Borehole Competence at Mohole Depth
Authors: Blenkarn K.A. 
Published: December 2007

Tests are described for evaluating the ability of crustal rock around the Mohole to withstand stresses imposed when sea water rather than heavy mud is used as a circulating fluid. Borehole competence can be assured if simple compression-type tests of crustal rocks show strengths greater than 40,000 psi. If lower strengths are observed, more complicated model tests of hole stability are in order.

Introduction

As the title implies, this paper is concerned with drilling of the Mohole. In its most elementary aspects, this scientific drilling venture calls for drilling a hole through the crust of the earth to reach the Mohorovicic discontinuity, commonly called the Moho. More specifically, the program will involve drilling some 25,000 ft below the ocean floor, and doing this from a vessel floating in perhaps 15,000 ft of water.

Needless to say, such an unprecedented undertaking gives rise to a host of problems, several of which could be greatly simplified if sea water could be used as a drilling fluid. However, using sea water to drill an extremely deep hole could lead to difficulties. It may be pointed out that it is sometimes necessary to use heavy mud in drilling deep wells to keep the hole from closing around the drill string. It is true that these troubles are associated with soft formations, while formations penetrated in drilling the Mohole are expected to be mainly hard basalt. But this does not completely resolve the question of borehole competence because geological pressures to be encountered in drilling the Mohole will be greater than any previously encountered.

The central question with which this paper is concerned is that of borehole competence. If sea water is used as a circulating fluid, will a hole drilled to a total depth of 40,000 ft remain stable and not close?

The final answer to the question of competence will come only when the drilling is performed, but partial answers to the question may be developed from knowledge at hand or from knowledge which can be easily obtained. The aim of the present paper is to discuss such partial answers in order to evaluate the feasibility of using sea water as the drilling fluid in the Mohole.

Expected Overburden Pressure and Bottom-Hole Pressure

It will be assumed in this paper that the stresses several thousand feet below the ocean floor are essentially hydrostatic and equal to the overburden pressure.

The overburden pressure p o and the bottom-hole pressure p b, at the Moho depth may be expressed as follows, since the hydrostatic gradient in salt water is approximately 0.5 psi/ft, and in basalt 1.2 psi/ft.

and

where Do = ocean depth, anddepth of the well belowthe ocean floor.

For the most exacting conditions expected in the Mohole, Do = 15,000 ft and D w = 25,000 ft, so that

p b = 20,000 psi

and

p o = 37,500 psi

Expected Bottom-Hole Temperature

On the Guadalupe site in the Pacific Ocean, where Mohole Project Phase I holes were drilled, the temperature at the ocean floor was found to be 34 F.

According to information furnished by C. A. Burk, AMSOC scientific officer, the heat flow rate through the earth's crust is expected to be 1.2 × 10(−6) cal/(sq cm sec) in geologically undisturbed areas. Moreover, the smallest value of the thermal conductivity of basalt (diabase) reported in Ref. 1 (page 149) and Ref. 2 (page 155) is 0.003 cal/(sec cm C). Based on this, the value of the geothermal gradient to be expected in the crust should be 2.2 F/100 ft. Thus, disregarding the drop of temperature in the soft sediments between the top of basalt and the ocean floor, the calculated temperature at the Moho is 34 F + (2.2 F/100 ft)× 25,000 ft = 584 F.

In the soft sediments, the geothermal gradient is greater than in the basalt, but since these sediments are only a few hundred feet thick, the increase of temperature in them is small. On the Guadalupe site this temperature increase was about 40 F. This is considered to be a rather high value in view of an exceptionally high heat flow at that location. However, if this value is assumed, the expected temperature at the Moho becomes 584 F + 40 F = 624 F.

JPT

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