Summary
This paper presents a discussion of the pressure-transient responses of
horizontal wells in anticlinal structures and curved and undulating wells in
slab reservoirs. It confirms that, in the absence of a gas cap, conventional
horizontal-well models may be used to approximate the flow characteristics of
the systems in which the trajectory of the well does not conform to the
curvature of the producing structure. If a gas cap is present, however, the
unconformity of the well trajectory and producing layer manifests itself,
especially on derivative characteristics when the gas saturation increases
around the well. In general, the most significant deviations from the
conventional horizontal-well behavior are observed during the buildup periods
following long drawdowns. In these cases, the pressure-transient analysis is
complicated and requires detailed numerical modeling of the well trajectory and
reservoir geometry in the vertical plane.
Introduction
Conventional horizontal-well pressure-transient models assume that the top
and bottom boundaries of the reservoir are horizontal planes; that is, the
producing stratum is a slab, and the well is straight and parallel to the slab
boundaries. Wells, however, may be drilled horizontally in anticlines and
domes, or they may be curved or undulating in a horizontal slab reservoir.
In the literature, several reservoir shapes have been considered in the
context of horizontal wells: infinite slab (Clonts and Ramey 1986; Ozkan et al.
1989; Goode and Thambynayagam 1987; Rosa and Carvalho 1989; Kuchuk et al. 1990,
1991; Ozkan and Raghavan 1990a), cylinder (Ozkan and Raghavan 1991a, 1991b),
rectangular parallelepiped (Ozkan and Raghavan 1991a, 1991b; Daviau et al.
1988; Odeh and Babu 1990), and vertical no-flow boundary at an arbitrary
orientation (Azar-Nejad et al. 1996a). The common feature of these reservoir
models is the assumption that the top and bottom boundaries are horizontal
planes. Despite the fact that the conditions at the top and bottom boundaries
strongly influence the pressure-transient characteristics of horizontal wells
(Clonts and Ramey 1986; Ozkan et al. 1989; Goode and Thambynayagam 1987; Ozkan
2001), the effect of the curvature of these boundaries, as in the case of
anticlines and domes, has not been discussed in the literature.
Similar to the curvature of the top and bottom boundaries, the curvature or
undulations of horizontally oriented wells (referred to as horizontal wells in
this paper) have not attracted much attention in the
pressure-transient-analysis literature. Two studies have addressed this issue
specifically. Azar-Nejad et al. (1996b) considered a curved well that was a
quarter of a circle (from vertical to horizontal) in a slab reservoir. They
showed that especially in anisotropic reservoirs, the pressure-transient
response of the curved well could not be approximated by that of a straight
horizontal well of equal drilled length. This study did not address the issue
of effective well length and the effect of the aspect ratio (the ratio of the
distance from the well to the closest boundary and thickness of the
formation).
Goktas and Ertekin (2003) discussed another common problem for horizontal
wells—undulations. Their study indicated that when the vertical window of
undulations becomes comparable to the formation thickness, undulations might
influence the characteristics of pressure-transient responses. For practical
windows of undulations that commonly result from standard drilling practices,
however, the pressure-transient responses could be closely approximated by that
of a straight horizontal well. This conclusion was different from that of
Azar-Nejad et al. (1996b). It also must be noted that Goktas and Ertekin (2003)
used the straight distance between the tips of the undulating well in the
comparisons with straight horizontal wells, as opposed to the total drilled
length used by Azar-Nejad et al. (1996b).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
10 May 2004
- Meeting paper published:
5 October 2003
- Revised manuscript received:
26 June 2006
- Manuscript approved:
5 November 2006
- Version of record:
20 February 2007
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