A new resistivity logging tool is described that improves the efficiency of drilling and logging operations by halving the length of traditional Dual Laterolog arrays to 16 ft. The reduced length and weight of the monolithic sonde lead to faster rig-up and can save several meters of rat-hole. The array, based on the configuration of a Dual Laterolog and Complemented by an azimuthal electrode array in the center, was designed for high vertical resolution and optimized for invasion response and borehole effect. The tool's operation is based on a number of independent and simultaneous measurement modes that are combined by software to yield a series of resistivity measurements with different depths of investigation and resolutions. The standard deep and shallow laterolog curves are output with improved resolution, but the flexible computed focusing also allows the substitution of different focusing conditions, which simultaneously yield deep and shallow resistivity curves with still higher resolutions of around 8 in. The passive azimuthal array at the center of the tool provides resistivity imaging of the formation in deep and shallow mode and an auxiliary ultra-shallow measurement sensitive to the radial distance from the borehole wall. An additional in-situ determination of the mud resistivity permits accurate real- time borehole corrections to be made. Examples from recent field tests show the invasion and thin- bed response of the tool together with applications of the resistivity images. Introduction The laterolog has become the standard formation device in salt drilling muds and in hard rock environments. Doll (1951) first presented the principles of the measurement, and two decades later the first simultaneous Dual Laterolog followed, described by Suau et al. (1972). The Dual Laterolog not only permitted the two measurements to be made simultaneously, but also was combinable with a microresistivity tool for a complete resistivity measurement suite. Since the introduction of the Dual Laterolog, few notable advances in laterolog devices have been made until Davies et al. (1994) presented the ARI* (Azimuthal Resistivity Imager tool), that introduced azimuthal resistivity measurements in addition to the standard deep and shallow resistivities. The azimuthal array further allowed a deep resistivity measurement to be made with increased resolution. In practice, resistivity tools are seldom run alone for complete formation evaluation. Reference has already been made above to the combination of laterologs with microresistivity tools and, in fact, operators seek to combine these tools with porosity measurements into the so-called "triple-combo." More complex and complete combinations result in tool strings of considerable length - often over 90 ft. While these combinations improve efficiency by reducing the number of logging runs, they also pose specific problems such as lengthy rig-up/rig-down, reduced logging speed and the need for more rat-hole. A new laterolog tool, the HALS* (High Resolution Azimuthal Laterolog Sonde), is presented here which was designed to overcome these problems. It is only 16 ft long, half the length of the Dual Laterolog and monolithic in construction. Like the ARI, it has an azimuthal resistivity array that gives measurements having manifold applications. Directional measurements aid understanding and interpretation in horizontal wells, while formation images provide visual presentation of fractured zones or other heterogeneities. P. 563
Summary A new generation laterolog tool, the Azimuthal Resistivity Imager (ARI) is described. The tool makes deep azimuthal resistivity measurements around the borehole with higher vertical resolution than the Dual Laterolog (DLL) tool. An array of twelve azimuthal electrodes is incorporated into the dual laterolog array so as to provide twelve deep, oriented resistivity measurements while retaining the standard deep and shallow laterolog measurements. To allow full correction of the azimuthal resistivities for borehole effect, a very shallow auxiliary measurement is incorporated on the azimuthal array. Though the full-coverage azimuthal resistivity image has much lower spatial resolution than borehole micro-electrical images, it complements these because of its lower sensitivity to shallow features. Fracture evaluation and computation of structural dip are applications of the tool's imaging capabilities which are discussed and illustrated with log examples. Other log examples cover thin-bed response, Groningen effect and borehole corrections, including that for eccentering of the tool in the borehole. Introduction The Laterolog technique was introduced in 1951, with the Dual Laterolog tool following some twenty years later. Though instrumentation has been upgraded as technology has developed, the Dual Laterolog deep and shallow measurements, LLd and LLs, have remained essentially unchanged since their introduction. Together with induction tools, the laterolog provides the key input for basic formation evaluation. While important advances have been made in the design of induction devices in the past ten years, few comparable developments have been made in the laterolog domain, despite known limitations to the laterolog measurements. Reference electrode effects have plagued deep laterolog measurements since their early days. Though effects such as Delaware and anti-Delaware effect have been overcome by repositioning the measure and current returns, Groningen effect remains a particularly complex problem which has yet to be satisfactorily resolved. It manifests itself as an increase in the LLd reading in conductive beds overlain by thick, highly resistive beds. The vertical resolution of the deep and shallow laterologs is two to three feet, with a typical beam width of around 28 inches. Thin beds are assuming increasing importance as potential reservoirs, and the vertical resolution of the deep and shallow laterologs is increasingly recognised to be insufficient for adequate evaluation of these beds. Development of a pad-mounted laterolog-3 has reportedly improved vertical resolution to two inches, though a consequence is reduced depth of investigation. Paradoxically, pad or skid devices suffer from a larger borehole effect than cylindrical tools. Though the effect of dip is much less severe than for induction devices, whose responses are perturbed drastically, dual laterolog response is affected significantly across dipping bed boundaries. A directional resistivity measurement around the borehole axis would provide a means of correction for the effects of dip. In one sense such measurements are already available in the form of high-resolution electrical borehole imaging tools, which have been shown to be very effective in evaluation of complex reservoirs.
