This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 183743, “Maintaining Injectivity of Disposal Wells: From Water Quality to Formation Permeability,” by Ali A. Al-Taq, Mohammed N. Al-Dahlan, and Abdullah A. Alrustum, Saudi Aramco, prepared for the 2017 SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, 6–9 March. The paper has not been peer reviewed.
An extensive laboratory study was carried out with two objectives: to evaluate the effect of water quality on injectivity of disposal wells with reservoir core plugs and to restore injectivity of damaged wells. In this paper, water-quality guidelines to minimize or prevent formation damage are recommended. On the basis of laboratory work, a novel chemical treatment was successfully applied to restore injectivity of several damaged disposal wells. This novel treatment reduced the long operation time and cost of a typical treatment practice while effectively stimulating the well.
Effect of Water Quality and Formation Permeability on Injectivity
Water quality has a major influence on the injectivity of injection and disposal wells. Poor injection or disposal-water quality can compromise the effective injectivity of even high-quality sandstone or carbonate formations. Source water used for injection often contains solids, which can reduce permeability of the formation around the wellbore.
Solids-particle damage depends on particle size of the solids, oil present in the injected water, and the average pore-throat diameter of the formation. If the particles are larger than the average pore-throat diameter of the formation, then the particles cannot penetrate the pores. As a result, an external filter cake with permeability lower than that of the formation will form (Fig. 1a). Another type of injectivity impairment occurs when the size of the particles present in the injected water is smaller than the average pore-throat diameter of the formation. These particles will invade the formation and bridge at some pores (Fig. 1b). As solids concentration in the injection water is increased, the rate of permeability decline becomes greater. Obviously, if the size of the particles is significantly smaller than the average pore-throat diameter of the formation (Fig. 1c), then the particles will flow through the formation without causing any damage. As a result, there will be no loss of injectivity for a long period of time.