Abstract

Stimulation of carbonate formations by acid dissolution of the rock has been an efficient and successful method of bimproving production in oil and gas wells. Hydrochloric acid is the normal fluid of choice. However, in high temperature applications corrosion issues limit usage, especially in chrome completions. Acetic acid has been used with some success and with adequate corrosion protection. But due to its low reactivity at higher temperatures, the efficiency with which a gallon of acid dissolves the formation is perceived as low. This perception comes from reaction efficiency of acetic acid reported in the literature ranging in values from 90% at 25 °C to 40% at 121 °C for 2 to 15 wt%, respectively. Acetic acid reaction on calcium carbonate is controlled by its small dissociation constant, 1.754E-05 at 25 °C (77 °F) and therefore is labeled a weak acid.

Abstract

One of the most effective methods of carbonate stimulation is acid fracturing. Development of fracture geometry sufficient to allow etched penetration to provide an economic production increase is dependent upon several factors, one of which is pump rate. High rates and/or lower hydraulic horsepower requirements can be critical to success to one of these projects.

Many times a wellbore lacks the integrity or flexibility to be treated down casing. Treating down tubing affords highly viscous fluid's opportunity for rate restrictions due to large friction pressures observed when pumping acid. In addition, fluids of high viscosity based on the crosslinking of polymers using zirconium are known to have shear limitations. Therefore, adequate fracture geometry in this type of a well has been unattainable.

Laboratory testing and case histories are presented regarding the evaluation and usage of zirconium crosslinked hydrochloric acid systems in the acid fracturing of several carbonates utilizing a variety of tubulars. Major emphasis is on the friction pressures observed on wells treated down 7.303 cm (2.875 inch) and 11.43-cm (4.5-inch) O. D. tubulars. Also, laboratory results on the effect of shear on the stability of zirconium crosslinked gelled hydrochloric acid viscosity is presented. In addition, the crosslink delay effect on ultimate viscosity is presented.

Background

Stimulation of carbonate reservoirs is typically the result of a need for restoration or enhancement of production to a more economic level. Acid Fracturing is the most widely used technique for stimulating limestone or dolomite formations.1 Fracture acidizing originally was conducted using non-reactive fluids as pads to create the fracture. Acid was then injected into the fracture to react with the formation to produce flow channels, after closure. During the 1970's, various techniques and new acid systems were developed.2–4 The fluid systems were designed to provide better penetration through leak-off control, fracture geometry control and reaction retardation of the hydrochloric acid. In general, leak-off control and retardation are found to be the two most significant parameters in achieving success in vugular and natural fractured dolomites.

The gelation of hydrochloric acid (HCl) for use in the oil industry began in the mid-1970's. Since, the distance reactive acid will penetrate along a fracture normally will increase as the flow velocity along the fracture is increased the higher the rates generally the better the treatment.1 Higher pumping rates were possible using gelled acid blends allowing for better acid fracturing of carbonates. The viscosity of the acid reduced the overall spending of the acid on the formation allowing for deeper pene

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