SPWLA 55th Annual Logging Symposium, May 18-22, 2014 WHAT IS Rt? LOGGING-WHILE-DRILLING AND WIRELINE RESISTIVITY MEASUREMENTS SPOTLIGHTED: AN OFFSHORE CASE STUDY IN ABUDHABI Amr M. Serry, Sultan A. Budebes, and Hassan Aboujmeih, ADMA OPCO; Ahmet Aki and Michael Bittar, Halliburton Copyright 2014, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors. This paper was prepared for presentation at the SPWLA 55th Annual Logging Symposium held in Abu Dhabi, United Arab Emirates, May 18-22, 2014. practices for both well-placement and petrophysical evaluation. INTRODUCTION ABSTRACT Recent technology improvements in logging-whiledrilling (LWD) electromagnetic wave propagation resistivity devices have provided dramatic improvements in well-placement applications. Azimuthal, deep-sensing measurements, coupled with other sensor measurements and significant software enhancements, have facilitated enhanced geosteering capabilities, which not only help maximize reservoir exposure, but also provide real-time updates of the local reservoir model. However, LWD propagation resistivity measurements in highly deviated and horizontal holes can also present challenges to the analyst in answering fundamental questions in relation to formation evaluation. Typically, it is not only problematic to correlate LWD resistivities to offset vertical and/or pilot resistivity data, but it is also difficult to deduce true resistivity (Rt) and the flushed zone resistivity (Rxo), particularly in thin beds, from the numerous multi-frequency and multi-spacing measurements available. This paper presents a case study from a thinly bedded offshore carbonate reservoir in Abu Dhabi. Two horizontal drains were drilled using LWD tools for the purposes of geosteering and formation evaluation. The available offset well data were from near-vertical wells, which were logged using wireline tools. The LWD propagation and laterolog resistivity measurements are compared to the offset wireline induction and laterolog resistivity measurements. Comparisons are also made between LWD propagation and laterolog resistivities acquired while drilling and while wiping after drilling. Differences between the various measurements are explored to identify the most appropriate choice of measurement in various circumstances. In light of the results, recommendations are made for data selection in future wells, with the intention of optimizing data acquisition The formation evaluation of carbonate Reservoir-A, located in one of the largest oil fields in offshore Abu Dhabi, has been historically performed using a variety of traditional logging tools. Beginning in the 1960s and continuing to the present day, many wells are cored and various formation evaluation techniques are used. For example, elemental spectroscopy, nuclear magnetic resonance (NMR), array sonic, dielectric, and image log data are used for benchmarking against core data analysis. However, some layers of ReservoirA have not yet been developed because of their lower porosity and permeability compared to other layers that have been major producers for many years. A hydrocarbon saturation assessment of such undeveloped carbonate layers is very critical. The saturation computation model (Archie) is applied as follows: ( ) = Where, ∅ ∗ ................................................(1) Sw = Water saturation in the uninvaded zone, % a = Tortuosity factor n = Saturation exponent m = Cementation exponent Rw = Formation water resistivity, Ohm-m Rt = True formation resistivity, Ohm-m. To obtain more control of the computed saturation values, measurements of a, m, n, and Rw are conducted at reservoir conditions at the core laboratory and applied for each layer of Reservoir-A. After calibrating the computed porosity with core porosity for cored wells across the field, the remaining element is to compute an accurate value of the Rt. 1 SPWLA 55th Annual Logging Symposium, May 18-22, 2014 Historically, this is achieved in the early phase of development by combining the resistivity seeking laterolog and the conductivity seeking induction array deep-resistivity log using a unique software package developed by service companies. To help minimize acquisition costs and reduce the length of the conventional logging string, while at the same time staying coherent with the low-salinity-mud highformation-water salinity contrasts, laterolog-type tools have been used later in the life of the field for modern wells solely to acquire resistivity data. Then, a resistivity inversion algorithm is applied to reconstruct much more reasonable values for Rt after correcting for invasion, shoulder beds, and sometimes for formation dip, assuming a piston-like mud filtrate invasion profile. hydrocarbon in place, based on log saturation (Rt driven) that must be to the highest possible accuracy, while during drilling, openhole log data acquisition must be performed with minimum operation time and cost