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Formation Damage Evaluation by Secondary & Tertiary Precipitation during Sand Stone Acidization: A Laboratory Study

Kumar, Ajay *1
(1) Institute of Oil & Gas Production technology, Panvel, India.

The Nandasan field is producing oil & gas since 1969 with little problem as it is having good porosity and permeability. Its multi layered mehsana formation comprises of 45%silicon, 9%aluminium, 18%iron, 0.23%calcium, 0.47%potassium ,12% other heavy metal shows presence Montmorillonite, Kaolinite and Chamosite clay. Several mud acid treatments have been performed successfully in the shallow depth but the same is not quite effective for deeper pay zone(1700-1800 m). Core flow study with mud acid indicates some precipitation occurred resulting in formation damage and its success in field is limited. Therefore, a greater understanding of downhole chemistry and assess the extent of the much slower secondary and tertiary reactions under reservoir conditions is critical for the acid treatment success. Many HF acids formulation are good in core flow study but it gives secondary precipitation in the laboratory study. Therefore, monitoring of metallic ion generation during treatments gives insight into actual chemical acid-spending processes that occur in the formation.

In laboratory study, four type of acid system has been reacted with core and resultant dissolution and metallic ion generation processes has been monitored for 3 hours using AAS followed by core flow study. Aluminium, Silicon and Iron precipitation observed during secondary & tertiary reaction has been successfully tailored by changing the ratio of HCl:HF and other additives in traditional mud acid system. Results showed that RA-1(AlCl3+HF) acid is little reacting with Silicon whereas RA-2(Phosphonic acid+ABF) creates Silicon & Calcium precipitation since beginning, thus not suitable. Similarly, RF (HCl+HBF4) creates Silicon precipitation after 150min in tertiary reaction, whereas, MHF(HCl-HF) acid creates Al precipitation in secondary reaction and Iron precipitation since beginning that is resolved by increasing HCl:HF ratio and adding iron sequestering agent. Core flow study confirms the success of MHF by observing permeability improvement, therefore, suitable for acidization of mehsana formation.

 

AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain