Maximizing Economic Return by Minimizing or Preventing Aqueous Phase Trapping During Completion and Stimulation Operations

Abstract

Abstract Hydrocarbon exploration continues to evolve toward lower quality reservoirs, particularly in North American natural gas reservoirs. In most cases, without hydraulic fracture stimulation treatments, many wells would have no hopes of being economic plays due to inadequate production volumes relative to the costs associated with drilling the wells. Many of these reservoirs are described as "water-sensitive", but are water-sensitive due to aqueous phase trapping rather than due to clay expansion or salinity shock issues. Aqueous phase trapping is a capillary imbibition effect which has been noted in both oil and gas reservoirs, and has been observed as a particularly severe problem in reservoirs where a sub-irreducible water saturation exists. Understanding the aqueous phase trapping phenomenon is important to the oil and gas industry due to its being the driving force behind many low-permeability reservoir stimulation decisions, which are aimed at reducing the impact of aqueous phase trapping. Diagnostic tools that utilize measured reservoir parameters are presented for use in evaluating a suspect reservoir's sensitivity to aqueous phase trapping. This paper details primary stimulation options designed to prevent production losses related to relative permeability decreases caused by aqueous trapping. Remedial treatment options where prior well treatments or production losses indicate aqueous trapping are also outlined. An example reservoir and current stimulation techniques are also described, where techniques used to reduce aqueous phase trapping problems are proving successful. Introduction Exploration in the mature hydrocarbon provinces of the North American continent has continued to evolve from oil reservoirs to natural gas reservoirs, and has further moved toward exploration in progressively lower reservoir-quality zones. The low quality, "tight gas" reservoirs commonly completed in the Permian Basin, Rockies, Mid-Continent, Appalachian, and onshore Gulf Coast regions are increasingly being relied upon as the primary energy source for industrial, commercial, and residential use. These low permeability reservoirs are typically stimulated by hydraulic fracture treatments to extend the wellbore away from the vertical hole and increase drainage. Refracturing tight gas reservoirs to drain more of the reservoir and extend the economic life of the well is becoming common practice. Wells in some parts of the Rockies are now being stimulated for the third time (tertiary fracs). In most cases, without hydraulic fracture stimulation treatments, many of these reservoirs would have little hope of being economic plays due to inadequate production volumes relative to the costs associated with drilling the wells. Throughout the history of the oil and gas industry, many hydrocarbon reservoirs have been described as "water-sensitive" based upon disappointing results when water-based treatment fluids are pumped into zone. Historically, the damage mechanism is believed to be due to low-salinity brines reacting with expandable clays to cause formation damage. Expandable clay damage is a well-documented phenomenon which commonly occurs in younger sediments, but it is not the only water-induced damage mechanism.