Rock Mechanics, Petrophysics, and Stratigraphy of the Tuscaloosa Trend
- Martin O. Traugott (Shell Oil Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- February 1982
- Document Type
- Journal Paper
- 428 - 432
- 1982. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 1.6 Drilling Operations, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.14.1 Casing Design, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.2.2 Geomechanics, 5.6.1 Open hole/cased hole log analysis, 5.2 Reservoir Fluid Dynamics
- 1 in the last 30 days
- 647 since 2007
- Show more detail
- View rights & permissions
|SPE Member Price:||USD 12.00|
|SPE Non-Member Price:||USD 35.00|
Tough drilling and evaluation problems fall to easy solutions in deep Tuscaloosa wells: Log resistivities in soft shales are linked to impending pore pressure reversals in deep sands; high-resistivity hard shales are linked to lost circulation intervals; and resistivity invasion profiles are linked to freshwater anomalies in boreholes with oil mud.
Several onerous things can happen during the 150 or 180 days it takes to drill a deep Tuscaloosa well in south Louisiana. As an example, consider this scenario: A Tuscaloosa well penetrates the overpressured Chalk formation. The well kicks and subsequently loses returns. The operator sets protective casing and drills ahead into the Eagleford shale. Gas in the mud increases. He sets a liner, drills into the Massive Tuscaloosa, loses returns, and reduces mud weight. Drilling continues. The well penetrates the Interbedded Tuscaloosa and kicks hard. Pipe sticks. The well is sidetracked. Apparent pay zones are tested and flow fresh water. The depth is 18,000 ft (5500 m). The cost: $5 million. But things are not so tough if casing and evaluation programs are based on geomechanical and petrophysical concepts. Understanding this depends on understanding four basic ideas: 1. Formations can be grouped into one of two classifications-soft or hard-based on resistivity logs. 2. Soft (viscoelastic) shales often have high fracture gradients and high pore pressures depending on proximity to massive sands. (Pore pressures can be derived from resistivity logs.) 3. Hard shales (and carbonates) often have lower pore pressures and lower fracture gradients depending on boundary conditions. 4. Fresh water often occurs in isolated, lower-pressure sands, near high-pressure zones containing salt water.
Stratigraphy and Shale Resistivity
The Tuscaloosa wells penetrate several different stratigraphic units, some hard and some soft (Fig. 1). An outline is given here of the important intervals:
A hard carbonate and potential lost-circulation zone.
A soft shale interbedded with limes and sands. Not over-pressured except where the Wilcox sands are not developed.
A sand interbedded with hard, resistive shales. Not developed at the east edge of the trend.
A semisoft shale. Shale resistivity is 2 to 3 Omega typical depths, and fracture gradients are high based on breakdown tests.
A thin, soft, overpressured shale. Pore pressures increase with depth as a function of distance from the lower Cretaceous shelf margin (Fig. 2).
A thick carbonate that often is drilled underbalanced.
A thick, soft, often overpressured shale.
|File Size||261 KB||Number of Pages||5|