A Shear-Thickening Fluid for Stopping Unwanted Flows While Drilling
- Charles L. Hamburger (Exxon Production Research Co.) | Yuh-hwang Tsao (Exxon Production Research Co.) | Betty Morrison (Exxon Production Research Co.) | Evelyn N. Drake (Exxon Research and Engineering Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- March 1985
- Document Type
- Journal Paper
- 499 - 504
- 1985. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.5 Drill Bits, 1.14.3 Cement Formulation (Chemistry, Properties), 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials, 5.6.5 Tracers, 1.14 Casing and Cementing, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.3.1 Hydrates, 4.1.5 Processing Equipment, 1.10 Drilling Equipment, 1.7 Pressure Management, 1.6.10 Running and Setting Casing
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Unwanted flows encountered while drilling, such as lost circulation, gas influx, or underground blowouts, can be costly and dangerous. All the traditional methods of stopping unwanted flows have their shortcomings. Lost circulation materials often fail to seal the loss zone; cement may require too much time to harden; gunk and other types of plugs are unreliable because they depend on the mixing of two or more components downhole. A fluid was needed that would thicken reliably immediately after it exited the drillstring or as it flowed into the loss zone. This paper describes a new fluid developed specifically for this purpose. At the low shear rates encountered while it is being pumped down the drillpipe, the fluid is a low-viscosity, pumpable liquid. Yet as it passes through the 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-thickening ability. The "before" picture shows the fluid being pumped out of an open-ended pipe. The "after" picture shows the 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 modeling clay and can be used to stop unwanted flows in a well. This fluid should not be confused with a typical drilling fluid for a well; it is used to cure specific flow problems in 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-swellable material (usually a clay) dispersed in an oil-external emulsion. The emulsion consists of a liquid oil, an oil-soluble surfactant, and aqueous-phase droplets containing dissolved polymer. A low-viscosity, paraffinic oil such as mineral oil is preferred. The oil-soluble surfactant is added to stabilize the aqueous-phase droplets and to prevent 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-swollen clay 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 by downhole water sources.
A typical formulation is listed in Table 1. It is prepared as follows. Polymer is added to the aqueous phase and allowed to dissolve. Surfactant is added to the oil, then the 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 slurry will 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 break up. The slow diffusion of water through the oil phase into the clay particles causes them to soften and swell slightly. This, in turn, increases the slurry viscosity.
When very high shear rates are encountered, such as those in the drillbit nozzles, the water droplets and the clay particles are shattered and the two phases mix. The clay quickly hydrates and swells, and is crosslinked by the dissolved, aqueous-phase polymers. The initial strength of the fluid exiting the bit can be controlled by the pressure drop (which controls the shear rate) across the bit. A high pressure differential causes a high shear rate with more drop breakup and mixing and, thus, results in a high initial strength. A low pressure differential results in low-strength material exiting the bit. Because this material is still liquid, it can be pumped into formation cracks and fissures, which thickens it further. This allows flexibility in different well situations while having a standard formula. For a given formula, the ultimate strength is the same, regardless of the initial strength.
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