Assessment of the interval heat flow and thermal resistance at the Faltu-1 well, Borno basin, NE Nigeria

Abstract

The heat flowing through horizons in the Faltu-1 well, Borno Basin, NE Nigeria was calculated from their thermal conductivities and geothermal gradients with the aim of determining whether or not it is uniform, and if not, the depths where it is diverted, and the possible heat diversion process. The interval heat flow was assessed to be non-uniform. While fluid convection is adjudged to be the major heat diversion mechanism within the Chad Formation with minor heat refraction, the reversed is adjudged to be the situation for the underlying Kerri Kerri Formaton within which increasing sand content with depth is also predicted, with the lower interval predicted to be the Gombe Formation. Patterns of disruptions to the vertical heat flow within the Fika Formation were inferred to suggest rhythmic bedding of shale and sand beds that could serve as both source and reservoir rocks. Magmatic intrusions that impacted the maturation of organic matter into oil and gas also provided necessary entrapment structures and possible migration pathways. The Gongila and Bima Formations each has single disruption of the heat flows that are attributed to refraction. In the case of the Gongila Formation, the disruption is between the early-deposited more sandy and laterdeposited more shaley lithologies in the marine transgression of the area, while in the case of the Bima, it is between the more shaley Upper and more sandy Middle Bima Formations. Analysis of the Bullard plots also revealed disruptions to the vertical heat flow that are attributed either to convecting fluids or to heat refraction and diffraction. Two such disrupting heat advections were identified within the Chad Formation with the first being attributed to convection, while the other is attributed to a combination of both. Two similar disruptions for the Kerri Kerri Formation were attributed largely to lithological variations with minor contributions from convection of fluids. While unable to discern the rhythmic bedding, the five disruptions of the Bullard plot for the Fika Formation and one each for the Gongila and Bima Formations were interpreted to indicate similar features inferred from interval heat flow plots.

Keywords: Interval heat flow, heat convection, heat diffraction, thermal resistivity, shaliness