and cannot be compromised because the estimation of hydrocarbons in place, based on log saturation, must be very accurate. WELL PLAN AND DESIGN The original deviated pilot hole, shown as a black line in Figure 2, was drilled at a 47° inclination. To determine the resistivity profile of the well, the well was logged with both wireline induction and laterolog resistivity tools. After evaluating the pilot hole, another sidetrack mother hole was drilled at a 63° inclination. LWD tools were used in the evolution of the sidetrack hole. Azimuthal deep resistivity (ADR) and gamma data were acquired in real time, and another wipe run was performed with ADR, gamma, azimuthal litho-density (ALD), and compensated thermal neutron (CTN) sensors, which are shown as a blue line in Figure.2. After evaluation, two horizontal drain wells were drilled. Drain 1 was drilled in Reservoir-A, Layer 2 (shown as a green line in Figure 2), and Drain 2 was drilled in Reservoir-A, Layer 1 (shown as a red line in Figure.2). To compensate for the limited vertical thickness and low petrophysical properties of the undeveloped layers in Reservoir-A, compared to other layers that have been producing for years, ambitious development plans include excessive drilling of horizontal and highangle wells into the undeveloped layers, which have a porosity of up to 9% and permeability ranges between 1 to 10 md, as highlighted in the red envelope of the crossplot of Figure.1. Figure.2 Well plan and design showing original deviated pilot and sidetrack mother holes together with two horizontal drains. Figure.1 Conventional Core Analysis (CCA) porositypermeability crossplot of Reservoir-A. CHALLENGES EVALUATION AND PLACEMENT OF DRAIN 1 IN LAYER 2 Consequently, there are many possible challenges to consider, from geosteering operations to the start of well production. Primarily Estimation of the Pilot Hole Wireline Evaluation. The original pilot hole drilled at a 47° inclination was logged with wireline 2 SPWLA 55th Annual Logging Symposium, May 18-22, 2014 gamma ray, density, neutron, and both induction and laterolog wireline resistivity tools (Figure 3). Reservoir-A, Layer 2 at a 47° inclination represented in true vertical depth (TVD) scale. Sidetrack Mother Hole LWD Evaluation. The sidetrack mother hole was drilled at a 63° inclination. LWD tools were used in the evaluation of the sidetrack. Figure.5 displays the LWD logs obtained in drilling and wiping mode. The gamma ray log is displayed in Track 1. The ADR log obtained in drilling mode is displayed in Track 2, and the ADR obtained during wiping mode is displayed in Track 3. ALD and CTN logs are displayed in Track 4. Both Track 2 and Track 3 display the 16-in., 32-in., and 48-in. phase resistivities at 2 MHz and 500 KHz frequencies. Figure.3 (A) Original deviated pilot hole wireline logs and petrophysical interpretation. (B) Horizontal Drains Logging While Drilling Data and Interpretation across Reservoir-A, Layers 1 and 2. The original deviated pilot hole wireline logs are further illustrated in Figure.4, together with target sublayers for Drain 1, Layer 2. The gamma ray log is displayed on Track 1. The wireline resistivity laterolog is displayed in red on Track 2, and the induction log is displayed in Track 3. Both resistivity tools show good resistivity readings across the target sublayers. However, it is evident that the laterolog tool reads higher than the induction tool. This is probably an indication that the zone is highly anisotropic. In a low angle and in the presence of anisotropy, laterolog tools read higher than induction tools; in a low angle, induction tools tend to read closer to the horizontal resistivity (Rh). Figure.5 Sidetrack mother hole LWD logs in drilling and wiping mode at a 63° inclination. The resistivity logs of Figure.5 show two important features. The first is that the drilling mode resistivity reads higher than the wiping mode across the target zone (Layer 2). This shows that the wiping resistivity log, which was obtained at a later time, is more affected by invasion, and that the drilling resistivity log reads higher because it is less affected by invasion. Note the separation between the phase resistivity in drilling mode, which indicates the presence of anisotropy. Also, note that the ADR reads somewhat higher than wireline induction because the inclination increased to 63°, and that the ADR appears to be more affected by anisotropy while the induction tool, at low angle, basically reads Rh. Figure.4 laterolog Original deviated pilot hole wireline and induction resistivity logs across Well Placement of Drain 1. After evaluating the original pilot and mother holes, the first lateral, Drain 3 SPWLA 55th Annual Logging Symposium, May 18-22, 2014 1 in Layer 2, was drilled. The well was geosteered using the ADR tool and StrataSteer® 3D (SS3D) steering software. A snapshot of the well placement, along with the corresponding