Abstract

The Lebada East oilfield - of the Upper Cretaceous formation on the Romanian Continental Shelf of the Black Sea, located at a depth of 2,100 m, is a microfractured type reservoir, with a matrix permeability in the order of tenths of milidarcies and with an equivalent matrix-fractures effective permeability with values comprised between 0.5 and 1.5 mD. The production tests of geological investigation wells have proved that this reservoir can be commercially produced by vertical wells only after performance of some hydraulic fracturing operations in view of obtaining large dimensions fractures, which are operations based on expensive, advanced technologies. In 1993, on this reservoir, the first horizontal well in Romania (Well LO1-Lebada East) was designed, drilled and brought in. The horizontal drilled length of Well LO1 is 650 m. The geophysical investigation logs carried out during drilling point out that only 250–350 m of the drilled length are porous-permeable. The well was completed with a 7 in EL liner, slotted over a length of 588 m. Well rocking to production required stimulation treatments. The efficiency of these new technologies on oil reservoirs with low effective permeabilities - that cannot be profitably produced with vertical wells - has thus been proved. The favorable horizontal well performance opens the prospect of reserves recovery from this productive formation.

Introduction

Horizontal drilling started to be applied in our country in 1993, when the first offshore well, LO1 - Lebada East, was drilled. The LO1 - Lebada East well started the development of the oil reservoir from the Upper Cretaceous on the Continental Platform of the Black Sea, a reservoir which is at an average depth of 2,100 m. The reservoir is characterized by low effective permeabilities, whose production by vertical wells is not efficient and results in a very low recovery. The reservoir is microfracturated (with a matrix permeability obtained during the production tests of the geological exploration wells). The production tests of the geological exploration wells led to the conclusion that this reservoir can be produced on a commercial scale by conventional wells only following the performance of some hydraulic fracturing aimed at achieving large sized fractures. These hydraulic fracturing operations are very expensive and their technique and technology are not available in Romanian oil industry. No significant difficulties were encountered during the LO1 horizontal well drilling and there were no significant differences between the designed and the achieved trajectory of the wellbore. The objective for the LO1 - Lebada East well was the production of the hydrocarbons located in the Upper Cretaceous (Santonian + Coniacian + Turonian) formations.

Drilling and completion

The completion program and the main trajectory elements are presented in Fig. 1. For the LO1 well drilling the designed drilling fluids were used, namely:

  • over the 150 – 1,281 m measurement depth (MD) interval, a seawater based fluid treated with encapsulation polymers, having a maximum density of a 1,250 kg/m3;

  • over the 1,281–1,758 m MD interval, an "INHIB-KCl" type fluid, having a maximum density of 1,550 kg/m3;

  • over the 1,758–3,172 m MD an "INHIB-KCl" type fluid, having a maximum density of 1,350 kg/m3.

P. 799

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