Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area,these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Summary The use of transient well testing for detemining reservoir parameters andproductivity of horizontal wells has become common because of the upsurge inhorizontal drilling. Initially, horizontal well tests were analyzed with timeconvention techniques designed for vertical wells. During the last decade,analytic solutions have been presented for the pressure behavior of horizontalwells. New flow regimes have been identified, and simple equations and flowregime existence criteria have been presented for them. The flow regimes arenow used frequently to estimate horizontal and vertical permeabilities of thereservoir, wellbore skin, and reservoir pressure. Although the existing tools and interpretation techniques may be sufficientfor simple systems, innovation and improvement of time present technology arestill essential for well testing of horizontal wells in many reservoirs withdifferent geological environments and different well-completionrequirements. Introduction This paper reviews testing and interpretation methods for horizontal wells.Since Renney's1 article in 1941, many articles dealing withreservoir engineering, PI, and well-testing aspects of horizontal wells haveappeared in the literature.1-12 In the last decade, many papers havebeen published on the pressure behavior of horizontal wells in single-layer,homogeneous reservoirs.1-29 Recently, numerous papers oninterpretation of horizontal well test data21-26 and on the behaviorof horizontal wells in naturally fractured27-29 andlayered30,31 reservoirs have appeared. Because of time uncertainty of regulating flow rate or keeping it constantfor drawdown tests in general and buildup tests (particularly at early-times),the use of production logging tools to measure downhole flow rate duringpressure well tests has increased in the last decade. These tools haveincreased the scope of pressure-transient well testing by providing newmeasurements. Drawdown tests, for which it has often been difficult to keep theflow rate constant, can now provide the same quality of information as builduptests. Thus, the possibility of obtaining reliable information about thewell/reservoir system by using characteristic features of both transient tests(drawdown and buildup) has increased considerably. This is particularly crucialfor horizontal wells, where the early-time transient data are the mostsensitive to the vertical permeability and skin if the wellbore storage effectis minimized. Recently, production logging and downhole shut-in have beencombined32 to acquire reliable pressure/rate data during drawdownand buildup tests. Nonaxisymmetric drilling-fluid invasion and the long, snakelike completedwellbore make the cleanup process difficult, particularly toward the tips ofhorizontal wells. Therefore, it is important to obtain flow profiles and theeffective well length, which is often much less than the drilled length, forthe interpretation of horizontal well tests. The effective well length isimportant for determining damage skin and the vertical permeability. Productionlogging for horizontal wells is now usually conducted with a coiled-tubingsystem32. The fluid profiles also provide information about standingwater and wellbore crossflow, both common phenomena.32Unfortunately, the wellbore crossflow during buildup tests makes interpretationdifficult. In many instances, time pressure data may not reveal any informationabout the wellbore crossflow. The wellbore temperature profiles are oftenuseful tools for determining wellbore crossflow for buildup tests. Significant progress has been made over the last decade in developingforward analytical models and interpretation techniques for horizontal wells.Many flow regimes predicted by the theory which are essential for systemidentification, have been observed in the field example. However, testinghorizontal wells is sill challenging in terms of measurements andinterpretation. The field experience documented in time last decade indicatesthat interpreting tests from horizontal wells is much more difficult than forvertical wells. The objective of this paper to present solutions and to describe problems inpressure-transient testing and interpretation for horizontal wells rather thanto 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 ananisotropic reservoir, which is infinite in the x and y directions. Theformation 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-lengthis Lw, the radius is rw, and the distancefrom the wellbore to time bottom boundary is zw. The boundaryconditions at the top and bottom (in the z direction) of time system areeither no flow and/or constant pressure. For this horizontal well in asingle-layer reservoir, we provide simple equations for obtainingpermeabilities and skins. There are usually several flow regimes with differentdurations because of the partially penetrated nature of horizontal wells andmultiple boundary effects. For instance, as Fig. 2 shows, we may observethree radial (pseudoradial) flow regimes for a horizontal well in a verticallybounded single-layer reservoir. The flow regimes for horizontal wells have beeninvestigated by many authors,14-18 and specific methods have beenproposed to identify flow regimes and their durations under idealconditions.