Damage mechanism of water-based fracturing fluid to tight sandstone gas reservoirs: Improvement of The Evaluation Measurement for Properties of Water-based Fracturing Fluid: SY/T 5107-2016

Abstract

At present, evaluation on reservoir damage induced by fracturing fluid mainly refers to The Evaluation Measurement for Properties of Water-based Fracturing Fluid: SY/T5107-2016 (referred to as the industry standard below). However, the fracturing fluid displacing core process stipulated in the industry standard is not consistent with the fast invading process of fracturing fluid into the reservoir under high pressure during the actual fracturing construction. Besides, the influences of fracturing fluid residues, gel breaking mode, original water saturation and other factors are not taken into consideration in the experiments to evaluate the damage of fracturing fluids. Thus, the accuracy of evaluation results is influenced. In this paper, tight sandstone cores of the Lower Jurassic Ahe Formation (J1a) in Dibei area of Kuqa Depression of the Tarim Basin were selected as samples. The invading process of fracturing fluid into a tight sandstone reservoir was simulated by modifying experimental process and method. Then, the damage degree of fracturing fluid to gas reservoir was evaluated and the damage mechanisms of fracturing fluid were analyzed systematically. And the following research results were obtained. First, the modified evaluation method takes into account the influences of several factors, such as the original water saturation of gas reservoir, the instantaneous “breakdown” effect of high-pressure during fracturing and the fracturing fluid residues, so it can evaluate the damage degree of fracturing fluid to tight sandstone gas reservoirs more objectively. Second, the evaluation results based on the industry standard show that the damage degree of fracturing fluid to the permeability of tight sandstone gas reservoirs is medium to strong, whereas the damage degree evaluated by the modified method is medium to weak. Third, the retention of fracturing fluid residues in fractures is the main cause of permeability damage. The residues can easily block fractures and fracture surface pores. Most of them retain in the pores in the surface layer of matrix cores (invasion depth less than 3 cm), so residues are filtered by matrix pores. Fourth, when fracturing fluid migrates inwards from the core surface, high molecular polymers retain in the form of thin-film lamellar, local flaky nodular and crystal inclusion in turn in the reservoir pores. Fifth, under the experimental conditions, salting-out crystals appear and are unevenly distributed in the cores. In fractures, salting-out crystals and high molecules are polymerized to form composite inclusions. In matrix pores, salting-out crystals and a small number of fragments (e.g. illite) are enclosed to form a complex. Sixth, migratory particles caused by speed sensitivity are usually combined with residues and high molecular polymers to form composite inclusions, thus blocking pores and fractures.