Abstract

Pre-frac pressure analyses were done on the Oriskany or Huntersville Chert/Oriskany combination in wells located in Ohio and Pennsylvania. The pre-treatment pump-in tests were designed to pump several thousand gallons of chemically treated fresh water at multiple pump rates. Upon pump shutdown at the end of this "mini-frac", the wellhead pressure decline was monitored for an extended time period. Using a commercially available computer software program, the pump rate vs. pressure data and the pressure decline data were analyzed. Pressure decline analysis incorporated a Horner plot for reservoir pressure and permeability along with regression analyses including the Nolte G Function to identify normal leak-off or pressure dependent leak-off and fracture height recession.

Subsequent to the above analyses, hydraulic fracturing treatments were performed on the wells. The treatment designs were tailored to each individual well incorporating the information obtained from the mini fracs about reservoir properties and hydraulic fracture behavior. Use of a Net Pressure Plot of the fracturing treatment was used to validate leakoff behavior identified in the mini fracs.

Introduction

The Oriskany has been a major source of natural gas production in the Appalachian Basin for more than sixty years. In 1949, in sixty-six fields throughout the Appalachian Basin, estimated total reserves for the Oriskany were 1.6 TCF. Subsequent discoveries have increased this figure substantially. The Elk-Poca Field in West Virginia which is the largest Oriskany Field in the basin has produced in excess of 1.0 TCF1

The overlaying Huntersville Chert is not always present with the Oriskany across the Appalachian Basin and in fact, there is a coincidence between no Oriskany production and overlaying Chert1. Diecchio1 postulates that the highly fractured Chert is a poor cap rock allowing the gas to escape. As a result in some places, the Chert is a better gas reservoir than the Oriskany. The following geological discussion will provide a brief description of the Huntersville Chert and a more detailed discussion of the Oriskany which is the more prolific gas producer across the Appalachian Basin.

The Huntersville Chert is a dense, impure microcrystalline chert interbedded with silicified shale or mudrock. Due to its brittle nature, the Chert was fractured during deformation thus natural fractures are prevalent throughout the Chert. The Chert occurs primarily in the central part of the Appalachian Basin from McKean County, Pennsylvania south to Smyth County, Virginia. The thickest section of Chert (+/−260') is found in north central West Virginia.

The lower Devonian Oriskany Sandstone as described by Bruner2 is an expansive sheet deposit in the Appalachian Basin extending from New York State to Kentucky (Fig. 1). The thickest section of the reservoir is 250 feet to 300+ feet. This section (+/− 10% of the aerial extent of the Oriskany) lies along the Allegheny Front in a six county area. The counties are Somerset and Bedford counties of Pennsylvania, Garrett and Allegany counties of Maryland, and Mineral and Hampshire counties of West Virginia. Nearly 40% of the Oriskany is less than 50 feet thick. Bruner2 describes the lithology of the Oriskany as a hybrid sandstone with variable quantities of detrial quartz and sand-sized carbonate detritus. Within the Oriskany, four basic rock units can be characterized.

Beginning at the base of the Oriskany, a brief description of these units or facies is as follows. Facies 1 can be described as basically a limestone section interbedded with fine-grain sandstone. The second facies is a medium-grained clean sandstone with decreasing carbonate matrix. The most widespread and heterogeneous is facies 3. In facies 3 the rock is a mix of bioturbated calcareous sandstones and fossiliferous sandy limestones with argillaceous and organic laminae. Facies 4 is at the top of the Oriskany. Here there is coarse-grained to pebble sized quartz overlain by laminated fine-grain sandstones with grain size increasing vertically.

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