Previous   |    Next

doi:10.2204/iodp.proc.308.203.2008

Method

The DVTPP and T2P penetrometers interface with the colleted delivery system (CDS). The CDS is lowered by wireline and engages with the bottom-hole assembly (BHA). Once the CDS is engaged in the BHA, the drill string is used to push the probe into the formation. The drill string is then raised 3–4 m and the CDS telescopes to decouple the probe from the drill string. The probe remains in the formation to measure pressure and temperature for 30–90 min. After measurement, the wireline pulls the CDS to its extended position and then pulls the penetrometer out of the formation. A detailed description of the deployment procedure is presented in “Appendix A.” The data are downloaded from the data acquisition unit when the tool is retrieved.

When the penetrometers penetrate the formation, the temperature (resulting from friction on the tool) and pressure (resulting from deformation of the soil) are raised relative to their in situ values. Subsequently, the tools are left in place in order to dissipate toward the equilibrium values (Fig. F5). Temperature decay can be used to infer the formation temperature and thermal conductivity (Davis et al., 1997; Villinger and Davis, 1987). Decay of the penetration-induced pore pressure can be used to infer formation pressure, hydraulic diffusivity, and permeability (Baligh and Levadoux, 1986; Gupta and Davidson, 1986; Long et al., 2007a; Whittle et al., 2001).

Rates of pressure and temperature decay are functions of the probe diameter and the hydraulic/​thermal diffusivity of the sediment (Bullard, 1954; Long et al., 2007a; Villinger and Davis, 1987). Pressure decay is much slower than temperature decay in low-permeability mudstones (Long et al., 2007a). Because of the restricted time available for deployment, we must interpret in situ pressure from partial dissipation records. If detailed soil properties are available, the in situ pressure and hydraulic diffusivity of the sediment can be inferred from modeling of soil behavior for different penetrometer geometries. However, in many cases soil properties are not available or there are insufficient resources to pursue soil modeling. In these cases, in situ pressure is inferred from simple extrapolation approaches such as inverse time (1/t) extrapolation (Davis et al., 1991; Lim et al., 2006; Long et al., 2007b; Villinger and Davis, 1987; Whittle et al., 2001) and inverse square root of time (1/√t) extrapolation (Long et al., 2007b).

Top of page   |    Previous   |    Next