.
2) Logging While Drilling (LWD): Tools that measure downhole formation parameters are LWD tools. These include gamma ray, porosity, resistivity and many other formation properties. The measurements fall into several categories that are discussed below. The oldest and perhaps most fundamental formation measurements are spontaneous potential (SP) and gamma ray (GR). Today one or both of these traces are used mostly for correlation between logs. Electric or formation resistivity logs are another class of logs used in oil and gas logging. Because of the long history of these logs, several varieties have evolved. The electrical basis of this class of logs is to measure the conductivity or resistivity of the various geologic materials and fluids in them. The resistivity of shales vs that of a clean sand set the limits for an ideal electric log. The fluids in the formation also are reflected in this measurement as water is conductive when found in boreholes and oil is not. The basic use of electric logs is to delineate bed boundaries and in combination with other logs to determine gas/oil/water contacts. Yet another class of logs is density logs. These logs are indicative of the formation density of the material in the well bore. These logs require either a neutron or a gamma source, and actually measure gamma ray flux differences. Porosity tools are another class of common logging tools. These tools normally use chemically or now more common electrically generated neutron to estimate formation porosity. Since these logs are normally calibrated in sandstone, limestone or dolomite care has to be taken when measurements are made in different rock types. Finally in the last few years a number of specialty tools have evolved, these include specialized formation pressure testing tools which can be run while drilling, nuclear magnetic resonance tools, and pulsed neutron spectroscopy tools to list only the most popular.
Rationale for use
In recent years the cost of an average oil and gas hole has increased dramatically, part of this cost increase has been driven by the need to go after much deeper and more complicated reserves. This increases the risk of failure of holes drilled into these reserves. As a reaction to increased risk, the use of LWD and MWD technology and techniques has increased. In the final analysis, the decision to use of LWD and MWD tools depends on managing risk. The EGS program moves the art of geothermal drilling into a new region of risk, the evaluation of the LWD and MDW technologies must be undertaken to determine the applicability of these technologies to the particular risks faced in this new effort. It is important to realize in the EGS model, in many cases we are not going to be setting our surface casing into igneous or metamorphic rock as we have in the past. These deeper holes may look more like the classic oil and gas hole at the shallower depths, with this in mind we begin to examine the possible uses of the LWD and MWD technologies. We begin by listing what is commonly available.
Measurements available from current LWD/MWD oil field tools.
Mention of companies and tool or service names does not imply endorsement by Sandia National Laboratories; it appears that most companies involved in MWD and LWD have a version of these tools. Searching the internet is a reasonable way to get most of this information.
| Measurement Name: Downhole Weight On Bit | Abbreviation: DWOB |
| Class: MWD | Measurement Function: – |
| Max Temp: 175ºC | Length: 25’ |
| Current Oil Field Use : This trace allows the determination of the actual weight on bit at the bit. |
Potential Geothermal Use: The DWD program has shown that this measurement can be used to detect bit-damaging events and prolong bit life |
| Special Conditions : None | Example Tool: Schlumberger TeleScope Baker Hughes Inteq CoPilot (service) |
| Measurement Name: Downhole Torque On Bit | Abbreviation: DTOB |
| Class: MWD | Measurement Function: – |
| Max Temp: 175ºC | Length: 25’ |
| Current Oil Field Use : This trace allows the determination of the actual torque on bit. |
Potential Geothermal Use: The DWD program has shown the use of this measurement in prolonging bit life and providing an effective drilling program. |
| Special Conditions : None | Example Tool: Schlumberger TeleScope Baker Hughes Inteq CoPilot (service) |
| Measurement Name: Downhole flow rate | Abbreviation: |
| Class: MWD | Measurement Function: – |
| Max Temp: 175ºC | Length: 25’ |
| Current Oil Field Use : This measurement allows the determination of the mudflow rate at or near the bit. |
Potential Geothermal Use: The rolling float meter program has shown that this measurement in combination with a good measurement of return flow is critical in detecting lost circulation events. The DWD program has shown the use of this measurement in detecting pipe washout and bit plugging conditions. |
