F. Brambilla, B. Corley, A. Garcia, H. M. Maurer, Baker Hughes. This paper was presented at the 11th Offshore Mediterranean Conference and Exhibition in ...
IMPROVED LATEROLOG RESISTIVITY MEASUREMENTS PROVIDE REAL-TIME TRUE FORMATION RESISTIVITY AND INVASION PARAMETERS F. Brambilla, B. Corley, A. Garcia, H. M. Maurer, Baker Hughes This paper was presented at the 11th Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, March 20-22, 2013. It was selected for presentation by OMC 2013 Programme Committee following review of information contained in the abstract submitted by the author(s). The paper as presented at OMC 2013 has not been reviewed by the Programme Committee.
ABSTRACT A newly developed multi-laterolog device, the Rt eXplorer™, has been introduced in the Mediterranean region. The device is based on the dual laterolog (DLL) principle, but is designed with four independent, focused measurements having different depths of investigation (ranging from 10 to 50 inches), with a common enhanced vertical resolution of one foot, providing a detailed high-resolution radial resistivity profile. A new hardware design, with symmetrical configuration, using a feedback loop ensures the required focusing, eliminating the need for a bridle or long isolation subs. This configuration greatly reduces Groningen effect, significantly improving resistivity measurements below evaporitic sequences or thick tight carbonate beds, which are typical conditions in several Mediterranean fields. These measurements, particularly the shallow ones, are subject to large borehole and eccentricity effects in very conductive muds and large boreholes and conventional borehole corrections using an assumed tool position become ineffective. To better remove borehole effect, an adaptive borehole correction has been designed, which accounts for the variable tool position. The technique includes a 1-D radial inversion that provides improved borehole corrections and Rt (true formation resistivity), Rxo (flushed zone resistivity), and Lxo (length of mud filtrate invasion). The inverted Rt may be significantly higher than the deepest laterolog measurement in situations of deeper invasion. In most cases a micro-laterolog (MLL) will be run in combination with this device to complement the four focused laterolog resistivities and used in the inversion process as flushed zone resistivity (Rxo), reducing uncertainty during the 1D-Inversion. The advantages of this multi-laterolog and its processed data are illustrated with field data examples.
INTRODUCTION The laterolog is a galvanic technique, introduced more than 50 years ago (Doll 1951), and its improvement, the dual laterolog tool, was developed two decades later (Suau et al., 1972). The dual laterolog became the standard formation resistivity device in logging environments characterised by conductive (salt-saturated) drilling mud and high formation resistivity (often hard rock). Advancements in formation evaluation techniques provide a better understanding of mud filtrate invasion effects in reservoirs but often require a greater numbers of resistivity measurements with varying depths of investigation as input values. For this reason, dual laterolog measurements are often insufficient for an advanced evaluation because this type 1
of device provides just two measurements, a deep and shallow, making it impossible to uniquely determine formation and invasion resistivities (Rt and Rxo) as well as the invasion length (Lxo). To overcome this limitation several high-resolution array laterologs were developed during the 1990s to improve the vertical, radial, and azimuthal resolution of the measured formation resistivity around the borehole. In moderate formation-to-mud resistivity contrast environments these devices provide logs with several depths of investigation. However, due to the software focusing approach used, at very high formation-to-mud resistivity contrast, the response of these tools becomes quite unreliable.
