Look-Ahead VSP” A Key Tool for Offshore Exploration and Development. Field Case – Shallow Waters Mexico

Author: John Arenas (Baker Hughes, a GE Company, Co-authors: Héctor Andrade (Baker Hughes, a GE Company), Gabino Cruz (PEMEX), Andrés Cabrera (PEMEX), Jesús Chico (PEMEX)

CMP2019_46 Artículo presentado en el CMP / 2019

Summary

Zero offset Vertical Seismic Profiling (ZVSP) is a very wellknown tool for time to depth calibration. This kind of geometry is specially demanded at the early stages of field development, when velocity models may still be under refinement. Borehole seismic surveys as such are usually scheduled with the wireline logs at the last section of the well, once the drilling program has already been completed. One of the questions from the ZVSP deployment is how this technology can provide decision support before the drilling is complete. Here we present a field case offshore Mexico, in the Tabasco coast shallow waters, where a ZVSP was successfully used as a geo-stopping technology in a “look-ahead” fashion, enabling the identification of an unexpected salt body ahead of the bit and a potential missing geologic formation in the sequence, which in turn allowed on-time well repositioning.

Introduction

The information acquired in exploratory and development wells is fundamental for the drilling design of new locations. Characterization carried out from well logs (i.e. MWD, LWD, PWD, SL, WL, etc.), and the subsurface models they allow to build or update, feed future drilling programs. It is during the generation of these programs that the need for information becomes more evident, since the uncertainty that models entail in terms of structural arrangement of the sequences to drill, for instance, is due to the quantity, representativeness and quality of the data these models were generated from.

Vertical Seismic Profiles (VSP) are among a set of technologies called to reduce the uncertainty in structural and velocity models, due to their capability to provide subsurface reflectivity, image and velocity information. They are necessary to consistently match well data with associated reflectors in 3D seismic volumes (i.e. deviation surveys, well logs, formation tops, drilling events, etc.), and essential in the early stages of field development for precise migration or time-depth conversion models construction. However, the acquisition of VSPs in offshore wells is commonly avoided, or it is just considered as optional on drilling programs, relegating them to the final stages once the well is complete, which limits the likelihood of technology applications while drilling.

How to extend its applicability to drilling operations? How this technology can provide decision support even while drilling? To emphasize the relevance of vertical seismic profiling applications for field appraising and development, and to illustrate why VSP acquisition is justified while drilling complex geological structures offshore during intermediate sections, a field case of the Tabasco coast shallow waters is presented. Here, a zero offset VSP was deployed with a look-ahead approach. VSP allowed on time well repositioning since it enabled the identification of a potential missing geologic formation in the sequence and an unexpected salt body ahead of the bit.

Look-Ahead Method

Vertical seismic profiling is a very well-known seismic technique (Hardage, 1985). VSP captures the underground formations’ reflectivity the same way surface seismic does in reflection method, but with a different configuration; receivers are placed in a wellbore rather than on the earth surface. This geometry advantage provides borehole seismic with unique attributes; closer reflections to the boundaries that generated them, precise source signature in downgoing wave trains, shorter travel paths, and less attenuation, which translates into higher resolution. The profile itself is composed of a series of complete seismograms recorded at multiple receiver stations downhole. Arrival times at each station enable seismic average and interval velocities calculations between the surface source and receivers

at known depths, which is very important for inversion purposes. Survey traces are also used to build a reflection image below each of the receivers and can be inverted to acoustic impedance above and below the deepest one.

Either 1D corridor stack or 2D image from VSP data may be inverted to acoustic impedance using sparse spike inversion (SSI) algorithms (Oldenburg et al. 1983). In this process, a couple of already known velocities and densities from a characteristic lithology change are used to calibrate a reflection coefficient. The calibrated value is used to generate the whole reflectivity series. The series is then combined with a low velocity trend (commonly derived from the VSP T/D relationship) to generate a pseudo acoustic impedance trace, which is then integrated from the calibration point or any other specific velocity contrast to predict the depth of seismic events below TD. This prediction is frequently cited as Look Ahead. (Stewart and Di Siena, 1989; Payne, 1994; Brewer, 2002; Planchart et al. 2017).

