Fines Migration and Formation Damage: Influence of pH and Ion Exchange
- R.N. Valdya (U. of Michigan) | H.S. Fogler (U. of Michigan)
- Document ID
- Society of Petroleum Engineers
- SPE Production Engineering
- Publication Date
- November 1992
- Document Type
- Journal Paper
- 325 - 330
- 1992. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 1.4.3 Fines Migration, 1.11 Drilling Fluids and Materials, 1.6.9 Coring, Fishing, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.4.1 Waterflooding, 4.1.5 Processing Equipment, 5.1.1 Exploration, Development, Structural Geology, 1.8 Formation Damage
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This paper elucidates the influence of pH and ion exchange on formation damage caused by fines migration. The experimental results affect waterflooding, design of drilling muds, and alkaline flooding. In-situ release of naturally existing fines (generally clays) results from changes in colloidal conditions of the permeating fluid. Such processes can cause extensive formation damage in sandstones, thereby reducing oil production. Our recent studies clearly indicate that the release process is started by a combination of high pH and low salinity. We present experimental results that suggest and confirm the interdependence between changes in salinity, cation exchange, and pH, leading to drastic permeability reductions. These results therefore provide new insight into the phenomenon of formation damage caused by water sensitivity or injection of incompatible brines. We also describe a unified approach to understanding these results and the findings of previous investigators. Predictions obtained from a physiochemical model based on ion exchange and colloidal chemistry agree well with experimental observations. The effect of different cations on formation damage also was investigated. This study can be extended to predict migration of bacteria and other particulates that cause formation damage.
Khilar and Folger1 observed that the permeability of an initially brine-saturated Barea sandstone declines rapidly and drastically when the brine flow through the core is abruptly switched to fresh (deionized) or low-salinity water. Fig. 1 shows typical results. The abrupt change in salinity that leads to this drastic decline in permeability is called a "water shock," and the phenomenon is called "water sensitivity." Lever and Dawe 2 reported similar findings on a different sandstone. Permeability reduction is a serious problem in oilfield operations, such as waterflooding, and its control is essential for successful economic operations of oil and gas wells.
Analysis of effluent samples from Barea sandstone after a water shock showed that permeability reduction results from blockage of fluid paths by submicron-sized fine particles, which are generally (kaelinite) clay.3 In the presence of brine, these particles (fines) are undisturbed and line the pore walls of the sandstone surface. When brought in contact with low-salinity water, the particles detach from the surface. The released fines migrate with flowing fluid and subsequently are captured at pore throats or pore constrictions, causing formation damage. The phenomenon of water sensitivity (i.e., reduction in permeability during a water shock) was explained by the Deryaguin-Landau-Verwey-Overbeek (DLVO) theory of colloids.3-6 such a permeability reduction also was observed when low-salinity brine replaced the initially permeating high-salinity brine. The concentration at which this permeability reduction begins is called the critical salt concentration and is analogous to the critical flocculation concentration of the DLVO theory for colloids.7 Such phenomena as migration of toxic waste from dump sites and failure of earthen embankments also are attributed to the process of fines migration,1,8 making the problem of water sensitivity of scientific and industrial importance.
The two widely studied parameters that characterize the ionic conditions of the permeating fluid are salinity and pH. For brines composed of a mixture of ions, the mold ratios and the valency of the ions also become important.5,7,9 In this study, however, we limit ourselves to a 1:1 electrolyte. While a detailed review of earlier results is not possible here, we discuss them briefly to understand and contrast the current results with earlier published work.
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