The Mechanism of Sand Production Caused by Pore Pressure Fluctuations

Abstract

Sandstone core samples cored from an outcrop layer of a Saudi sand producer reservoir were tested under simulated bottomhole stress-fluid flow conditions to investigate the process of increased sand production after well shut-in or work-over jobs (i. e. pore fluid pressure fluctuation). Bottomhole stress-state was simulated by using an experimental set-up consisting of a stiff compression machine equipped with a high-pressure Hoek cell, a servo-controlled confining pressure system and a pore pressure and flow generation system. This set-up enables the measurement of fluid flow and sand movement under simulated in situ stresses. In this work, 3% saline water and 1. 5 cP light oil were used as pore filling fluids as well as displacement fluids. Displaced fluid and sand were produced through the outlet port of the Hoek cell (diameter = 4. 25 mm). It has been found that an increased amount of sand was produced when the production process was restarted after a shut-in period of 24 h. Higher amounts of sand were produced when successive shut-in processes were performed. Furthermore, the amounts of produced sand decreased when the pore fluid pressure was brought to its initial value (i. e. immediately before the next shut-in). This decline is attributed to the increase of the effective confining pressure which tends to hold sand grains together as a replacement for the damaged cementing material. As indicated by the compressive tests, about 8% to 15% reduction in the strength of the tested sandstone was recorded after the conduction of three successive cycles of production using light oil and saline water. This reduction in rock strength was caused by the fluctuation in the pore fluid pressure during sand production process.