MULTI -LATEROLOG This paper describes the data quality obtained by an array laterolog device developed starting from the dual laterolog configuration and focusing principle, with four independent current injection modes at different operating frequencies. The new device is about half the overall length of the traditional dual laterolog (tool length: 14 ft or 4.3 m). It consists of four coil spacings, providing resistivity curves with depths of investigation ranging from very shallow to deep. The precise depth of investigation of a laterolog device is a function of borehole size and the contrast between mud and formation resistivity. Depth of investigation (DOI) for a laterolog measurement is traditionally defined as the radius at which 50% of the signal is received from the formation within that radial depth. This value can be determined by modeling a tool in a formation with just two radial zones; a flushed zone (Rxo) and a non-invaded zone (Rt) as the radius of invasion increases. The percentage of total response contributed by the invaded zone is expressed as a radial geometrical factor. Figure 1 shows the radial geometric factors of the dual laterolog and multi-laterolog tools in a thick bed as a function of the invasion radius. This model consists of three radial zones where Rt = 10 ohm-m, Rxo = 1 ohm-m, borehole diameter Bhd = 8 in. and Rm = 0.1 ohm-m. The corresponding depth of investigation for each spacing of the multi-laterolog is summarized in Table 1. The DOI of the multi-laterolog covers the range from about 9 in. to 40 in. The DOI of MLR2 is close to the DOI of a traditional dual laterolog shallow measurement (RS); the DOI of MLR4 is slightly less than that of the dual laterolog deep (RD) measurement.
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Fig 1: Radial geometric factors of the dual laterolog and multi-laterolog tools in a thick bed as a function of the invasion radius
Tab 1: Curves investigation depth Curve
Investigation radius (in)
Investigation depth (m)
MLR1 MLR2 MLR3 MLR4
9 13 19 37
0.23 0.33 0.48 0.94
The vertical resolution of each depth of investigation is function on the dimension of the current injection electrode and is approximately 1 ft. One of the major changes in this multi-laterolog design regards the current flow. The current for resistivity measurements, including the deep measurement, is designed to return locally to the tool body instead of the bridle electrode/cable, as was the case with the dual laterolog. This configuration eliminates the need for long isolator subs during logging and reduces Groningen effects. It also provide a larger operating range about Rt, from ~ 0.2 ohmm to ~ 100.000 ohmm with Rm =1 ohmm, compared to the traditional dual laterolog.
INSTRUMENT CONFIGURATION The multi-laterolog configuration, shown in Figure 2, consists of a 14-ft mandrel, plus other tool mandrels immediately above and below it. This configuration reduces the length of the tool string while still achieving a deep measurement. The central survey current electrode is A0, the bucking electrodes are Ai and Ai’ (i=1,…,4; Ai and Ai’ are internally connected) symmetrical located around the center electrode. The 3
monitor electrodes are labeled as Mi and Mi’. The basic configuration comprises a central electrode emitting the survey current, and multiple guard electrodes above and below it to achieve focusing. Focusing (bucking) current is sent between various guard electrodes to achieve shallower-to-deeper focusing. The longer guard electrodes provide greater focusing, therefore the greater depth of investigation. Electrodes A0, M1, M2, A1, A2 and A3 are located on the mandrel. The instruments above and below the mandrel become part of the electrode system and are referenced to as electrodes A4, A4’, A5 and A5’; Most often, a micro-laterolog tool (MLL) would be run below the multi-laterolog tool, which serves as electrode A4’.
Fig 2: Schematic representation of the multi-laterolog tool logging string and electrode mandrel. The central survey current electrode is A0, the bucking electrodes are Ai and Ai’ (i=1,…,4; Ai and Ai’ are internally connected). The monitor electrodes are labeled as Mi and Mi’. Fig 1: Write Caption here A feedback-loop-based, hardware-implemented method is employed throughout the measurements to ensure the focusing conditions are satisfied, even at extremes of mud and formation resistivities and their contrasts. 4
CONVENTIONAL BOREHOLE CORRECTION The traditional dual laterolog resistivities are corrected using a chart or a table to adjust the apparent resistivity measured in the formation, accounting for that part of the signal that is generated by the borehole. These charts provide correction based on two parameters: the borehole size, and the resistivity contrast between formation and borehole fluid assuming the tool is totally centered in the borehole, or it is totally eccentered in the borehole except for a fixed rubber standoff. Each measurement (shallow and deep) from the tool has its own correction and is independent from each other. The shallow measurement has the largest borehole effect and the conventional borehole corrections become unstable and inaccurate for higher Rt/Rm contrasts. Simple assumptions about the tool position, centered or eccentered, can cause a large over- or under-correction for these shallow curves.
