RESPONSE OF LOW-PERMEABILITY, ILLITIC SANDSTONE TO DRILLING AND COMPLETION FLUIDS

The effect of ion exchange and fluid flow on formation damage of reservoirs containing illitic clay minerals has been studied. Based on results of a number of novel experiments, a mechanistic description of the process of pore plugging by both clay particles and filtrate has been developed. The petrophysical properties of reservoirs containing illite depend significantly on the core preparation technique. Illite collapses upon air drying resulting in high porosity, high permeability and low capillary pressure. The illite rebounds, however, on contact with fresh water and projects across the pores, giving rise to low porosity, low permeability and high capillary pressure. Illite has also shown high susceptibility to migration: in fresh water it remains dispersed and is carried with the flowing fluid until the particles are trapped in pore restrictions. Above a critical salt concentration, however, the illite remains attached to the pore walls. A dynamic filtration of drilling fluid also indicates that an effective bridge (both external and internal filter cake) can be obtained by properly matching the particles of the drilling and completion fluids to the formation pores. This limits the invasion of solids to between 1 and 2 cm from the wall of the drilled hole. Polymers, however, can be carried deeper into the formation due to the filtration process. This study can be extended to medium- to high-permeability sandstone reservoirs which are also adversely affected by migration of fines and other particulates.