Abstract

This paper discusses the uses, properties, and early field results of a gelling agent that increases the viscosity of hydrochloric acid solutions. Gelling hydrochloric acid for use in fracture acidizing is desirable for several reasons:

  1. Increased viscosity provides better control of fluid leak-off.

  2. Increased viscosity creates wider fractures resulting in deeper acid penetration.

  3. Retention of viscosity provides better cleanup of fines.

A limitation encountered with most gelling agents (such as natural polymers and gums) is that they chemically degrade upon contact with hydrochloric acid, with little or no viscosity increase if the acid temperature exceeds 38 degrees-65 degrees C (100–150 degrees F). This gelling agent exhibits complete chemical stability in all strengths of hydrochloric acid. It is a liquid which mixes readily with acid to produce a viscous solution, and the viscosity is maintained regardless of time or shear conditions. Although the material is completely stable in any strength HCl, a breaker is not required because the acid loses over 80% of its viscosity through exposure to calcium or magnesium salts formed when the acid reacts with the formation. Also, the gelling agent remains in solution and does not form any insoluble residue.

Other properties include friction reduction comparable to conventional polymer gels, and effective suspension of fines that are released during the acidizing operation. The fines suspension ability enables solid materials to he removed that could plug conductive passages created by the acid. Early field results confirm these passages created by the acid. Early field results confirm these properties. properties

Introduction

This paper examines the properties of a new gelling agent for use in fracture acidizing of acid soluble formations. It presents data from the laboratory and field tests. The first part, a discussion of gelling agents in general, covers both positive and negative aspects of existing acid gelling materials. The next section is an analysis of the gelling agent.

GELLING BENEFITS

Gelled acids provide several advantages for oil and gas wells being stimulated by fracture acidizing. While many beneficial results may be realized with non-gelled acid, the following considerations cause a gelled acid to be desirable.

First, it has been shown that the rate of fluid leak-off is directly related to viscosity. Increased viscosity means improved fluid efficiency because more acid stays within the fracture. And if fluid efficiency is improved, more live acid can be pumped deeper into the formation, resulting in increased fracture flow capacity.

A second benefit of a gelled acid is that deeper penetration of the formation is achieved due to decreased mass transport of the hydrogen ion to the formation face. This is influenced by two characteristics:

  1. a viscous fluid creates a wider fracture, and,

  2. the fluid tends to remain in laminar flow at higher pump rates.

If the gel creates a wider fracture, the ratio between the volume of acid and the area of the fracture face is increased. This decreases contact between the acid and the rock surface. If the gel is in laminar flow, the acid travels a greater distance before spending on the rock surface (as compared to turbulent flow).

A third advantage of a gelled acid is the potential for better cleanup of fines (particulate matter) released by the acid. Due to uneven dissolution of rock by the acid, most formations release some fines, even if the formation is 100% acid soluble. Some formations containing non acid soluble materials (silicas, clays, and feldspars) have been shown in laboratory tests to release over 227 Kg (500 lb) of fines per 3.79m3 (1000 gal) of acid. And these fines carry the potential for clogging the etched fracture flow channels, especially potential for clogging the etched fracture flow channels, especially during migration back to the well bore. If the spent acid could retain a degree of viscosity, fines removal would be greatly improved.

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