Well-Log-Based Analysis of Pore Pressure and Minimum Effective Stress (Fracture Gradient) in the Penobscot B-41 Well, Offshore Nova Scotia

Henri Jeni Tua Baringin Sitohang; Abdul Baris

doi:10.33541/edumatsains.v10i3.7792

Authors

Henri Jeni Tua Baringin Sitohang Universitas Indonesia
Abdul Baris Universitas Indonesia

DOI:

https://doi.org/10.33541/edumatsains.v10i3.7792

Keywords:

Pore Pressure; Fracture Gradient; Normal Compaction Trend (NCT); Eaton’s Method; Bowers Velocity Model; Penobscot Field

Abstract

This study analyzes pore pressure () and minimum effective stress (fracture gradient) in the Penobscot Offshore Field using well log data from wells Penobscot B-41. The Eaton and Bowers methods were applied to estimate pore pressure and minimum effective stress (fracture gradient), while vertical stress () and minimum horizontal stress () were calculated from density data.

The results indicate the presence of a top overpressure zone beginning at approximately 8,376 ftTVD, marked by a significant deviation from the normal compaction trend. A reversal depth is also identified around 9,000-9,200 ftTVD, indicating a shift in the overpressure mechanism from normal compaction to unloading. Pore pressure values derived from both Eaton and Bowers methods show overall consistency, although Bowers tends to yield more conservative estimates in the deeper sections.

Unloading is particularly evident in the deeper interval of well Penobscot B-41, where elevated values are observed without corresponding low density, suggesting stress release caused by fracturing or fluid migration. Effective stress () calculations reveal high-risk zones for fracturing at depths greater than 9,500 ftTVD, which should be carefully addressed in mud weight design.

Overall, the integration of Eaton and Bowers methods with effective stress evaluation successfully characterizes the geopressured system of the Penobscot Field. These findings provide an essential basis for drilling risk mitigation and casing design optimization.

References

Athy, L. F. (1930). Density, porosity, and compaction of sedimentary rocks. AAPG Bulletin, 14(1), 1–24.

Bachu, S., & Underschultz, J. R. (1995). Large-scale underpressuring in the Western Canada Sedimentary Basin. AAPG Bulletin, 79(4), 551–572.

Bowers, G. L. (1995). Pore pressure estimation from velocity data: Accounting for overpressure mechanisms besides undercompaction. SPE Drilling & Completion, 10(2), 89–95.

Bruce, C. H. (1984). Smectite dehydration—its relation to structural development and hydrocarbon accumulation in Northern Gulf of Mexico Basin. AAPG Bulletin, 68(5), 673–683.

Dutta, N. C. (2002). Geopressure prediction using seismic data: Current status and the road ahead. Geophysics, 67(6), 2012–2041.

Eaton, B. A. (1975). The equation for geopressure prediction from well logs. SPE Paper 5544. Society of Petroleum Engineers.

Fertl, W. H., & Chilingarian, G. V. (1987). Abnormal formation pressures. Amsterdam: Elsevier.

Horsrud, P. (2001). Estimating mechanical properties of shale from empirical correlations. SPE Drilling & Completion, 16(2), 68–73.

Jansa, L. F., & Wade, J. A. (1975). Geology of the continental margin off Nova Scotia and Newfoundland. In Canadian Society of Petroleum Geologists Memoir (Vol. 4, pp. 79–102).

Kidston, A., Brown, D. E., Smith, B. M., & Altheim, B. (2002). Geology of the Sable Subbasin offshore Nova Scotia. Canada-Nova Scotia Offshore Petroleum Board Reports.

Law, B. E., & Spencer, C. W. (1998). Abnormal pressure in hydrocarbon environments. U.S. Geological Survey Bulletin 2164. Denver, CO: USGS.

Osborne, M. J., & Swarbrick, R. E. (1997). Mechanisms for generating overpressure in sedimentary basins: A re-evaluation. AAPG Bulletin, 81(6), 1023–1041.

Rider, M., & Kennedy, M. (2011). The geological interpretation of well logs (3rd ed.). Rider-French Consulting Ltd.

Sayers, C. M. (2010). Geophysics under stress: Geomechanical applications of seismic and borehole acoustic waves. The Leading Edge, 29(12), 1494–1502.

Swarbrick, R. E., & Osborne, M. J. (1998). Mechanisms that generate abnormal pressures: An overview. In B. E. Law, G. F. Ulmishek, & V. I. Slavin (Eds.), Abnormal pressures in hydrocarbon environments (pp. 13–34). AAPG Memoir 70.

Swarbrick, R. E., Osborne, M. J., & Yardley, G. S. (2002). Overpressure in petroleum exploration. Geological Society, London, Special Publications, 207(1), 1–12.

Terzaghi, K. (1943). Theoretical soil mechanics. New York: Wiley.

Wade, J. A., & MacLean, B. C. (1990). The geology of the southeastern margin of Canada. Geological Survey of Canada, Geology of Canada Series No. 2.