Abstract

Foamed fluids with nitrogen percentages or qualities greater than 60 percent have been used to provide greater efficiency in fracturing fluid design than other conventional types of frac fluids. When acid rather than water is used for 15–40% of the fluid volume, the fluid retains the same efficiency characteristics of a stable foam with one additional dimension. The added dimension is that the acid will remain virtually unreacted while being pumped into even the most soluble formations, and only begins to react when its leak-off/reaction controlling movement is stopped. It is suspected that bubble configuration and structure play an important role in preventing reaction to any great degree before the fluid ceases to be pumped. However, to place the acid a given distance into the formation, with conventional foam qualities or gas percentages requires fracturing rates and pressures which limit application of such a retardation system in many reservoirs where concern for break-through via fracturing into water-bearing zones exists. Therefore, this technique is possibly limited to fracture acidization, but testing is presented to show the effects of acidization of soluble cores with lower quality foam (25%-50% N2 gas). Actual job results are presented with foamed acid in the lower quality ranges.

Introduction

The use of stable foam stimulation has been well documented in the literature in recent years. Its use has been in the realm of stimulation via fracturing and proppant placement. Blauer, Mitchell, Kohlhaas et. al., have described the mechanisms of foam rheology and efficiency within the realm of high quality or low quality foams (those with high gas volume to total foam volume ratios). Others have suggested foam properties in porous media. The ranges with the prescribed efficiencies for fracturing include those qualities between 0.5236 and 0.95 theoretically, and practically, those between 0.65 and 0.85. Low fluid loss with spurt losses approaching zero, and CI values of 1 × 10(-4) are the known benefits with foamed fracturing systems. Actual field demonstrations and laboratory demonstration of this efficiency were the first steps in the establishing of a mechanism for the extended control of HC1 acid reaction on soluble formations. Thus extended penetration of "live" HCl further away from the immediate well bore can be realized. Retardation of acids by a variety of means has been extensively investigated.

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