Production Forecasting Methods for Horizontal Wells

Abstract

Abstract This paper summarizes production forecasting methods for horizontal wells. The forecasting methods are based on analytical solutions and correlations of numerical model results. Methods are suggested for single phase flow, solution gas drive, naturally fractured, and bottom water drive reservoirs. These forecasting methods could be used for an initial evaluation of horizontal well drilling prospects. A horizontal well can be looked upon as a controlled infinite-conductivity vertical fracture of limited height. Therefore, this paper compares horizontal well productivity with finite and infinite-conductivity, and uniform-flux fractures. A choice between horizontal wells and stimulated vertical wells should be determined by considering local fracturing experience and local costs for stimulation treatments and horizontal drilling. Introduction During the last eight years, technology to drill horizontal wells has advanced significantly. Presently, various commercial techniques are available to drill horizontal wells. Table I lists these techniques, which are classified on the basis of their turning radius. Turning radius is defined as the radius of curvature of a well that is required to turn the well from a vertical to a horizontal direction. Horizontal wells are usually new wells and are 300 m (1000 ft) to 1000 m (3000 ft) in length. Drainholes are usually drilled through existing vertical wells and are typically 50 m to 200 m (150 to 700 ft) in length. In this paper the term horizontal well applies to both types of completions unless noted otherwise. From the reservoir standpoint, a horizontal well represents a controlled vertical fracture. The fracture height is equal to the well bore diameter. Moreover, horizontal wells offer an infinite conductivity flow path, i.e., the pressure drop within the well bore from one end to the other is minimal. In contrast, in a conventional fracture job, it is difficult to obtain infinite-conductivity. Additionally, the conductivity may drop over time, resulting in loss of well productivity. In the future, horizontal wells and fracturing techniques will compete with each other. Therefore, this paper includes comparisons of productivities for vertical stimulated wells with horizontal wells. The comparisons are restricted to thin reservoirs, i.e., reservoir thicknesses up to 50 m. Figure I shows a schematic diagram of a horizontal well, drainhole, fully penetrating fracture, and an induced fracture. Horizontal wells have a distinct advantage over vertical wells in reservoirs with gas and water coning problems, where fracturing could pose potential difficulties. In stimulation treatments, control of fracture direction and height is difficult. A fracture treatment may create a direct communication with the top gas or bottom water P. 303^