The CaraCara field in the Los Llanos area of Colombia is a highly permeable (1 − 3 Darcy) sandstone with more than 10% total clay content, producing heavy crude. Gravel packing is required to avoid sand production. However, the Productivity Index after conventional gravel packing is only 50% of the open-hole potential due to drill-solids—calcium carbonate —plugging the matrix and fines migration. Non-acid and acid based preflushes have been used to dissolve the drill-solids and fines in the critical near wellbore matrix respectively. However, corrosion, uncontrolled filter cake dissolution, sludging with heavy crude and disposal of spent acid limits the use of acid pre-flushes.
A recently developed non-acid based fluid capable of stimulating high temperature sandstone formations was taken as the starting point to develop a customized solution to remove formation damage and control fines migration for this particular application. The base fluid is chelant such as diammonium EDTA (DAE) to which a low concentration of HF acid is added, most commonly in the form of ammonium bifluoride. The HF acid greatly increases the dissolution of aluminosilicates, while the chelant prevents the precipitation of silica gel. The addition of boric acid effectively retard the dissolution rate of the fluid as boric acid the reacts with the HF acid to form fluoroboric acid (HBF4) which slowly hydrolyses to release HF acid. Fluroboric acid also provides an effective means of fines migration control. By adjusting the concentration of acid salts in the fluid as a function of temperature the fluid can provide effective controlled filter cake removal, matrix stimulation and fines migration control, at temperatures between 120 and 300°F.
The newly developed non-acid fluid system provides some unique advantages for gravel packing and matrix stimulation applications in highly permeable heavy oil reservoirs 1) A single carrier fluid, eliminating the need for preflushes 2) Controlled filter cake dissolution ensuring circulation is maintained while gravel packing horizontal wells. 3) Effective fines migration control. 4) Scale inhibition. 5) Crude compatibility and negligible corrosion rates.
In one field the average productivity index of vertical wells completed using this fluid increased from 0.2 bbl/psi to 0.9 bbl/psi. The final skin values were close to zero and the production remained stable for an extended period of time. A similar fluid when used in horizontal well applications reduced the cleanup time from days to hours. The ability to stimulate while gravel packing optimizes the productivity of dirty sandstones while minimizing the cost.