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Kelsall, Neil R (Schlumberger) | Euranie, Ludivine C (Schlumberger) | Kocsis, Gábor (MOL Norge AS) | Jørgensen, Torsten (Aker BP) | Adestal, Victoria (MOL Norge AS) | Kallhovd, Anders (Schlumberger) | Liman, Tunc (Schlumberger) | Stoia, Irina-Tulia (Schlumberger) | Rajan, Digvijay (Schlumberger)
This paper demonstrates how an operator mitigated a hazardous pore pressure drilling risk using a technology that was new to the operator. The Seismic While Drilling (SWD) increased safety and efficiency at lower cost over commonly used alternatives. In 2019 MOL Norge AS drilled a deviated exploration well in the southern part of the Norwegian North Sea. The large 300 bar pore pressure uncertainty in the reservoir made it necessary to land the 9 7/8-in casing shoe as close as possible to the reservoir to maintain a safe kick tolerance. The pre-drill depth uncertainty of +/- 75m at the reservoir made it necessary to find a reliable technology to significantly decrease this depth uncertainty.
Borehole seismic technology enables measurement of seismic velocities, which is the main culprit in seismic depth uncertainty. Measuring the time taken for a seismic pulse from a surface airgun to reach a downhole sensor provides velocity information which can provide a significant reduction in depth uncertainty while drilling. The closer the seismic sensor is to the formation of interest then the more accurate the depth prediction of the formation top is. In addition, seismic reflections from the formation top can provide a lookahead capability extending to hundreds of meters. SWD technology coupled with 3D ray-trace modelling allowed a survey to be designed for this deviated well using rig deployed airguns rather than deployment from a costly additional vessel. The survey design included real time data during drilling to significantly reduce the depth uncertainty and also image the reservoir top.
The 12.25in hole section was drilled with a seismic logging while drilling (LWD) tool in the BHA, acquiring data during the drilling connections. Vertical Seismic Profile (VSP) processing of the real time data allowed the drilling team to see seismic reflections from the reservoir top during the approach from over 600m away. The decision to stop drilling occurred when the lookahead image indicated the reservoir was at a depth of 3,426 m TVD (+/- 13m), 50m shallower than the predrill plan. The casing point was set at a depth of 3,402 m TVD, approximately twice the depth uncertainty above the lookahead prediction. The actual reservoir top was subsequently logged at 3,425.6m TVD, less than 1m from the predicted depth.
The Schlumberger Oilfield Glossary states that Q is ‘the ratio of the peak energy of a wave to the dissipated energy. As waves travel, they lose energy with distance and time due to spherical divergence and absorption. Such energy loss must be accounted for when restoring seismic amplitudes to perform fluid and lithologic interpretations, such as amplitude versus offset (AVO) analysis’.
In surface seismic, we face the problem of characterizing the effect of anelastic attenuation on the seismic signal during propagation. The Q-factor is sought after because it is useful for amplitude analysis, phase compensation and for improving resolution. Though the Q-factor can be estimated from surface seismic data, traditionally the Q-factor is extracted using acquisition methods relying on accelerometers and/or geophones during borehole seismic surveys such as zero-offset vertical seismic profile (VSP), 2D walk-away VSP (2D WVSP) or 3D VSP campaigns. The reason why borehole seismic is the preferred option to estimate the Q-factor is because VSP surveys sample the wavefield by sensors in-situ (depth control), and also it gives robust measures using the direct arrivals. The Q-factor can then be, used to derive a Q-filter, which can be applied to the surface seismic to diminish the effect of anelastic attenuation and can compensate for the loss of amplitude and frequency in the data as well as to correct for the phase difference.
Several methods of Q-factor determination may be used, each with its pros and cons (e.g., continuous (log) Q-factor using spectral ratio, multi-spectral ratio, continuous interval Q-factor).
With recent advances in the fiber optic field, we anticipate (and already see) a surge in VSP surveys being acquired with distributed acoustic sensing (DAS) or heterodyne distributed vibration sensing (hDVS) as this acquisition method is extremely cost-effective from a deployment point of view and required acquisition time.
We demonstrate that with minimum work, one can provide a robust Q-factor estimation using fiber-acquired datasets.
The PDF file of this paper is in Russian.
There is almost universally a demand for data inversion, both to data from borehole seismic and by analogy with land seismic. In this paper we have considered the feasibility of inversion in the main modifications of the borehole seismic. In case of zero-offset VSP, inversion can be performed to predict the geological section below the bottomhole. The solution of the problem requires the highly skilled performer. It is also possible only if there is data from acoustic and density logging in the drilled part and brute information about the structure of the environment under the bottomhole. The offset VSP modification refers to use methods of full-fold data inversion known in the 2D seimic, but the task is not correct due to large incidence angles varying in a wide range and changes in the recording environment, the effect of which on the wave field far exceeds the effect of changes in the properties of formations. The reliability of the results is, at best, limited to a small near-wellbore zone, which has no practical significance. Modifications of walkaway VSP and VSP-3D are multiple observation systems, but, unlike land seismic, the use of inversion of sums partial multiples is impossible, since the vast majority of space cells correspond to a narrow range of stack angles. The use of inversion of full-fold stacks is also incorrect due to changes in the angles of incidence and the complexity of taking into account the changes in the structure of the upper part of the section. Thus, the inversion of offset VSP, walkaway VSP and VSP-3D data is hardly appropriate.
