ABSTRACT

This paper addresses the problem of failure and fracture initiation from deviated boreholes, with emphasis on the added complications introduced by the inclination of the borehole with respect to the in situ principal stress directions. The paper also discusses the implications of deviated wells with regard to completion and stimulation.

1 Introduction

  • In thin reservoirs where horizontal wells provide a larger contact area; hence, a better immediate return upon investment.

  • In irregular reservoirs as opposed to large blanket reservoirs, where inclined boreholes allow access to isolated pockets, without the need to drill a number of additional wells, or to add new platforms in offshore situations.

  • In strongly anisotropic formations, where the ratio of the vertical to horizontal permeabilities hinders the production from a classical vertical completion.

  • In naturally fractured reservoirs, where directional drilling provides a communication channel intersecting the family of dominant discontinuities.

  • In situations where water/gas coning is of concern; highly inclined boreholes will definitely delay the "breakthrough time", allowing greater hydrocarbon recovery.

  • In secondary waterflooding schemes where directional drilling technology allows the placement of a man-made "curtain" , resulting in a more efficient drive.

Although extended reach drilling is by no means a new idea, only recently has the topic generated renewed interest in the petroleum industry as it could provide substantial economic enticements in the following situations: The reasons why most boreholes are "dropped back" to vertical, even from off-shore platforms at great expense, is due to the present lack of confidence in success-fully completing and stimulating highly inclined wellbores.

The present day drilling technology allows a vertical hole to be deviated to horizontal within a 15 ft. milled-out section of the casing (build rate of 6 degrees/foot), using high angle whipstocks and articulated drive assemblies. Horizontal sections in excess of 3,000 feet have been successfully drilled under open hole completion schemes. The major issues, specific to drilling inclined holes are: (i) cutting removal and, (ii) borehole stability. Indeed, in deviated wells exceeding 60 degrees from vertical, chunks of rock usually remain at the highest bend location, rather than dropping to the bottom of the borehole and be re-ground.

Once the hole is drilled, a steel casing is usually cemented into the well to iso-late the various producing intervals. Such a treatment is performed via pumping cement inside the casing and displacing the wellbore fluids in the annulus. In highly deviated holes, the concern is the potential of leaving an unbound "channel" along the upper generatrix of the borehole, due to liquid segregation. Special theological cement properties (e.g. turbulent flow regime, controlled setting time, expansion during setting,...) have taken care of most of these problems.

What still remains unresolved is the question of hydraulically fracturing an in-

clined borehole, especially the created geometry. On one end of the spectrum, a longitudinal .fracture could be initiated, resulting to "traditional" design and field eatments. On the other end, if a transverse .fracture is initiated, only a limited amount of existing perforations will "participate" in subsequent production.

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