Determination of formation damage in limestone reservoirs and its effect on production

Reduction of injectivity of wells by formation damage can be of considerable magnitude during waterflooding of oil reservoirs. Variation of pH of the flowing fluid is an indicator of chemical activity occurring in the formation. High pH promotes formation damage by particle deposition within the porous media and consequently particle bridging at the pore throats. Permeability reduction is used as quantitative measure of formation damage. In this paper, the results of core flow experiments in limestone formations are presented. Effluent pH and Cl concentrations are measured at the outlet of the porous medium. Brines are prepared with sodium, calcium and potassium salts (NaCl, CaCl2, KCl). In limestone formations, formation water, which is used in injection, and artificially prepared brine in various concentrations, caused the permeability reduction. High pH values in produced water caused the permeability reduction and consequently the pores are plugged and formation damage is observed. In alkaline flooding experiments, permeability reduction is observed as a result of high pH alkaline fluids. The permeability reduction is minimized using brines of NaCl, CaCl2 and KCl mixtures and high oil recoveries are obtained. Suspended solid particles are released and moved with injection water when salt concentration drops below the critical salt concentration, causing the permeability reduction and formation damage. Experiments below the critical salt concentrations, resulted in a reduction in permeability values in consolidated/unconsolidated limestone samples, whereas, high pH value solutions caused a reduction in permeability values, plugged the pores and resulted in formation damage.