Fracturing in Tight-Gas Reservoirs: Application of SCAL Methods To Investigate Formation-Damage Mechanisms

Abstract

Abstract Gas well productivity in tight reservoirs is greatly impeded by the fracturing fluid interactions with the formation. New simulators introduce formation damage mechanisms to calculate gas well productivity. However, equations describing formation damage must be supported by experimental data obtained in conditions representative of fracturing operations. The purpose of this work is to derive the absolute permeability damage and multiphase flow upon return gas permeability after core invasion by a fracturing fluid by methods used in Special Core Analysis Laboratory (SCAL). The core permeability is in the microDarcy range. Experimental data of absolute permeability damage due to the fracturing fluid filtration and water sensitivity of this illitic sandstone as well as water saturation profiles measured by X-Ray in two phase flow experiments are interpreted. The methodology of interpretation provides the petrophysical data specific to the rock-fluid system: the absolute permeability, the relative permeability damage due to hysteresis, and the capillary pressure curve. In addition, simulations are presented for the evaluation of the impact of various operational parameters like pressure draw-down on the gas productivity. It is shown that permeability hysteresis must be considered to explain the low gas recoveries at short term. On the long term, the natural clean-up is very slow. The results, derived from a real rock-fluid system, are used to provide recommendations for improving back flow procedures. This methodology can be applied to any case of damage due to the alteration of rock-fluid properties. Introduction Following Holditch definition, a tight reservoir is a reservoir that cannot be produced at economic flow rates and in which economic volumes of natural gas cannot be recovered unless the well is stimulated by a large hydraulic fracture treatment. There is a significant literature on the productivity of fractured tight reservoirs and the way to minimize fractured well productivity impairment1–4. Two main factors are affecting fractured well productivity: fracture face damage and fracture conductivity impairment. The effect of fracture face damage on well productivity is mainly investigated by numerical simulations of gas production during back flow. The simulators are taking into account common physical processes known as affecting gas productivity5–8. However, most of the physical laws introduced in the simulators use coefficients which are not validated for the conditions of the simulatons. In some cases, various damaging scenarii are tested and the results in terms of return permeability are calculated9. Although this exercise is very interesting, it does not give useful information unless physical parameters for a particular rock-fluid system are known. Particularly, there is a number of very specific questions concerning tight rocks. Regarding absolute permeability damage and the filtration of fracturing fluids the following questions remain with no definitive answer as far as low permeability rocks are considered: Is there a filter cake to control the filtration rate and therefore the fluid invasion distance ? Does the polymer macromolecules (incase of a polymer based fracturing fluid) enter the pores ?