Tests Detail Effects of Weighting Materials on Performance of Thermal Cements

Abstract

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 163544, ’Performance of Thermal Cements With Different Weighting Materials,’ by Jean-Philippe Caritey, SPE, and Jason Brady, SPE, Schlumberger, prepared for the 2013 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 5-7 March. The paper has not been peer reviewed. To maintain zonal isolation throughout the life of a high-pressure/high-temperature (HP/HT) well, the cement sheath must perform reliably at temperatures that can exceed 315°C. Weighting materials have largely been assumed to be inert with respect to set Portland cement; however, the present study reveals that this assumption may be false in HP/HT environments in which well temperature exceeds 260°C. Some weighting agents may react with the set Portland cement, causing strength loss and increased permeability. Fortunately, this effect can be prevented, and the set-cement integrity can be preserved. Experimental Procedures Cement Blends. Five solid cement blends were prepared; their compositions are presented in Table 1. The blends were formulated according to the engineered-particle-size concept, wherein the volumes of coarse, medium, and fine particles are optimized to maximize the packing volume fraction (PVF). An increase in the PVF reduces the amount of mix fluid required to pre-pare a stable and pumpable slurry, and it increases the strength and reduces the permeability of the set cement. The five blends contained 50% by volume of blend (BVOB) silica, which is sufficient to allow formation of the calcium silicate hydrate mineral xonotlite [Ca6Si6O17(OH)2] upon curing at temperatures above approximately 160°C. The weighting-agent concentration was held constant at 15% BVOB. The slurries were prepared according to the recommended American Petroleum Institute (API) procedure. Strength. The five slurries were placed in an ultrasonic cement analyzer (UCA) fitted with a dual-cylinder high-pressure pump that provided pulseless metering. They were cured at a final temperature and pressure of 302°C and 122 MPa, respectively, for periods of up to 100 days. Two additional tests were conducted at 274 and 288°C at a final pressure of 122 MPa. It should be kept in mind that the UCA does not provide absolute strength values at such elevated temperatures because the transit time increases significantly during the heating period; therefore, one should consider strength trends vs. time. When possible, the cured- cement samples were crushed with a hydraulic press to obtain an accurate compressive strength. The recommended API procedure was followed. Permeability. When possible, water-permeability measurements were per-formed on 5.08-cm-long, 2.54-cm-diameter cement cores with a liquid permeameter operating at a confining stress of 2.76 MPa.