Petroleum and Coal Pet Coal (2019); 61(4) 749-756 ISSN 1337-7027 an open access journal Article Open Access A SENSITIVITY ANALYSIS FOR THE EFFECTIVE PARAMETERS DISPARITY ON PRESSURE AND PRESSURE DERIVATIVE OF WELL-TESTING IN A HORIZONTAL WELL Arash Pourabdol Shahrekordi 1, 2* , Mohammad Behnood 1 , Armin Hosseinian 3 , Sajjad Mozaffari 1, 2 , Seyed Jamal Sheikhzakariaee 1 1 Department of Petroleum and Chemical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran 2 Young Researchers and Elite Club, Science and Research Branch, Islamic Azad University, Tehran, Iran 3 Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada Received May 2, 2019; Accepted June 13, 2019 Abstract In this study, the pressure and pressure derivative of a drawdown test for a horizontal well was inves- tigated. The examined well, it is located in the middle of a homogeneous oil reservoir and the calculation has been obtained using an industry standard PTA software known as Saphir software. Afterward, from the pressure derivative curve, initial radial, linear and final radial flow periods were observed, respectively. Then, the variation of the effective three parameters was inspected using the sensitivity analysis tool provided in Saphir software. The considered parameters are the ratio of the vertical to horizontal permeability (Kz/Kr), skin factor (S) and the horizontal length of the well ( Lw). that the results reveals that by increasing the ratio of Kz/Kr and Lw the pressure curve is lowered, and it shifts upward with growing the S parameter. Meanwhile, by increasing the value of parameter Kz / Kr ratio, the initial radial flow becomes shorter in the pressure derivative curve and the linear flow starts earlier. In the other hand, within a pressure derivative diagram, as the value of S parameter rises, the hump becomes larger resulting a delay in the start of the first radial flow. Besides, the other consequences will be shifting downward the curves of the first radial flow and linear flow periods, increasing the duration of the linear flow, and also the final radial flow starts later. The analysis outcomes of this study were in conjunction of what it will be expected in well-testing procedure. Keywords: Horizontal Well; Skin Factor; Permeability; Well-Testing; Saphir well-testing Software; Sensitivity Analysis. 1. Introduction Horizontal drilling is one of the most advanced technologies which has been used in the last thirty years in oil and gas industry. In fact, the horizontal wells are type of directional wells and they have many different practical applications. Those can be categorized in different aspects. The horizontal wells are being employed to reach the reservoirs located below the sea, lake, river, residential areas and mountainous terrain, where vertical access to these reservoirs is not possible [1] . In oil reservoirs with a high ratio of vertical to horizontal perme- ability, oil has a much tendency to move in a vertical direction. Therefore, horizontal wells cause an increase in oil production from this type of reservoirs [2] . These kinds of wells reduces the pressure drop due to production in the area around the well, slows down and delays water and gas coning phenomena, which is the main application of horizontal wells [1] . In reservoirs with thin pay zone, horizontal wells cause a greater degree of the well to reservoir contact, thus the production from this type of reservoirs increases, significantly [2] . Moreover, in the 749