ADR and TVD logs, is shown in Figure.6. From this figure, it is apparent that the ADR resistivity values are closer to the pilot hole wireline laterolog than the induction log, as the inclination in this hole section is approximately 87 to 89°. Again, it can be observed that the laterolog reads much higher than the induction log. This is also an indication that the target zone is highly anisotropic. Sidetrack Mother Hole LWD Evaluation. Figure.8 shows the LWD logs obtained in drilling and wiping mode. Good resistivity readings are observed across the target sublayer of Layer 1. Again, the phase resistivity separation in Track 2 obtained in drilling mode indicates the anisotropic nature of the zone. Figure.6 Well placement of Drain1 in Layer 2. EVALUATION AND PLACEMENT OF DRAIN 2 IN LAYER 1 Pilot Hole Wireline Evaluation. Figure.7 displays the wireline gamma ray log, the induction wireline resistivity log, and the laterlog wireline resistivity logs, together with the target sublayer for Drain 2. Figure.8 Sidetrack mother hole LWD logs in drilling and wiping mode at a 63° inclination. WELL PLACEMENT OF DRAIN 2 After evaluation of the original pilot and mother holes, the second lateral, Drain 2 in Layer 1, was drilled. This time, the azimuthal focused resistivity (AFR) and SS3D software were used to place the well. A snapshot with the corresponding AFR TVD logs is shown in Figure.9. The AFR (deep laterolog in red) is clearly much closer to the wireline induction log from the original deviated pilot hole. The inclination in this drain is 88 to 90°. The AFR shows good promise in these thin beds for well placement as well as for determining Rt for formation evaluation. Figure.7 Original deviated pilot hole wireline laterolog and induction resistivity logs across Reservoir-A, Layer 1 at a 47° inclination represented in TVD scale. 4 SPWLA 55th Annual Logging Symposium, May 18-22, 2014 Figure.9 Well placement of Drain 2 in Layer 1. Rt, Rxo, AND ADJACENT BED EFFECTS Obviously, there are other issues to consider when determining Rt, such as invasion and adjacent bed effects. However, in horizontal wells, deep readings that have a much greater diameter of investigation are somewhat disadvantageous for formation evaluation purposes. Much shallower measurements, such as AFR, can be used as long as the deep laterolog is not affected by invasion. It is also advantageous to be close to Rh in horizontal holes because of the robustness of Archie-based Sw algorithms. Figure.10 Formation volumetrics and fluid analysis from Drain 1 and Drain 2. CONCLUSIONS The Rxo result is an interesting comparison. Although the invasion effects are evident in the wipe runs for both the ADR and AFR, there seems to be more control over Rxo with the AFR. For Drain 1, the shallow phase 16-in. resistivity (RH16P) from drilling was used as Rt, and the same RH16P was used as Rxo from the wipe run. For Drain 2, the AFR deep was used for Rt, and the AFR shallow from the wipe run was used as Rxo (Figure 10). Therefore, combining the ADR and AFR in both the drilling and wipe mode proved beneficial for obtaining Rt and Rxo. Determining Rt in horizontal or highly deviated wells is challenging because of anisotropy effects and adjacent bed boundaries effects. In vertical and lowangle wells, wireline induction and LWD propagation tools tend to have no sensitivity to anisotropy and instead read the Rh. On the other hand, wireline laterolog and LWD toroidal resistivity tools have sensitivity to formation anisotropy in both dipping and non-dipping formations. Sensitivity to Rv increases with a higher dipping angle but at a much slower rate than with traditional induction and wave propagation resistivity tools. In horizontal wells, deep reading tools with a greater depth of investigation are somewhat disadvantageous for determining Rt because of adjacent bed effects. Much shallower measurements from both wave propagation and laterolog-type tools can be used as long as they are not affected by invasion. This case study emphasized that LWD laterolog- and LWD propagation-type tools complement each other when used for evaluation and for placing lateral wells, and they help reduce uncertainty in the determination of Rt in horizontal and highly deviated wells. 