Rock Mechanics, Petrophysics, and Stratigraphy of the Tuscaloosa Trend

Abstract

Summary 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. Introduction 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 over pressured 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:Formations can be grouped into one of two classifications-soft or hard-based on resistivity logs.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.)Hard shales (and carbonates) often have lower pore pressures and lower fracture gradients depending on boundary conditions.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: Het Lime A hard carbonate and potential lost-circulation zone. Claiborne Group A soft shale interbedded with limes and sands. Not over-pressured except where the Wilcox sands are not developed. Wilcox A sand interbedded with hard, resistive shales. Not developed at the east edge of the trend. Midway A semisoft shale. Shale resistivity is 2 to 3 Omega typical depths, and fracture gradients are high based on breakdown tests. Clayton A thin, soft, over pressured shale. Pore pressures increase with depth as a function of distance from the lower Cretaceous shelf margin (Fig. 2). Chalk A thick carbonate that often is drilled underbalanced. Eagleford A thick, soft, often over pressured shale. JPT P. 428^