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Abstract

Use of F-overlays to evaluate air-drilled Devonian shale wells simplifies recognition of subtle variations in log response between productive and nonproductive intervals. Combinations of F-curve overlays, called F-pairs, are plotted on a logarithmic grid and normalized in an interval thought to be unfractured. Anomalous response behavior becomes useful in choosing fracturing intervals.

Introduction

Most Devonian shale production in the eastern United States consists of gas, but in Wirt, Ritchie, and Pleasants counties of northwestern West Virginia, considerable quantities of oil also are being produced from this unit. Potentially oil-productive produced from this unit. Potentially oil-productive Devonian shale covers approximately 300 square miles (777 sq. km.) in these counties. The prospective pay interval is generally from 2500 to 3500 feet pay interval is generally from 2500 to 3500 feet thick (762–1067 m), and consists of interbedded shales and siltstones.

Within the study area, shales and siltstones are characterized by low primary porosity and permeability, and most production is believed to come permeability, and most production is believed to come from naturally occurring fractures associated with the Burning Springs anticline. The reservoir quality of these naturally occurring fractures can be enhances by hydraulic fracturing to increase production rates.

Most wells are air-drilled to minimize formation damage, and a typical logging suite consists of gamma ray, induction, density, sidewall neutron, and temperature logs.

Log interpretation of fractured reservoirs is difficult. Fracture porosities tend to be very low, normally in the .5 to 1.5 percent range, and only subtle differences, if any, in log responses may distinguish a productive from a nonproductive interval. Some operators simply "shoot in the dark". They pick a shallow zone where oil is likely, or they try to "frac" all the zones at once. This procedure is subject to error and can be needlessly expensive.

Eliminating those intervals where production is unlikely would reduce the number of perforations needed, as well as the number and size of the fracture treatments. This can result in considerable savings without raising fears of having missed commercial intervals.

This paper presents a qualitative, quick-look evaluation technique using gamma ray, induction, density, and sidewall neutron logs. It points to a means of identifying intervals in any given well which are most likely to support commercial hydrocarbon production. It also attempts to identify those intervals from which only oil production is probable. probable."F-overly techniques are devised for use in conventional log analysis to quickly eliminate from further detailed evaluation all nonproductive intervals. Applied to fractured shale reservoirs, the method uses all F's available to take advantage of different tool responses to lithology and fluid changes.

TOOL RESPONSES

The induction log measures conductivity which then is converted to resistivity for log presentation. Measurements are made in horizontal loops, presentation. Measurements are made in horizontal loops, and the tool is thought of as being relatively unresponsive to vertical fractures and to hydrocarbon filled horizontal fractures will provide an easy current path and should register as intervals of reduced resistivity.

Devonian shales in the area under consideration produce hydrocarbons without any water, so if newly produce hydrocarbons without any water, so if newly logged wells show any fractures at all, it can be assumed that the section will contain hydrocarbons.

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