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doi:10.2204/iodp.proc.301.206.2009

Hydrogeologic experiments

Single- and crosshole pressure responses were interpreted from data collected during and after Expedition 301 to resolve hydrogeologic properties in the crust (Becker and Fisher, 2008; Fisher et al., 2008). Interpretation of single-hole tests is based on fitting pressure-time observations to an equation of the form

ΔP = f(Q,t,T),

where Q is fluid pumping rate, t is time, and T is formation transmissivity. Transmissivity is the product of aquifer thickness and hydraulic conductivity, K, within a horizontal, tabular aquifer, where the latter is related to permeability by

k = Kµ/ρg,

where µ is the viscosity and ρg is the specific weight of the fluid. Two independent variables are added in the case of a borehole pressure response during a crosshole test:

ΔP = f(Q,t,T,r,S),

where r is the radial distance between perturbation and observation wells (known), and storativity, S, is

S = ρgb(α + nβ),

where α is aquifer compressibility, n is porosity, and β is fluid compressibility. Thus, results from crosshole tests theoretically provide more information, but interpretation of these tests is also subject to additional assumptions (Becker and Fisher, 2008; Fisher et al., 2008).

If there is sufficient understanding of aquifer geometry and properties, one can account for deviations from ideal conditions such as heterogeneity and anisotropy (vertical and/or azimuthal), near-borehole damage resulting from drilling, leakage through overlying and/or underlying confining layers, and partial penetration of the well into the aquifer. Transmissivity and permeability are heterogeneous tensor quantities (particularly within fractured rocks), having values that vary with the temporal and spatial scales of testing. Formation storativity is a bulk (volumetric) scalar quantity, but it too can vary considerably with location, the spatial scale of testing, and the frequency of pressure perturbations. The different scales inherent in short-term (≤60 min) single-hole testing and longer term (13 month) crosshole testing provide some insight with regard to property scaling, but as discussed later, there may be influences of heterogeneity and anisotropy on different test responses as well.

Packer experiments during Expedition 301 were conducted with the same system and methods used during Leg 168 and numerous earlier drilling expeditions (e.g., Becker and Fisher, 2000). The packer is made up as part of specialized bottom-hole assembly, lowered into a borehole to the desired depth, and inflated within casing or in the open formation (hole conditions permitting). A "go-devil" is dropped down the pipe to enable packer inflation. This device also carries autonomous pressure gauges that monitor conditions beneath the packer during pumping experiments. Digital gauges used during Expedition 301 were serviced and recalibrated prior to sailing and recorded pressures and temperatures throughout the experiments. Packer experiments such as these are limited in several ways. It is not possible to use pressure data from a pumping well to determine formation storage properties. In addition, the short testing time (≤60 min) results in estimates of formation transmissivity in the region immediately surrounding the borehole. One advantage of this latter restriction is that testing of multiple depth intervals during packer experiments provides insight to the layering of crustal hydrologic properties, at least close to the borehole.

Packer tests were attempted in Hole U1301A during Expedition 301, but the packer had to be set in 0.27 m casing above the open hole because the hole was oversized and in danger of collapsing (see the "Site U1301" chapter). Because the 0.27 m casing was not sealed at the base or inside the 0.41 m casing, these tests provide no useful information. In contrast, packer tests were run in Hole U1301B with the packer inflated in open hole, allowing determination of formation properties below the packer element. Only packer tests in Hole U1301B were interpreted (Becker and Fisher, 2008).

Packer setting depths in Hole U1301B were selected where basement rocks were relatively massive and the hole diameter was sufficiently small for the packer element to seal the hole and hold the packer in place, at 472, 442, and 417 mbsf (207, 177, and 152 msb, respectively) (Figs. F3A, F4). The deepest packer setting depth corresponds to an abrupt change in the character of basement geophysical logs. Above 470 mbsf (205 msb) the borehole diameter is highly irregular and there are intervals 10–50 m thick having very low bulk density, electrical resistivity, and P-wave velocity (Bartetzko and Fisher, 2008). Below ~470 mbsf (~200 msb), the borehole diameter is more consistent with the drill bit diameter, and zones of low bulk density are thinner and more widely spaced (Fig. F3A). This change in geophysical properties within the uppermost extrusive crust is similar to changes seen in other upper crustal sections (e.g., Bartetzko et al., 2001; Jarrard et al., 2003; Pezard et al., 1992).

Packer test records required several processing steps prior to interpretation, including corrections for superimposed perturbations related to tides, barometric pressure changes, and differences in fluid temperatures (Becker and Fisher, 2008). Examples of pressure-time records and interpretations from packer tests show typical formation and instrument responses and model fits (Fig. F4). Pressure tends to rise rapidly for the first few minutes of a pumping test, and then to rise more slowly for the duration of the test. In general, data are well fit by a conventional aquifer model, including assumptions of isotropic and homogeneous conditions; horizontal, radial flow into the formation; and laminar flow conditions at the borehole wall and within the aquifer (i.e., Darcy's law applies). The good fit of the data to an idealized model does not prove that such a model applies, but it suggests that more complex models may be difficult to justify on the basis of observational data. The use of a simple model also allows comparison of Expedition 301 test results to results from earlier packer tests at other locations, based on similar assumptions.

