Well-Control Operations in Horizontal Wells

Abstract

Summary This paper analyzes well control for horizontal wells. It presents a computer model for predicting the pressure behavior in a horizontal well during gas-kick removal and analyzes the simulation results for several field conditions. The paper also analyzes the drillpipe-pressure schedule, the kick-tolerance concept, and the swabbing effect during tripping out of the hole. Introduction Horizontal drilling has quickly become one of the most successful technologies of the oil industry. During the past 5 years, this technology has proved to he an efficient means to improve production rates and recoveries. The importance of this technology is also confirmed by the large number of papers recently published on this topic. Although these papers discuss several aspects of horizontal drilling, one topic papers discuss several aspects of horizontal drilling, one topic remains unexplored: well control operations. This paper investigates the important aspects of well control for horizontal drilling. The first part of this paper describes a computational procedure to predict the pressure behavior in the annulus during a gas-kick circulation out of the well. The second part presents another comutational procedure for establishing the drillpipe-pressure schedule to be followed by the rig personnel during the displacement of the old mud by the kill mud. The third part discusses kick tolerance and its applications in horizontal wells. Finally, the paper presents a simplified theory for the swabbing effect during presents a simplified theory for the swabbing effect during tripping out of the hole and demonstrates the hazards of taking a kick during this operation. Horizontal Wells Fig. 1 shows the geometry of the example horizontal well. The well comprises three sections: the vertical, buildup, and horizontal Sections. The radius of curvature, r, of the 90deg. of arc of circle is defined by the buildup rate, Rbu: r=5729.58/Rbu....................(1) The length of the buildup section, Lbu, can be calculated by the following equation: Lbu=1.5708r.....................(2) In Fig. 1, the true vertical depth (TVD) is the sum of the vertical section length and the radius of curvature, and the total measured depth (MD) is the sum of the vertical, buildup, and horizontal lengths. MD's can easily be converted to vertical depths, or vice versa, with simple trigonometric relationships. Mathematical Model for the Annular Space This section describes a numerical procedure for modeling the pressure behavior inside the annular space of a horizontal well during pressure behavior inside the annular space of a horizontal well during gas-kick removal. With this numerical procedure in a FORTRAN computer program, it was possible to simulate many field conditions where pertinent drilling variables were varied to analyze their effects on pressure behavior. Later in this section, the results of the simulations are analyzed and discussed. Assumptions and Considerations. Previous studies have shown that if the gas-kick region is considered to be a plug or a single bubble, then predicted wellbore pressures will be unrealistically high. More realistic results are obtained when the gas-kick region is modeled by a two-phase zone. This study assumes that the gaskick zone is a two-phase mixture of gas and water-based mud flowing under unsteady-state conditions.