К материалам скважинной сейсморазведки, как и наземной, практически повсеместно предъявляется требование инверсии данных. Рассмотрена целесообразность инверсии в основных модификациях скважинной сейсморазведки. При продольном вертикальном сейсмическом профилировании (ВСП) инверсия может быть выполнена с целью прогноза строения геологического разреза под забоем. Решение задачи требует высокой квалификации исполнителя и возможно только при наличии данных акустического и плотностного каротажа в пробуренной части, а также априорных сведений о строении среды под забоем. В модификации непродольного вертикального сейсмического профилирования (НВСП) принято использовать известные в методе общей глубинной точки (МОГТ) способы инверсии полнократных данных. Однако эта задача некорректна из-за больших углов падения, изменяющихся в широком диапазоне, и изменения среды регистрации, влияние которых на волновое поле намного превышает влияние изменения свойств продуктивных пластов. Достоверность результатов в лучшем случае ограничена небольшой ближней зоной околоскважинного пространства, не имеющей практического значения. Модификации «метод обращенного годографа» (МОГ) и ВСП-3D являются кратными системами наблюдений, но в отличие от наземных сейсмических исследований применение инверсии частично кратных сумм невозможно, так как подавляющему большинству ячеек пространства соответствует узкий диапазон углов суммирования. Применение инверсии полнократных сумм также некорректно из-за изменения углов падения и сложности учета изменения строения верхней части разреза. Таким образом, инверсия данных НВСП, МОГ и ВСП-3D вряд ли целесообразна.
Gao, Yongde (CNOOC Zhanjiang) | Chen, Ming (CNOOC Zhanjiang) | Du, Chao (CNOOC Zhanjiang) | Wang, Shiyue (CNOOC Zhanjiang) | Sun, Dianqiang (CNOOC Zhanjiang) | Liu, Peng (Schlumberger) | Chen, Yanyan (Schlumberger)
Drilling in Ledong field at Yinggehai basin of South China Sea faces challenges of high-temperature and high-pressure (HTHP). The high pore pressure and low fracture gradient results in a narrow mud weight window, especially when drilling close to overpressured reservoir. Well LD10-C was the first exploration well targeting at reservoirs in Meishan formation. Well LD10-A and LD10-B were offset wells in a distance of 15-20km drilled for reservoirs in Huangliu formation, which is above Meishan formation. During drilling, both wells encountered severe gas kick, mud loss and did not reach target.
In order to drill and complete well LD10-C safely, a real-time pressure monitoring solution was introduced with integration technique of logging while drilling (LWD) and look-ahead vertical seismic profile (VSP). It helped to monitor pore pressure and fracture gradient while drilling and predicted top of the overpressured reservoir. This enabled to keep the mud weight and equivalent circulation density (ECD) within a safe margin to avoid kick and mud loss, helped to set casing as close as possible to the top of reservoir. The reservoir section was drilled with a manageable mud weight window.
The main achievements of this task were: 1) accurately monitor and predicted pore pressure coefficient at reservoir. The predicted pore pressure coefficient was 2.25 SG versus 2.24 SG from actual measurement. 2) accurate prediction of reservoirs top. The predicted top depth of Sand C was 2m error with accuracy of 0.05%. The top depth of Sand D was 10m error with accuracy of 0.2%. 3) 12.25in section and 8.375in section was successfully drilled deeper with pressure monitoring. The 9 5/8in casing was set 491m deeper and 7in line was set 80m deeper than plan. As a result, well LD10-C was drilled and competed without any drilling complexities.
This was first application of LWD and VSP together for pressure monitoring while drilling in Yinggehai basin. The successful completion of well LD10-C confirmed that this integrated solution was an efficient technique to predict and reduce drilling risks, optimize mud weight and casing diagram, improve operational safety and save cost in HTHP offshore drilling.