A Zero Offset VSP 3C dataset needs to undergo significant signal processing before a representative suitable-for-inversion trace above and below TD can be attained. From the time picking of wave front incidence, to the wave field separation and deconvolution, each filter and processing methodology must be carefully selected to assure upcoming primary reflected energy is extracted, seismic amplitudes are properly treated, and true subsurface reflectivity response can be retrieved in a zero phase corridor stack trace. Inversion itself involves several assumptions, and therefore depth prediction reliability decreases significantly with depth below the bit, hence preprocessing stages must be defined in such a way that no more uncertainty/bias is added.

Field Case - VSP Shallow Waters Mexico.

In 2018 the first two development wells of an offshore block in the Tabasco coast shallow waters were spudded; Well-A and Well-B. Drilling started in both wells almost simultaneously from the same octopod. These wells shared upper Jurassic Kimmeridgian carbonates as the target and for the intermediate 12 ¼” section they both were planned to cross an abnormal pressure zone, mainly made up of shale sequences of late Miocene to early Eocene age, finalizing once a transition zone into lower Paleocene carbonate rocks had been reached.

Contrary to finding the lithology changes described at the depths expected by prognosis, Well-B, the deepest of the two 12 ¼” sections, ran into an unexpected halite body, more than a hundred meters above the transition zone, before getting even close to section’s TD. Drilling at Well-B continued through salt to a planned Cretaceous marker depth without evidence of carbonates. Before Well-A had yet reached the upper Paleocene top, both drilling operations were temporally suspended (Figure 1). The idea of a VSP to evaluate relocation chances for Well-A was suggested.

A ZVSP with 210 downhole levels was then conducted at Well-A for corridor stack and look-ahead purposes (Figure 2). Seismic traces were recorded with a five 3C geophone receiver array along an open section of 2,535.0 m MD and 19.78° of inclination (azimuth of 324°), in the 12 ¼” section. Twenty-one additional stations were also acquired in the shallower cased sections for velocity survey control.

The processing sequence included time break picking, stacking, source-receiver geometry assignment, first arrival picking (onset), true vertical and horizontal rotations, velocity analysis, spherical divergence compensation and trace balance, wave field separation by parametric inversion (Lou et al. 2013) and median filters, deterministic deconvolution and wave field enhancement (Figure 3).

Lower Eocene top was chosen as SSI calibration point. The VSP spectral response was limited to a bandwidth of 12 - 50 Hz and -56.4 dB, while the low frequency velocity model was constrained to 4.7 Hz and -56.6 dB, based on data quality. Recursive integration started from the same calibration point until the whole trace velocities and depths were inverted above and below Eocene marker. Once inversion was completed a 2D VSP-CDP image was also generated for an integrated interpretation (Figure 4).

VSP inversion for Well-A predicted a possible top of salt 400 m below TD, with an uncertainty of +/- 21.4 m. The sequence from the Upper Jurassic Kimmeridgian to the late Paleocene was interpreted as incomplete, since the number and shape of the events observed in the VSP stack trace and 2D image didn’t match expected response. JSK was apparently absent, and salt event might have been responsible for this. Well-A planned survey seemed to take it directly to the salt body, and so a side track was required to reposition the well, to a different azimuth away from the risk zone, decision made based on Look-Ahead results.

Figures

Figure 1. Schematic location map.

Figure 2. Raw 3C data from Well-A zero Offset VSP. Vertical (top), horizontal HY (middle) and horizontal HX components (bottom). Data aligned at 200 ms.

Figure 3. P wave reflections enhanced after deconvolution. Well-A VSP. a) Frequency spectrum. b) Trace amplitude display.

Figure 4. Well-A SSI composite display. Corridor stack trace (left), surface seismic and VSP-CDP transform image overlay (center), and VSP sparse spike inversion (right). Eocene calibration event, 12 ¼” section TD, and possible top of salt markers are pointed out. Jurassic Kimmeridgian (JSK) to upper Paleocene (PS) sequence interpreted is also circled.

