As more gas wells have been completed in East Texas on 160 acre spacing, fracturing down casing has become more prevalent as the best way to stimulate these wells. Field trends have been toward higher sand concentrations. Most wells were fractured earlier through 2 7/8" tubing with maximum concentrations of 3 lbs./gal. 20–40 Mesh Sand. However, as wells were produced, it became evident that longer fractures and higher flow capacities were needed. The casing fracturing technique permitted higher pumping rates which allowed higher sand concentrations to be placed. These began to be placed at 4 lbs. and 5 lbs./gal. and have now successfully been placed at concentrations in the 7 lbs. to 10 lbs./gal. range. Recently several jobs have been placed at 15 lbs./gal. These higher sand concentrations as compared to a lower sand concentration in an equal predicted propped fracture length seem to be giving improved clean-up of fracture fluids and longer stabilized flow rates. This improved clean-up is probably due to the higher fracture flow capacity to a multi-phase fluid which is present in the fractures immediately after fracturing. The higher fracture flow capacity also would be maintained even with fines and residue from treatment fluids thus yielding the better stabilized flow rates.
The casing fracturing jobs generally have started with a treated water prepad or in some cases a low concentration crosslinked gel system. The prepad or in some cases a low concentration crosslinked gel system. The normal volumes have ranged from 10,000 – 30,000 gals. with some jobs being pumped with only 6000 gals. of prepad. All these volumes are assuming that pumped with only 6000 gals. of prepad. All these volumes are assuming that the casing is full at the time the fracturing treatment starts. A 25 – 30% of the total crosslinked fluid is then pumped as a pad ahead of the sand ladened fluid. The sand ladened fluid has generally begun with one casing volume of one lb./gal. of 20–40 Mesh Sand then proceeded to 1 – 1 ½ casing volumes of 2 lbs./gal. sand as in Figure 2 and Figure 3. The reason for the small stage volumes of the 1 lb. and 2 lbs./gal. sand is that the resultant proppant concentration per square foot in the fracture is very low. The 3 lbs./gal. stage usually ranges from 30,000 – 60,000 gallons of fluid volume and will yield adequate concentrations in the fracture to allow the tip area to be drained even with sand crushing. The four and five lbs./gal. stages have undergone the most changes as the higher sand concentrations are being pumped. When the casing fracturing treatments first started, the 4 lbs. and 5 lbs./ gal. were the highest concentrations to be placed downhole. An early treatment schedule is depicted by Figure 1. The volumes of these stages were not large by any means as compared to the total volumes of treating fluid. As placement of these concentrations became more routine, the volumes began to increase to a substantial percentage of the total volumes. The production history of these early wells have allowed some conclusions to be drawn from the decline curves. Most wells showed a rapid decline for the first 3 to 6 months and then a leveling off at a substantially reduced rate as compared to the initial production. In an attempt to decrease this rapid decline, the use of 6 – 7 lbs./gal. sand stages, not just as a tail-in if there was some extra sand on location, but as designed stages of fairly large volumes. The need for and the designing of these 6 – 7 lbs./gal. stages forced a rapid change in surface technology. The sand handling equipment which could meet these demands became a reality.
