Many carbonate and sandstone formations are completed with horizontal and extended reach wellbores. Drill-in fluids are generally water-based muds containing a viscosifier, a fluid-loss reducer, sized salts or and calcium carbonate particles. Filter cake is a major cause of restricted flow from the target zone. Effective filter cake removal during well completion is very difficult to achieve, especially in deep heterogeneous reservoirs. The obstacles encountered are the difficulty of zonal coverage of cleaning fluids with the filter cake throughout the whole interval, and controlling the rate of removal of the filter cake to ensure uniform coverage of the whole interval. The cleaning solutions can be acids, enzymes, oxidizers, chelating agents, or combinations of these materials. Enzymes are only specific for biopolymers and will not attack the calcium carbonate particles. Acids are highly reactive and corrosive fluids.
A new in-situ acid system can deliver the acid to long interval wells. The treatment fluid contains an acid precursor that provides time-controlled downhole acid release. Several lab experiments were conducted to examine thermal stability of fluids at high temperatures. The kinetics of acid producing reactions were examined in detail. Modified dynamic and static HPHT fluid loss cells were used to assess the effectiveness of the system in cleaning drill-in fluid filter cake. The surface tension of the acid precursor and the spent acid was measured as a function of temperature and acid concentration.
No phase separation of precipitation was noted when the precursor solution what heated to 250F for 24 hours. Unlike conventional HCl acid-based systems, the slow reaction kinetics ensures better distribution of the new fluid across long sections with minimal losses during pumping. The HPHT experiments showed a significant improvement in the efficiency of removing acid soluble portions of the filter cake. Surface tension measurements indicated that the spent acid solution had low surface tension, which will accelerate lifting the spent acid from tight formations.