Controlled variable density fluid for wellbore operations

Abstract

Fluid systems may contain elements to provide changes in bulk fluid density in response to various environmental conditions. One environmental driver to the variable density is pressure; other environmental drivers include, but are not limited to, temperature or changes in chemistry. The variable density of the fluid is beneficial for controlling sub-surface pressures within desirable pore pressure and fracture gradient envelopes. The variability of fluid density permits construction and operation of a wellbore with much longer hole sections than when using conventional single gradient fluids.

Description

CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims the benefit of U.S. provisional application No. 60 / 582,687 filed Jun. 24, 2004.FIELD OF THE INVENTION [0002] The present invention relates to wellbore operations fluids, and more particularly relates, in one embodiment, to wellbore operations fluids having variable density. BACKGROUND OF THE INVENTION [0003] Prior variable density drilling fluids primarily concerned the use of a highly compressible gas (e.g. air or nitrogen) as a free phase in the fluid. Limited, if any, efforts are made during conventional air, mist or foam drilling to control the expandability of the bulk fluid or to adjust or engineer the compressibility of the fluid other than managing the ratio of air or other gas to the fluid. Other proposals to employ a virtual multiple gradient fluid include so-called dual gradient drilling. This method would use two columns of different density fluids. One column would be essentially static, while the sec...

AI Technical Summary

Benefits of technology

The present invention provides a method for drilling a wellbore using a variable density fluid that changes its density by design as a function of external parameters that vary along the depth or length of the well. This allows for the construction and operation of a wellbore with longer hole sections than when using conventional single gradient fluids. The variable density fluids have unit densities that can be deliberately changed, and they include a base fluid and a plurality of elements that change their volume / weight ratio in response to a condition such as pressure, temperature, and / or chemical composition of the base fluid. The invention also provides an element that changes its volume / weight ratio in response to a condition that includes a non-deformable core, a compliant skin surrounding the core, and at least one gas-filled space between the non-deformable core and the compliant skin, and a non-limiting embodiment that involves a hollow rigid external shell having at least one cavity therein and at least one opening into the cavity and an inner material within the cavity that changes its volume / weight ratio in response to a condition.

Problems solved by technology

Limited, if any, efforts are made during conventional air, mist or foam drilling to control the expandability of the bulk fluid or to adjust or engineer the compressibility of the fluid other than managing the ratio of air or other gas to the fluid.

While these density changes are often accounted for during the mathematical modeling of the fluid pressures in the wellbore, the density changes resulting from this behavior is not sufficient to change the design of the wellbore with respect to pore and fracture pressure profiles, as well as position and number of casing strings.

No effort is known to be made to intentionally modify the compressibility (density) of classic drilling fluids.

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