Foamed Acid An Effective Stimulation Fluid
Authors: Ford William G.F. 
Published: December 2007

Ford, William G.F., SPE, Halliburton Services

Summary
Dynamic laboratory testing of foamed acid on limestone cores has established the effectiveness of foamed acid as a stimulation fluid. The effects of foam quality, foam stability, and chemical compatibility on fluid loss and fracture flow capacity were investigated. Recommendations are presented for deriving maximum benefits from a foamed acid treatment. Field results are presented that show the effectiveness of foamed acid in the stimulation of both oil and gas wells.

Introduction
The use of foam in fracturing treatments has gained widespread acceptance in the past few years. Low liquid content, good fluid loss control, and quick cleanup are just a few reasons why foams are being used. The fluid loss properties of foam as a fracturing fluid and the flow of foam through porous media have been investigated by several authors. The use of foamed acid in fracture acidizing has been reported to give the same benefits as foam in hydraulic fracturing treatments. This paper describes work where fluid loss of foamed acid has been measured directly and the effect of foamed acid on fracture conductivity has been studied. This paper presents laboratory data describing the effects of foam quality, foam stability, chemical compatibility, formation permeability, and pressure differential on. fluid loss and fracture flow capacity with foamed acid on limestone.

Apparatus and Test Procedure

Fluid Loss Tests
The system used to study foamed acid fluid loss is illustrated in Fig. 1. The liquid and gas portions of the foam were maintained at a constant volume by separate control mechanisms. A 1,500-mL volume Ampcoloy floating piston cell was used to hold the acid solution. The driving fluid for this cell was supplied by a positive displacement Jaeco pump. Flow rates were set mechanically on the pump and tested to be 20 mL/min at 1,500 psi. The flow rate of nitrogen at 1,500 psi was measured with an integral orifice meter equipped with a digital readout supplied by Fisher Porter with a No. 2 orifice. The meter was calibrated for various nitrogen flow rates at 1,500 psi through the use of a GCA/Precision Scientific wet test meter. The pressure of the system and nitrogen flow rates were adjusted manually by the use of a back pressure regulator for coarse settings and a needle valve for fine settings. Once pressure and flow rate were stabilized at the start of the test, very little adjustment was needed to maintain the proper pressure and flow rate.The foam generator consists of a 180-mL volume Ampcoloy cell and impeller driven at a speed of 2,200 rpm. This type of foam generator was chosen over the wire screen in a pipe type because of the reactive nature of the test solution. With time, the acid would react with and erode the screen, thus limiting the chances of reproducing a foam with the same texture (bubble size) and consistency from test to test.Foamed acid was generated and allowed to pass through two visual flow cells and out to the waste trap through a high-pressure backpressure regulator. The visual flow cells were used to check the condition of the foamed acid before the fluid loss test began, with uniform small bubbles and no gas pockets conditional to a stable foam. Once this condition was reached, a portion of the foamed acid was allowed to flow into the fluid loss cell while continuing to flow foamed acid through the system to the waste trap.
JPT
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