Sand production experiments run on outcrop and reservoir cores reveal that the cavity of hollow cylinders fails in three distinct patterns depending on the sandstone type. The sandstones are classified according to the cavity failure pattern in three classes, (i) Class A - Brittle, (ii) Class B ? Ductile, and (c) Class C - Compactive. Criteria are established for the classification of sandstones on data that are often available, such as data from uniaxial/triaxial tests and/or petrophysical tests. Sand measurement results show that the failure type has a direct consequence on the observed sand rate. Class A shows rapid sand rate increase with increasing stress, Class B shows a slow down period after initial sand production and Class C shows a slow increase of sand rate with increasing stress. The sand mass measurements and the sandstone classification were used for the development and calibration of improved analytical functions for sand mass quantification under given in situ stress conditions and production scenarios.
Sand production tests were performed on hollow cylinder specimens with radial fluid flow. The main objective was to establish the relations between the sand rate over time and the other tests parameters, i.e. the stresses and the fluid flow rate. The results from the hollow cylinder tests are used to calibrate the volumetric a sand production quantification model. The tests were performed with oil and brine as flowing fluid, in order to investigate the effects of water breakthrough on the amount of produced sand. Tests were performed on four outcrop sandstones, the Castlegate, Red Wildmoor, Saltwash North and Saltwash South, a Synthetic sandstone, and four reservoir sandstones from Field S, Field SN, Field O and Field T. Tests with oil flow were performed in all sandstones. Tests with brine flow were performed on Saltwash North and Field S sandstone. These sandstones do not show significant water sensitivity on their mechanical properties and thus the test results are rather related to the hydromechanical erosion part of the sand production problem.
2.1. Sand production test setup
Sand production has been investigated in the past with tests on 100- and 200-mm external diameter hollow cylinder sandstone specimens [e.g. 1, 2]. Similar equipment and test procedures are used in the tests presented here. An exception is the installation of an improved sand trap for continuous and accurate measurements of the produced sand during testing. A schematic of the sand production test setup with the pressure cell and the sand trap is given in Figure 1. The figure shows the interior of the cell with a 100-mm external diameter hollow cylinder specimen mounted in position. The base plate and bottom-loading piston have been specially designed to enable cavity failure and sand production studies. They are equipped with center holes, through which the specimen inner hole may be accessed and exposed to ambient pressure. The maximum specimen diameter that can be accommodated in the apparatus is 200 mm. The test specimens are placed on the bottom-loading piston, which is mounted on the base plate of the cell.