Novel Ways of Optimizing Production of Gas Wells Suffering from Downhole Salt Deposition

Abstract

Abstract Downhole salt precipitation is an increasing problem in the mature gas fields of the North Sea. With depletion and relatively high drawdowns, excessive evaporation of production water leads to salinities exceeding the solubility limits and thus salt deposition. This leads to significant losses of production and reserves, and increased costs as wells are traditionally shut-in, waterwashed regularly and/or choked back heavily. Two novel solutions have been conceptualized and applied, the first being installation of velocity strings. Besides increasing gas velocities to eliminate liquid loading, a smaller tubing has a downhole choking effect and as such strongly increases flowing pressures. A case study is presented wherein four velocity strings were set, leading to a strong increase in platform flowrate and extension of asset life. The major advantage of this solution is that it is relatively simple, extremely robust and does not involve any increased OPEX. As it involves increased flowing pressures, it is best applied to lower quality producers. Due to their steep inflow performance relation, there is only a very limited decrease in flowrate upon installation of the smaller tubing. For good to excellent quality reservoirs, the choking effect by a velocity string causes a flowrate decrease of often 40 to 50% - too significant to yield attractive economics. As such, it is a better option to compensate for the evaporation rate by external supply of freshwater. Proof of concept has been delivered for 1) using recycled low salinity/oxygen process water for water washing and 2) continuous downhole freshwater injection by a successful three-week pilot test. During the latter, injection rates of approximately 1 m3/d have been achieved, which is shown to effectively eliminate downhole salt deposition for the presented case.