Aspects of Deep Drilling in The Carpathian Basin
Authors: Alliquander Odon 
Published: December 2007

Presented here are some experiences regarding oilwell drilling activity in Presented here are some experiences regarding oilwell drilling activity in Hungary, with reference to improved jet-bit drilling techniques, jet-bit design, drilling hydraulics, drilling control and blowout preventer systems for very high-temperature, high-pressure formations.

Introduction
In Hungary an especially large program of oilwell drilling has been realized in the past 10 years. As a result of certain geological and geothermal conditions prevailing in the Carpathian Basin, valuable prevailing in the Carpathian Basin, valuable experience has been gained in the field of drilling technology. Those conditions arethe occurrence of strata composed of unconsolidated young Upper Pliocene sand and plastic clay that locally are over 2,000 m thick, andanomalous formation pressure and formation temperature values that have been recorded below 2,000 m.
Some of the more important achievements in the field of rotary drilling in Hungary over the past 30 years have involved increasing the speed of drilling and preventing blowouts.
Drilling rates have been increased in much the same way throughout the world; i.e., drilling hydraulics have been developed, at first unsystematically, then on a carefully planned basis, until an optimum value is reached; simultaneously more weight has been p I laced on the bit and the rotary speed has been raised to reach a level of "power drilling".
Increasing the average hole depth, as well as drilling deeper and deeper holes, raised problems of drilling safely and rapidly through high-pressure formations as well as of preventing blowouts. In the course of solving these, the possibility arose of performing underbalanced drilling, which could enable the drilling of very deep holes faster and at lower cost than ever before.
Some factors leading to the need for blowout preventers are that thick and unconsolidated strata under preventers are that thick and unconsolidated strata under tensile stress tend to fracture vertically, and that at depths of about 2,000 m they are exposed locally to overburden pressures on the order of 50 to 60 percent of normal formation pressures; in addition, they percent of normal formation pressures; in addition, they exhibit relatively extreme temperature values. These factors demand a state of balance, at medium depth, between formation pressure and circulating-mud pressure. This could be achieved only with the help of pressure. This could be achieved only with the help of controlling devices, and steps have been taken to re-establish the state of balance that was lost.

Oil and Gas Well Drilling Activities in Hungary
Compared with its areal extent, which is less than 100,000 sq km, a relatively large amount of oil- and gas-well drilling now is taking place in Hungary (Fig. 1).
The resulting hole network, expressed in m/sq km, is equally remarkable. This has been achieved through the use of the jet bit (introduced in 1951 and steadily improved since then), which has been effective even for increasingly deeper holes.
The footage drilled for oil and gas wells (Fig. 1) has exceeded 350,000 m/year (1.15 million ft/year) for nearly a decade, and in fact in 1962 it approached half a million meters (1.64 million ft). 21 This has led to a drilling density exceeding 4 m/sq km for the past decade. (Drilling density for the U. S. is reported past decade. (Drilling density for the U. S. is reported to be about 7 m/sq km, and as much as 8.25 m/sq km for the past 10 to 12 years; for Trinidad, it is about 8 m/sq km; while in Rumania it is on the same level as in Hungary.)
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