Deep Drilling In Po Valley : Planning Criteria And Field Results
Authors: Pietro B. , Raffaele D. , Diego G. 
Published: December 2007

The paper briefly reviews, in the first section, the criteria, the methods and the technologies, that AGIP actually applies in planning and drilling deep wells in the Po Valley basin. The discussion, that regards this part, will be necessarily summarized, because a complete and detailed presentation of these subjects appeared recently in literature. In this occasion, anyway, it is worthwhile to recapitulate at least the main concepts, concerning:the methods, that allow accurate prediction and evaluation of the overburden, pore pressure and fracture gradients from the analysis and elaboration of seismic data, drilling parameter sand electrical logs;the criteria of application of this information to well planning, in particular for what adequate and justifiable selection of casing points, casing design, fracture jobs, mud and cement technologies etc., is concerned.

In the second section, the paper describes in detail the field results of three particularly significative and among the deepest wells drilled by AGIP in the Po Valley. The cases discussed put into evidence the satisfactory agreement observed between well planning and field results, in spite of the depth, planning and field results, in spite of the depth, abnormally high pressures, temperatures, hole problems implied, that make the Po Valley basin among the most difficult and interesting areas in the world.

Introduction

Oil exploration in Italy has always to face remarkable technical problems due to the particularly complex geological situation of the country. This complex situation, of which the Alps and the Appennines are the most significative outcropping examples, continues also underground in plain areas, as in the case of the Po Valley basin.

The Po Valley basin, with an area of 46,000 km2, a length or 400 km and a width varying from 80 km to 270 km, is the largest plain in Italy; it is crossed, by the Po river and is almost completely covered by a thick alluvium. The main deposition is formed by clastic sediments from Oligocene to Quaternary, originated by the erosion of ancient and recent relieves. These sands, marls and shales are lying on deeper Mesozoic carbonates, the most interesting of which, from the hydrocarbons point of view, are the dolomites of the Triassic "Dolomia Principale" formation.

Up to the last decade, AGIP activity in Po Valley was essentially directed to Pliocene and/or Miocene targets; only recently, when new geophysical techniques of acquisition and processing of seismic data were introduced, the deeper Mesozoic targets started to be faced. At this point, AGIP had to solve such problems and situations as to be obliged to review most problems and situations as to be obliged to review most of criteria and methods, known from the traditional literature.

Tn the 1970-72 period, sixteen wildcat wells were drilled at depths varying from 4,000 to 6,000 m; in 1972 the drilling of the well Malossa 1 led to the discovery of the first field in the Po valley basin producing from the "Dolomia Principals" formation. producing from the "Dolomia Principals" formation. The Malossa field is at present under development; thirteen wells have been already drilled or are under way to be completed on the structure, at depth from 5,500 to 6,500 m about.

AGIP deep drilling programme, on the basis of the Malossa success, received a very strong impulse; up to now, more than 40 wells reached depths higher than 5,000 m.