| Special Conditions : None | Example Tool: Schlumberger TeleScope |
| Measurement Name: 3-D Shock | Abbreviation: |
| Class: MWD | Measurement Function: – |
| Max Temp: 175ºC | Length: 25’ |
| Current Oil Field Use : This trace used in combination with 3-D vibration is used to monitor bit conditions. Avoiding shock loads has been shown to increase bit life |
Potential Geothermal Use: Same as oil field but more critical in harder formations. |
| Special Conditions : None | Example Tool: Schlumberger TeleScope Baker Hughes Inteq VSS (service) |
| Measurement Name: 3-D Vibration | Abbreviation: |
| Class: MWD | Measurement Function: – |
| Max Temp: 175ºC | Length: 25’ |
| Current Oil Field Use : This trace used in combination with 3-D shhock is used to monitor bit conditions. Avoiding damaging vibrations has been shown to increase bit life, and increase ROP. Also used to determine RPM at bit |
Potential Geothermal Use: Same as oil field but more critical in harder formations. RPM determination critical to avoiding several bit damaging situations. |
| Special Conditions : None | Example Tool: Schlumberger TeleScope Baker Hughes Inteq CoPilot (service) |
| Measurement Name: Direction and Inclination | Abbreviation: D&I |
| Class: MWD | Measurement Function: |
| Max Temp: 175ºC | Length: 25’ |
| Current Oil Field Use : These traces are used in directional drilling. Both are required to control bit position |
Potential Geothermal Use: Would be used in directional drilling applications. |
| Special Conditions : None | Example Tool: Schlumberger TeleScope |
| Measurement Name: Azimuthal Natural Gamma Ray | Abbreviation: GR |
| Class: LWD | Measurement Function: Gamma Ray |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : This trace measures the naturally occurring gamma radiation in several directions from the borehole. The trace is used to identify shales and clays as opposed to sands in lithologic sequences. Processed trace is a primary correlation trace between logs run at differing times. |
Potential Geothermal Use: Gamma ray is primary correlation trace particularly in cased hole. |
| Special Conditions : None | Example Tool: Schlumberger EcoScope |
| Measurement Name: Multi-frequency resistivity | Abbreviation: |
| Class: LWD | Measurement Function: Electric |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : This measurement in oil and gas logging provide bed boundary and gas/oil/water contact information. |
Potential Geothermal Use: Could be used in sedimentary sequence for bed boundary identification. Usefulness in metamorphic and igneous formation is undocumented. |
| Special Conditions : None | Example Tool: Schlumberger EcoScope |
| Measurement Name: Sonic | Abbreviation: |
| Class: LWD | Measurement Function: Sonic |
| Max Temp: 150ºC | Length: 23’ |
| Current Oil Field Use : This measurement set provides information on porosity, mechanical rock properties and borehole stability. |
Potential Geothermal Use: The borehole stability information would be a greatest value |
| Special Conditions : None | Example Tool: Schlumberger sonicVision Baker Hughes Inteq SoundTrak |
| Measurement Name: Multi-frequency, multi-depth resistivity | Abbreviation: |
| Class: LWD | Measurement Function: Special/Electrical |
| Max Temp: 150ºC | Length: 11’ |
| Current Oil Field Use : This tool is intended formation imaging. Used for fracture identification and finding stress orientation. Use in non-sedimentary formation is undocumented. Also provides temperature data. |
Potential Geothermal Use: This tool may find use in advanced directional drilling applications. If fracture imaging can be done in non-sedimentary geologies may be useful for fracture mapping and stress orientation |
| Special Conditions: Requires use of conductive mud system. | Example Tool: Schlumberger GeoVision Baker Hughes Inteq StarTrak; AziTrak |
| Measurement Name: Annular pressure | Abbreviation: APWD |
| Class: LWD | Measurement Function: Pressure |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : This trace measures the pressure near the BHA-open hole interface. |
Potential Geothermal Use:This trace would be used to determine areas where lost circulation may be occurring. |
| Special Conditions : None | Example Tool: Schlumberger EcoScope Baker Hughes Inteq PressTEQ (service) |
| Measurement Name: Azimuthal Density | Abbreviation: |
| Class: LWD | Measurement Function: Density |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : This trace measures the formation density in multiple directions out from the bore hole. Used in combination with other measurements for formation lithology identification |
