Effect of Salt Cement Filtrate on Subsurface Formations

Abstract

Abstract A search was made of the literature to determine the erect of the ion content of cement filtrates on shale and clay mineral formations. The influence of the ions on solubility by the addition of salt is shown. Using formation samples, and applying a variety of brines and cement filtrates, flow rates and permeabilities are compared. Discussed are factors that limit the penetration of formations by cement filtrate. Introduction Salt has many unique properties for use in oilwell cementing. Ludwig described the general effects of salt on cement and the basic chemistry involved when cement reacts with sodium chloride in concentrations ranging up to saturation of the mixing water. Some qualities characteristic of salt, used in unspecified quantities for oilwell cementing, are that itimproves flow properties,is a good dispersing agent,is compatible with most other additives,acts as an accelerator in low concentrations and as a retarder in high concentrations,aids in expansion of set cements, andincreases slurry weight. Slagle and Smith reported, "the principal benefit of salt evolves not from its accelerating, dispersing or retarding properties but, rather, because of its influence on clay minerals, which represent a predominant portion of shales and exist in various quantities in sands and other producing formations". More recently a question was raised as to formation permeability damage by leak-off of salt cement filtrate. A search for answers to this question took the form of a survey of available literature and laboratory studies. The literature survey shows the effects on shale and other clay mineral-bearing formations of ions normally found in cement slurry filtrate. Several filtrates were used to determine the relative effect cement filtrate invasion can have on formation permeability, and whether or not deleterious reactions exist between formation constitutents and the ionic components of a filtrate. Effect of Ionic Environment On Bentonitic Formations Most researchers agree that permeability reduction in bentonitic sandstone occurs from two sources: swelling of clay minerals, and migration of dislodged particles to a constriction in the pore system. In his work with permeability of sandstone cores, Mungan concluded that the primary cause of permeability reduction is blocking of the pore by dispersed particles, and that permeability reduction due to salinity changes occurs regardless of the type of clay. The changes referred to here are changes in solutions containing no divalent cations such as calcium, and abrupt changes in concentration. Jones stated that an increase of salt concentrations in water tends to prevent clay blocking. He pointed out that clays in the calcium form do not disperse so easily in fresh water as do those in the sodium form. Jones concluded that cores capable of even severe clay blocking generally can conduct water as fresh as drinking water without significant change in permeability if at least one-tenth of the salts dissolved are calcium or magnesium salts, and if the salinity is decreased very gradually rather than abruptly. Hewitt concluded that a reduction in cation concentration by dilution promotes dispersion, and that the amount of swelling of each clay generally increases with a decrease in salinity of the surrounding fluid. He stated, "Therefore, a concentrated brine would cause the least damage; fresh water would cause the most." Van Olphen showed how Van der Waals attractive force is sufficient to overcome the repulsive forces of the electric double layer in bentonitic sols at high electrolyte concentrations, and thereby collapse the expanded clay particles. Foster, Savins and Waite showed this particle collapse by means of X-ray measurements of the distance between platelets, made at increasing salinities of solution (Fig. 1). JPT P. 259ˆ