Experimental study of volumetric sand production with gas flow

Abstract

The presence of sand in a production stream has been a common issue for wells drilled in high permeability sandstone formations. In the case of gas producing reservoirs, sand production can be particularly hazardous due to the high gas velocities. It can among others lead to erosion of downhole components and surface equipment and to significant production downtime. A comprehensive experimental study was conducted to investigate the effects of gas flow on sand production. This was accomplished by comparing gas flow tests to tests with oil or water flow. In these tests, hollow cylinder sandstone specimens, initially wet or at ambient humidity conditions, were isotropically compressed under simultaneous fluid injection. One-phase as well as two-phase flow experiments were carried out to simulate important processes encountered in gas fields, like water capillary flow or water-cut. The results showed that sand onset was delayed in the gas flow experiments, which agrees with experimental observations in the literature. The delayed sand onset, i.e., higher sand onset stress, was related to water evaporation due to gas flow, termed flow-through drying, which strengthens the rock. The water-sensitive strength of sandstones was also examined by completing a series of uniaxial compression tests at various water saturations. The analysis of the volumetric sand production data and the X-ray CT-scans of the tested specimens showed that the decreased capillary bonds between grains when only one fluid is present in the pores, e.g., one-phase water or gas flow, results in continuous sand production of individual grains or small-sized flakes, creating large voids in the vicinity of the hole. Under such conditions, the constantly enlarging failure zone eventually causes the total collapse of the specimen. In contrast, specimens with higher capillary cohesion due to a water-oil or water-gas interphase show post sand onset stability and lower sand rate where, due to higher grain to grain cohesion, chunks of failed rock material remain in the hole and provide additional support that prevents the rapid collapse of the specimen even as the applied stress increases.