Potential Geothermal Use: This trace could be used in directional holes to drill along a bed boundary |
| Special Conditions : None | Example Tool: Schlumberger EcoScope Baker Hughes Inteq LithoTrak |
| Measurement Name: Compensated Neutron | Abbreviation: CN |
| Class: LWD | Measurement Function: Porosity |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : This measurement is used to estimate porosity. This trace is integrated in most logging tools as a part of a triple combo. |
Potential Geothermal Use: Same as oil field use |
| Special Conditions : None | Example Tool: Schlumberger EcoScope Baker Hughes Inteq APLS |
| Measurement Name: Photoelectric Factor | Abbreviation: |
| Class: LWD | Measurement Function: Special |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : This trace measures the average atomic number of the formation constituents, used with density to determine mineralogy. |
Potential Geothermal Use: Same as oil field use |
| Special Conditions : None | Example Tool: Schlumberger EcoScope |
| Measurement Name: Ultrasonic Caliper | Abbreviation: |
| Class: LWD | Measurement Function: Borehole |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : Measures the hole size directly behind the bit. Used to determine size of hole and rugosity |
Potential Geothermal Use: Same as oil field use. |
| Special Conditions : None | Example Tool: Schlumberger EcoScope Baker Hughes Inteq LithoTrak |
| Measurement Name: Porosity | Abbreviation: |
| Class: LWD | Measurement Function: Porosity |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : This trace measures the apparent porosity of the formation based on fast neutrons emitted by a neutron source. Neutron source may be chemical or electrical in nature. |
Potential Geothermal Use: This trace could be used to in the estimation of formation porosity |
| Special Conditions : None | Example Tool: Schlumberger EcoScope Baker Hughes Inteq LithoTrak |
| Measurement Name: Sigma | Abbreviation: Ѕ |
| Class: LWD | Measurement Function: Special |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : This trace is a measure of the macroscopic absorption cross section for thermal neutrons, used to determine formation water saturation. | Potential Geothermal Use: |
| Special Conditions : None | Example Tool: Schlumberger EcoScope |
| Measurement Name: Pulsed neutron spectroscopy | Abbreviation: |
| Class: LWD | Measurement Function: Special |
| Max Temp: 150ºC | Length: 26’ |
| Current Oil Field Use : This measurement utilizes a pulsed neutron source, and gamma ray detectors to estimate formation oil content, salinity, lithology, porosity and clay content. | Potential Geothermal Use:This trace could be used to in the estimation of porosity and the determination of lithology |
| Special Conditions : None | Example Tool: Schlumberger EcoScope |
| Measurement Name: Nuclear magnetic resonance | Abbreviation: |
| Class: LWD | Measurement Function: Special |
| Max Temp: 150ºC | Length: 39’ |
| Current Oil Field Use : This measurement is used to determine free and bound fluid volumes, fluid type, porosity and permeability estimation. | Potential Geothermal Use:This trace could be used to in the estimation formation porosity and permeability. |
| Special Conditions : Informal conversations with persons who have used this tool indicate that its use is best understood in sandstone. |
Example Tool: Sperry MRIL-WD Baker Hughes Inteq MagTrak |
| Measurement Name: Seismic | Abbreviation: |
| Class: LWD | Measurement Function: Special |
| Max Temp: 150 ºC | Length: 14’ |
| Current Oil Field Use : Data derived from this tool is used to look-ahead for formation changes, pore pressure changes and faults. | Potential Geothermal Use: Same as oilfield uses |
| Special Conditions : Requires active seismic source on surface | Example Tool:Schlumberger seismic Vision |
| Measurement Name: Formation Pressure | Abbreviation: |
| Class: LWD | Measurement Function: Pressure |
| Max Temp: 150ºC | Length: 31.5’ |
| Current Oil Field Use : This tool measures the formation pressure and fluid mobility while drilling. This measurement can replace some drill-stem (DST) formation test measurements. Device seals against borehole wall and isolated formation from drilling fluids for testing. This measurement is also used to drilling optimization. | Potential Geothermal Use:Sam e as oil field use |
| Special Conditions : None | Example Tool: Schlumberger Stethoscope Baker Hughes Inteq TesTrak |
Commentary
As one can see from the list, most tools are rated to 150ºC. Some measurements can be made up to 175ºC. These tools provide limited bandwidth data to the surface via mud pulse telemetry and higher bandwidth data via large onboard memories, which can be unloaded as a part of a bit trip.
