The Nandasan field discovered in 1969 is a multi-layered reservoir in western part of India and producing oil & gas with little problem as it is having good porosity and permeability. Deeper multi layered mehsana formation comprises of 45% silicon, 9%aluminium, 18%iron, 0.23%calcium, 0.47%potesium, 12%other heavy metal having 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 (1700-1800m). Core flow study with mud acid through indicates some precipitation resulting in formation damage and its success in field is limited. Therefore, greater understanding of down hole chemistry and assess the extent of the much slower secondary and tertiary reactions under reservoir conditions is critical for the acid treatment success. Monitoring of metallic ion generation during / after various acidizing treatments gives insight into actual chemical acid-spending processes that occur in the formation.

In the laboratory study, four type of different acid system has been tried with representative core and the resultant dissolution and metallic ion generation processes has been monitored for 3 hours using atomic absorption spectrometry followed by core flow study. Aluminium, Silicon and Iron complexes precipitation observed during secondary and tertiary reaction has been successfully tailored by changing the ratio of HCl:HF and other additives in traditional mud acid system. Results show 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 150 min 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 matrix acidization of mehsana formation.

This paper presents a new, effective and economical technique to assess and understand the extent of secondary & tertiary reactions under reservoir conditions that will help in additives selection and making acid formulation for effective matrix acidization of sand stone reservoir that leads to improved field practice.

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