In very thinly bedded sand-shale sequences, sand counts and sand/total ratios can be determined from high resolution electrical images of the borehole wall. This technique is illustrated here by a case study ~·n turbidites with sand laminae down to 1 em in thickness. The numerical modeling of the tool response and a comparison with fullbore core data help assess the accuracy of bed thicknesses derived from electrical images for this type of formations.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe analysis of deep-reading electromagnetic measurements is critical to the evaluation of hydrocarbon reserves. However, in thin bed formations, poor tool vertical resolution and corresponding low sensitivity to hydrocarbon presence make interpretation in the virgin zone difficult. A priori knowledge such as the formation geometry or auxiliary petrophysical information is necessary to overcome these difficulties. This paper presents a prototype code developed by Schlumberger S-RPC in collaboration with AGIP. Using this code, wireline or LWD, laterolog and induction measurements can be more correctly analyzed in thinly bedded environments (2-D geometry, fluid invaded layers perpendicular to the borehole).This code has been implemented in a software framework that provides a common environment specifically designed for electrical tool interpretation. Processing modules have a common interface and share common functionalities. Their organization reflects an implicit processing methodology, with progressive refinements that provides the interpreter with a robust and simple to use product, to better quantify reserves.A preliminary step is to determine the formation geometry, which is carried out by detecting bed boundaries and representing the formation as a vertical sequence of layers. Petrophysical analysis can be invoked to characterize certain formation properties such as shale volume and porosity. These steps are performed prior to resistivity log measurement analysis and serve as a form of a priori knowledge.Once the formation is described as a sequence of layers, wireline l ogging or logging while drilling (LWD) tool response can be computed using fast 2D simulators. The estimation of resistivity and the subsequent estimation of saturation will correspond to the minimization of a cost function, defined as the weighted squared difference between the measurement and the simulated response. Confidence outputs can be related to the local shape of the cost function at the end of the processing.Two important advantages of the new code must be emphasized: (1) the possibility to choose among several petrophysical models to better describe the environment and determine directly parameters such as hydrocarbon saturation, and (2) the possibility to group together beds which are too thin or too close to each other to be analyzed independently, into a so called single optimization interval described by a reduced set of parameters.This paper presents results obtained on selected benchmarks extracted from real data and compares them with those obtained through more traditional approaches.

Thla'paper wea aekcted for praaentalion by an SPE Program Committee followingreview of informatkmcontained in en ebatract aubmltted by the author(a).Contente of the paper, aa presented, have not been rwlewed by the society ot Petroleum ErrOwem Ondare autiwt towxmction Wthe aUtfIONO). Thomat.fid M -td * not~k~ã ny poeitiorrof the Sociaty of PetroteumEngineers, Itaofficers,or members. Pepera praeented at SPE meetingsare subject to publicetkonreview by EditorialCommltteea of the &roiaty of PstroteumEnginaara Permieeionto copy is reefriotedto en abstractornotmc+ethen S00 words.Illuetratbnamay motbe copied The abetrecfehoutdcontaincon@cuw acknowMpmt of where and by whom the paper lapraeented. Writs Publications Manager, SPE, P.O. *X -, RichardeorhTX 7~. Telax, 7S09S9 SPEDAL. ABSTRACTIn very thinly bedded sand.abate sequence.r, uand count8 and imnd/total ratioo can be determined from high resolution elec. trical imagea of the borehole wall. Thi6 technique ia ilhdrated here by a caae study in turbiditea with sand laminae down to I cm in thickneaa. The numerical modeling of the tool response and a comparison m"th jullbore core data help asaess the accuracy of bed thicknesses derived from e!ectricrd images for this type of{ormationa.

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.

No abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe analysis of deep-reading electromagnetic measurements is critical to the evaluation of hydrocarbon reserves. However, in thin bed formations, poor tool vertical resolution and corresponding low sensitivity to hydrocarbon presence make interpretation in the virgin zone difficult. A priori knowledge such as the formation geometry or auxiliary petrophysical information is necessary to overcome these difficulties. This paper presents a prototype code developed by Schlumberger S-RPC in collaboration with AGIP. Using this code, wireline or LWD, laterolog and induction measurements can be more correctly analyzed in thinly bedded environments (2-D geometry, fluid invaded layers perpendicular to the borehole).This code has been implemented in a software framework that provides a common environment specifically designed for electrical tool interpretation. Processing modules have a common interface and share common functionalities. Their organization reflects an implicit processing methodology, with progressive refinements that provides the interpreter with a robust and simple to use product, to better quantify reserves.A preliminary step is to determine the formation geometry, which is carried out by detecting bed boundaries and representing the formation as a vertical sequence of layers. Petrophysical analysis can be invoked to characterize certain formation properties such as shale volume and porosity. These steps are performed prior to resistivity log measurement analysis and serve as a form of a priori knowledge.Once the formation is described as a sequence of layers, wireline l ogging or logging while drilling (LWD) tool response can be computed using fast 2D simulators. The estimation of resistivity and the subsequent estimation of saturation will correspond to the minimization of a cost function, defined as the weighted squared difference between the measurement and the simulated response. Confidence outputs can be related to the local shape of the cost function at the end of the processing.Two important advantages of the new code must be emphasized: (1) the possibility to choose among several petrophysical models to better describe the environment and determine directly parameters such as hydrocarbon saturation, and (2) the possibility to group together beds which are too thin or too close to each other to be analyzed independently, into a so called single optimization interval described by a reduced set of parameters.This paper presents results obtained on selected benchmarks extracted from real data and compares them with those obtained through more traditional approaches.