High Pressures Encountered While Drilling In Greece

Causes of High Formation Pressures in Deep Drilling in Western Greece A. Mavromatidis, Petroleum Development Oman LLC , Muscat, Sultanate of Oman, [email protected] V. C. Kelessidis, Technical University of Crete, Polytechnic City, Chania, Greece, [email protected] D. Monopolis, Technical University of Crete Paper presented at the AAPG International Energy Conference & Exhibition, Athens, November 18-20, 2007.

ABSTRACT High formation pressures while deep drilling thick evaporitic (mainly anhydrite) sections occurred in northwestern Greece. The stratigraphy of the area is composed of Jurassic to Tertiary carbonates underlain by Triassic evaporites. The well structure was a sub-evaporite high, with uncertainty as to the nature of the interval beneath the Triassic evaporites. During drilling at the depth of ~3,900 m in the evaporitic section high pressures were encounted and the well was killed. The well was sidetracked at shallower depth (~2,900 m) but at ~3,600 m, high formation pressures were encountered again, the well was found flowing and finally it was shut, plugged and abandoned. Possible causes of the high pressures could be: flysch overpressure, laying beneath the evaporites, which could somehow charge dolomitic lenses within the evaporates via faults; intra evaporite overpressure due to earlier isolation of carbonate lenses/rafts within anhydrite while they were at greater depths, followed by uplift of those lenses to the present (shallower) depths; and/or the result of formation water migration from under laying potential oil-gas bearing formations which has accumulated due to the evaporites seal. Suggestions are made for possible actions to be taken, as implemented in similar situations, in order to overcome these difficulties for future drilling activity, which must continue because viable play may exist underneath the evaporites. INTRODUCTION The main objective of this article is to emphasize the drilling difficulties experienced for the first time while drilling a deep exploration well (EW-1) in the Ionian Zone in northwestern Greece in an evaporitic section (Figure 1). Abnormally high pressures in thick evaporites are common and difficult to predict drilling hazards due to these pressures can cause exploration delays and eventually abandonment of fields. This article discusses possible causes of high pressures, and presents drilling techniques for future deep exploration wells in order to eliminate the danger and risks that high pressures can cause to exploration projects. The Ionian Zone has a long history of exploration, but with few exploration wells, and hosts the Katakolon oil-gas field (Mavromatidis, 2005) as well as numerous oil seeps on its surface (Figure 1).

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Figure 1. Simplified surface geology of western Greece (significant modification after IGME, 1983). Chronostratigraphic summary of different areas in Ionian Zone and pre-Apulian is also shown. Summary was derived from well reports, outcrop sections and seismic data. Major wells and surface oil seeps are also shown (Ait-1 = Aitolikon-1, As-1 = Astakos-1, De-1 = Demetra-1, Fl-1 = Filiates-1, Ga-1 = Gastouni1, Ka-1 = Katakolon-1, Ke-1 = Kelevi-1, Lky-101 = Loutra Kyllinis-101, Pa-2 = Parga-2, Px-1 = Paxi-1, So-1 = Sosti-1, SK-1 = South Katakolon-1, WK-1 = West Katakolon-1). REGIONAL SETTING The location of the well is in the Ionian Zone (Figure 1). The Ionian Basin is located in the western part of the Hellenide fold and thrust belt, which was developed during the collision and continued convergence of the African and Eurasian Plates from the Mesozoic. Deformation associated with this convergence was expressed as a progressive westward migrating deformation front, which compressed the previous predominantly extensional basin and platform morphology (Clement et al., 2000). 2

