SPE 164547 Evaluation of Well Performance for the Slot-Drill Completion in Low and Ultra-Low Permeability Oil and Gas Reservoirs

Low to ultralow permeability formations require "special" treatments/stimulation to make them produce economical quantities of hydrocarbon and at the moment, multi-stage hydraulic fracturing (MSHF or MHF) is the most commonly used stimulation method for enhancing the exploitation of these reservoirs. Recently, the slot-drill (SD) completion technique was proposed as an alternative treatment method in such formations (Carter 2009). This paper documents the results of a comprehensive numerical simulation study conducted to evaluate the production performance of the SD technique and compare its performance to that of the standard MSHF approach. We investigated three low permeability formations of interest, namely a shale-gas, a tight-gas, and a tight/shale-oil formation. The simulation domains were discretized by using Voronoi gridding schemes to create representative meshes of the different reservoir and completion systems modeled in this study. The results from this study indicated that the SD method does not, in general, appear to be competitive in terms of reservoir performance and recovery compared to the more traditional MSHF method. Our findings indicate that the larger surface area to flow that results from the application of MSHF is much more significant than the higher conductivity achieved using the SD technique. However, there may exist cases, e.g., lack of adequate water volumes for hydraulic fracturing, or very high irreducible water saturation that leads to adverse relative permeability conditions and production performance, in which the low-cost SD method may make production feasible from an otherwise challenging (if not inaccessible) resource. Introduction Shale gas and other low/ultralow permeability hydrocarbon reservoirs have recently emerged as very important energy sources. Such reservoirs, which are often referred to as "unconventional" resources, have now become the hub of exploration and production (E&P) activities in several areas, but mainly in North America. As at 2005, more than 25 percent of the daily natural gas production in the United States was derived from unconventional reservoirs (Naik 2005). The SD is a completion technique that can be used to recover hydrocarbons from such low permeability reservoirs. The Slot-Drill (SD) completion method The proposed SD technique is an advanced cable-saw method that works like a "downhole hacksaw", and is suitable for application at depths ranging from 1,000 to 10,000 ft (Carter 2009). It involves the use of a tensioned abrasive cable, which is attached to the drill-pipe, to create a slot perpendicular to the wellbore up to 100 ft. deep into the target formation (Fig. 1). The SD is purported to provide a large surface area and a high-conductivity conduit in the low-permeability formation, thus enhancing hydrocarbon flow and production. To create the SD completion in a target formation, the following process is followed. First, the well is drilled (to a predetermined kick-off depth in the target formation) and cased. This kick-off depth is usually a few feet into the target formation to ensure that the slot is formed within it. Next, the horizontal section of the well is drilled, but with the tip pointing upward in a mirrored J-like manner (as illustrated in Fig. 1). The drill string is then retrieved and the cutting