Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol.30, No.7, pp. 719-722. 1993 Printed in Great Britain 0148-9062/93 $6.00 + 0.00 Pergamon Press Ltd Constitutive Mechanical Behaviour of Synthetic Sandstone Formed Under Stress R. M. HOLT + T. E. UNANDER tt C. J. KENTER ttt ~TRODUCTION Synthetics are superior to natural rocks in studies per- formed to control effects of specific parameters. A disad- vantage, however, is that microstructure of synthetics may be different from that of a natural rock, since the di- agenetic processes are difficult to simulate in the labora- tory. The main motivation for the work presented here has been to devise a method whereby coring effects on rock mechanical parameters can be evaluated, e.g. for pe- troleum applications such as predictions of possible res- ervoir compaction or sand production. We have developed a method whereby sandstones are cemented under stress. In a previous Paper [1], we fo- cussed on the effects of unloading (simulating coring) on compaction predictions for a very weak sandstone, simu- lating the mechanical behaviour of a particular North Sea oil and gas reservoir. In this paper, we present a detailed description of the procedure developed for manufacturing of this synthetic sandstone. We will further present meas- urements of the static mechanical behaviour from a set of triaxial tests, supplemented by measurements of acoustic wave velocities (AV) and acoustic emissions (AE). The data are discussed in terms of i) the micromechanics of the unloading - reloading process, ii) the possible appli- cation of the results in order to re-establish in situ stresses, ,and iii) quantification of core damage. FORMATION OF SYNTHETIC SANDSTONE UNDER STRESS The goal of this work was to create a synthetic sand- stone that resembles as close as possible (with respect to mechanical ,and petrophysic,'d par~uneters) a given reser- voir rock. It was therefore decided to look for a brittle cement, like that found in natural sandstones. The process selected employs sand ,and a sodium sili- cate solution. These components ,are mixed into a slurry, ,and then precompacted at a low stress sufficient to allow handling of the sample. After mounting in the triaxial cell, the vertical (,axial) ,and horizontal (confining) stresses are increased to the selected "in-situ" level. The sample is then evacuated, and CO 2 gas is flushed through. This causes the sample to harden, as can be in- IIKU Petroleum Research / NTH Norwegian Institute of Tech- nology, N-7034 Trondheim, Norway IJIKU Petroleum Research, N-7034 Trondheim, Norway tt?KSEPL Shell Research, 22886D Rijswiik ZH, The Nether- lands 719 spected by monitoring amplitude and velocity increases of transmitted acoustic waves. These measurements indi- cate that cementation is completed within 5 minutes. The chemical process leading to cementation can be written: Na2Si307 + 2 H20 + 2 CO 2 --4 3 SiO2xH20 + 2NaHCO 3 The cement is amorphous silicic acid, formed by polym- erization. The sodium salt is not bound to the rock matrix and appears as a white powder that is not expected to in- fluence the mechanical properties. The introduction of CO 2 has been used in the foundry industry for mould and core production for about 35 years [2]. Combining this process with stress, however, appears to be new. The up- per limit to the unconfined strength of silicate-cemented sandstones formed by this process is around 10 MPa. There are several parameters to play with in order to alter the strength, such as precompaction stress, grain size dis- tribution vs. silicate content, amount of water and ratio of Na to Si in the cement, and the ratio of silicate to sand. In our case, we chose the grain size distribution representa- tive of the target reservoir and 4 ml of a 35% sodium sili- cate solution to 160g sand per sample. The samples are 38mm (1.5") in diameter and 75mm (3") in length. Alter the samples are cemented, they are left at con- stant stress conditions (15 MPa vertical ,and 7.5 MPa hori- zontal stress) for 1 hour. Then, coring is simulated during unloading, by removing the vertical stress as completely as possible before the horizontal stress is taken off. After unloading, the samples are left for (2 hour, ,after which a series of standard triaxial tests is performed at confining pressures 0,1,2.5,5,7.5 ,and 15 MPa. During these experi- ments, we recorded vertical and horizontal stresses and strains, in addition to acoustic velocities (vertical and horizontal P, plus vertical S) ,and acoustic emissions. We did not correct axial stress for change in sample area dur- ing the tests, nor correct volumetric deformation for change in sample shape (e.g. barrelling). STATIC BEHAVIOUR Figure 1 shows the measured axial stress vs ,axial ,and volumetric strain during the triaxial segment of the tests (strains ,are taken as positive in compaction). The stress- strain curves do not show any clear peak stress. At the highest confining pressures, there is a tendency of strain hardening. The volumetric strains show that there is a transition from dilatant behaviour below 7.5 MPa confin- ing pressure to contractant behaviour above 7.5 MPa.