RIASSUNTO
Abstract:
Due to a limited number and poor quality of the available core samples, sandstone analogues are required for a research project on sand production in Kazakhstan. A consistent sample preparation method is essential to achieve repeatable test results in terms of the deformational characteristics and the failure strength of the rock. This paper presents results of the work conducted to produce synthetic sandstone samples by firing clay and sand mixture. The samples replicate the reservoir sandstones in terms of the particle-size distribution, cement content, mineralogy and porosity. The reservoir sand was used as a material for the sandstone samples. The samples with 20% and 30% clay content were prepared. Effect of temperature and firing duration was studied by firing the samples at 200°C for 5 hours, 200°C for 8 hours and 500°C for 5 hours. Scanning Electron Microscopy analysis was conducted on the samples to study the chemical transformation during firing and its effect on clay cementation. Triaxial test behavior of the reservoir sand and sandstone samples was investigated.
Introduction
Sand production is one of the main problems associated with weak reservoir sandstones at shallow depths in Kazakhstan oilfields. The reservoir sandstones of the oilfield from the Ustuyrt-Buzachi Sedimentary Basin are confined to the Lower Cretaceous formations of Hauterivian and Barremian ages. The reservoir depth varies from 200 m to 500 m. The paleoclimate was warm and humid during Hauterivian period, sedimentation occurred at deltaic depositional environment, followed by sea transgression during Barremian period and deposition of shallow-marine sediments (Tashliyev and Tovbina, 1992). As a result of such climatic and depositional conditions, the reservoir is composed of fine-grained sandstone with weak clay cement (Worden and Morad, 2003). Clay content of the reservoir rock varies from less than 10% up to 35%, reservoir properties diminishing with higher clay contents.
Due to a limited number and poor quality of the available core samples, sandstone analogues are required for the experiments. A consistent sample preparation method is essential to achieve repeatable test results in terms of the deformational characteristics and the failure strength of the rock. Different synthetic sandstone preparation methods have been developed and reported in the literature. Usage of Portland cement and gypsum as a bonding material has been widely practiced (Coop and Atkinson, 1993, Nouri et al., 2004, Consoli et al., 2012). Another series of techniques can be grouped under the method employing silicates to bond the sand particles. Cementation by silica glass was achieved by Bernabe et al., 1992 by mixing Ottawa sand with very fine powder of pyrex borosilicate glass, compacting at 14 MPa and heating to 950°C. Holt et al., 1993 mixed sand with sodium silicate solution, and flushed the mixture with CO2 under 'in-situ' stress. Mixture of fine Fontainnebleau sand and alkaline silica gel was exposed to high pressure and temperature by Broke et al., 1997. Calcite In-situ Precipitation System (CIPS) is employed in several works (Ismail et al., 2000, Sherlock and Siggins, 2003), where calcite solution is injected into the sand sample, and as a result of chemical processes within the fluid crystalline calcite precipitates, coating the sand grains and forming calcite bridges at particle contacts. Calcite precipitation can also be initiated and mediated by biological processes. Several studies (Rong et al., 2012) made use of alkalophilic microbe and calcite ion solution to achieve particle bonding, the technique is known as Microbial Induced Calcium Carbonate precipitation (MICP).