A Review of Horizontal Well and Drainhole Technology

Abstract

SPE Member Abstract This paper reviews the state-of-the-art of horizontal well and drainhole technology, and provides guidelines for the initial evaluation of horizontal well and drainhole drilling prospects. The primary emphasis of the paper is reservoir engineering. Since drilling and completion methods influence a horizontal well's performance, a brief review of these aspects of technology is also included. The paper also reports the field production histories available in the literature. Introduction During the last decade, significant advances in drilling technology have made it possible to drill horizontally. Although the major emphasis of this paper is a reservoir engineering review, for completeness a brief description of drilling and completion of horizontal wells is also included. Moreover, for a successful field operation, a drilling method should be chosen based upon reservoir considerations. Horizontal wells are normally new wells, 1000 to 3000 ft long, which are drilled from the surface. Drainholes are generally drilled from the existing vertical wells and are 100 to 700 ft long. One can drill either a single drainhole or multiple drainholes through a single vertical well. In this paper the term horizontal well refers to both new horizontal wells and drainholes, unless noted otherwise. Horizontal wells and drainholes represent wells with limited fracture height, where fracture height is equal to the wellbore diameter. A properly designed horizontal well would be equivalent to a vertical well with a fully penetrating fracture. A horizontal well represents a long, controlled vertical fracture. In most fracture jobs it is difficult to obtain infinite conductivity and, moreover, fracture conductivity decreases over time. In contrast, a horizontal wellbore offers an almost permanent infinite conductivity fluid flow path. Additionally, in reservoirs where the bottom water or top gas cap renders fracturing difficult, a horizontal well offers an alternative to obtain high production rates without gas and water coning. A horizontal well offers a viable completion option and will compete with fracturing in the future. In general, horizontal wells are found effective in thin reservoirs, some naturally fractured reservoirs, tight reservoirs, and in reservoirs with gas and water coning problems. In this paper these horizontal well characteristics and applications are discussed. In addition to a literature review, the paper provides guidelines for oil production forecasting. Moreover, available field data are also discussed. DRILLING AND COMPLETION As listed in Table 1, the presently available horizontal drilling methods can be classified in four broad categories, depending upon the turning radius required to turn from a vertical to a horizontal direction. Figure 1 compares turning radii of different drilling methods. A detailed discussion of various drilling methods is given elsewhere, but a brief description is given below. Ultra Short Turning Radius (1 to 2 ft) This method utilizes water jets to drill 100–200 ft long drainholes with a turning radius of 1 to 2 ft. The process involves underreaming the vertical wellbore and then drilling several radials from the underreamed zone. The first drilling system requires a 48 inch diameter underreamed zone while the improved second system requires a 24 inch diameter zone. The underreamed zone length varies from 6 to 10 ft depending on the system utilized. The drainhole diameter varies from 1-1/2 to 2-1/2 inches. Usually two or more drainholes are drilled. For sand control, the drainholes are completed using either slotted liners or gravel packing. After completing the drainhole, the pipe is severed. P. 613^