Characterizing borehole fluid flow and formation permeability in the ocean crust using linked analytic models and Markov chain Monte Carlo analysis

Abstract

[1] Thermal records from boreholes in young oceanic crust, in which water is flowing up or down, are used to assess formation and borehole flow properties using three analytic equations that describe the transient thermal and barometric influence of downhole or uphole flow. We link these calculations with an iterative model and apply Markov chain Monte Carlo (MCMC) analysis to quantify ranges of possible values. The model is applied to two data sets interpreted in previous studies, from Deep Sea Drilling Project Hole 504B on the southern flank of the Costa Rica Rift and Ocean Drilling Program Hole 1026B on the eastern flank of the Juan de Fuca Ridge, and to two new records collected in Integrated Ocean Drilling Program Holes U1301A and U1301B, also on the eastern flank of the Juan de Fuca Ridge. Our calculations indicate that fluid flow rates when thermal logs were collected were $2 L/s in Holes 504B, 1026B, and U1301A, and >20 L/s in Hole U1301B. The median bulk permeabilities determined with MCMC analyses are 4 to 7 Â 10 À12 m 2 around the uppermost parts of Holes 504B, 1026B, and U1301A, and 1.5 Â 10 À11 m 2 around a deeper section of Hole U1301B, with a standard deviation of 0.2 to 0.3 log cycles at each borehole. The consistency of permeability values inferred from these four holes is surprising, given the range of values determined globally and the tendency for permeability to be highly variable in fractured crystalline rock formations such as the upper oceanic crust.

Hydrogeologic Properties, Processes, and Alteration in the Igneous Ocean Crust

“…There may be slow fluid convection within the upper 750 msb in this crustal section today, helping to steepen the geothermal gradient in the upper half of Hole 1309D (Blackman et al, 2014). There is no rapid flow down Hole 1309D today, as this would suppress the borehole thermal gradient much more than observed (e.g., Winslow et al, 2013), but slow flow of water down the hole and into thin zones at depth (fractures, faults), could also explain the observed curvature.…”

“…Temperature logs in cased, crustal boreholes have quantified downflow and upflow conditions, and researchers have used these observations and estimated or measured differential pressures to infer near-borehole hydrogeologic properties (e.g., Becker & Davis, 2003;Becker, Langseth, Von Herzen, & Anderson, 1983;Fisher, Becker, & Davis, 1997;Winslow, Fisher, & Becker, 2013). Flow down crustal boreholes is initiated during drilling by pumping cold (dense) seawater in the borehole adjacent to warm (less dense) formation fluid.…”

“…This creates a positive differential pressure that can drive borehole fluid into the formation, even if the formation is naturally overpressured, and this flow can continue for days to years (e.g., Becker, Bartetzko, & Davis, 2001;Gable, Morin, & Becker, 1992). Downflow can be recognized from curvature of borehole temperaturedepth logs and modeled based on heat balance considerations to estimate the flow rate (e.g., Becker et al, 1983;Lesem, Greytok, Marotta, & MecKetta, 1957;Winslow et al, 2013). If a natural formation overpressure exceeds the differential pressure created during downflow, the flow direction can reverse Fisher et al, 1997).…”

“…These analyses suggest that the initial flow rate down Hole U1301A (before flow reversal) was ∼2 L/s, whereas ∼20 L/s flowed down Hole U1301B. Flow may have been more rapid down Hole U1301B, compared to Hole U1301A, because Hole U1301B extends deeper into basement (which imposes a taller column of cold, dense bottom water), and because the formation permeability around Hole U1301B appears to be somewhat greater ( Winslow et al, 2013). Wheat et al (2010) evaluated the thermal and geochemical response of Hole 1301A before, during, and after the reversal from downflow to upflow.…”

“…These loggers recorded thermal conditions during a long period of fluid downflow. These thermal data and additional information (borehole geometry, completion details, and basement stratigraphy) were used to assess upper basement hydrogeologic properties with linked analytical models and a statistical analysis using a range of borehole parameters (Winslow et al, 2013). These analyses suggest that the initial flow rate down Hole U1301A (before flow reversal) was ∼2 L/s, whereas ∼20 L/s flowed down Hole U1301B.…”