To investigate the effect of vesicular property on mechanical characteristics of basalts a series of uniaxial and triaxial compression tests were conducted for basaltic intact rocks sampled in the northeastern onshore and offshore, southeastern offshore and northwestern offshore of Jeju Island, South Korea. The uniaxial compressive strength and parameters used in the More-Coulomb failure criterion, namely cohesion and internal friction angle, estimated from the results of the uniaxial and triaxial compression tests were compared and analyzed with effective porosity, a parameter representing the vesicular property of basalts. The results demonstrate that the uniaxial compressive strength and cohesion with respect to the effective porosity can be classified clearly as two different non-linear regression curves in accordance with two different linear relationships between bulk specific gravity and effective porosity. As the effective porosity increases, the uniaxial compressive strength and cohesion decrease exponentially. On the other hand, the internal friction angle decreases gradually with the effective porosity, regardless of the relationships between bulk specific gravity and effective porosity.
Basalt is one of the most common rock types of volcanic lava area, and has a fine-grained mineral texture. In addition, basalt has various shaped and sized vesicles formed by dissipation of gaseous phases in lava decompressed in the process of erupting onto the surface of the earth or flowing on the surface.
Vesicular structures of basalt have important effects on the physical and mechanical properties of the intact rock itself as well as the stability of rock mass which are crucial for the design of diverse foundation structures, tunnels, and other projects.
There are many studies of the effects of vesicular property on the physical and mechanical properties such as permeability, uniaxial compressive strength, elastic modulus, Poisson's ratio and ultrasonic velocities of vesicular basaltic intact rocks (Kelsall et al., 1986; Kim and Choi, 1991; Kwon et al., 1993; Al-Harthi et al., 1999; Saar and Manga, 1999; Eum, 2002; Kim, 2006; Gates, 2008; Cho et al., 2009; Moon et al., 2014; Yang, 2014; Yang, 2015a; Yang, 2015b; Yang, 2016; Yang and Sassa, 2016). Most studies on the basaltic intact rock revealed the relationship between physical parameters representing vesicular property and mechanical characteristics estimated from the results of uniaxial compression test. There are, however, very few studies about the strength parameters, such as cohesion and internal friction angle, which can be estimated directly from the results of three or more triaxial compression tests on basaltic intact rocks.
Tollefsen, Edward (Schlumberger) | Goobie, Roger Bisham (Schlumberger) | Noeth, Sheila (Schlumberger) | Sayers, Colin Michael (Schlumberger) | Den Boer, Lennert (Schlumberger) | Hooyman, Pat (Schlumberger) | Akinniranye, Goke (Schlumberger) | Cooke, Jay (Helis Oil &Gas) | Thomas, Ron (PPI Technology Services) | Carter, Edgar (PPI Technology Services)
Remote real-time pore pressure monitoring using a combination of Logging-While-Drilling (LWD) services coupled with a predrill pore-pressure model provides significant insight into wellbore stability and allows for optimizing casing points. This paper presents the results of a job in the Gulf of Mexico (GoM) that allowed an operator to drill confidently in a very tight hydraulic envelope and eliminate a string of casing.
The real-time use of data from the LWD formation pressure and sonic tools provides confidence in geopressure predictions. These LWD measurements allow the predrill velocity-to-pore-pressure transforms established during predrill modeling to be updated while drilling using the velocities from the sonic tool and pressures from the LWD formation pressure tool. This calibrated transform is then applied to revise the predrill pore-pressure model while drilling, thus reducing the uncertainty in the pore-pressure prediction ahead of the bit. In this case, the predrill model used velocities extracted from a 3D mechanical earth model of the northern GoM based on velocities derived from checkshots and sonic logs. These velocity data are kriged to give a 3D velocity model with uncertainty estimates.
Using state of the art Logging-While-Drilling (LWD) technologies, a new methodology was initiated to optimize drilling performance on a Vermillion 338 well. Continuously updated LWD annular pressure measurements effectively gauge wellbore pressures and help the driller rapidly intervene in pressure and/or geomechanical wellbore stability issues. A complete understanding of the hydraulic forces on a borehole can increase the rate of penetration, provide greater safety, minimize casing strings, reduce or eliminate kicks and formation fracturing, and allow faster and less expensive completions.
The technique described in this paper allows for incorporating real-time measurements into a pre-drill model, thus reducing the uncertainty ahead of the bit and allowing the operator to extend both the 9-5/8-in. intermediate casing and 7-in. liner to TD. As a result, a critical casing string was pushed 1,287 ft deeper than planned and a pre-planned 5-in. liner eliminated. The reduction in casing expense, as well as slim-hole drilling and completion costs resulted in a savings to the operator of approximately $1.7 million.