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

Reduction of injectivity of wells by formation damage can be of considerabl e magnitude during waterflooding of oil reservoirs. Variation of pH of the flowing fluid is an indicator of chemical activity occurring in the formati on. High pH promotes formation damage by particle deposition within the por ous media and consequently particle bridging at the pore throats. Permeabil ity reduction is used as quantitative measure of formation damage. In this paper, the results of core flow experiments in limestone formations are pre sented. Effluent pH and Cl concentrations are measured at the outlet of the porous medium. Brines are prepared with sodium, calcium and potassium salt s (NaCl, CaCl2, KCl). In limestone formations, formation water, which is us ed 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 form ation damage is observed. In alkaline flooding experiments, permeability re duction is observed as a result of high pH alkaline fluids. The permeabilit y reduction is minimized using brines of NaCl, CaCl2 and KCl mixtures and h igh oil recoveries are obtained. Suspended solid particles are released and moved with injection water when salt concentration drops below the critica l salt concentration, causing the permeability reduction and formation dama ge. Experiments below the critical salt concentrations, resulted in a reduc tion in permeability values in consolidated/unconsolidated limestone sample s, whereas, high pH value solutions caused a reduction in permeability valu es, plugged the pores and resulted in formation damage. (C) 2000 Elsevier S cience B.V. All rights reserved.