Abstract

In most reservoir engineering studies, the vertical fracture height and the formation height are assumed to be equal, which may or may not be true. This study investigates the effect of vertical fracture penetration and horizontal anisotropy in the penetration and horizontal anisotropy in the development of production decline curves for a low permeability gas reservoir. permeability gas reservoir. This study shows that the productivity for a given vertical fracture decreases as the fracture position departs from the center of the formation interval. Horizontal anisotropy significantly impacts production decline curves. Up to approximately 70 percent of cummulative recovery is affected significantly regardless of the fracture penetration ratio.

Introduction

Hydraulic fracturing methods for commerical production of natural gas from low permeability reservoirs production of natural gas from low permeability reservoirs have become of great interest to the petroleum industry. Fracturing treatments are routinely performed to help stimulate the well's productivity. In addition to the desire for increased production, the capability to predict the increases expected from the fracturing predict the increases expected from the fracturing treatment is also helpful. This knowledge is useful in designing and planning the most economical fracturing treatment to accomplish the desired production goals for the well.

In this study, prediction of production increase due to vertical fractures was obtained from a general purpose three-dimensional numerical gas reservoir purpose three-dimensional numerical gas reservoir simulator. In most studies, it is assumed that the vertical fracture height and the formation height are equal, which may not be true. Also, the formation is often considered to be a homogeneous isotropic porous medium, an assumption which may also often not be true. It should be recognized that these situations do not exist in most reservoirs. This study investigates the effect of vertical fracture penetration and anisotropy in the development of production decline curves.

Production decline data for vertically fractured wells has several important applications. First, the data provides the petroleum engineer with an approach to supplement or validate his theoretical prediction of production increases. Using this information, he may design and plan the most economical fracturing treatment to accomplish the desired production goals for the well. He may use production decline curves to calculate future production rates and also for determining total reserves left after initial production. This analysis provides the engineer with an production. This analysis provides the engineer with an approximation of the production time remaining for a well. Another important application of this analysis is estimating the time necessary to recover the initial investment of drilling and completing the well.

The first factor to be discussed, which affects production decline curves, is the significance of production decline curves, is the significance of anisotropy in vertically fractured wells. An anisotropic reservoir has permeability that varies with flow direction. In other words, the maximum and minimum permeabilities are oriented 90 degrees apart. Most studies performed deal with anisotropy in the vertical and horizontal directions. This study will look at the effects of anisotropy, specifically in the x and y directions of the horizontal plane.

Anisotropy can be caused by many factors but is primarily caused by sedimentary processes, such as primarily caused by sedimentary processes, such as channel fill deposits or by tectonics. Tectonics is very significant when one considers horizontal anisotropy. Due to tectonic effects, the stress patterns in the rock can change orientation. The greatest stress should be approximately vertical, while the least stress should be horizontal. Flow patterns in the horizontal plane depend upon the predominant orientation of natural fissures. The natural fracture orientation depends predominately upon preexisting stress patterns which may have changed over geologic time.

P. 69

This content is only available via PDF.
You can access this article if you purchase or spend a download.