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Sun, Keli (Schlumberger) | Omeragic, Dzevat (Schlumberger) | Minh, Chanh Cao (Schlumberger) | Rasmus, John (Schlumberger) | Yang, Jian (Schlumberger) | Davydychev, Andrei (Schlumberger) | Habashy, Tarek (Schlumberger) | Griffiths, Roger (Schlumberger) | Reaper, Graham (Schlumberger) | Li, Qiming (Schlumberger)
Omeragic, Dzevat (Schlumberger) | Bayraktar, Zikri (Schlumberger) | Thiel, Michael (Schlumberger) | Habashy, Tarek (Schlumberger) | Wu, Peter (Schlumberger) | Shray, Frank (Schlumberger) | Antezana, Victor Hugo Goitia (EY)
Triaxial induction measurements were originally designed for the evaluation of resistivity anisotropy in vertical and medium- to low-deviation wells. Because of measurement complexity and difficulties in handling the large amount of data acquired, interpretation is based on model-based inversion to determine a 1D anisotropic resistivity profile and formation dip and azimuth. More recently, these measurements have been used for fracture interpretation to detect near-vertical fractures and estimate their strike, assuming a homogenous background.
Historical use of triaxial induction measurements was in exploration and development wells that mostly accessed laminated siliciclastic formations. The use of triaxial induction tools in high-angle (HA) and horizontal (HZ) wells has been limited chiefly because few of these wells required them for formation evaluation. However, demand has recently sharply increased in HA/HZ wells in both unconventional reservoirs and conventional laminated shaly sand formations. This demand has stimulated the concurrent development of new answer products that more fully use the rich data set from the triaxial induction tool. We have built upon our experience with inversion approaches used for well placement real-time interpretation of deep directional resistivity data. Such inversion methods account for variation of trajectory inclination, formation dip, and azimuth and lateral changes in layer properties and thickness.
We applied a minimally biased adaptive multilayer inversion to triaxial induction data. The methodology is validated on synthetic data for an anisotropic Oklahoma test formation at a relative dip of 85° with conservative noise level several times above the electronic noise applied to data. Only deep-array data were used assuming that the shallow measurements are of limited use because of potential environmental effects. We observed fairly consistent structure reconstruction for this dataset.
We propose the use of cross-dipole measurement symmetrization in interpretation. The antisymmetrized measurements have enhanced sensitivity to anisotropy and dip and reduced sensitivity to nearby bed boundaries. They also provide information about the anisotropy azimuth relative to the tool axis. In fractured reservoirs, the equivalent anisotropy orientation corresponds to the fracture orientation. We used 3D modeling to demonstrate that this orientation is unaffected by the influence of nearby boundaries.
We applied the inversion to a HA/HZ well data set from a West Siberia. The results of the inversion were useful in establishing the location of the well with respect to the adjacent, tightly cemented layer. Visualization of this layer provides confidence in believing that the well is in the right place, and that there is a low-permeability barrier between the target oil zone and the gas cap. We also processed a data set from the Gulf of Mexico with high relative formation dip. Borehole correction was applied to the data, and only the two deepest reading arrays were processed. The inversion is able to consistently image thin, low-contrast layers that are not seen in standard resistivity logs and are not easy to identify from transverse and cross-dipole couplings.
Constable, Monica Vik (Statoil) | Antonsen, Frank (Statoil) | Stalheim, Stein Ottar (Statoil) | Olsen, Per Atle (Statoil) | Fjell, Øystein Zahl (Statoil) | Dray, Nick (Statoil) | Eikenes, Sigurd (Statoil) | Aarflot, Haakon (Statoil) | Haldorsen, Kjetil (Statoil) | Digranes, Gunnar (Statoil) | Seydoux, Jean (Schlumberger) | Omeragic, Dzevat (Schlumberger) | Thiel, Michael (Schlumberger) | Davydychev, Andrei (Schlumberger) | Denichou, Jean-Michel (Schlumberger) | Salim, Diogo (Schlumberger) | Frey, Mark (Schlumberger) | Homan, Dean (Schlumberger) | Tan, Sarwa (Schlumberger)
AbstractA vision in the oil industry for decades is becoming a reality - we can now finally drill and react pro-actively to formation resistivity properties identified several meters ahead of the drill-bit, instead of drilling reactively on resistivity measurements at or behind the bit. Through a technology collaboration with Schlumberger, Statoil supported a targeted technology development for measuring resistivity contrasts ahead of the bit in real-time to reduce cost and risk during drilling operations.
