Abstract

Sand production in gas reservoirs is often associated with severe erosion of topside installations, due to high velocity impact of sand grains on tubing bends and choke restrictions. Dedicated laboratory testing on various field and outcrop sandstone specimens surprisingly show that sand production onset is delayed relative to oil or brine flow through the same rocks. This delay is observed even when care is taken to match the drag force on the sand grains to that observed in tests with liquid flow. One proposed explanation is that desiccation of the sandstone occurs, due to the expansion of the gas as it emerges into the borehole; drying of many sandstones is accompanied with an increase in strength, which may be responsible for the observed sand onset delay. Another hypothesis is that non-linear gas flow in the porous rock results in losses due to acceleration and deceleration from pore to pore, such that only part of the flow energy is translated into drag force on the grains. Careful analysis of the laboratory tests seems to confirm the latter hypothesis.

1. INTRODUCTION

Gas reservoirs are often produced with high rates in wells and topside pipes [1]. For weak sandstone formations, this entails the risk that any sand particles co-produced with the gas also have high velocities. Such high velocity sand grains cause extensive erosion damage, especially in bends and when passing through choke restrictions [2]. This erosion propensity explains the widespread practice in the oil and gas industry of restricting gas production rates so as to avoid any risk of sand production. On the other hand, accurate prediction of the conditions for sand onset, in terms of allowable drawdown to the wells and depletion of in-situ pressure in the reservoir, is vital in order to preserve the economic interests, i.e. maximizing fluid productivity. Even if no sand management approach is adopted [3], a closer investigation of sand rate evolution after sand onset is still needed, so as to better quantify the needed safety factor when designing well production throughout expected operation lifetime.

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