The Ionian Zone consists of three main stratigraphic unit components. The Triassic evaporites and carbonates, the Jurassic-Cretaceous carbonates with shaly units and the Tertiary clastics (mainly flysch and molasses) and carbonates (mainly limestones) (Figure 1). Representative thickness derived from drilled sections and outcrops are averaged in Figure 1. Detailed examination of numerous wells that penetrated the Ionian Zone show that Mesozoic thickness varies from 1.2 to 3.8 km in the north, between 1.1 km to 3.5 km in the central region and from 1.5 to 3.9 km in the south. However, data from seismic sections (mainly from north and central Ionian Zone) show that thickness may be up to 8 km (Mavromatidis et al., 2004; Mavromatidis, 2005). Similar maximum values (of 10 km) have been reported in Albania (Velaj, 2001). Little is known about the pre-Mesozoic evolution of western Greece owing to the fact that pre-Mesozoic rocks are neither exposed at surface, nor penetrated by boreholes. The oldest documented rocks are the Triassic evaporites, which are strongly tectonised and dominated by anhydrite, gypsum and halite in some wells (e.g. Astakos-1). These evaporites have been assigned to the pre-Ladinian age (Jenkins, 1972). DRILLING OPERATIONS AND HIGH FORMATION PRESSURES The EW-1 well was drilled in the eastern part of the Ionian zone, within the Hellenide fold and thrust belt (Figure 1). The well structure was prognosed as a sub-evaporite high, with uncertainty as to the nature of the interval beneath the Triassic evaporites. There were reports that while drilling the vertical well EW-1, high pressures were encountered, of equivalent mud densities of 16.5 ppg which resulting in a kick at ~3900 m, eventually killed with a mud weight of 17.5 ppg. A side-track well was drilled, EW-1S, with a kick-off 1,000 m above, but while drilling at ~3,500 m the well kicked again and was finally killed with a mud weight of 17.6-17.9 ppg (10,000 to 11,000 psi). The well was finally plugged and abandoned. We set here below to examine factors that may have generated these high pressures in such a geological environment and propose procedures for future drilling activity, which should continue as possibilities for a reservoir beneath the evaporites do exist. Causes of overpressure in geological sequences are numerous but may be classified into three general categories (Young and Lepley, 2005): undercompaction, fluid expansion and tectonic activity. The first category, occurs in young, clay–sand sequences that had experienced rapid sedimentation loading. Fluid expansion may be due to thermal effects, clay diagenesis or hydrocarbon maturation. Tectonic events may include cross flow along faults, overthrust faulting or compressional loading. Swarbrick et al. (2002) similarly classify two causes of overpressure, compaction and hydrocarbon generation. It appears that in EW-1 and EW-1S, fluid expansion and tectonic activity could be the causes of high pressures since formations are not of young age nor of a clay-sand sequence. Seven previous wells had drilled through the evaporites but never penetrated them. Furthermore, there were no reports of pressures of the magnitude encountered in EW1. Delvinaki-1 (Figure 1) did encounter “lenses of overpressured dolomite” but with a maximum pressure of 11.0 ppg equivalent mud weight. 3

There may be two ‘external’ causes: (a) Dolomitic lenses within the evaporites which, at some stage, may have become charged, say via faults, from flysch overpressure due to rapid burial. Flysch can be met below the evaporites according to major or nonmajor shortening. In this flysch, smectite to illite transformation (clay dehydration) expels water resulting in generation of pore fluids and leads to overpressuring if the permeability is sufficiently low to retard the fluid escape (Swarbrick et al., 2002). (b) Intra-evaporite overpressure due to earlier isolation of carbonate lenses/rafts within anhydrite (& minor halite) while they were at greater depths, followed by uplift of those lenses to the present (shallower) depths & hence overpressure. The initial cause of the overpressure (prior to isolation) may or may not have been part due to dilation and fracturing of evaporites during the thrusting events creating the possibility of fluid intrusions via connected systems of micro-cracks. After confining, the pressure in these inclusions may have been even higher than in the subevaporitic reservoir and may be close to lithostatic. The evaporitic layers controls the development and distribution of the ovepressures (Figure 2a).

Figure 2. Sketch of EW-1 and EW-1S including possible lithology and faults of the surrounding area when (a) pressures appear inside the evaporites away from their bottom and (b) pressures appear close to the bottom of the evaporites. More causes can be reported if the depths of the high pressures appear close to an environment where there is change of lithology from evaporites to other lithologies.

