Long horizontal and multilateral oil and gas wells provide an attractive option for maximizing reservoir contact. Formate-based brines have been used in drilling and completion operations for more than 10 years. Advantages of these fluids include high density, solid-free brines, better compatibility with XC polymer, starches, and less potential for formation damage. These advantages were reported in several previous publications.
Filter cakes that are generated by drilling fluids can act as a barrier to the fluid flow in both cased and openhole wells. Calcium carbonate particles are frequently used as weighting material to maintain the pressure that is required for well control and minimizing leakoff rate. These solid particles become consolidated and trapped in a polymeric material, and this makes the filter cake a very effective permeability barrier.
The conventional method for cleaning filter cake is by using solids-free formate brines, either by soaking or circulating for many hours at high flow rates. This mechanical technique removes only external filter cake. Chemical means like acids, oxidizers, chelating agents, enzymes, or combinations of these chemicals are usually used as an alternative method for dissolving both the external and internal filter cake. Most of these fluids cannot give a full coverage to the wellbore because of the formation heterogeneity.
A new precursor (ester of an organic acid) can generate an acid downhole at a low release rate, which results in uniform fluid distribution through the wellbore. Compatibility and thermal stability tests between the precursor solution and formation brine were studied in detail. Return permeability experiments were conducted by using high-pressure/high-temperature (HP/HT) dynamic fluid loss cells. The ester solution was effective in cleaning the filter cake in comparison to the formate brines.
A field application, where formate brine was used to drill and complete a gas producer in a sandstone reservoir, is included in this paper. The gas well was drilled in a weakly consolidated reservoir and was completed with expandable sand screens (ESSs). The bottomhole temperature was 285F. The produced gas contained 23 vol% carbon dioxide and no hydrogen sulfide. Full analysis of flowback samples indicated that most of the returned solids were calcium carbonate. Laboratory tests indicated that the ester solution can be used to restore well productivity by removing damage induced by the drilling mud filter cake.