Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Summary The use of transient well testing for detemining reservoir parameters and productivity of horizontal wells has become common because of the upsurge in horizontal drilling. Initially, horizontal well tests were analyzed with time convention techniques designed for vertical wells. During the last decade, analytic solutions have been presented for the pressure behavior of horizontal wells. New flow regimes have been identified, and simple equations and flow regime existence criteria have been presented for them. The flow regimes are now used frequently to estimate horizontal and vertical permeabilities of the reservoir, wellbore skin, and reservoir pressure. Although the existing tools and interpretation techniques may be sufficient for simple systems, innovation and improvement of time present technology are still essential for well testing of horizontal wells in many reservoirs with different geological environments and different well-completion requirements. Introduction This paper reviews testing and interpretation methods for horizontal wells. Since Renney's1 article in 1941, many articles dealing with reservoir engineering, PI, and well-testing aspects of horizontal wells have appeared in the literature.1–12 In the last decade, many papers have been published on the pressure behavior of horizontal wells in single-layer, homogeneous reservoirs.1–29 Recently, numerous papers on interpretation of horizontal well test data21–26 and on the behavior of horizontal wells in naturally fractured27–29 and layered30,31 reservoirs have appeared. Because of time uncertainty of regulating flow rate or keeping it constant for drawdown tests in general and buildup tests (particularly at early-times), the use of production logging tools to measure downhole flow rate during pressure well tests has increased in the last decade. These tools have increased the scope of pressure-transient well testing by providing new measurements. Drawdown tests, for which it has often been difficult to keep the flow rate constant, can now provide the same quality of information as buildup tests. Thus, the possibility of obtaining reliable information about the well/reservoir system by using characteristic features of both transient tests(drawdown and buildup) has increased considerably. This is particularly crucial for horizontal wells, where the early-time transient data are the most sensitive to the vertical permeability and skin if the wellbore storage effect is minimized. Recently, production logging and downhole shut-in have been combined32 to acquire reliable pressure/rate data during drawdown and buildup tests. Nonaxisymmetric drilling-fluid invasion and the long, snakelike completed wellbore make the cleanup process difficult, particularly toward the tips of horizontal wells. Therefore, it is important to obtain flow profiles and the effective well length, which is often much less than the drilled length, for the interpretation of horizontal well tests. The effective well length is important for determining damage skin and the vertical permeability. Production logging for horizontal wells is now usually conducted with a coiled-tubing system32. The fluid profiles also provide information about standing water and wellbore crossflow, both common phenomena.32Unfortunately, the wellbore crossflow during buildup tests makes interpretation difficult. In many instances, time pressure data may not reveal any information about the wellbore crossflow. The wellbore temperature profiles are often useful tools for determining wellbore crossflow for buildup tests. Significant progress has been made over the last decade in developing forward analytical models and interpretation techniques for horizontal wells. Many flow regimes predicted by the theory which are essential for system identification, have been observed in the field example. However, testing horizontal wells is sill challenging in terms of measurements and interpretation. The field experience documented in time last decade indicates that interpreting tests from horizontal wells is much more difficult than for vertical wells. The objective of this paper to present solutions and to describe problems in pressure-transient testing and interpretation for horizontal wells rather than to provide a scholarly review of time literature on time subject. Flow Regimes for Horizontal Wells Let us consider a horizontal well (Fig. 1) completed in an anisotropic reservoir, which is infinite in the x and y directions. The formation permeabilities in the principal directions are denoted bykx=ky=kH, andkz=kV, with a thickness, h, porosity, f, compressibility, ct, and viscosity, µ. The well half-length is Lw, the radius is rw, and the distance from the wellbore to time bottom boundary is zw. The boundary conditions at the top and bottom (in the z direction) of time system are either no flow and/or constant pressure. For this horizontal well in a single-layer reservoir, we provide simple equations for obtaining permeabilities and skins. There are usually several flow regimes with different durations because of the partially penetrated nature of horizontal wells and multiple boundary effects. For instance, as Fig. 2 shows, we may observe three radial (pseudoradial) flow regimes for a horizontal well in a vertically bounded single-layer reservoir. The flow regimes for horizontal wells have been investigated by many authors,14–18 and specific methods have been proposed to identify flow regimes and their durations under ideal conditions.