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Indeed, overpressured fluid accumulations are usually encountered immediately underneath the evaporitic section which is drilled easily and at high rates (Zilberman et al., 2001). This part of evaporites could be a transition zone from evaporites to a carbonaceous lithology. Stringers may have been deposited originally as large carbonate platforms that were subsequently deformed or completely fragmented due to salt or evaporites movement. Dolomitic lenses have created high pressures in Delvinaki-1 although these pressures were much lower. Overpressures in the stringers occur as a result of intra-stringer charge. This means that the source rock along with the reservoir is within the stringer. Stringers can be very rich in organic material and together with the source rock, coming from deeper pre-evaporitic unit, provide the fluids (Figure 3b). If stringers are completely encased in evaporites the pressures have built up within the stringers during charge. Unfortunately stringers are difficult to be seen in seismic images. Amongst the possible causes of abnormal pressures could be hydrocarbon generation (Swarbrick et al., 2002). The depths where high pressures were encountered indicate that hydrocarbon generation could be a probable reason of overpressure generation since drilling was through old sediments. However, there were no hydrocarbons in both wells. But if we are close to the base of the evaporites, then gas can exert such pressures to the overlying evaporites. If there is gas in a carbonaceous and/or clastic reservoir section then we would expect to see overpressures in similar depths. Hydrocarbons can be transferred into the reservoir, which is flysch or carbonates, via fault or fracture permeability. And while it is true that such high pressures have not been encountered in other wells in western Greece, wells EW-1 and EW-1S were the deepest ones that have ever been drilled within the evaporitic section and it may be probable that the wells reached the closest ever depth to a possible gas play. If we are close to the base of the evaporites then, we may have reached the detachment level, that occurs in overpressured sediments beneath the impermeable evaporites and is characterized by extreme fluid overpressuring (Reston et al., 2002). Fluids in such environment creating such pressures are expected to be water and/or gas, as previously described. CONCLUSIONS AND RECOMMENDATIONS Despite all pre-drilling preparations, unexpected high pressures can be encountered, as it happened in western Greece, and it can result in abandonment. Proper postdrilling analysis could allow for assessment of causes of overpressure and better preparation in future drilling activity. Drilling through the thick evaporitic zone in western Greece and into the underlying potential hydrocarbon reservoir could be achieved with the use of advanced drilling technology and the better formulation of drilling fluids, capable of operating with minimal problems under these harsh conditions. The availability of better detection pore pressure mechanisms, like ability to see ahead of bit as well as improved well control procedures could enhance drilling success. But for this to happen, access to drill data from recent drilling excursions in western Greece should become available for full evaluation of the challenges ahead. No well has ever penetrated the whole Triassic evaporitic strata in the Ionian Zone. It is suggested that detailed geophysical studies with specially designed parameters tailored for deep prospects are important to be undertaken not only for the Ionian Zone but generally for western Greece, such as the pre-Apulian Zone, which has 5

similar lithology to the Ionian Zone, and the Gavrovo Zone. Furthermore, geochemical analyses of outcropping rock samples well cuttings and existing oil shows will provide a further insight of oil generation and migration. These studies must trace the deep evaporitic strata and target areas where the evaporites will be fully penetrated. Drilling in western Greece should continue because, while there are some 25 oil and gas fields in Albania, only one discovery has been made so far in western Greece, that at west Katakolon, in offshore Peloponnesos region. This is highly significant as it proves the existence of a viable play and its continuation throughout western Greece. REFERENCES CITED Clement, C., A. Hirn, P. Charvis, M. Sachpazi, and F. Marnelis, 2000, Seismic structure and the active Hellenic subduction in the Ionian islands: Tectonophysics, v. 329, p. 141-156. IGME (Institute for Geology and Mineral Exploration), 1983, Geological map of Greece, Scale: 1:500000, in J. Bornovas and Rondogianni-Tsiambaou, eds., Division of general geology and economic geology: Institute of Geology and Mineral Exploration. Jenkins, D., 1972, Structural development of western Greece: AAPG Bulletin, v. 56, p. 128-149. Mavromatidis, A., 2004, Review of the sub-evaporitic lithology in the Ionian Basin, Western Greece and hydrocarbon prospectivity, in S. Pavlides and A. Chatzipetrou, eds., 5th International Symposium on Eastern Mediterranean Geology: Thessaloniki, Greece, v. 3, p.1435-1438. Mavromatidis, A., V. C.Kelessidis, D. G. Monopolis, 2004, A review of recent hydrocarbon exploration in Greece and its potential, in Z. Agioutantis and K. Komnitsas, eds., 1st International Conference on Advances in Mineral Resources Management and Environmental Geotechnology, 7-9 June, Hania, Greece, Heliotopos, p. 187-194. Mavromatidis, A., 2005, Hydrocarbon potential of western Greece: First Break, v. 23, p. 37-39. Reston, T.J., R. Von Huene, T. Dickmann, D. Klaeschen, and H. Kopp, 2002, Frontal accretion along the western Mediterranean Ridge: the effect of Messinian evaporites on wedge mechanics and structural style: Marine Geology, v. 186, p. 59-82. Swarbrick, R. E., M. J. Osborne, and G. S. Yardley, 2002, Comparison of overpressure magnitude resulting from the main generating mechanisms, in A. R. Huffman, and G. L. Bowers, eds., Pressure regimes in sedimentary basins and their prediction: AAPG Memoir, v. 76, p. 1–12. Velaj, T., 2001, Evaporites in Albania and their impact on the thrusting processes: Journal of the Balkan Geophysical Society, v. 4, p. 9-18. Young, R.A., T. Lepley, 2005, Five things your pore pressure analyst won’t tell you. Paper: AADE-05-NTCE-12 presented at the AADE 2005 National Technical Conference and Exhibition, Houston TX, April 5-7. Zilberman V.I., V.A. Serebryakov, M.V. Gorfunkel, and G. V. Chilingar, 2001, Prediction of Abnormally High Formation Pressures (AHFP) in petroliferous salt-bearing sections: Journal of Petroleum Science and Engineering, v. 29, part 1, p. 17-27.

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