Abstract

Fracture acidizing of deeper Delaware Basin gas producing carbonate rocks, employs a number of techniques and acid formulations. A recently developed technique makes use of a stabilized foamed acid to achieve increased stimulation along with fast clean up. The quick recovery of treating fluids results in early stabilized flow permitting accurate testing and early gas sales.

Deeper gas zones with abnormally low bottom-hole pressures present a special problem when acid fracturing. New replacement wells and new wells drilled in older producing reservoirs, such as those in the deep Gomez Ellenberger Field, encounter partially depleted pressure conditions. Conventional fracture acidizing requires extensive, costly jetting with nitrogen and coil tubing to remove treating fluids and 'kick off' the well. Foamed acid stimulation treatments have not required jetting to initiate gas production.

Once acid in a stable foamed conditions gets into the created fracture it provides reduced leak off and reduced acid reaction rate with the soluble carbonate formation. This allows greater fracture penetration along with increased fracture conductivity throughout the fracture length. Adequate injection rates are possible, even with the reduced hydrostatic pressure of the foam column because of the low friction properties and reduced frac gradients in these reservoirs.

A number of foamed acid stimulation treatments in carbonate formations are detailed along with tabulation of production increases.

Introduction

The stimulation of deep carbonate reservoirs is greatly affected by acid spending times and fluid leakoff rates. To achieve adequate stimulation, a deep penetrating fracture with high conductivity must be created. Abnormally low bottom-hole pressure conditions, relative to depth, present a special problem for the return of stimulation fluids.

Foamed acid provides a new approach to reducing acid reaction rates with highly soluble carbonate rocks, along with achieving control of acid leakoff into existing hairline fractures. A great deal of energy, in the form of nitrogen gas, is induced into the reservoir to aid in the return of fluids to the well bore. A foamed flush fluid will greatly reduce the hydrostatic pressure of the fluid column in the tubing. Wells under these conditions will 'kick off' and flow, where otherwise they would not.

Fracture Aciding with Foamed Acid

The deeper gas zones with abnormally low bottom-hole pressures present a special problem when fracture acidizing. Fracture acidizing with conventional fluids requires jetting with coil tubing and nitrogen to remove treating fluids and 'kick off' the well. This involves a great deal of time and money. The evaluation of the well is delayed due to long cleanup time. Gas sales and subsequent revenues are delayed. If the rig is still on the well, costs are increased even more.

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