Fluid-Rock interactions were evaluated using formation rocks and fluids from Carito Field at East of Venezuela, in order to establish better criteria for the design of more efficient cement spacers. Rock and formation fluids were analyzed before and after exposure to overbalanced drilling and cementing filtrates through formation cores. Rock changes were evaluated by X-ray diffraction, SEM, permeability and wettability, whereas fluids were analyzed by fluid and gas chromatography, GC - MS, NMR and IR. Results indicate that 100% oil based mud filtrate produces a large permeability reduction. Spacer A showed greater mud removal efficiency than spacer B, whereas the cement slurry filtrate generates lighter formation permeability reduction than the oil-based mud, after either spacer exposure. This can be explained due to insoluble salts' precipitation when the cement filtrate is in contact to the residual formation water.
The molecular analysis of crude samples indicates that the absence of heavy molecules from hopane compounds in the crude saturated fraction could be related to adsorption of these compounds on the rock surface, which is partially oil wettable. In addition, absence of C15 - C20 molecules from crude oil is related to molecular distribution in the gasoil from mud and spacers evaluated. The results of these tests indicate interaction between crude oil and rock composition with drilling and cementing filtrates. Overall results of this study will allow achieving a more efficient approach for design of spacer fluids.
Introduction
Formation damage during drilling and cementing operations have became important in the oil industry lately because of severe chemical and mineralogical changes observed to the rock and fluids from reservoirs during these processes. Oil based mud is a technology relatively new, thus few studies (Refs. 1 through 3) on the formation damage these fluids cause have been published only recently.
Physicochemical equilibrium in the reservoir is altered when rock formation is exposed to drilling and cementing fluid filtrates, especially on rock permeability, wettability, water saturation and crude-oil properties. These alterations can directly cause impairment and loss in reservoir productivity (Refs. 4 through 8).
Cementing, as drilling, is an important process in all phases of the well development. This process also may require oil-based fluids as washer and spacers that contain different kinds of surfactants and polymers useful to improve cement adherence to the casing and formation, when oil based mud are used. Cement slurries also have high calcium concentration and pH, which results in insoluble salts precipitation in the porous media that reduce considerably the rock permeability. Other additives are commonly added to cement formulations in order to disperse cement particles, modify the setting time under temperature and pressure conditions, control fluid losses from the cement slurry during and after placement and control influx and migration of fluid into the cement column. These additives usually are organic and polymeric materials, lignite and cellulose derivatives, which can cause severe formation damage by fines' migration (Refs. 9 through 13).
Well construction in North of Monagas area is done using oil-based fluids, highly densified to provide physicochemical stability to critical rock formations (deep formations, swelling clays and high pressure and temperature). Spacers to remove oil based mud of 18 ppg and densified cement slurries are commonly used (Ref. 14). This kind of materials can contribute significantly to severe formation damage by fines' migration, wettability alterations, insoluble solids precipitation and emulsion formation among others.
