Abstract

The Cotton Valley Group is an Upper Jurassic to Lower Cretaceous sequence of sandstone, shale, and limestone that underlies much of the northern Gulf of Mexico coastal plain from eastern Texas to Alabama. A great many wells have been drilled into this tight gas formation over the years. With the advent of new technology, wells can benefit both from improvements in horizontal well-drilling efficiency and horizontal-well stimulation efficiencies. New horizontal wells are currently being drilled into Cotton Valley sands for gas, oil, and natural gas liquids. Lateral lengths, stimulation volumes, and production results continue to increase substantially.

A great deal of information has been gathered from vertical wells in many fields that can yield critical insight into the evaluation of horizontal well placement. This paper will demonstrate the use of legacy data in the evaluation of Cotton Valley field areas for the placement of horizontal wells into the more productive Cotton Valley intervals.

The U.S. Geological Survey (USGS) has stated: “...the difficulties with wireline logs in tight Cotton Valley sandstones is that logs are of limited value in differentiating between gas-productive and wet intervals, and therefore in identifying gas-water contacts on the flanks of Cotton Valley fields.”

Major factors contributing to the abnormally low resistivities in tight Cotton Valley sandstones include bound water (micro-porosity) associated with pore-filling clays or clay grain-coatings and conductive authigenic minerals such as pyrite and ankerite. By using conventional core and rotary core plugs, magnetic resonance logs, and after-stimulation production logs on vertical wells, it is possible to optimize targets for horizontal well production and overcome a number of these problems.