Polymer Flooding in Unconsolidated-Sand Formations: Fracturing and Geomechanical Considerations

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 121840, ’Polymer Flooding in Unconsolidated-Sand Formations: Fracturing and Geo mechanical Considerations,’ by Mohamad Khodaverdian, SPE, Tibi Sorop, SPE, Sophie Postif, SPE, and Paul Van den Hoek, SPE, Shell E&P, prepared for the 2009 SPE Europec/EAGE Annual Conference and Exhibition, Amsterdam, 8-11 June. The paper has not been peer reviewed. A study was carried out to determine the geomechanical effects of polymer flooding in an unconsolidated-sand reservoir. The work provided a geomechanical perspective on the generally complex problem of polymer flooding unconsolidated formations containing viscous oil. The work offers insights into critical issues that must be examined in such situations to avoid catastrophic failures and highlights the existing technological gaps in the current predictive capabilities. Introduction Some unconsolidated-sand formations, both onshore and offshore, contain high-viscosity oil and could be considered for polymer flooding to improve recovery. Flooding unconsolidated sand could lead to “fracture” propagation. Although such formations typically have permeability on the order of several darcies, minute quantities of impurities and solids in the injection fluid can plug the sandface over time and lead to fracturing if the injection rate is maintained. Even in the absence of fines and solids in the fluid, the high in-situ oil viscosity and low polymer mobility could instigate fracture propagation if the injection rate is sufficiently large. Polymer flooding was proposed as an option for an offshore field to enhance sweep efficiency and production of viscous 200-cp oil. Aside from the relatively high oil viscosity, the formation characteristics include unconsolidated and highly compressible sand, relatively thick pay zone (gross nominal thickness of 100 m), and relatively small stress contrast between the sand and the caprock (potentially leading to containment loss). The primary goal of the laboratory experiments was to identify the fracturing mechanisms and establish a net propagation pressure in such formations. The findings from the laboratory tests provided empirical input (e.g., equivalent fracture toughness) for numerical simulators. Injectivity Mechanisms Previous R&D. Industry and research institutions have carried out several research projects to address injectivity and hydraulic-fracturing issues in unconsolidated sand. Investigations have focused on several types of field activities, including cuttings reinjection, hydraulic fracturing, frac packing, produced-water reinjection, and water and chemical flooding. Polymer-Injection Tests. In this work, several experiments were conducted in which polymer was injected into oil-saturated sand to investigate the injectivity mechanisms and equivalent fracture toughness under the specific field conditions. Test samples were cubic (45 cm) sand blocks.