Pressure Analysis of an Unstimulated Horizontal Well With Type Curves
Authors: Duda J.R. , Salamy S.P. , Aminian Khashayar , Ameri Samuel 
Published: December 2007

Horizontal wells are effective alternatives to fractured vertical wells in several reservoir types, including low-permeability sandstones and shales. An understanding of fluid flow relationships is a prerequisite for efficient use of horizontal technology. The pressure-buildup data analyzed here were recorded in a horizontal completion in a low-permeability gas-bearing shale. The well was drilled and tested to verify laboratory and computer modeling research. After the general solutions were developed, the data were analyzed to describe reservoir properties and the effectiveness of the completion. Results of the type-curve analysis indicate that the contributing well length was shorter than the actual drilled length and that the contributing reservoir thickness was less than the typical net pay. Numerical modeling was used to verify the calculated parameters. Results fully support the well length and reservoir thickness determined through type-curve analysis, indicating that an unstimulated horizontal well completed in a heterogeneous formation may be insufficient to link the full vertical extent of the reservoir to the wellbore.

Introduction

The use of horizontal wells to exploit oil and gas reservoirs has continued to increase in recent years. One operator planned 30 horizontal wells as part of its 1989 drilling program. The target formation for much of this drilling program is the Bakken shale, which underlies several western counties in North Dakota's Williston basin. The Austin chalk play of south-central Texas is also host to a horizontal well drilling boom. Other areas where horizontal wells have been used to exploit petroleum-bearing rocks include Canada and Alaska's North slope.

Most horizontal drilling has targeted oil-bearing formations, although interest in using horizontal wellbores to improve the economics of natural gas plays is increasing. The gas-bearing Devonian shale underlying southwestern West Virginia has been the target for several horizontal well operations. The U.S. DOE sponsored drilling, completion, and testing of the first horizontal well, the Recovery Efficiency Test No.1 (RET No.1), as part of the Unconventional Gas Research Program.

This paper reports the analysis of the prestimulation pressure-buildup test for RET No.1 using general solutions (type curves). Families of general solutions for rate-constrained horizontal well flow were generated for field data analysis. The type curves include wellbore-storage effects, which mask the true reservoir response at early times. These effects can extend for considerable lengths of time, especially in very long horizontal wells and in those shut in at the wellhead.

Joshi reported pressure responses of horizontal wells flowing at a constant rate. Mutalik et al. developed skin factors for rate-constrained problems involving horizontal wells. A finite-difference, 3D gas simulator was used to generate the pressure responses for this study. In the end, the field data were matched with a general pressure response to characterize the reservoir and the well.