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American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. This paper was prepared for the SPE-European Spring Meeting 1976 of the Society of Petroleum Engineers of AIME, held in Amsterdam, The Netherlands, April 8–9, 1976. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal, provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Exploration drilling in the Carpathian Basin is extremely risky in view of the presence of overpressured formations. This can presence of overpressured formations. This can easily cause blowouts. The simultaneous use of drilling rate and modified "d" factor logs combined with a calculated log of the drillability factor has proved to be a suitable method for detecting relatively thin, overpressured transition formations. A complementary method exists in observing change in flowline mud temperature. Bringing the deeper, heavily overpressured reservoirs into production is also a most difficult job. Perforating the casing at a depth of 4,000 m will easily result in a rapid production decline because of the early closing production decline because of the early closing of the overpressured reservoirs and because rock debris fills the well. Acid fracturing and opening only the most permeable intervals seems to be the best method of maintaining well productivity. productivity Introduction The statement that use of balanced drilling technology and avoidance of blowouts are key problems for economically drilling deep and very problems for economically drilling deep and very deep wells is considered valid on a world-wide basis. Doing this in the Carpathian Basin is difficult. This is because some Pliocene formations constitute a thick sequence of rock layers and some lower-Pannonian sandy formations, as well as Pliocene-Miocene base conglomerates and Miocene or older formations lying below them are largely overpressured; and partly because the geothermic gradient there is partly because the geothermic gradient there is of abnormally high value. Fighting against potential kicks-threatening blowouts—is also potential kicks-threatening blowouts—is also difficult because, over the points where kicks occur, the Pannonian formations are in a relaxed state. They tend to be fissured nearly vertically and the fissures starting from the borehole may reach the surface even when formed at great depth. For designing the structure of boreholes, therefore, it is very important to locate initially any overpressured formation at least on a correlation basis, or at a new site, drilling wild-cat holes, by evaluating adequately the obtained seismic data, respectively.
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, and anomalous 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.) P. 125