Analytical solutions are presented for pressure-drawdown and -buildup analyses of horizontal wells in anisotropic reservoirs with tectonic, regional, or contractional natural fractures. (The reservoirs might be limited by parallel sealed boundaries.) These solutions have led to the identification of various flow periods, including radial in a vertical plane, transitional as a result of flow from the matrix into the fractures, linear when the transient pressure reaches the upper and lower boundaries of the reservoir, pseudoradial toward the wellbore in a horizontal plane, and linear when the transient reaches the outer parallel boundaries. Recognition of these flow periods leads to the calculation of Pi or p*; permeabilities in the x, y, and z directions; storativity ratio, w; average distance between natural fractures; fracture porosity; and fracture aperture, skin, and pseudoskin caused by vertical and horizontal partial penetration. Calculations are illustrated with an example.

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IN1RODUCT:ION Type curves have been developed to facilitate reservoir Characterization. Their use is illustrated with an example.

Analytical solutions that include real gas pseudo pressures are presented for pressure drawdown and buildup analysis of pressure drawdown and buildup analysis of horizontal wells in anisotropic reservoirs with tectonic, regional or contractional natural fractures. The reservoir might be limited by parallel sealed boundaries. The solutions have been obtained with the use of Fourier and Laplace transforms. Wellbore storage has been handled using a finite difference scheme of the convolution integral. These solutions have led to the identification of the following flow periods: periods:1. Wellbore storage which might mask many of the subsequent flowperiods.2. A first radial flow period at early times from the natural fractures into the horizontal well. This occurs in a verticalplane.3. A transition period due to flow from the matrix into the naturalfractures.4. A second radial flow period, still in a vertical plane, which starts when pressures in matrix and fractures reach an equilibrium. This pressure equilibrium can also be attainedat a later flow period in which case the second radial period would not be present.5. A linear flow period when the pressure transient reaches the upper and lower boundaries of the reservoir.6. A pseudo radial flow period towards the wellbore in a horizontal plane.7. A linear flow period when the pressure transient reaches the outer parallel boundaries. It is concluded that recognition of these flow periods can lead to calculation of p i or p*, permeabilities in the x, y and z directions, storativity ratio omega, average distance between natural fractures, fracture porosity, fracture aperture, skin, and pseudo skin due to vertical and horizontal partial penetration. Calculations are illustrated with an example. Introduction Horizontal wells are coming of age and they should prove very important in the development of naturally fractured reservoirs. Previous publications by these authors considered transient pressure analysis of horizontal wells in anisotropic naturally fractured reservoirs. This paper extends our previous work to the case of horizontal gas wells. A literature survey indicated that only a few papers have been published on the area of horizontal gas wells and most of them deal with type curves or numerical simulation work, for single-porosity systems. P. 155

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