ABSTRACT

A new detailed analysis of floating drilling operations to determine the proper mud densities required for sub sea wildcat wells leads to the development of an equation for calculating fracture gradients in open hole below the first casing string. The effect of penetration rates and the resulting cuttings load on mud density is discussed. The penetration rate is related to fracture gradient in an equation derived for this section of the hole.

Because mud density requirements change instantaneously when the riser is removed to run casing, an equation has been derived to calculate the mud density required to keep the mud column pressure constant in the drilled hole prior to and after riser removal. The procedure presented will prevent a well kick when the riser is removed.

Kicks can seldom be contained without fracturing the formation while drilling the surface pipe hole; therefore, the heaviest mud density possible should be considered to allow high pressured zones to be drilled with maximum safety.

The mathematical analyses regarding kick tolerances and mud density are given using actual drilling conditions which indicates that sometimes a gas kick cannot be taken and contained without fracturing the formation at the last casing shoe. Therefore, depending on the kick tolerance the lowest or highest possible mud densities should be considered in preference to normal densities.

INTRODUCTION

Throughout oilfield history man hasdrilled wildcat wells. The first wells were drilled without regard for the mud densities required to safely seek total depth. Many times this resulted in loss of control of the well.

Drillers hypothesized that wells could be controlled if mud densities exerted pressures that were equal to or greater than formation pressures. This theory was valid; however, in most cases formation pressures were unknown until the formations were penetrated.

Hence, well control loss still occurred but less frequently. Some drillers thought that blowouts could be entirely eliminated if high mud densities were used. Drillers sometimes used such high mud densities that weak formations were fractured causing whole mud to flow into these zones. Drillers found that lost returns could be prevented by placing casing in the hole and cementing the casing-hole annulus thus isolating formations behind the pipe from the hole drilled below the casing.

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