Distributed acoustic sensing (DAS) is a rapidly evolving fiber optic technology for monitoring cement curing, perforation performance, stimulation efficiency, and production flow and, more recently, for performing vertical seismic profiling (VSP). VSP data can be acquired and processed to determine velocity models that are used in surface seismic imaging for reservoir characterization, or for microseismic monitoring of hydraulic fracturing operations. The limitation of conventional VSP data acquisition has been well accessibility, with wireline-conveyed tools deployed during openhole or casedhole logging campaigns before well completion or during workovers. Fiber optic cable conveyance by coiled tubing (CT) expands the opportunity for VSP data acquisition during planned CT interventions. This paper presents an example of a CT DAS VSP acquisition. The processing steps are shown to overcome some of the noise challenges inherent in CT DAS data, such as persistently strong borehole tube waves induced from the surface operations activities. A case study is shown for the depth tie between surface seismic data and the CT DAS VSP derived corridor stack image, demonstrating the viability of CT deployed fiber to acquire DAS VSP data.
Ferla, Maurizio (Eni) | Miranda, Francesco (Eni) | Nutricato, Giacomo (Eni) | Galli, Giuseppe (Eni) | Moriggi, Sara (Eni) | Malossi, Alfio (Eni) | Guglielmo, Carmelo (Eni) | Tesconi, Marcello (Eni) | Mangione, Alessandro (Eni) | Stocchi, Donatella (Eni) | Naouar, Aymen (Eni) | Bettinelli, Pierre (Schlumberger) | Manai, Nabil (Schlumberger) | Slail, Osama (Schlumberger) | Pagnin, Andrea (Schlumberger)
Efficiency is a key factor on any operation. In this paper, we introduce the heterodyne Distributed Vibration Sensing (hDVS), which is an innovative technology based on fiber optic system to improve the duration of borehole seismic operations.
We designed a survey aimed at comparing standard downhole geophone accelerometers measurements to i) optical fiber seismic installed inside the hybrid Wireline cable and ii) optical fiber clamped permanently to the well completion tubing. This comparison was conducted using a standard rig source VSP in association to advanced Offsets VSP. The purpose of the study was to evaluate this innovative technology and to assess the feasibility of drastic operation time reduction without compromising output data quality.
To better evaluate the readiness of the technology, we decided to compare three distinct types of downhole measurements and designed a specific advanced acquisition which allowed us to compare various configurations. Consequently, the borehole seismic acquisition performed in the MR-SE1 well located in Makhrouga field (Tunisia) was split into two phases. Phase #1: during open-hole Wireline logging, using the standard downhole geophone accelerometers (VSI) and fiber optic seismic cable (single-mode cable) installed inside the Wireline logging cable (called hybrid Wireline cable). Phase #2: at the departure of the drilling rig, using a fiber optic seismic cable (single-mode cable) installed permanently along the intelligent completion. The results highlight the effectiveness of the hDVS technology with a proven decrease on operation timing, with reliable and good SNR recorded data.
Nowadays, efficiency is a key requirement for any data acquisition process. The heterodyne Distributed Vibration Sensing (hDVS) is an innovative technology designed to achieve such effectiveness by making the Vertical Seismic Profile (VSP) a matter of minutes instead of hours, as using standard downhole equipment, without compromising output data reliability and allowing the measurements repeatability (no well interventions required).
Finally, based on the quality of the dataset acquired, further analysis can be conducted for imaging purpose by analyzing the reflected waveforms, which could bring additional information and could change the way we are operating.
From well planning to well execution, the record breaking Mississippi Canyon 822#14 3D Vertical Seismic Profile (VSP) at the Thunder Horse Field in the Gulf of Mexico has been extremely valuable for overcoming resolution challenges and bridging illumination gaps in the surface seismic data. The VSP has been influential in key decisions, such as future well targeting, calling total depth (TD) of hole-sections in a challenging Pore Pressure/Fracture Gradient (PPFG) environment, and framing volumetric upside & downside potential for future wells. The VSP accomplished its primary goals of providing a high-resolution image compared to surface seismic and proving the viability of Independent Simultaneous Source (ISS®) technology for future offshore VSPs, a potential huge cost savings for future acquisitions. The key uncertainties targeted by the well bore survey were thin beds below detection and sub-seismic faults. Additionally, the unique acquisition geometry of the VSP provides illumination uplift over surface seismic that is negatively impacted by current infrastructure and the salt canopy, which overlies approximately one third of the Thunder Horse South structure.
Presentation Date: Wednesday, October 17, 2018
Start Time: 8:30:00 AM
Location: 212A (Anaheim Convention Center)
Presentation Type: Oral
A 3D vertical seismic profile (VSP) was collected in the Stocker Field to help characterize a field in the Bend-Arch Fort Worth Basin. We applied the local phase slowness method to the data to estimate slowness across the receiver array. The local phase slowness method allows for the estimation of anisotropy around the borehole without the need to model the overburden. The constructed slowness curve is then inverted for components of the stiffness tensor. Preliminary results show that the inverted vertical velocities match well with nearby well data. However, the horizontal estimates were less reliable, possibly due to the lack of wide angular coverage.
Presentation Date: Tuesday, October 16, 2018
Start Time: 9:20:00 AM
Location: Poster Station 16
Presentation Type: Poster