Interference tests provide valuable information about reservoir characteristics such as aerial average transmissibility, storativity, and degree of communication between wells. Due to the larger radius of influence and larger contact area of a horizontal well with the formation than its vertical counterpart, the transmissibility and storativity obtained from interference testing of horizontal wells would be more representative of the formation characteristics than those obtained from interference testing of vertical wells.

Very few studies have been made about interference testing of horizontal well. Malekzadeh and Tiab1 were the first to analyze and give solution for interference test of horizontal wells. They presented type curves with corresponding equations for the analysis; they also presented a direct method to analyze these tests. Their work however is limited to the study of an isotropic medium. To date, a solution to the problem of interference testing of horizontal wells in an anisotropic medium and the corresponding type curves have not been presented.

In this study, a method, based on the pressure derivative function, is presented for analyzing the interference testing of horizontal wells in an anisotropic medium. Type curves of dimensionless pressure and dimensionless pressure derivative have been presented, with the corresponding equations to calculate the directional permeability and the storativity. It has been found that type curves presented in this study for the case of an isotropic medium match well those obtained by Malekzadeh and Tiab under the same conditions.

A direct method using pressure derivative curve, based on the Tiab's Direct Synthesis (TDS) technique, is also presented to calculate the permeability in both directions and the storativity. The model was validated by matching the numerical and the analytical curves of pressure and pressure derivative. A sensitivity analysis was performed to investigate the effect of the following parameters on the proposed technique: permeability anisotropy, distance between the interfering wells, and the length of the horizontal drains. It has been concluded that the horizontal permeability anisotropy and the distance between the interfering wells have a significant effect on the behavior of the pressure and pressure derivative, whereas the length of the horizontal drain only slightly affects these responses.


Due to significant breakthroughs in drilling technology in the early 1980's, horizontal well drilling became an attractive technology and was later proven to be more advantageous over vertical wells in certain cases, such as: tight reservoirs, especially if vertical fractures are suspected; naturally fractured reservoirs containing vertical fractures; unconventional low permeability gas reservoirs; thin formations; thin oil columns, especially when bottom water, and/or gas cap is present; old reservoirs that no longer have adequate drive mechanisms; producing reservoir with extremely high dip angles; secondary recovery operations (increased injection area and improved sweep efficiency); exploration and development of inaccessible locations.

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