Salt Precipitation in Gas Reservoirs

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 122140, "Salt Precipitation in Gas Reservoirs," by Q.T. van Dorp, SPE, Delft University of Technology (now with OMV Austria Exploration & Production GmbH); M. Slijkhuis, SPE, Wintershall; and P.L.J. Zitha, SPE, Delft University of Technology, originally prepared for the 2009 SPE Formation Damage Conference, Schevenin gen, The Netherlands, 27-29 May. The paper has not been peer reviewed. Examination of productivity decline in several wells revealed that halite precipitation most likely is the result of water evaporation with pressure drop in the vicinity of the wellbore. Hence, a model was developed in an attempt to capture the main features of this phenomenon. In addition to the classical mass- and momentum-conservation equations, the model includes equations describing the evaporation and salt-precipitation kinetics and the modification of porosity and permeability as a result of pore restriction caused by halite precipitate. Introduction Production wells in gas reservoirs occasionally experience rapid performance decline as recovery progresses. In many cases, this behavior may be attributed to halite scale in the near-wellbore region around the perforated pay zone or within the wellbore. For certain gas wells in the North Sea, regular downhole freshwater treatments lasting half a day are required to restore production. Fig. 1 shows production rates as a function of time. From the end of March to the end of May, a rapid decline in production rates is observed. A water wash at the end of May was sufficient to restore production rates. Additional water treatments at the beginning of July and the end of August were required to maintain high flow rates. It is clear that these treatments allowed overall production decline from the end of March to the end of August to be limited to 100 000 m3/d, which is related to natural depletion. Salt accumulation has been indicated by mechanical wireline surveys, by a video-camera survey (Figs. 2 and 3), and by well-performance data (Fig. 1). Most salt was found in the upper perforations. A salt sample revealed almost pure halite scale. A better understanding of salt-precipitation phenomena as well as the conditions under which this takes place is needed for better productivity control, and may reduce or even eliminate the necessity for downhole freshwater washes. The study detailed in the full-length paper is concerned with modeling the salt-precipitation process. The objective of the current work is to build a simple model, integrating classical multiphase-flow theory and relations for describing water evaporation and salt precipitation to predict the resulting decrease in porosity and permeability. The full-length paper begins by describing the physical model and the main assumptions upon which it relies. Transport equations and constitutive relations are formulated. Next the evaporation and precipitation model, including the effect of precipitation on porosity and permeability, is described. A numerical-solution method using finite differences and the implicit-pressure/explicit-saturation method for solving pressures and saturations then is presented.