A reduction in near wellbore permeability can dramatically decrease hydrocarbon production. This study examines the extent of formation damage caused by 1) incompatibility between the formation and injected fluids, 2) exposure times and 3) leakoff volumes. Dynamic fluid loss studies are presented to illustrate the effects of some drilling muds, spacer fluids and cement slurries on typical Berea sandstone. The studies were conducted using 2×5" inch long, pre-saturated, 500 millidarcy core specimens to evaluate the damage potential from fines and filtrates. These tests simulated applications at typical bottom hole circulating temperatures and differential pressures. Permeability was measured at depths of 0.5, 3.0, 5.5 and 8.0 inches from the exposed formation faces.
Prior industry studies have concentrated on using filtrates and small core specimens to measure changes in permeability. Although beneficial, these tests do not simulate filter cake deposition or exposure time for different fluid phases during a cementing application. Drilling muds may be exposed to a formation for many days, while most spacer fluids only contact the formation for 5 to 15 minutes, and cement slurries for a few hours.
Test results indicated that lignin or PHPA muds cause minimal formation damage to Berea sandstone. They also built very dense, low permeability filter cakes which restricted invasion of fines and filtrate from spacers and cement slurries. Most formation damage was confined to within 1–2 inches of the exposed core face.
The studies also showed that fresh water spacers were much more damaging than those containing small concentrations of potassium chloride (KCl). Tests incorporating API cement slurries demonstrated their excellent bridging and cake building characteristics. The addition of fluid loss additives such as hydroxyethylcellulose (HEC) polymer or polyvinyl alcohol (PVA) latex decreased filtrate loss and formation damage. These tests also revealed that the selection of the polymer for fluid loss control is important. Duplicate tests using styrene butadiene resin (SBR) latex resulted in excessive formation damage at penetration depths up to eight (8) inches. Scanning electron microscopic (SEM) analysis was conducted to further illustrate the results obtained from the dynamic fluid loss studies.
While these results are not considered quantitative, the apparatus and procedure permitted dynamic fluid loss testing under simulated downhole conditions using whole treating fluids. The extent of formation damage caused by some drilling muds, spacer fluids and various cement slurry formulations were evaluated. These test methods can be used to improve critical job designs for problematic reservoirs and to screen for damaging additives.