A Shear-Thickening Fluid for Stopping Unwanted Flows While Drilling
Authors: Hamburger Charles L. , Tsao Yuh-hwang , Morrison Betty , Drake Evelyn N. 
Published: December 2007

Introduction

Unwanted flows encountered while drilling, such as lostcirculation, gas influx, or underground blowouts, can becostly and dangerous. All the traditional methods ofstopping unwanted flows have their shortcomings. Lostcirculation materials often fail to seal the loss zone; cement may require too much time to harden; gunk and othertypes of plugs are unreliable because they depend on themixing of two or more components downhole. A fluidwas needed that would thicken reliably immediately afterit exited the drill string or as it flowed into the losszone. This paper describes a new fluid developed specifically for this purpose. At the low shear rates encounteredwhile it is being pumped down the drillpipe, the fluid isa low-viscosity, pumpable liquid. Yet as it passes throughthe drill-bit nozzles, the resulting high shear rate causes the fluid to thicken irreversibly into a high-strength viscous paste, hence the name shear-thickening fluid(STF). Fig. 1 illustrates this fluid's shear-thickeningability. The "before" picture shows the fluid being pumpedout of an open-ended pipe. The "after" picture showsthe same fluid being pumped through a partially closed valve. The high shear rate encountered by the fluid as it flows through the valve has caused it to thicken. The thickened paste has about the same consistency as modelingclay and can be used to stop unwanted flows in a well. This fluid should not be confused with a typical drillingfluid for a well; it is used to cure specific flow problemsin and around the wellbore. A description of its unique properties, formulation, and field applications follows. properties, formulation, and field applications follows. Formulation

STF is a multicomponent system composed of a water-swellablematerial (usually a clay) dispersed in an oil-external emulsion. The emulsion consists of a liquid oil, anoil-soluble surfactant, and aqueous-phase droplets containingdissolved polymer. A low-viscosity, paraffinic oil such asmineral oil is preferred. The oil-soluble surfactant isadded to stabilize the aqueous-phase droplets and toprevent premature mixing with clay particles during pipe prevent premature mixing with clay particles during pipe flow at low shear rates.

The polymer dissolved in the aqueous phase, typically polyacrylamide, serves an important function. It reacts polyacrylamide, serves an important function. It reacts with clay after high shear rate mixing to form a higher-strength paste than would be possible with clay alone. This strength results from the crosslinklng of water-swollenclay particles by the polymer.

The water-swellable component can be any material that will swell to form a high-viscosity, solid mass in the presence of the aqueous polymer solution. Wyoming presence of the aqueous polymer solution. Wyoming bentonite is the material generally used. After thickening, the STF paste remains as an oil-external system, which prevents it from being washed out or weakened bydownhole water sources.

A typical formulation is listed in Table 1. It is preparedas follows. Polymer is added to the aqueous phase andallowed to dissolve. Surfactant is added to the oil, thenthe aqueous phase is mixed into the oil/surfactant solution to form an oil-external emulsion. Finally, clay is added to the emulsion to form a slurry.

The slurry has a low viscosity and remains pumpable for 4 to 6 hours even with gentle agitation. Water slowly diffuses into the clay particles with time, so the slurrywill not remain pumpable indefinitely. However, the diffusion rate is slow enough that premature thickening is not a serious problem in field applications.

Under low shear rates, the aqueous droplets remain separated from the clay particles by the oil/surfactant phase. The droplets may deform in the flow field, but the phase. The droplets may deform in the flow field, but the shear rate is not high enough for the droplets to breakup. The slow diffusion of water through the oil phase intothe clay particles causes them to soften and swell slightly. This, in turn, increases the slurry viscosity.

When very high shear rates are encountered, such asthose in the drillbit nozzles, the water droplets and theclay particles are shattered and the two phases mix. Theclay quickly hydrates and swells, and is crosslinked bythe dissolved, aqueous-phase polymers. The initial strength of the fluid exiting the bit can be controlled bythe pressure drop (which controls the shear rate) acrossthe bit. A high pressure differential causes a high shear rate with more drop breakup and mixing and, thus, resultsin a high initial strength. A low pressure differential resultsin low-strength material exiting the bit. Because this material is still liquid, it can be pumped into formationcracks and fissures, which thickens it further. This allows flexibility in different well situations while having astandard formula. For a given formula, the ultimate strengthis the same, regardless of the initial strength.

JPT

P. 499