The Sandia DWD program provided a limited MWD tool for testing. While this tool was much more difficult to operate than the mud pulse telemetry tools listed above it did provide a great deal of data and demonstrated the usefulness of a MWD tool in optimizing drilling parameters with respect to drilling performance and extending bit life. Extensive reviews of this work have been published and presented by others, and may be of interest to some readers. One class of measurement that was not present in the DWD tool that is present in most current MWD tool is direction and inclination, which are critical in directional drilling. The case for MWD tools in EGS is relatively clear. The costs associated with damaged bits and BHA components clearly indicate the need for MWD. In the longer term, if the use of directional drilling is contemplated MWD is a requirement.
The case for most LWD tools or traces will have to be made on a well-by-well basis. As a minimum, one must comply with the individual states requirements for logging and MWD may be a viable option to meet these requirements for deeper holes. MWD may not be required for the full duration of the drilling effort but certainly may be a requirement in deeper directionally drilled holes to assure that the hole is completed in the target area. As has been noted most LWD traces and tools are optimized for drilling sedimentary formations; some experience will be required to determine the optimal MWD suite for EGS drilling programs. With this in mind, the first few EGS programs should be funded and scheduled to provide for the evaluation of a wide variety of available MWD tool. The basic limitation on the available tools is temperature, and accommodations will need to be made in the mud system and operational procedures to maintain the tools within their operational temperature ranges long enough to allow a reasonable evaluation. After some experience is gained with the available tools efforts to construct tools suitable for higher temperatures could begin.
While the least complex of these measurements can be telemetered to the surface via mud pulse technology, the more complex, data dense applications will require a higher speed telemetry systems and indeed may require two way communication. Here the use of a wired pipe system such as the intelliserv system will be required. Looking back on the previous drilling efforts such as the Long Valley drilling project, there has always been a desire by the scientific community to use these precious drilling opportunities to test new and different ideas and tools. The adoption of a standard telemetry system such as the intelliserv network early in the life of the drilling program would encourage a well disciplined scientific program and provide a framework on which the scientific community could plan their programs in a way that would minimally interfere with the drilling program.
Summary
Just as it would not be reasonable to try to up the efficiency of the countries truck fleet by looking solely at replacing all tires in use today with tires that lower rolling friction; rather one needs to step back and look at the entire system including routing and deadheading for a quick example; the selection of MWD and LWD tools must be driven by the entire EGS system. The selection of tools that is currently available at lower temperatures is more that sufficient. One needs to look at the needs as generated from a broad system outlook and then upgrade the temperature capabilities of the necessary MWD and LWD tools. A far more challenging task is to evaluate how these tools mostly designed and used in sedimentary environments will react in the EGS environment. This project should be undertaken early in the development of EGS wells and will require a theoretical as well as a practical side. If as is expected that directional drilling is an important part of the overall EGS effort, understanding how to control and direct these drilling tools with the available MWD and LWD modules will be a major effort.
