Borehole measurements beneath the seafloor.
The Nature of Downhole Measurements
Downhole measurements used in scientific ocean drilling programs can be grouped into three categories. The most common are wireline logs, spatially continuous records of the physical and chemical properties of the formations penetrated by a borehole. The wireline is a cable that extends from a ship down to a probe or sonde in the borehole; it comprises one or more conductors that allow real-time communication between the probe and the surface. Logs or depth records are made as the probe is pulled up the length of the hole at constant speed to provide continuous measurements of the surrounding formation. Some tools are lowered on a mechanical line, or slickline, that provides no digital communication with the surface: These are known as memory tools, and they are deployed where cable specifications would be inadequate, for example, in very hot holes where temperatures are greater than 400 |degrees~ C. Logging tools are available for measuring a wide range of formation properties, including electrical resistivity (laterologs and induction logs), sonic velocity, natural radioactivity (gamma-ray log), porosity (neutron log), density, susceptibility, magnetic field orientation and strength, and temperature. The primary sources of tool technology have been the oil-field well-logging service companies, which have provided and run the majority of drilling program logging tools on a contractual basis.
The second category of downhole tools includes formation testers and fluid samplers, which provide spatially discrete data. These tools are designed to respond to formation-behavior-induced mechanical disturbance, such as an applied stress or a pressure drawdown in the wellbore. They can be deployed on a wireline, a slickline, or as part of the drill string. Primary objectives of such tools are the in situ measurement of dynamic parameters such as permeability, temperature, and pressure; determination of stress distributions; and the acquisition of pristine pore-fluid samples.
The third type of downhole tool is a long-term sensor placed in a drill hole to record natural data over a period of time. In this case, the borehole is used not as a laboratory but rather as an observatory. Sensors can be designed to record variations in local microseismic activity or in fluid properties such as temperature, pressure, or chemistry. In some cases, the long-term measurement of fluid properties requires that the borehole observatory be sealed to prevent direct flow between the deeper pore fluids and the sea above.
The History of Downhole Measurements in Scientific Ocean Drilling
Although logs had been run in offshore wells in established petroleum sedimentary provinces such as the Gulf of Mexico for many years, the deep water sites and basement rocks encountered by the scientific drilling programs presented new operational and technological challenges that once again made borehole measurements a pioneering venture. The first DSDP borehole logs were run northwest of Bermuda in September 1968. These used natural gamma-ray and neutron-porosity tools run in the drill pipe. From this point the use of borehole logs increased erratically, drawing eventually on most of the branches of classical physics, but there were extended periods when no logs were run at all. Two basic open-hole tool suites gradually emerged. These can be described retrospectively as a seismic-stratigraphic tool suite (to make resistivity, sonic, and gamma-ray measurements) and a lithology-porosity tool suite (for density, neutron porosity, and gamma-ray measurements). The gamma-ray log is run with all tool combinations to facilitate reconciliation of depth scales between logging runs.
When DSDP was succeeded by ODP in 1985, the logging program became more formalized. The standard logging suite encompassed the seismic-stratigraphic and lithology-porosity tool sets from DSDP days, but also took advantage of subsequent developments in tool technology by oil-field service companies. A geochemical tool set was added to provide the elemental concentrations of formations surrounding a borehole. In 1988 the formation microscanner was added to the standard suite. This is a high-spatial-resolution microresistivity tool that provides an electrical image of the borehole wall. The formation microscanner allows matching of cores and logs in terms of both depth and orientation.
In addition to the standard tools, several other oil-field tools have been deployed from time to time. These essentially comprise in-hole seismic tools for vertical seismic profiling, a borehole televiewer for obtaining an acoustic image of the borehole wall (analogous to the formation microscanner), and, more recently, a susceptibility/magnetometer tool for resolving magnetic reversals in sediments, which are weakly magnetized compared to basement rocks. All of these are examples of technology originally developed for oil-field applications. However, several downhole tools have been built by scientists in the DSDP/ODP community specifically for scientific use.
They include an ultra-deep-sensing resistivity tool for hydrogeological studies of rock porosity; a high-resolution temperature tool for heat-flow studies; a probe tool for taking water samples and measuring temperature and pressure in sediments ahead of the drill bit; and a thermistor string for long-term deployment in a borehole observatory.
Scientific Applications of Downhole Measurements
Downhole measurements have made a major contribution to all the principal scientific themes of DSDP and ODP. The first key area is that of global environmental change. Logs are especially well suited to addressing these problems because the solutions require a continuous depth record so that cyclic variations in sediment composition and texture can be evaluated in terms of paleoclimate and ocean circulation. The second area is crustal composition and structure, which must be known in order to understand the origin and evolution of oceanic lithosphere. For example, the sharper spatial resolution of logs relative to surface seismic data has led to a better understanding of the acoustic characteristics of different layers of Earth's crust. The third area is hydrogeology, with all its implications for the global geochemical budget. The two key parameters are porosity and permeability. Porosity can be evaluated from density, neutron, sonic and (in the absence of hydrocarbons) resistivity logs. Permeability is determined from downhole pressure tests over an interval of the borehole that has been isolated using packers or seals. The fourth key area is the global stress regime. Our understanding of the forces that drive tectonic plates and determine their motions can be advanced through knowledge of in situ stresses. Stress orientations can be inferred from failures of the borehole wall, known as "breakouts," which can be imaged using the borehole televiewer or formation microscanner. Changes in stress orientation can be depicted with depth or mapped regionally.
Scientific borehole logging is entering a new era. It is no longer sufficient to rely on oil-field technology to meet ODP logging needs. A major ODP objective is to drill in the young brittle crust near spreading centers. This will require high-temperature tools that are less than 5 centimeters in diameter and rated to 400 |degrees~ C. Since these specifications exceed the capabilities of commercial logging tools, ODP will have to develop its own tools, possibly in conjunction with other scientific programs in order to share the considerable engineering costs. At present, resistivity, temperature, and fluid sampling tools are being developed for high-temperature slim hole deployment. In this respect, scientific borehole logging is at a watershed. The scientific community is responding to the technical challenges positively so that the downhole measurements of the future will constitute as effective a scientific legacy as their present counterparts.
Paul F. Worthington served as Chairman of the ODP Downhole Measurements Panel from 1987 to 1992. He is an environmental and resource evaluation consultant, based in Ascot, Great Britain, and a visiting research professor at the Lamont-Doherty Earth Observatory of Columbia University.