Abstract
The leakage of reservoir fluids into a wellbore during perforation, workover, or other completion operations is a substantial concern, especially in high temperature wells where existing fluids utilized to mitigate reservoir fluid influx offer poor performance. To maintain the hydrostatic integrity of the wellbore during these operations, a thermally-stable, solids-free fluid loss pill has been developed.
The pill is composed of synthetic polymers and cross-linkers in monovalent high-density brine. Under ambient pressure and temperature conditions, the pill flows as a viscous liquid and remains fluid for over six months. Upon exposure to elevated temperatures (200°F to 375°F), the pill cross-links to form a robust gel that can act as an appropriately-weighted, low fluid loss pill. This pill can temporarily seal the reservoir, enabling well control during completion operations.
Gelled samples of the fluid loss pill have maintained integrity for up to three weeks under HTHP conditions in laboratory testing with minimal signs of collapse. Fluid loss tests at different temperatures were conducted to assess the fluid loss performance. Gel could be easily removed without a breaker by the formation pressure of the reservoir upon well completion due to the solids-free nature and lower lift-off pressures required for this particular gel. However, depending on the specific requirements for a formation, this gel could be readily destroyed with high temperature oxidative breaker systems. In such circumstances, the timing of gel removal could be completely controlled by the timing of the breaker addition.
The gel was also evaluated for any formation damage issues if used in a high temperature gas reservoir by using regain permeability experiments with Berea sandstone cores. Necessary precautions were taken to eliminate the permeability impairment effects caused by the change in water saturation of the tested core.
Average fluid loss in the core flooding test over 3 days at 320 °F was about 50 ml. Restoration of the Berea core permeability after the treatment exceeded 95% of the initial permeability with low lift-off pressures observed. This paper presents full laboratory data on the fluid loss performance and regain permeability of a novel thermally-stable, solids-free fluid loss gel.
