Development of an Acid Degradable Drill-In Fluid for Fractured Reservoirs
Authors: Singh Tarlochan , Sharma Mukul M. 
Published: December 2007

Abstract
This paper investigates the use of acid soluble sized fibers as an additive for drill-in fluids when drilling through fractured formations. Experiments were conducted with a dynamic filtration core holder on both fractured and unfractured cores. 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 oil mobility impairment across different sections of an 8 inch long core were monitored. Both drill-in fluids were found to have excellent fluid loss characteristics and were relatively non-damaging. The sized calcium carbonate drill-in fluid resulted in a oil mobility reduction of 71% while the sized salt drill-in fluid showed a reduction of 18%. In both cases the damage was limited to the first section of the core (2.5 inches),
On the basis of these observations the sized salt drill-in fluid was chosen to be the less damaging fluid under our experimental conditions and experiments were conducted on fractured cores using sized salt as the base fluid. The overall return oil mobility was found to decrease by 83 % when a granular additive (sized salt with a particle size distribution of 15-300 m) was used as the LCM (loss circulation material) in the sized salt drill-in fluid. Acid soluble fibers (Fibersol) were used as a LCM additive in the sized salt drill-in fluid. The overall return oil mobility after circulating the drill-in fluid across the face of the core for 10 hours was found to be 55% of the original oil 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 indicate 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. The concentration, length and diameter of the Fibersol fibers was varied over a wide range in these rheological measurements.
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 polymer precipitate with spent hydrochloric acid. These findings suggest that the breakdown of filter cakes formed with sized salt drill-in fluids supplemented by Fibersol is achievable by using hydrochloric acid in sandstones or formic acid in carbonates. Based on our results of return oil mobility as well as the solubility of the filter cake, the use of Fibersol supplemented sized salt drill-in fluids in fractured formations is recommended.

Introduction
Horizontal wells are increasingly being used to maximize productivity. In many instances horizontal wells are designed to intersect natural fracture systems which provide most of the production from tight formations. Horizontal wells are typically completed openhole, without cementing and perforating. Openhole completions include prepacked screens or slotted liners (Ryan et al., 1995). Production from openhole completions takes place through the mud filter cake and the damaged zone resulting from mud solids/filtrate invasion. It is, therefore, vital to limit mud solids/filtrate invasion and to obtain acid degradable mud filter cakes which require low differential pressures to lift off. Brine based muds with acid or water soluble additives are commonly used as drill-in fluids to limit mud solid/filtrate invasion. The deposited filter cake is then dissolved by using acid breakers (McLarty et al., 1993 and Ali et al., 1993). Use of high salinity brines also reduces the mud solids loading compared to bentonite based muds. Sized salt and sized calcium carbonate drill-in fluids are the two commonly used water based muds. P. 85^