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doi:10.2204/iodp.proc.348.103.2015

Downhole measurements

Leak-off test

After Hole C0002P was sidetracked out of Hole C0002N from 1936.5 mbsf (3904.0 m BRT), two LOTs were carried out at 1954.5 mbsf (3922.0 m BRT). The tests were conducted to determine the maximum mud density that could be used for the drilling of Hole C0002P. Additionally, LOTs can in principle be used to provide constraints on the magnitude of the least horizontal principal stress (see “Downhole measurements” in the “Methods” chapter [Tobin et al., 2015]). The sidetrack section leading to Hole C0002P was ~18 m in length from the kickoff point at 1936.5 mbsf. This exposed a zone from a few meters to 18 m of formation outside the cement during the two LOTs (Fig. F107).

The two tests were conducted at 0.20–0.32 and 0.7–0.8 bbl/min injection rates (Fig. F108A, F108B). The first pressure cycle injected 4.1 bbl of drilling mud for ~14 min and reached a downhole pressure of ~53 MPa. The downhole pressure increased steadily throughout this test until shut-in. In the second test, a downhole pressure of ~54 MPa was achieved after injecting 4.7 bbl of drill mud over a period of 6.5 min. The pressure slowly increased during the last 10 s of the second test, and a peak pressure value was possibly reached at 54 MPa, which is expected to be fairly close to the formation breakdown pressure. (Fig. F108C).

The pressure-volume curves show that the two tests follow a similar trend (Fig. F109A, F109B). This suggests that there is little rate dependence in the pressure-volume relation, that there is no significant time-dependent fluid leakage during the tests, and that the elastic response of the wellbore system was repeatable to first order. However, a nonlinear trend is observed with the pressure-volume relation in the pressure build-up rate data of the second test (dP/dV; Fig. F109B, F109C), which makes it difficult to pick a distinct LOP. The linear decrease in dP/dV with volume, as observed in the first test and most of the second test, suggests that there is slight mud fluid loss into the formation at a constant permeability (Todd and Mays, 2005). dP/dV decreases linearly with volume because the rate at which the mud fluid is lost is proportional to the increase in downhole pressure observed at a constant formation permeability. In such cases, the LOP can be recognized as the point where the linearly decreasing trend breaks in the dP/dV record. In the first pressure cycle (Fig. F109B), we do not observe any change in the decreasing trend of dP/dV, suggesting that the LOP was not reached during this test. In the second pressure cycle (Fig. F109C), dP/dV starts to decrease faster at an injected volume of around 3.3 bbl. We interpret this as the onset of a sudden increase in system volume, which corresponds to a LOP of 52.2 MPa.

The LOP pressure of 52.2 MPa corresponds to an equivalent mud density of 1.36 specific gravity (sg), higher than the 1.15 sg mud density value derived from the LOP pressure of 32 MPa observed at 872.5 mbsf in Hole C0002F (Strasser et al., 2014b). Although the exact relation between LOP pressures and least horizontal principal stress is still unclear (Raaen et al., 2006; Zoback, 2007), if we consider a stress gradient based on the LOP pressure observations, we see an increase from 14.9 to 17.1 MPa/km relative to the seafloor in the interval between 872.5 and 1954.5 mbsf (Fig. F110).

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