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doi:10.2204/iodp.proc.348.102.2015

Downhole measurements

Leak-off test

A leak-off test (LOT) is used by drilling engineers to determine the maximum mud weight that can be used for drilling without damaging the formation by hydraulic fracturing. If properly conducted, results of LOTs may also become useful for inferring the magnitude of the least principal stress in the formation. In a LOT, pumping drilling fluid into a closed wellbore at a constant rate pressurizes a small section of the formation. By observing the pressure history during fluid injection, one can identify changes in pressurization rate, formation breakdown, and steady-state pressures to infer the progress of hydraulic fracture formation/propagation and, ideally, the magnitude of the least principal stress (e.g., Zoback, 2007).

Injection of mud into an elastically responding borehole leads to a linear relationship between the injected mud volume and the borehole pressure. The leak-off point (LOP) corresponds to the pressure and volume at which the relationship deviates from a linear-elastic trend as a result of fracture volume creation. At the LOP, the gradient of pressure versus injected volume decreases because drilling mud escapes into the formation. If injection continues beyond the LOP, the formation may break down as the peak pressure (formation breakdown pressure) is reached and a hydraulic fracture is created. If enough fluid volume is injected to cause a hydraulic fracture to propagate away from the near-wellbore region, pressure at shut-in or fracture closure pressure can be observed to infer the magnitude of the least principal stress (Zoback, 2007). Even if hydraulic fracture propagation is not achieved, pressure at the LOP is commonly thought to give the magnitude of the least principal stress (S3), especially if there is a clear LOP. However, the mechanical correspondence between the pressure at LOP and S3 is not well understood, and the pressure at the LOP is often observed to be higher than S3 (Raaen et al., 2006). When the LOP is not clear, it is especially difficult to interpret the data other than to establish drilling engineering limits.

LOTs were carried out at the top of the 12¼ inch Hole C0002P borehole after a sidetrack hole was drilled out of Hole C0002N at 1936.75 mbsf (3904.25 m BRT). The test was conducted after 18 m of the formation was exposed (1936.75–1954.5 mbsf; 3904.25–3922.0 m BRT) in the sidetrack hole. The LOT was conducted with the outer annulus closed by the blow-out preventer, and mud pressure was measured at the cement pumps. The pressure at the bottom of the hole was calculated by the recorded pressure plus the static pressure of the mud column (mud density = 1.16 specific gravity). Two cycles of pressurization were conducted at 0.25–0.32 and 0.7–0.8 bbl/min injection rates. After shut-in, mud pressure was observed for ~10 min to monitor the pressure decay behavior. The drilling engineer provided pressure, volume, and pump-rate data after the test was conducted. Volume and pump-rate data, recorded based on counts of pump strokes, were known to be inaccurate, so they were corrected based on the consumed mud volume directly observed at the mud tank during the LOT operation.

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