Case History of a Low Permeability Cretaceous Offshore Reservoir Developed Using Horizontal Wells-Lebada East, Romania First Part: Geology, Reservoir Description and Performance Prediction
Authors: Deliu Gabriel , Baleanu Ion , Vasiliu Grigore , Supeala Gheorghe , Adamache Ion 
Published: December 2007

Through the oilfields located on the Black Sea Continental Platform can be found Lebada East oilfield. The Upper Cretaceous reservoir consists of microfractured limestone with very poor flowing properties.

Due to a very low permeability, this oil field cannot be developed using conventional vertical wells. For this reason, a team of specialists from our company, has made a study in order to make the exploitation feasible. The solution was to drill a horizontal well.

This oilfield was put into production in 1993 using the first horizontal well that has been drilled in Romania.

In the first part of our article, we present the geological aspects of the oilfield.

The second part of the article proposes a new model for inflow performance calculations. The contribution of the model, consist in taking into account the real shape of the well - snake like - having as background the model developed by Babu and Odeh, and adapted to the new conditions.

The wellbore splitting into small cells and taking into account of geological factors, leads us to a more accurate prediction of the horizontal well productivity.

The model offers the possibility to calculate the fluid influx along the wellbore being useful in discovering of the intervals from which most of the fluid is coming from. Also, can be determined the intervals in which the water will breakthrough first due to coning effects.

In the third part are presented the economics calculations for this well and the plans for future development of this oilfield using horizontal wells.

Introduction

On the western Romanian Black Sea Shelf, in 1979, the third off-shore well drilled, discovered Lebada oil and gas bearing field, at 60 kilometers distance from the shoreline, in a 50 meters water depth., Fig. 1.

The Upper Cretaceous reservoir was tested first time in '80 using vertical wells. Production flowing tests and reservoir studies proved a inefficient production. Later, the horizontal drilling technology, gave the opportunity to test again the economic efficiency of oil production from this reservoir. As a result, in 1993, a horizontal well was drilled and since 1994 the Upper Cretaceous reservoir has been in production.

In this article we present how it was estimated the initial oil rate for this horizontal well using two classical methods, Joshi and Babu & Odeh, and after that, based on the geometry of the borehole and geologic information, a new method for initial rate estimation immediate after the well is drilled (but not completed) is presented.

Comparisons between these methods and their impact to the economic aspects are also presented.

Geological settings

Lebada Field is located on the continuation of the onshore orogenic North Dobrogea zone.

Sedimentary column of the field is composed from Triasic to Quaternar aged formations. Reverse and normal faults affects Triasic to Oligocene deposits.

Upper Cretaceous reservoir.

Upper Cretaceous reservoir is situated at about 2000 meters depth and has an anticline shape. A reverse fault is crossing over the northern, more tilted flank of anticline, Fig. 2.

Lithologically, the reservoir mainly consists of limestone (micritic, biogenic and pelitamorphic). Quartzitic sandstones with calcitic, quartzitic or argilitic matrix and conglomerates are locally interbedded with limestone.

The reservoir quality is generally poor. Core analysis showed several types of porosity: fractural, fissural, intergranular, moldic, vuggy and intragranular. The values of core porosity ranged between 3 and 23 %. Related to this porosity variation, permeability showed very small values from 0.1 mD to a few mD. The analysis performed, showed a good correlation between the zones with low porosity and the zones with low permeability.

The log analysis showed an important secondary porosity.

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