Present address: Laboratory for Extraterrestrial cistances of up to 90 km from the source loop were Physics, NASA/GoddarC Space Flight Center, made at several discrete frequencies in the range Greenbelt, Maryland 20771. 0.05 to 400 Hz. The source loop was located on the Huntington Research Station of Syracuse
Electromagnetic (EM) tomography can be used to estimate the distribution of electrical rock properties between boreholes. For typical oil field well spacings and reservoir electrical characteristics, crosswell EM measurements are sensitive primarily to the electrical conductivity, not the permittivity. Ray‐tracing methods normally associated with undamped propagating waves can also be applied to the rapidly attenuated, diffusive EM waves encountered in the petroleum reservoir environment. Thermal noise considerations place an upper limit of approximately 20 skin depths on the separation between wells.
This note describes a simple method for converting transient electromagnetic (EM) sounding data into profiles of conductivity versus depth, based on an approximate image representation for the decaying induced ground currents. The method can provide one‐dimensional (1-D) inversion for any kind of time‐domain EM sounding data; the discussion here is limited to the case of central‐loop sounding. In particular, I apply the inversion to some time‐derivative central‐loop sounding data and demonstrate that essentially the same interpretation of the geoelectric section is obtained with the simple image method as is obtained using traditional iterative least‐squares fitting to layered models. This simple inverse can be computed much more quickly than an iterative least‐squares inverse, making it possible to estimate the geoelectric section concomitant with data acquisition.
Controlled source magnetic induction experiments in the Adirondack Precambrian shield region of northern New York State indicate that the electrical conductivity in both the upper and lower crust are in conformity with laboratory studies of moist, igneous rock and in serious discord with such measurements on dehydrated rock. A two order of magnitude increase in the conductivity found at 20 km depth suggests a structural or phase change there. A single turn loop of wire 1.5 km in diameter was used to generate an oscillating magnetic dipole source field over the frequency range 0.4 to 390 Hertz. The amplitude and phase of the resulting fields were measured as a function of distance to 90 km from the source. Analysis of these measurements indicates approximate horizontal stratification of the electrical conductivity. The uppermost layer, which is no more than a few hundred meters thick, has a conductivity thickness product of 0.4 (ohms)−1. It is underlain by a conductivity of about 8 × 10−5 (ohm meters)−1 to a depth of about 15 km. The conductivity between 15 and 20 km is approximately 10−3 (ohm meters)−1; the conductivity is greater than 10−2 (ohm meters) −1 below 20 km.
Downhole connections between multiple wellbores have applications in many operations such as extended-reach drilling, multilateral completions, subsurface pipelines, and downhole fluid separation. An R&D project was undertaken to develop and validate an electromagnetic ranging concept for enabling cost-efficient downhole connections—namely, Rotating Magnetic Ranging Services and Single Wire Guidance. After extensive testing that found the ranging technology suitable, an existing offshore well jacket located in Southeast Asia, in 5.1 m of water and 1.3 km from shore, was identified as a good candidate for field validation of the concept. The electromagnetic-ranging technology facilitated the successful intersection of the target well according to plan, including by-pass within specified proximity (< 40 cm) and in the correct sand (+-1.5 m TVD window). This paper discusses some of the many applications of downhole well connections. Furthermore, it explains the Rotating Magnetic Ranging Service and Single Wire Guidance ranging concept, which enables efficient downhole well connections. Introduction Determining the distance and direction to adjacent wellbore(s) is a critical task while drilling relief wells or preventing wellbore collisions. Because of the cumulative and systematic errors inherent in MWD or gyroscopic tools, the measured survey coordinates of a wellbore will have increasing uncertainty with depth, which is referred to as the "cone of uncertainty." For example, a vertical well drilled using a surveying tool with an error cone of 1.5 m/1000m, the radius of uncertainty at a true-vertical depth (TVD) of 10,000 m is 15 m. Hence for blowout intervention, to accurately steer a relief well to a deep intersection by relying on the survey data of the target well alone is practically impossible—that is, without a considerable amount of luck. Instead, the homing-in process must be accomplished by a downhole ranging technique. Although some unique ranging technologies are currently being researched, the most common methods are passive-magnetics or active-electromagnetic ranging, which both depend on steel, such as casing or drillstring, in the target well. Over the last decade feasibility studies, field trials, and implementation of downhole ranging technology have been performed for additional applications. This paper explain examples of applications for downhole well connections and describe the Single-Wire Guidance (SWG) and Rotating Magnetic Ranging Service (RMRS) technology developed and tested for these types of applications. History of Well Connections Downhole well intersections are common for relief wells and have been performed regularly for decades as a last-resort well-intervention method when other surface kill efforts have failed. The original purpose of a relief well was to relieve pressure on a blowing formation by drilling a vertical well and producing the formation at high rates. In 1933 a directionally drilled relief well intersected the flowing reservoir below the surface location of a cratered blowout in Conroe, Texas, marking the first milestone in relief-well development. The first application of electromagnetic ranging to achieve a downhole well intersection was performed on blowout in the Gulf of Mexico in 1980 (Kuckes et al. 1984). Furthermore, in 1982, Kuckes et al. used a modified technique with downhole current injection to demonstrate that casing could be detected in a blowout at a range of at least 200 ft. The technique showed great efficiency in locating blowout tubulars for a direct intersection. Casing detection, along with additional developments in surveying and MWD, provided a technique of triangulating the blowing well, reducing plugging and sidetracks. This again changed the basic strategy for designing relief well trajectories.
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