Abstract

Horizontal wells are thought to be necessary in formations with low-permeability such as the Devonian shale to increase natural gas recovery and to reduce the risk of drilling a dry hole. In a horizontal well, the bore hole crosses multiple natural fractures in the reservoir. Stimulation data from a 2,000 ft (609.6 m) horizontal well drilled into the Devonian shales in Wayne County, West Virginia, was used in this study. Inflatable packers and casing port collars were used so that individual zones could be tested or stimulated along the wellbore.

This paper focuses on an analysis of hydraulic fracture design and geometry predictions for the above horizontal well. Current hydraulic fracture modeling theories address failure mechanisms and the propagation of a single crack from a vertical wellbore. These theories have been adapted to predict the pressure, flow rate, and induced fracture geometry for each natural fracture intersected by the hydraulic fracturing fluid in the horizontal wellbore. A tubing/annulus flow model was coupled with a hydraulic fracture model that predicts the three-dimensional geometry of multiple natural fractures propagating from a horizontal well. Additionally, a closed-form solution was developed to predict the pressure and flow rate distribution along the predict the pressure and flow rate distribution along the lateral extent of the wellbore.

Predicted results were compared with in situ fracture Predicted results were compared with in situ fracture diagnostics from gas (nitrogen and CO2) and foam stimulation treatments. Radioactive-tracer with spectralgamma-ray logging confirmed that both fluid pressure and stress perpendicular to the fracture affect the injection flow rate distribution along the wellbore. Both of these factors were used as governing mechanisms for fracture geometry predictions in the simulation model. Predictions based on these models and tracer logs confirm that the single crack theory for fracture propagation is not applicable for stimulations that propagation is not applicable for stimulations that are initiated along an isolated part of a horizontal borehole.

Background

Recent investigations at the U.S. Department of Energy's Morgantown Energy Technology Center have addressed the potential of horizontal wells to increase the gas recovery from low-permeability formations. A 2000 ft (609.6 m) horizontal well was drilled into the Devonian shale formation in Wayne County, West Virginia, to a measured length of 6,020 ft (1,835 m) and up to a true vertical depth of 3,403 ft (1,037 m).

A schematic of the well configuration is shown in Figure 1. The fracture spacing and locations of casing packers were determined with a downhole video camera packers were determined with a downhole video camera and geophysical well logs. Seven zones were isolated along the horizontal section, with external casing packers and port collars as part of the casing string. packers and port collars as part of the casing string. The port collars and packers were used to isolate stimulation intervals with existing perforations. Fracturing fluids were injected through the port collars into the wellbore tubing and annulus to pressurize the natural fracture system. Stimulations were performed in Zone 1 (see Figure 2) with nitrogen, carbon performed in Zone 1 (see Figure 2) with nitrogen, carbon dioxide, and sand-laden nitrogen foam to determine the most effective fracturing fluid for the shale formation.

Introduction

The objective of this study on a horizontal wellbore as to determine the recovery effectiveness of the natural fracture system and the impact of stimulating the well by hydraulic fracturing. Five stimulations ave been performed. Multiple fractures were propagated simultaneously during these stimulation propagated simultaneously during these stimulation treatents. The well was drilled in the direction of the minimum principal stress and orthogonal to the major fracture system in the reservoir. Six natural fracture orientations were identified with the downhole video camera and geophysical well logs. Figure 2 depicts he natural fracture pattern and orientations in Zone 1.

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