The Cased Hole Formation Resistivity Tool (CHFR*) that measures formation resistivity through casing has detected and evaluated bypassed hydrocarbon, tracked reservoir fluid movements, performed contingency logging, and has complemented the operating envelope of nuclear saturation monitoring tools. It has to be run stand-alone and due to its 3 3/8-in. outside diameter, the completion had to be pulled before logging. This paper will present some of the field test results of the third-generation cased hole formation resistivity tool. It has a 2 1/8-in. outside diameter and is specifically designed for conveyance through tubing. The tool is also fully combinable with production logging tools and nuclear saturation-monitoring tools. It can be run conventionally on wireline, or deployed on coiled tubing or by tractor in difficult borehole conditions. Field tests have verified the performance of the slimhole tool. Some of the field test data collected from different locations around the world are presented in this paper. They include log examples acquired in different well completions using various toolstring configurations. The small-diameter version adds new benefits to the industry: increased versatility, improved performance, conveyance under dynamic well conditions, and cost reduction. Introduction Formation resistivity measurements through casing long had been a request by the industry. The basic measurement principles were first proposed in a USSR patent issued to Alpin in 1939 (Alpin, 1939). Other patents from 1956, 1959 and 1972 got into details on how to make the measurement but were facing challenges in measuring the nanoVolt range voltages accurately and reliably. The subject was revisited in the late Eighties by Kaufman (1989, 1990), Vail (1989) and Schenkel (1991). First field data were published by Vail in 1993 and 1995. The year 2000 marked the worldwide commercial introduction of the CHFR wireline tool, followed by the second-generation CHFR-Plus tool in 2002 (Benimeli, 2002), using a dedicated monitoring and control system. Its dynamic range and error cancellation principle resulted in a large reduction in sensitivity to measurement errors and also doubled the effective logging speed. Since then the technology has gained industry acceptance and hundreds jobs have been acquired to identify bypassed pay in old wells, for reservoir monitoring and management, and for formation evaluation in wells with no open hole data. Traditionally formation evaluation through casing was mainly accomplished with nuclear measurements, such as a thermal-decay time measurement or a carbon-oxygen ratio measurement. These nuclear tools work best in medium to high porosities, and the thermal-decay time measurement further requires saline formation water. The CHFR tool provides of a deep formation resistivity measurement through metal casing, allowing a direct comparison with the open hole acquisitions and offers a distinct alternative to nuclear-based tools. The CHFR tool is not combinable with any other tool other than gamma ray and casing collar locator (CCL) and due to its 3 3/8-in. outside diameter, most completions had to be removed prior to logging which usually requires the presence of a workover rig. Driven by industry needs, the increasing focus on cost effective solutions and developments on brownfields technology, the Slimhole Formation Resistivity Tool (CHFR-Slim*) was developed in the Schlumberger Riboud Product Centre, Clamart. It was field-tested, starting from late last year. This paper presents the key features of this logging instrument and also shares the results obtained during this period. Three field test examples will be discussed in more detail.
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