Conclusions

The field case presented demonstrated the relevance and applicability of VSP technology in drilling operations and how it can help define from casing points to side tracks. In this case, Look-Ahead VSP enabled on time JSK absence identification at Well-A and a salt body ahead of the bit, which in turn supported well repositioning decision.

Acknowledgments

We would like to thank Pemex for allowing us to write and present this field case, and everyone involved in the preparation of this work from Baker Hughes, a GE company

References

Brewer, R. J. [2002] The Look Ahead VSP Survey: Its Utility and Future. Search and Discovery Article 40059.

Hardage, B.A., Toksöz, M.N., and Stewart, R.R. [1985] Vertical seismic profiling, part A: principles. Geophysical Press, London.

Lou, M., Campbell, M., Cheng, D., and Doherty, F. [2013] An improved parametric inversion methodology to separate P and SV wave fields from VSP data. SEG Technical Program Expanded Abstract, p. 5087-5091.

Oldenburg, D., Scheuer, T., and Levy, S. [1983] Recovery of the acoustic impedance from reflection seismograms. Geophysics, 48 (10), p. 1318-1337.

Payne, M.A. [1994] Looking ahead with vertical seismic profiles. Geophysics. 59 (8), p. 1182-1191.

Planchart, C., Sierraalta, R., Saadan, S., Brown, J., and Mengual, J.F. [2017]. Pore Pressure Prediction Using Look-Ahead Vertical Seismic Profile VSP - A Case Study from the Red Sea, Saudi Arabia. SPE Middle East Oil & Gas Show and Conference.

Stewart, R.R., and Di Siena, J.P. [1989] The values of VSP in interpretation. The Leading Edge, 8 (12), 16-23.

Authors

John Arenas. With more than 9 years in the Oil and Gas Industry mainly in borehole seismic data processing and acoustic log processing, John currently supports the Geoscience and Petroleum Engineering group of Baker Hughes in Latin America as Lead Geoscientist. He holds an undergraduate degree in Petroleum Engineering and a M.Sc. in Geophysics. He started his career in the Laboratory of Petroleum at the National University of Colombia for two (2) years, and has worked for Occidental Petroleum – OXY, Schlumberger, and the National Hydrocarbons Agency of Colombia. In 2013 he joined Baker Hughes in Colombia as geophysicist for borehole seismic processing, also supporting commercial and sells area for this segment too while in the country. In 2016 he moved to Mexico where he keeps performing as a processor for GPE.

Héctor Andrade. Exploration and production geophysicist with 12+ years of experience on G&G projects in: Gulf of Mexico, Brazil, Ecuador, Colombia, Argentina, Australia and Venezuela. His expertise goes from seismic interpretation to reservoir geophysics, and exploration and development of structural-stratigraphic traps in classics and carbonates, salt tectonics and geologically complex settings. Before joining Baker Hughes in 2014, he worked for PDVSA in design and processing of 2D & 3D seismic data, specially focused in noise suppression for pre-stacked seismic data conditioning, tomographic static solution and velocity analysis and modeling for Pre/Post-Stack Time migration, as well as integrated velocity modelling for Geomechanical applications and pore pressure prediction.

Gabino Cruz. Graduated as a Geophysicist in 1995 and joining Pemex the same year, Gabino has had a leading part in appraising and development of many of the Pemex offshore assets in Litoral de Tabasco, the shallow waters region in front of the Tabasco State coastline. He has worked in seismic interpretation and geophysical characterization supporting new location definition in Xanab, Yaxche, May, Bolontiku, Sinan fields, among many other. His current role is Superintendent for near fields and development new locations definition.

Andrés Cabrera. Part of the young professional workforce in Pemex, Andres graduated as Geophysicist from UNAM in 2014 and one year later started his career as seismic interpreter in this national oil company. Since then he has been supporting Litoral de Tabasco production asset of Pemex.

Jesús Chico. Another member of the young professionals’ team in Pemex, Jesús graduated as Geophysicist from IPN in 2014 and one year later started his career as seismic interpreter in Halliburton, from where he jumped to the National Hydrocarbons Commission in 2016. He joins Pemex in 2018 supporting Litoral de Tabasco production asset of Pemex as well.

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