Recent Advances in Laboratory Test Protocols to Evaluate Optimum Drilling, Completion and Stimulation Practices for Low Permeability Gas Reservoirs

Abstract

Abstract Low permeability gas reservoirs often exhibit high resistivity and low initial water saturations. Reservoirs of this type may also be susceptible to problems associated with the retention of water or hydrocarbon based drilling and completion fluids, as well as the accumulation of hydrocarbon liquids from the production of rich retrograde gas systems. These phenomena are referred to as aqueous and hydrocarbon phase trapping and have been extensively discussed in the literature as a major source of reduced productivity in low permeability gas reservoirs. This paper briefly describes specific laboratory procedures which have been developed to diagnose problems with phase trapping for given reservoir applications. This allows for the selection of the optimum drilling and completion fluids and practices prior to the costly execution of a potentially ineffective or damaging treatment in the reservoir. Specific examples of tests results and detailed protocols based on extensive testing and refinement of lab procedures are presented. Introduction Phase trapping or adverse relative permeability effects has been discussed extensively in the literature (Refs. 1 to 13) from both a theoretical and field production perspective. This mechanism of formation damage is becoming increasingly recognized as a significant issue in the reduced productivity of low permeability gas wells, and may occur in a number of different areas during drilling and completion operations and subsequent production operations. Most notably, with respect to low permeability gas reservoirs, the areas which are often evaluated on an experimental design basis include:Water-based phase trappingOil-based phase trappingRetrograde condensate dropout trapping and removal. Water-Based Phase Trapping (Water Blocking, Aqueous Phase Trapping) Due to the relative preponderance of water-based fluids that are used as drilling and completion media on a worldwide basis, this particular type of phase trap tends to be more common in most situations, and a considerable effort has been made to diagnose and evaluate its effect. Water-based phase trapping occurs when a water-based fluid is introduced into the reservoir matrix in the region surrounding the wellbore (or in certain situations a natural or induced fracture face), and a portion of this fluid in retained in the rock matrix upon commencing production (or in some cases when initiating injection). The severity of a water trap is highly influenced by:Initial fluid saturations in the reservoirRock wettabilityPore system geometry