Selected Items of Interest in Drilling Technology - An SPE Distinguished Lecture
Authors: Goins W.C. 
Published: December 2007

Editor's Note: With this article on drilling technology, SPE offers readers of JPT an insight into the wide range of technical and nontechnical topics presented to the local sections as a part of the Distinguished Lecturer Series. Selected Distinguished Lectures, approved by the Society's editorial committees, will be published periodically in the Journal of Petroleum Technology.

Introduction

When the Society asked me to be a Distinguished Lecturer on the subject of Drilling Technology, I was quite honored and, with company permission, gladly accepted. At that time, presentation of the talk was several months away and the task seemed simple. But as the time approached, the problem of what to talk about became of increasing concern. What does one talk about as an SPE Distinguished Lecturer?I have been privileged to present technical papers, but these have been on specific technical developments, and relatively easy to write. No such specific development was at hand; anyway such a presentation would be better suited for a technical meeting. It was possible to write some generalized, pious statements about Drilling Technology, but these would have been boring to me, and probably to my audience. The varied interests of my listeners prohibited the presentation of a drilling-course lecture. This analysis, frankly, was almost tantamount to concluding that I could not give a talk at all. But there are a few divers points of interest in the field of Drilling Technology that I felt were worth discussing and that might lead the listener to feel he had received something of value. I decided to limit my lecture to those few points. I hope my audience finds them of interest.

Preventing Formation Rupture to Surface

There have been a number of incidents in which formations ruptured from below a casing seat to the surface during or subsequent to a blowout, creating a most difficult condition to control. The Santa Barbara incident is not a subject for discussion here, but it is a painful remainder of costs and troubles associated with such blowouts. While this surface rupture can be associated with existing natural fractures can also be expected should a well blow out under conditions frequently existing during drilling, and where natural fractures do not exist. There are no known methods of predicting when a ruptured formation would extend to the surface; but since this is known to happen when fracturing occurs below surface strings, attention to preventing fracturing seems warranted. These circumstances can be analyzed, at least to some extent. If a well should blow out and be closed in, how much pipe would be necessary to prevent formation fracture? Let us make some reasonable assumptions. Most formations would be expected to fracture with about 1 psi per foot of pressure gradient. Normal formation pressure can be represented by a gradient of 0.465 psi/ft. When the well blows out after being drilled below the casing seat and is closed in on dry gas, there is no mud remaining to balance bottom-hole pressure, so maximum possible pressure at the casing seat occurs. Actual borehole pressure would be reduced from bottom-hole pressure by a minimum gas gradient (0.1 psi/ft). These conditions are shown in Fig. 1. In this example, bottom-hole pressure is

Pbh = GfD;

pressure at any casing seat is

Pc = Pbh - Gg(D - Dc) = GfD - Gg(D - Dc);

breakdown pressure at the casing seat is

Pbd = GbdDc.

JPT

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