Unconventional Fracturing Fluids for Tight Gas Reservoirs

Abstract

Abstract Many tight gas formations are water-wet and under-saturated where the initial water saturation in the reservoir is less than the capillary equilibrium irreducible water saturation. The use of water-based conventional fracturing fluids causes water to be trapped in the near-wellbore region, thereby significantly impairing the ability of gas to flow. Formations with sub-irreducible water saturation can be stimulated with fluids that minimize the interfacial tension (such as surfactant gels), minimize the amount of water used in the fluid (such as energized or foamed fluids), dehydrate the formation (such as alcohol-based fluids) or completely eliminate water (such as hydrocarbon-based or liquid carbon dioxide-based fluids). Since the rheology and proppant-carrying properties of these fluids vary, the uses of these fluids are different and will be discussed in detail in the paper. The paper will also present guidelines, based on formation properties, to indicate the need for considering unconventional fluids. Introduction As the industry moves to tighter and tighter formations, particularly formations such as shales or coalbeds where production is controlled by desorption of the gas rather than matrix flow, fluids that are non-damaging to the proppant pack and formation are becoming increasingly important. Wells with adverse capillary effects due to sub-irreducible water or hydrocarbon saturation also require different fluids to minimize those effects or mitigate effects caused by drilling with the wrong fluid. Several unconventional fluids have been developed and successfully used for these unconventional formations in the last decade. Adverse saturation in the formation can contribute to productivity impairment. Production has been successfully achieved in formations with matrix permeability as low as 10-3 md. However, adverse capillary forces, which result in high in situ saturation of trapped water or liquid hydrocarbons even in very low-permeability formations, make economic production difficult. Low-permeability formations are typically tolerant of only minimal saturation damage due to the sensitivity to capillary retention effects, and rock-to-fluid and fluid-to-fluid compatibility issues. In these wells, the damage from drilling and completion can be overcome by a properly designed frac treatment, which can penetrate beyond the zone of induced invasion and damage.