R. Woodhouse
BP Exploration, London
E. A. Opstad
Sperry Sun Drilling Services, Anchorage
A. Bryce Cunningham
BP Exploration, Anchorage
Abstract
The vertical migration of water and oil based mud filtrates has been observed for many years in conventionally drilled wells. This work investigates the nature of these same phenomena in horizontal wells. Here, the vertical migration of invaded fluids is not along the well axis, but rather moves normal to, and away from the borehole.
In formations with moderate to high permeability and high Kv/Kh ratios, invaded filtrates will migrate vertically, propelled by density contrasts between the mud filtrate and formation fluid. In horizontal wells this process acts to pull water-base filtrates down and away from the well, allowing hydrocarbons originally present in the formation to migrate back toward the wellbore. Density contrasts similarly influence the migration of oil-base mud filtrates which rise in water bearing formations and fall in gas bearing formations. Cross-sections taken through a theoretical well model for a series of increasing formation exposure times illustrate expected fluid migration patterns.
Field, examples of neutron and other Formation-Evaluation-While-Drilling (FEWD) logs support fluid saturation changes predicted by our model. As expected, oil based filtrate invasion in the oil column did not alter reservoir fluid saturations above the transition zone, and showed no gravitational segregation effects. In gas bearing formations, near wellbore gas saturation initially decreases due to filtrate invasion, then increases through time as oil or water-base filtrates drop away from the horizontal wellbore. This dynamic behavior is documented by time-lapse MWD logging.