Assessment of Conductor Setting Depth

Abstract

ABSTRACT This paper reviews the methods generally used by oil companies to determine the conductor setting depth required to avoid hydrofracture of cohesive soils during drilling for the first casing string. Traditional approaches are compared with an approach developed by the authors, and the results of each method are compared with teat data obtained during geotechnical site investigations offshore. A case history is presented which shows the effect of the authors' design method on the required conductor setting depth, and indicates the considerable possible cost savings and safety benefits available from effective conductor design. INTRODUCTION The advancement of any kind of borehole is dependent on the cuttings being continually cleared from the bit face. This is usually achieved by direct circulation drilling, circulating fluid to the bit through the drillstring with the returning fluid and cuttings passing up the annulus between the drillstring and the borehole or casing. If the marine casing is not set deep enough, the pressure of the drilling fluid may lead to formation breakdown and loss of circulation. Apart from difficulties in then advancing the borehole, this may also result in not being able to monitor and control shallow gas effectively. Formation breakdown can also lead to wash out and lees of support for the foundation of a structure. Correct assessment of the required conductor setting depth may therefore have not only economic but also safety implications for the well-drilling operation. One possible method of determining required setting depth is to perform hydraulic fracture testing (HFT's) in the field. This may however prove to be both costly and in some cases difficult to perform. Where fracture test data are not available, analytical methods have traditionally been based on excess fluid pressures not exceeding the minor principal coil stress, These traditional methods are suspected to give unnecessarily deep setting depths, but no theoretically sound and practically proven method of calculating shallower setting depths is known to the authors. From an oil company's point of view, the derivation of a reliable analytical tool that is less conservative than the traditional methods could lead to considerable savings. It is, however, important that any such method should not underpredict the required setting depth, since the cost and safety implications of such underprediction may be considerable. A detailed review of field tests has therefore been performed to assess the reliability of the new approach proposed in this paper. FIELD TESTS During the geotechnical site investigations performed for platforms in the North Sea hydraulic fracture tests (HFT's) are often performed in order to determine "in situ" the fracture pressure which causes formation breakdown at various depths below mudline. The test is most frequently performed in hard clays using the type of apparatus presented on Fig. 1. The typical procedure for performing such tests in the North Sea is as follows:The borehole is advanced to the required depth using open hole drilling with returns to mudline, with the bottom assembly including the test apparatus as shown in Fig. 1.