ABSTRACT

Pores in sedimentary rocks may be lined or filled with a variety of different clay minerals. These clays can greatly reduce permeability, increase acid or fresh water sensitivity, totally alter the electric log response, and increase irreducible water saturations. The composition of the clays is of great importance in reservoir management. Different clays have different compositions, and thus will react differently to various drilling and completion fluids. As a result, these fluids should be designed for the specific variety of clay present in the pores.

Four families of clay minerals exist and each causes different reservoir problems: 1) kaolinite is primarily responsible for the migration-of-fines problem associated with many reservoirs, 2) smectite can be extremely sensitive to fresh water, 3) illite increases pore tortuosity, and 4) chlorite is very acid sensitive. If well stimulations are designed without a knowledge of the type of clay minerals present in the pores, rapid production declines may occur after treatment. In some instances, the damage is permanent. In other instances a new, properly designed acid job may result in dramatic increases in flow. Therefore, in designing a mud system, a frac job, or even a waterflood project, it is vital to know what sort of clays occur in the pores of the reservoir rock.

Many Gulf Coast reservoir sandstones suffer from serious clay problems--the Frio, Hackberry, Hosston, and Wilcox. Clay problems vary from formation to formation, and from area to area. Individual formations contain regional clay trends which may be used to predict problems which may be encountered in wildcat wells. Clay technology has been refined to a degree where old side-wall cores or cuttings may be used for analysis. The new clay technology is a powerful tool in defining optimum exploration strategies and should be a vital factor in the planning of efficient reservoir management.