Tarlochan Singh's thesis
by
Tarlochan Singh, MSE
University of Texas at Austin, 1997
Supervisor: Mukul M. Sharma
Water based drill-in fluids are increasingly being used to limit mud
solids/filtrate invasion in present day drilling. This thesis investigates
the use of acid soluble sized fibers (Fibersol) as an additive for drill-in
fluids when drilling through fractured formations. Experiments were conducted
with a dynamic filtration core holder on sandstone and limestone cores,
both unfractured and fractured. Two existing drill-in fluids were used
as a starting point, a sized salt drill-in fluid and a sized calcium carbonate
drill-in fluid were evaluated for unfractured formations. The fluid loss
as well as the mobility impairment across different sections of a 6 or
8 inch long core were monitored. It was found that the mobility impairment
for both the sized salt and the sized calcium carbonate drill-in fluid
was limited to the first 2 to 3 inches of the unfractured core.
Dynamic fluid loss and return mobility experiments were conducted on
fractured sandstone cores using sized salt as the base fluid. The overall
return mobility was found to decrease by 83% when a granular additive (sized
salt with a particle size distribution of 15-300 microns) was used as the
LCM (loss circulation material) in the sized salt drill-in fluid. When
Fibersol was used as the LCM additive in the sized salt drill-in fluid,
the overall return mobility after circulating the drill-in fluid across
the face of the core for 10 hours was found to be 55% of the original mobility.
It was found that extremely low concentrations of Fibersol in the sized
salt drill-in fluid were sufficient to reduce the extent of mud solids
penetration and formation damage in the fractured cores. Visual observations
on the fractured cores indicated particle invasion was significantly less
in the case of the drill-in fluid containing Fibersol.
Rheological measurements made on the sized salt drill-in fluid containing
Fibersol showed excellent rheological properties. Addition of Fibersol
improved the cuttings carrying capacity of sized salt based drill-in fluids.
Acid solubility tests were conducted on the filter cake formed by the
sized salt drill-in fluid containing Fibersol. It was found that the filter
cake was completely soluble in hydrochloric acid and formic acid. The presence
of calcium carbonate resulted in the formation of a "plastic like" precipitate
with spent hydrochloric acid. The spending of Fibersol in formic acid in
the presence of calcium carbonate yielded a non-damaging very fine grained
precipitate. On the basis of these observations a formic acid wash was
followed by a hydrochloric acid squeeze treatment was carried out on a
fractured limestone core damaged by calcium carbonate based Fibersol drill-in
fluid. The formic acid wash and the hydrochloric acid squeeze treatment
both resulted in significant improvements over the original undamaged fracture
conductivity. These findings suggest that the breakdown of filter cakes
formed with sized salt or calcium carbonate drill-in fluids supplemented
by Fibersol is achievable by using hydrochloric acid in sandstones or formic
acid in limestones. Based on our results of return mobility as well as
the solubility of the filter cake, the use of Fibersol supplemented sized
salt and sized calcium carbonate drill-in fluids in fractured formations
is recommended.
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