ADAPTIVE BOREHOLE CORRECTION To better remove the borehole effect from the resistivities provided by the multi-laterolog, an adaptive borehole correction (ABC) has been designed, which accounts for variable tool position, exploiting the data from the four subarrays with four different depths of investigation from very shallow to deep. Due to the nature of shallow depth of investigation, measurements of the two shallowest sub-arrays are significantly affected by the borehole in very conductive muds and large holes. In addition, the shallow measurements become sensitive to the radial position of the tool in the borehole (tool eccentricity). The adaptive borehole correction technique overcomes the limitation of the conventional borehole correction. This technique is based upon an inversion that properly corrects shallow laterolog measurements for the borehole effect, including determining and accounting for unknown tool eccentricity. The algorithm is based on simplex radial 1-D inversion where at every logging depth we can determine up to four unknowns, tool eccentricity, Rt, Rxo, and Lxo. The simplified radial 1-D model used in ABC includes the following four parameters: Rm – mud resistivity, Bhd – borehole diameter, Ecc – tool eccentricity, Rt – formation resistivity, Rxo – invasion resistivity, and Lxo – depth of invasion. If we assume the first two parameters are known from mud resistivity log and caliper measurements, we are left with four unknowns that, in principle, can be determined from four independent measurements (MLR1, MLR2, MLR3, and MLR4). Quite often the MLL curve is acquired together with multi-laterolog, and we can incorporate its response into ABC. If the MLL is reliable (which is not always the case due to problems with pad contact), we can use it as Rxo in “invaded” models, so we have only three unknowns in ABC inversion. Another option is to use the MLL to constrain Rxo in inversion. The tool response for a model with the four parameters is compared with the tool response, where the mud resistivity Rm has been replaced by a “virtual” mud resistivity equal to Rxo (or Rt, if formation is not invaded).
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The measurements are corrected by multiplying with the ratio of the two responses. After this correction, all four laterolog curves “stack” in uninvaded intervals, and they show a consistent resistivity profile in invaded intervals. By processing all four laterolog measurements simultaneously, the shallow measurements can be corrected for borehole and eccentricity effects in large boreholes and high Rt/Rm contrasts. This increases the effective depth of investigation of the tool by recovering true Rt and reliably reconstruct Lxo and Rxo.
EASTERN MEDITERRANEAN OFFSHORE The Eastern Mediterranean offshore has been highlighted by discoveries of huge natural gas fields, in the subsalt sequence that includes all reservoirs within and below continuous Messinian-age salt, characterised by clastic reservoirs, below thick sequences of evaporitic rocks (halite and anhydrite) , usually drilled with salty saturated mud. The gas-earing reservoirs reach high resistivity, up to 200 Ohm.m, while the water legs the resistivity drops to 1.5 ohm.m, interbedded in claystone with occurrence of limestone stringers.
Fig 3: Sketch of stratigraphy of Eastern Mediterranean offshore 6
FIELD EXAMPLES The example illustrated as Figure 4 is a well where the multi-laterolog has been run in a vertical 8.5-in. borehole, with a salt-saturated (potassium) polymer glycole mud with a resistivity of Rm = 0.0264 ohm.m at 170 °F (BHT), MW 11.6 lb/gal. The curves from the adaptive borehole correction result in a clear invasion profile that is in good agreement with lithology derived from gamma ray logs. The model and 1-D inversion based process leads to an optimal estimation of Rt in the hydrocarbon bearing sands with shallow invasion. In addition, invasion length (Lxo) and flushed resistivity (Rxo) are obtained. The curves are stacked in the impermeable shales indicating no invasion. They are also stacked in the cleaner lower porosity zones that exhibit higher resistivity and no invasion. All curves show a high vertical resolution in both the low- and high-resistivity range. The example illustrated as Figure 5 is a deeper logging interval in the same well. The depth interval consists of a series of water-bearing sands and shales. The invasion profile in the sands indicates the formation water (Rw) has a higher resistivity than the mud filtrate (Rmf) that is invading these formations. A more accurate Rt, which could be used for the determination of Rw, is obtained in these sands. The length of invasion (Lxo) is similar to the hydrocarbon-bearing sands with similar porosity. The curves are again stacked in the impermeable shales, indicating no invasion. They are also stacked in the cleaner lower porosity zones that exhibit higher resistivity and no invasion. The example illustrated as Figure 6 is a shallow logging interval in the same well near casing where the bit size has changed. The depth interval consists of a series of impermeable shales and clean, lower porosity formations that are not invaded. Borehole size varies from 8.5 to 16 in. and the borehole is rugose and enlarged in many places. The curves all stack properly, indicating no invasion throughout the entire interval even though borehole conditions are poor. A curve indicating the amount of eccentricity (distance the tool is off center) determined by the adaptive borehole correction is shown in the first track. A standard borehole correction assuming a fixed value of eccentricity would have produced misleading curve separation and indications of invasion.
