Well Configurations in Anisotropic Reservoirs
- M.J. Economides (Texas A&M U.) | C.W. Brand (Mining U. Leoben) | T.P. Frick (Mining U. Leoben)
- Document ID
- Society of Petroleum Engineers
- SPE Formation Evaluation
- Publication Date
- December 1996
- Document Type
- Journal Paper
- 257 - 262
- 1996. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing, 1.6 Drilling Operations, 5.6.1 Open hole/cased hole log analysis, 5.1.1 Exploration, Development, Structural Geology
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Horizontal wells have emerged as a new means for well productivity enhancement. Simultaneously, they have brought forward the need to recognize and account for permeability anisotropies, including vertical-to-horizontal and horizontal-to-horizontal directions. In addition, there is the possibility of multiple horizontal drainholes emanating from the same vertical well. Performance relationships for the most interesting well configurations are presented including both early-time and late-time differences rather than only bounded flow regimes. Solutions for arbitrarily oriented single or multiple horizontal wells are introduced along with a discussion of well known existing relationships.
It is a foregone conclusion that horizontal wells will capture an ever increasing share of all petroleum wells drilled. The performance of these wells depends greatly on appropriate reservoir selection, substantial predrilling formation evaluation and optimized completion and stimulation practices.
There have been several attempts to describe and estimate horizontal well productivity and for injectivity indexes and several models have been employed for this purpose. Following the tradition of vertical well productivity models, analogous well and reservoir geometries have been considered. A widely used approximation for the well drainage is, conveniently, a parallelepiped model with no-flow or constant-pressure boundaries at the top or bottom and either no flow or infinite-acting boundaries at the sides.
One of the earliest models was introduced first by Borisov (1964) assuming a constant pressure drainage ellipse whose dimensions depend on the well length. This configuration evolved into a widely used equation presented by Joshi (1988) accounting for vertical-to-horizontal permeability anisotropy and, adjusted by Economides et al. (1991) for a wellbore in elliptical coordinates.
This model, while useful for first approximations and comparisons with vertical well productivity indexes, does not account for either early-time or late-time phenomena nor, more importantly, realistic well and reservoir configurations.
Babu and Odeh (1989) used an expression for the pressure drop at any point by integrating appropriate point source (Green's) functions in space and time. Their solutions for various no-flow boundary positions include infinite-sum expressions, accounting for individual pseudosteady-state pressure drops. These forms are rather complicated and cumbersome to calculate.
Using vertical well analogs, Babu and Odeh (1989) grouped their solutions into reservoir/well configuration shape factors and a (horizontal) partial penetration skin effect.
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