Two Electro-Magnetic Look Ahead (EMLA) prototypes have been developed for 12 ¼" to 14" borehole diameter. The EMLA tool is modular and consists of a low frequency EM-transmitter inserted in the rotary steerable drilling tool about 1.8 m behind the bit. The transmitter induces currents at multiple frequencies around and ahead of the bit and the resulting induced magnetic field is recorded with 2 to 3 receivers spaced out in the drillstring. The formation structure ahead of the bit is interpreted by inversion of the bulk signals to differentiate sensitivity around the tool from effects ahead of the bit. The look ahead capability of the EMLA tool is dependent on the transmitter-receiver spacings, frequencies, resistivity around the tool, thickness of the target, and the resistivity contrast ahead of the bit.
The EMLA tool provides a step change with regards to the precision we now can detect changes in rock properties ahead of bit, enabling the well placement teams to “see” several meters ahead of the bit and to react before drilling into potential hazardous situations, even in near vertical wells. One key use of the EMLA technology on the Norwegian Continental Shelf is to drill and set casing for the 12 ¼" section much closer to top reservoir than we do today. This can reduce the risk of collapse in the overburden, especially for depleted reservoirs which require a significantly lower mud weight for drilling the reservoir than the optimal weight for stabilizing the overburden. The technology can also be used to pick the coring point more precisely and prior to drilling into the zone of interest. This will enable coring of the transition between overburden and reservoir which is often missed when using near bit measurements and also prevent costly coring of thin sand stringers mistaken as the main zone of interest.
Statoil has recently tested the tool in a sub salt play in the Gulf of Mexico with great success. One of the main objectives was to detect bottom salt before drilling through it. The highly resistive salt formation offers a very favorable environment for EM applications. The bottom salt was detected 30 m ahead of bit, which gave the drillers an early warning of the salt exit and potential drilling challenges.
In the near future, with the technology already there, simultaneous look around and look ahead (LALA) while drilling will be available. Interpretation of measurements in 2D and 3D environment is the main challenge to overcome to make LALA happening. To do so, a tight integration between inversions and geological scenarios will be a necessity.
Constable, Monica Vik (Statoil ASA) | Antonsen, Frank (Statoil ASA) | Stalheim, Stein Ottar (Statoil ASA) | Olsen, Per Atle (Statoil ASA) | Fjell, Oystein Zahl (Statoil ASA) | Dray, Nick (Statoil ASA) | Eikenes, Sigurd (Statoil ASA) | Aarflot, Haakon (Statoil ASA) | Haldorsen, Kjetil (Statoil ASA) | Digranes, Gunnar (Statoil ASA) | Seydoux, Jean (Schlumberger) | Omeragic, Dzevat (Schlumberger) | Thiel, Michael (Schlumberger) | Davydychev, Andrei (Schlumberger) | Denichou, Jean-Michel (Schlumberger) | Salim, Diogo (Schlumberger) | Frey, Mark (Schlumberger) | Homan, Dean (Schlumberger) | Tan, Sarwa (Schlumberger)
A vision in the oil industry for decades is becoming a reality. Finally, we are able to drill and react proactively to formation resistivity properties several meters ahead of the drill bit, instead of reacting to measurements behind the bit. Through a technology collaboration between operating and service companies, a targeted technology development for measuring resistivity contrasts ahead of the bit in real time to reduce cost and risk during drilling operations was developed. Two electromagnetic look-ahead (EMLA) prototypes have been developed for 12¼- to 14-in. boreholes. The EMLA tool is modular and consists of a low-frequency transmitter inserted in the rotary steerable drilling assembly 1.8 m behind the bit and two to three receivers spaced out in the drillstring. The EMLA tool uses the same sensor technology and operates with the same multispacing and multifrequency measurements as the commercial ultradeep “look-around” directional resistivity tool. The formation structure ahead of the bit is interpreted by inversion to differentiate sensitivity around the tool from effects ahead of the bit. The look-ahead capability is dependent on the transmitter-receiver spacing, frequency, resistivity around the tool, thickness of the target, and the resistivity contrast ahead of the bit.
