The key to formation damage control is minimizing the depth of contact/impact of the formation and drilling fluid by minimizing solids and liquid invasion into porous (fractured) formations. Analysis of downhole filtration conditions and particle size (distribution) measurements were used to select “fit to pore size” fluid loss control materials in order to maximize pore/fracture plugging efficiency. Further tests served to predict potential alteration of rock properties. Particle size measurements have been performed for commonly used (natural and synthetic) polymeric fluid loss and other additives dispersed in water. High-pressure Hg porosimetry was used to determine the pore size distribution of core samples. Core flow tests served to measure permeability and permeability damage caused by fluid loss additives. Spontaneous fluid absorption rate was studied using modified tensiometer and wettability of the cores were determined by ‘sitting drop’ technique.
In low permeability porous (tight and shale gas) formations consisting of macro to nano pores most filtration studies are limited to provide reliable filtration data for all cases due to the fact that drilling fluids solids particle sizes relative to pore sizes. Particle size measurements of commonly used (natural and synthetic) polymeric fluid loss additives in water have provided typical size range by particle number in the 25-35 nm range, and 120-700 nm by volume. Such particles can play a significant role in internal plugging of micro to nano size pores and microfractures. Core flow/filtration tests using low permeability (<5 mD) core samples and model fluids containing fluid loss additives resulted in rapid permeability reduction (to 100%), typically in close to core face region. Results proved the achievable pore plugging efficiency and invasion control into low permeability rocks during drilling. Particle size data gained for CPG (Cloud Point Glycol) solutions proved that this system could also be used to form efficient barrier within the low permeability formation, showing certain analogy between mitigation of formation damage and wellbore instability. Better insight into downhole filtration mechanism and conditions, especially for initial filtration phase in low permeability formations can lead to designed “shallow bed” instead of practically uncontrolled “deep bed” filtration, where external filter cake is essentially not formed. Such internal pore plugging technique can be the key to the minimization of solids/liquid invasion and associated formation damage.