The geometric mean of results from packer tests at 472 mbsf are T = 0.0034 m²/s and k = 1.7 × 10^–12 m² (standard deviation [sd] = 2.3 × 10^–13 m²), whereas geometric mean values for tests at 442 and 417 mbsf are about twice as great (T = 0.0064 m²/s and k = 3.2 × 10^–12 m² [sd = 5.1 × 10^–13 m²]). This suggests that permeability may be significantly greater above 472 mbsf (207 msb), consistent with observed changes in geophysical logs (Fig. F3). If the consistent properties determined during the final four tests (setting depths of 442 and 417 mbsf) apply to the uppermost 207 m of basement, the geometric mean permeability within this interval is k = 5 × 10^–12 m². In fact, the consistency of properties determined with setting depths of 442 and 417 mbsf suggests that most of the formation transmissivity may be concentrated between these depths. If we assume that the transmissivity occurs within this 30 m interval, the bulk permeability is k = 2 × 10^–11 m² (Becker and Fisher, 2008).

A larger scale assessment of basement hydrogeologic properties was made from the long-term pressure perturbation observed in Hole 1027B that resulted from leakage of cold bottom seawater into the crust at Site U1301 (Fisher et al., 2008). The raw Hole 1027C pressure record was corrected for the influence of tides and other local oceanographic processes and instrument drift. The residual signal shows the clear influence of basement operations during Expedition 301 (Fig. F5). Basement operations in Hole U1301B caused the greatest pressure response in Hole 1027C, whereas operations in Holes U1301A and 1026B had little or no influence. The packer experiments in Hole U1301B caused the greatest immediate perturbation, despite modest pumping rates, because this was when pumped fluids were forced to enter basement, rather than being allowed to flow back up the borehole to the overlying ocean. These observations suggest that shallow basement surrounding Hole U1301A may be less well connected hydrologically to the uppermost oceanic crust at the base of Hole 1027C than is deeper basement in Hole U1301B. In addition, the rate of pressure rise following emplacement of the subseafloor observatory in Hole U1301B is less than the rate of pressure rise following packer experiments or associated with drilling, coring, and other upper basement operations. These observations and consideration of the pressure-time response associated with flow down Hole U1301B allowed the flow rate into Hole U1301B during the 13 months following Expedition 301 to be estimated as Q = 2–5 L/s (Fisher et al., 2008).

Given this information, the basement aquifer transmissivity around Hole U1301B is inferred to be 0.005 to 0.012 m²/s, suggesting bulk permeability within the upper 300 m of crust of k = 0.7 × 10^–12 m² to 2 × 10^–12 m². These values correspond to basement aquifer storativity of S = 1 × 10^–3 to 3 × 10^–3, which implies aquifer compressibility of α = 3 × 10^–10 Pa^–1 to 9 × 10^–10 Pa^–1, a value close to or somewhat larger than that of seawater. The transmissivity and permeability values are at the lower end of estimates based on single-hole packer experiments, even though they result from testing of a much larger rock volume, extending perhaps 10–30 km from the borehole (assuming isotropic and homogeneous conditions) (Fisher et al., 2008). In addition, basement hydrogeologic properties estimated from this crosshole response are 1 to 3 orders of magnitude lower than estimates based on numerical modeling and calculations based on tidal responses and drainage following tectonic strain events, which test similar crustal volumes (Fig. F6). One explanation for this difference in inferred properties is that properties in the crust around Site U1301 may be anisotropic.

Anisotropy in the seismic properties of oceanic basement rocks is thought to result from preferential orientation of cracks, faults, and fractures (i.e., the crustal "fabric") (e.g., Sohn et al., 1997; Stephen, 1981). The dominant crustal fabric is generally thought to be subparallel to the orientation of the mid-ocean ridge where the crust was created. This fabric may favor fluid flow in the crust in the "along-strike" direction (Delaney et al., 1992; Haymon et al., 1991; Wilcock and Fisher, 2004), an interpretation consistent with geochemical and thermal data from the Expedition 301 field area (Fisher et al., 2003; Hutnak et al., 2006; Walker et al., 2007; Wheat et al., 2000). Calculations show how azimuthal anisotropy could influence the permeability apparent from a crosshole experiment involving a single observation borehole (Fig. F5B). If the angle of measurement is oblique relative to the direction of greatest permeability, the measured value will be very close to that in the lowest permeability direction, even for a large anisotropy ratio.

Unfortunately, there are insufficient data in this area at present to quantify crustal permeability anisotropy, but such an assessment should be possible after the next drilling expedition, as discussed below. If the crust in this area is shown to be azimuthally anisotropic, this will require reinterpretation of data analyzed in many earlier studies that assumed primarily one-dimensional or two-dimensional (across-strike) geometries for fluid flow in basement.

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