Factors Affecting Clean-up of Water-Blocks: A Laboratory Investigation
- Jagannathan Mahadevan (U. of Texas at Austin) | Mukul M. Sharma (U. of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 2005
- Document Type
- Journal Paper
- 238 - 246
- 2005. Society of Petroleum Engineers
- 5.1 Reservoir Characterisation, 3.2.4 Acidising, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.6 Drilling Operations, 4.3.4 Scale, 4.6 Natural Gas, 3.2.6 Produced Water Management, 1.6.9 Coring, Fishing, 3 Production and Well Operations, 5.4.2 Gas Injection Methods, 5.8.1 Tight Gas, 5.2.1 Phase Behavior and PVT Measurements, 1.8 Formation Damage, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.1.2 Separation and Treating
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Water-blocked low-permeability gas formations with drawdown pressurescomparable to capillary entry pressures can take a very long time to clean up.This work aims to study the effect of permeability, wettability, temperature,and drawdown on the cleanup of cores containing brine.
Gas displacement experiments were conducted on cores fully saturated withbrine. Addition of methanol, increasing temperature, and increasing corepermeability by changing rock type resulted in faster cleanup afterapproximately 50 to 100 pore volumes (PV, the volume of vacant pores in the drysample) of gas flow. The change of wettability of the rock from water-wet tooil-wet also resulted in faster recoveries in gas relative permeability.
The cleanup of water blocks in gas wells occurs in two regimes: displacementof the fluids from the formation, followed by vaporization by the flowing gas,which becomes undersaturated as the pressure decreases.Our observationsshow that the cleanup of water blocks can be improved by: (a) Influencing thedisplacement regime (i.e., by changing the wettability) and (b) increasing therate of vaporization by introducing volatile solvents such as methanol.
The study quantifies the effects of factors such as rock type(permeability), wettability, surface tension/volatility, and temperature on gasrelative permeability. The results of this study will help in selectingstrategies for cleanup of water blocks created by various operations such asdrilling, acidizing, and fracturing, as well as making recommendations for theuse of surfactants or solvents for well treatments to remove water blocks.
Invasion of aqueous drilling, completion, or fracturing fluids during wellcompletion, workover, or stimulation operations can reduce the relativepermeability to gas and thereby cause a water block. The gas phase relativepermeability depends on the water saturation in the porous medium and thefractional flow characteristic of gas in the presence of water. The water blockcan be removed by reducing the saturation of the invaded fluid in the wellboreregion and/or by affecting the fractional flow characteristics of the gas.
Simulation studies show that the cleanup of water blocks in gas wells isfaster if the absolute permeability of the formation is high.1 In the case oflow-permeability formations, the capillary pressure tends to be high because ofthe smaller pore sizes. However, Holditch2 showed that when the drawdownpressures are very high, capillarity is not important. Studies by Parekh andSharma3 show that the ratio of pressure drawdown to the capillary pressure andthe relative permeability curve exponents has a significant effect on thecleanup of water blocks, with faster cleanup for high values of Coreyexponents. Abrams and Vinegar4 show that additives such as alcohol and/orsurfactants do not significantly improve the final gas flow when the drawdownpressures are significantly greater than the capillary entry pressures.
|File Size||255 KB||Number of Pages||9|
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