Mud-filtrate invasion during coring operations has been a major factor affecting the validity of in-situ saturations in reservoir rock. Laboratory studies have documented the possible effects of the drilling mud filtrate invasion on the rock wettability. Considerable effort has been devoted to the measurements of relative permeability by the laboratories of major oil companies or commercial laboratories. Accurate data is quite important in the projection of future reservoirs. Many different attempts have been made to preserve the in-situ wettability by use of special coring fluids and preservation techniques at the drilling site. Considerable improvement in data reliability could be achieved if the coring mud-filtrate invasion is eliminated during coring operations.
Mud filtrate invasion of the core occurs by three mechanisms. First, filtrate invasion occurs ahead of the bit. A filtrate bank builds up at low coring rates. The invasion is relative to the core filtrate invasion velocity and the core bit velocity. Second, filtrate invasion occurs at the core bit. Filtrate is generated at high rates because of the bit cutting action. Third, static filtration will occur on the core after the core enters the core barrel. Filtrate cake permeability controls filtrate invasion in the inner barrel for all types of sandstones, especially for high permeability sandstones. During the mid 1990’s, a consortium of oil and service companies sponsored a program to try to resolve the problem. A series of experiments were performed at the Terra-Tec laboratory at Salt Lake City to resolve this problem. In the past, specially designed PDC coring bits were found to provide high coring rates .These bits and low fluid loss oil muds were found to provide cores with significant intervals with uninvaded centers. The objective of these experiments was to find if low invasion coring fluids could be designed using water-based drilling fluids.
The results of the series of experiments indicated that a newly-designed polycrystalline-diamond compact (PDC) coring bit with a specially designed water-based mud with bridging solids could consistently minimize filtrate invasion and provide cores with uninvaded centers. One of the mud systems which provided the best results was a bland mud system using Bore-Plate®( a patented blend of specially-treated Gilsonite grades with other additives) . It was found that this type of system would generate a very low spurt loss of filtrate ahead of the bit which was eliminated by the core bit velocity. The insolubility of the Bore-Plate sealed off the pore spaces more effectively than the more soluble asphalts.
Chevron conducted several laboratory tests using this specially designed mud system in high porosity sandstone prior to several field tests. The bland mud system consisted of bentonite, CMC or PAC for fluid loss, slight amounts of caustic soda to maintain pH at 8.0 or less, and Bore-Plate in concentrations of 6-10 pounds per barrel (ppb), depending upon the anticipated porosity of the sand interval. This combination proved to be successful in the lab as the insoluble Bore-Plate was very effective in plugging the sandstone pores at the core surface. Several successful coring operations were conducted on wells in Western Offshore Australia using this system. Invasion of the sandstone was held to a minimum with no more than 0.25 inch invasion of the core. At first, the geologists and petroleum engineers were disturbed when they found the entire core coated with a black film. However, they learned that the film was the Bore-Plate plugging the surface pore space and penetration of filtrate into the core was not deep. The system sealed off filtrate invasion. As a result, the native wettability of the cores were obtained and reliable data was obtained by the operator. Bore-Plate was found to be the determining additive to accomplish this objective.