--> Designing A Real-Time Geomechanics Program For A 7Km Borehole To Intersect The Nankai Subduction Zone In Japan

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Designing A Real-Time Geomechanics Program For A 7Km Borehole To Intersect The Nankai Subduction Zone In Japan

Abstract

Drilling a deep borehole that intersects an active subduction zone plate boundary is a technological and operational challenge, which if successful, would produce a profound contribution to the earth sciences for better understanding the mechanics and dynamics of plate boundary processes. The successful construction of this deep science well also provides an equally important opportunity to install real-time monitoring instruments to measure pressure, temperature, strain, and ground acceleration as part of an earthquake early warning system to protect lives who may be vulnerable to severe structural damage and devastating tsunamis.

This drilling project is part of an intensive Nankai Trough drilling campaign with relatively shallow and moderately deep boreholes roughly perpendicular to the strike of the plate boundary. This drilling, sampling, and monitoring project (NanTroSEIZE) is managed by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Center for Deep Earth Exploration (CDEX), in partnership with the International Ocean Discovery Program (IODP). Sixteen sites have been drilled during the 10+ year NanTroSEIZE period, with some of the sites containing up to 10 boreholes drilled using the research vessel Chikyu (The Earth in Japanese). Although there are several boreholes monitoring in situ conditions, none of these boreholes have reached the plate boundary.

In 2013-2014, Expedition 348 drilled at Site C2 drilled through the accretionary prism to a total depth that is ~2,200 metres from the tectonically active mega-fault that splays from the plate boundary. This campaign drilled C0002N (and side-track C0002P) but not without experiencing an excessive amount of non-productive-time (NPT) largely due to instabilities of the borehole wall. Although borehole instabilities did not occur while drilling, there was an extensive degree of rock failure along originally horizontal bedding planes that have been tilted up to 60 degrees during the convergence process. Because the C0002N/P wells are vertical, the far-field stresses resolved around the borehole imposed an added stress on the weak bedding planes sufficient to induce spalling of blocky, planar, and angular cavings. The size of these cavings coupled with a mud rheology inadequately designed to remove the additional cavings resulted in inefficient hole cleaning experiences. The inability to efficiently remove the extra cavings created repeated obstructions in the annulus during circulation that resulted in very high bottom-hole-pressure spikes or pack-offs. The instantaneous and repeated pack-off events had the unintended consequence of forcing pressured drilling fluids further into the weakened bedding planes that spalled more rock debris into the open hole, and further exacerbated hole cleaning and hole instabilities. The severity of these cascading drilling problems resulted in an unplanned sidetrack (C0002P). Despite successfully setting casing in C0002N well and the C0002P, the casings were set ~300 m and ~100 m shallow of their section total depths due to cavings hole fill, respectively.

The geomechanical postmortem concluded that insufficient mud weight and inadequately designed mud rheology were the primary reasons for the NPT and excessive Exp 348 cost. Interestingly, the geomechanical postmortem revealed a stress state at 2,200 and 3,100 m from the mega-splay to be a non-critically-stressed state that is transitional between a normal faulting and strike-slip stress regime. This is in contrasts to previous notions that the Nankai subduction zone environment that is associated with recurring mega earthquakes should be driven by a critically-stressed reverse faulting stress regime.

Deepening the C0002P well an additional ~2,200 m to intersect the mega-splay may or may not encounter similar normal/strike-slip stresses or near hydrostatic pore pressures conditions. The JAMSTEC engineering program for this well must plan for stress/pore pressure scenarios that includes a rapid ramping up of stresses consistent with a critically-stressed reverse faulting stress regime, and high pore pressures potentially approaching lithostatic along a fault zone that is plumbed to the deeper seismogenic environment.

These harsh drilling conditions with pronounced uncertainties have driven well designs that included drilling the last 2,200 m of the accretionary prism in four hole sections. Expandable casing and underreamers as part of the bottom hole assembly will enable the JAMSTEC engineering team to systematically quantify probable changes in stresses and pore pressures as the mega-splay is approached without resorting to hole sizes less than 8.5 inches.

These four hole sections provide an unprecedented opportunity to collect important geomechanical information such as logging-while-drilling (LWD) image data for borehole condition monitoring and stress modeling, azimuthal acoustic data for rock strength and rock anisotropy, and extended-leak-off tests at each casing depth point to quantifying changes in the least principal stress (S3). Knowledge of S3 estimates compared to vertical stress (SV) calculated from density data will help quantify any potential stress regime transition. Modeling wellbore failure detected in the LWD image data during the initial drill pass and planned repeat passes will help document the time-dependent performance of the mud weight and mud rheology program. A real-time geomechanics program may include transmitting LWD images in real-time (continuous or at strategic sampling depths) to verify the operational plans are performing efficiently downhole (or not). At the very least, there would be ample time between downloading the LWD data at the rig floor and running in the hole to drill the next hole section to provide useful mud design information for the JAMSTEC engineering team.