The EMLA tool provides a step change with regard to precision in detecting changes in resistivity properties ahead of the bit in vertical and low-angle wells. The ability to react to resistivity contrasts ahead of the bit has a direct impact on how wells are drilled. The main application of EMLA to date has been to drill the well section above the reservoir closer to top of reservoir to avoid complications in the shale above the reservoir, as presented in two case studies. Challenges related to salt drilling are addressed in another case study where the salt exit was detected 30 m ahead of the bit. Improved precision in coring-point selection is another potential application.
Omeragic, Dzevat (Schlumberger) | Habashy, Tarek (Schlumberger) | Chen, Yong-Hua (Schlumberger) | Polyakov, Valery (Schlumberger) | Kuo, Chih-hao (Schlumberger) | Altman, Raphael (Schlumberger) | Hupp, Douglas (Schlumberger) | Maeso, Carlos (Schlumberger)
Recently introduced deep directional electromagnetic (EM) measurements, along with real-time borehole imaging, enable proactive geosteering, which allows drillers to optimize the well position within the pay zone. The wells are now routinely steered along a path defined by observed reservoir boundaries and fluid contacts rather than by preconceived geometries. This change typically results in increased production, fewer sidetracks, and managed drilling risk through better control of wellbore stability. With increasing operator confidence in these new measurement technologies, they are being used more often in complex reservoirs with uncertain geological structures.
This paper presents detailed sensitivities of azimuthal LWD measurements in complex scenarios. We studied sensitivities of directional EM responses to multiple effects including: nearby boundaries and faults; crossbedding; invasion shape and size; and the effect of invaded fracture swarms with the presence of nearby boundaries. Multiple effects can cause inconsistencies in model-based real-time interpretation and may affect accuracy of inverted distance to boundaries and formation resistivities. Sensitivities of laterolog LWD measurements were also analyzed, proving that bit resistivity may be used to identify faults and to estimate their apparent dip.
The new highly efficient and accurate 3D EM modeling codes are able to handle general 3D problems, going beyond standard layered media plus invasion models used for conventional interpretation in vertical and deviated wells. The modeling library is part of a high-performance computing (HPC) environment, fully integrated with reservoir models. It can be used in pre-job planning, allowing better understanding of measurements. It can also be used to assist in geosteering in these scenarios as well as for detailed post-job analysis and model refinements for improved reservoir characterization in high-angle and horizontal (HA, HZ) wells.
If a simple three-layer inversion model is used, a fault can cause a wrong estimate of the distance to boundaries that are in proximity. We show examples of subseismic fault identification during drilling, and interpretation is refined based on 2.5D modeling, including one or more faults and layering blocks. Excellent reconstruction of directional EM data is achieved by adjusting fault and layering dips and formation resistivities. We also present an example from a deepwater turbidite reservoir, where laterolog LWD measurements and images were used to estimate the fault azimuth and to refine the model built using real-time inversion of directional EM, reducing the uncertainty in structure interpretation. The final fault and layering models were validated using 3D modeling, reproducing all button, ring, and bit resistivities, as well as directional EM responses.
Accurate wellbore placement is vital to the success of any drilling program. Recent introduction of deep directional electro-magnetic (EM) tools has revolutionized well placement (Li et al., 2005). Complemented by real-time borehole images, the new measurements enable proactive geosteering and optimization of wellbore placement within the pay zone. Instead of being drilled geometrically based on preconceived reservoir structure, the boreholes are kept within the sweet spot using real-time inversion-based interpretation and mapping of reservoir boundaries. The new directional EM tool facilitates the capture of attic oil, lessens the risk of exits from the reservoir, and helps to increase production at reduced drilling costs (Wiig et al., 2008).