CONCLUSIONS A newly developed multi-laterolog device, the Rt eXplorer, has been introduced into the Mediterranean area. A newly developed hardware design, coupled with a model-based adaptive borehole correction technique, provides four depth-of-investigation resistivity measurements that have high vertical resolution and are properly corrected for borehole and tool eccentricity effects. This instrument provides higher vertical and more radial resolution measurements than the conventional dual laterolog. The new borehole correction method based on an inversion approach properly corrects shallow laterolog measurements, including determining and accounting for unknown tool position. At the wellsite while the data is being acquired, this model-and-inversion-based interpretation technique is used to provide true formation resistivity, and the length and resistivity of the invaded zone (Rt, Lxo, Rxo) for thick beds. This approach ensures that in uninvaded intervals all laterolog curves overlay properly, while in invaded intervals they show a reliable and consistent resistivity profile. Therefore, this new device and associated processing technique enables accurate and reliable characterization of the radial resistivity profile in invaded zones as well as accurate estimation of formation resistivity in both invaded and non-invaded intervals.
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Fig. 4: Multi-laterolog adaptive borehole corrected curves from the reservoir completed down to the transition to the water leg. Curves on left track are gamma ray, caliper and invasion length, on center track are plotted the four resistivity curves, Rxo and Rt, on right track the resistivity profile is plotted. 8
Fig 5: Multi-laterolog adaptive borehole corrected curves from a series of waterbearing sands. Curves on left track are gamma ray, caliper and invasion length, on right track are plotted the four resistivity curves, Rxo and Rt.
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Fig. 6: Multi-laterolog adaptive borehole corrected curves from an interval with poor borehole conditions. Curves on left track are gamma ray, caliper and tool eccentricity determined by the adaptive correction, on right track are plotted the four resistivity curves, Rxo and Rt.
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ACKNOWLEDGEMENTS The authors thank the management of ATP Oil & Gas and its partners for the release of their data for publication. They also wish to thank the management of both ATP Oil & Gas and Baker Hughes for permission to publish this work.
REFERENCES Doll, H.G., 1951, The laterolog: A new resistivity logging method with electrodes using automatic focusing system: Petroleum Transactions, AIME, 192, 305-316 Suau J., Grimaldi P., Poupon A., and Souhaite P., "The dual laterolog-Rxo tool", Paper SPE 4018, 1972, SPE Annual Technical Conference, San Antonio, TX. Zhou, Z., Corley, B., Khokhar, R., Maurer, H., Rabinovich, M., 2008, A New Multi Laterolog Tool with Adaptive Borehole Correction, SPE Annual Technical Conference and Exhibition, Denver, CO, Paper SPE 114704 Maurer, H., Antonov, Y., Corley, B., Khokhar, R., Rabinovich, M., Zhou, Z., 2009, Advanced Processing For A New Array Laterolog Tool, SPWLA 50th Annual Logging Symposium, Woodlands, TX, Paper SPWLA 56708. T. Abdel-Shafy, A. Fattah, B. Corley, R. Khokhar, H. Maurer, 2011, Comparison of a New Multi Laterolog Tool and a Formation Resistivity Imager in the Phiops Field of Egypt, SPE Middle East Oil and Gas Show and Conference Bahrain, 6–9 March 2011SPE-140692-PP
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