Inflow Performance of Horizontal Wells in Multilayer Reservoirs

Abstract

SPE Members Abstract The inflow performance formulas for horizontal wells presented in the literature are all for single-layer reservoirs. This paper presents formulas for evaluating the inflow performance of horizontal wells in vertically layered reservoirs with crossflow. The system has a rectangular drainage region in the x and y directions and is bounded at the top and the bottom by horizontal planes with no-flow and/or constant pressure boundary conditions. The well is located anywhere within the drainage volume and could be of any length. Analytical models for dynamic (transient), pseudosteady-state, and steady-state productivity indices for horizontal wells in layered and bounded reservoirs are developed. They may also be used for the analysis of transient pressure or flow rate data, and both from horizontal wells in bounded systems. We present a method for the selection of the well location in a layered system for maximizing the productivity index. We also show that in a layered system with variable vertical permeability distribution, the proper selection of the well location is important for maximizing the productivity. The layering effect for most systems may not be treated as a single layer with average properties. The vertical permeability of each layer is the controlling parameter for the productivity. The productivity model developed may be used as a screening tool before or during drilling to obtain the best well location for a horizontal well. It can also be used to fine-tune numerical simulators for horizontal wells in layered systems. Introduction Horizontal wells have recently become highly popular for producing oil and gas reservoirs and obtaining information about lateral variations of reservoir properties. The inflow performance formulas for horizontal wells presented in the literature are all for single-layer reservoirs. It is well-known that many oil-bearing formations, the world's most prolific reservoirs, consist of many different lithological units. In these reservoirs, rock properties vary considerably in the vertical direction. Depending on the depositional environment, formation properties vary gradually or rapidly. For example, layers consisting of siltstone or chert are usually low-permeability formations. Conglomerate formations are usually highly conductive. The objective of this paper is to examine the effect of layering on well productivity. In other words, in which layer and at what length should a horizontal well be drilled in order to have the maximum productivity if layer properties are known before the drilling (they can be obtained from nearby wells and/or the vertically drilled section)? In this paper, we present analytical solutions for obtaining a dynamic (time-dependent) and pseudosteady-state or steady-state productivity index for horizontal wells in layered systems. The model includes the no-flow or constant-pressure boundary effects in the x, y, and z directions. As shown in Fig. 1, the horizontal well may be placed anywhere within the drainage volume. P. 929^