5 SPWLA 55th Annual Logging Symposium, May 18-22, 2014 Wu, P., Barber, T., Homan, D., Wang, G., Johnson, C., Heliot, D., et al. 2010. Determining Formation Dip from a Fully Triaxial Induction Tool. Paper presented at the SPWLA 51st Annual Logging Symposium, Perth, Australia, 19–23 June. ACKNOWLEDGEMENTS The authors thank the management of ADNOC, ADMA-OPCO, and Halliburton for their support, encouragement to publish this work, and for reviewing the manuscript and providing helpful comments and suggestions. REFERENCES ABOUT THE AUTHORS Bittar, M. and Rodney, P. 1996. The Effect of Rock Anisotropy on MWD Electromagnetic Wave Resistivity Sensor. Paper PP presented at the SPWLA 35th Annual Logging Symposium, Tulsa, Oklahoma, USA, 19–22 June. Amr Serry works as a petrophysicists for Abu Dhabi Marine Operating Company (ADMA-OPCO), Umm Shaif field development division since 2010. Bittar, M. 2000. Electromagnetic Wave Resistivity Tool Having a Tilted Antenna for Determining The Horizontal and Vertical Resistivities and Relative Dip Angle in Anisotropic Earth Formations. US Patent No. 6,163,155. Mr. Amr has acquired his BSc. Degree of Petroleum Engineering at Cairo University in 2004 before joining AlMansoura Oil Company, as a petroleum engineer in the Gas development operations division. Bittar, M., Klein, J., Beste, R., Hu, G., Wu, M., Pitcher, J., et al. 2007. A New Azimuthal DeepReading Resistivity Tool for Geosteering and Advanced Formation Evaluation. Paper SPE 109971 presented at the SPE Annual Technical Conference and Exhibition, Anaheim, California, USA, 11–14 November. He has joined Baker Atlas in 2005 as a petrophysical engineer, conducting a variety of open and cased hole log analyses, providing operations support and technical sales for Egypt, UAE and various locations within the Middle East Region. He is an active SPE Member since 2005 and a member of SPWLA, Abu Dhabi local chapter. Bootle, R., Waugh, M., Bittar, M., Hveding, F., Hendricks, W., and Pancham, S. 2009. Laminated Sand-Shale Formation Evaluation Using Azimuthal LWD Resistivity. Paper SPE 123890 presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, USA, 4–7 October. Sultan Budebes works as a senior petrophysicist for Abu Dhabi Marine Operating Company (ADMA-OPCO), Umm Shaif field development since 2004. Hu, G., Bittar, M., and Hou, J. 2006. Evaluation of Horizontal Wells Using LWD Propagation Resistivity and Laterolog-Type Resistivity Logs. Paper SPE 103150 presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 24–27 September. Mr. Sultan worked for Abu Dhabi National Oil Company (ADNOC) from 1995 to 2004 as a petrophysicist in the Umm Shaif Khuff gas project providing petrophysical analysis. Rodney, P., Mack, S., Bittar, M., and Bartel, R. 1991. An MWD Multiple Depth of Investigation Electromagnetic Wave Resistivity Sensor. Paper presented at the SPWLA 32nd Annual Logging Symposium, Midland, Texas, USA, 16–19 June. Prior to that he worked as a wellsite geologist, for Al Ain Ground water Research Project, a joint project of the National Drilling Company (NDC) and the United States Geological Survey(USGS). Tabarovsky, L., Epov, M., and Rabinovich, M. 2001. Measuring Formation Anisotropy Using Multifrequency Processing of Transverse Induction Measurements. Paper SPE 71706 presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, USA, 30 September–3 October. He graduated from the United Arab Emirates University of Al Ain in 1990 with a BSc degree in Geology and Chemistry and has a wide range of SPW/SPWLA publications. 6 SPWLA 55th Annual Logging Symposium, May 18-22, 2014 including Chief Technical Advisor, Halliburton Technology Fellow, Director of Research and Senior Director of Formation Evaluation. Hassan Aboujmeih is an Operational Petrophysicist at ADMA-OPCO, Managing the data gathering operations and wireline logging and perforation contracts. Dr. Bittar received his BS, MS and PhD degrees in electrical engineering from the University of Houston and has more than 100 patents and publications. He is a long term member of SPE and SPWLA and serves on King Abdullah University of Science and technology (KAUST) Industry Advisory Board. Dr. Bittar was as well the recipient of the 2006 SPWLA Technical Achievement Award and the 2009 Halliburton Outstanding Commercialized Invention of the Year Award. Mr. Hassan Aboujmeih holds a Mechanical Engineering degree from the University of Western Ontario in Canada. He started his oil career with Schlumberger Wireline and formation evaluation as a field Engineer working in North America and the Middle East. He also provided management and technical support in high activity volume locations. Ahmet Aki is the Regional Technical Sales and Marketing Manager with Halliburton Sperry Drilling for Middle East and North Africa. Mr. Aki has obtained his B.Sc. and M.Sc. degrees from University of Birmingham in the UK in 1981. He has worked for Schlumberger and Halliburton in field operations and management positions in West and North Africa, North America, North Sea and the Middle East, prior to moving into log analysis in 1994. Mr. Aki has worked in petrophysical consulting and technical support positions for both Wireline and LWD since 1994 with numerous technical publications. Mr. Aki has been a member of SPWLA and SPE since 1986, and is currently serving on the board of SPWLA Abu Dhabi Chapter. Dr. Michael Bittar is a Senior Director of Technology for Halliburton. Dr. Bittar joined Halliburton in 1990 and since then has held various technical and leadership roles, 7
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