Abstract

Fracture acidizing has been a dominant practice in the industry to enhance well productivity in low permeability carbonate reservoirs. Many acid systems have been developed to improve this stimulation process. The most desirable characteristics for an acid system to be suitable for fracture acidizing are leakoff control and retarded reaction rate. These characteristics are required for deep acid penetration so that when fracture closes, long flow channels are etched on the fracture surfaces. Leakoff control is normally achieved by a pad containing viscosifying or solid bridging agents to plug wormholes generated by acid dissolution. Reaction retardation is normally attempted by lowering the effective diffusivity of hydrogen ion.

It is well known that during an acid fracturing operation the overall reaction rate of hydrochloric acid with limestone is mass transfer limited. Designing the treatment requires knowing the effective diffusivity of the acid fluid system. This parameter does not exist for viscoelastic surfactant gelled acids. Due to its combined leakoff control and retardation capabilities, surfactant-based acids have been used in acid fracturing treatments. As more carbonate reservoirs to be treated by the surfactant-based acid, it is important to obtain the effective diffusivity of H+.

The rotating disk device has been used to investigate the reaction kinetics between a reactive solution and carbonate rocks because the thickness of the boundary layer is uniform throughout the disk surface. This paper discusses the reaction rate data recently generated for surfactant-based acid using a rotating disk apparatus and presents the methodology used to quantitatively extract the effective diffusivity from the measurements.

The results obtained indicated that the viscoelastic surfactant examined (Betaine-type) reduced the dissolution rate of calcite with HCl acid. The surfactant reduced the diffusion coefficient for H+. The effect of temperature on the diffusion coefficient did not follow Arrhenius law.

Introduction

Carbonate reservoirs are heterogeneous, with large variations in rock permeability even in the same productive zone. A wellbore, vertical or horizontal, may not provide effective drainage of the reservoir by connecting the high permeability channels. Fracture stimulation is therefore needed to extend the reservoir contact from the wellbore. Performing hydraulic fracturing treatment in a carbonate formation encounters two major difficulties. The first is that stimulation fluid leaks off in the direction perpendicular to the fracture faces, reducing the fluid needed to extend the length of the induced fracture. The second is that the acid-carbonate reaction rate is very fast so that the acid has spent before it reaches the tip of the fracture. In addition, the high reaction rate further aggravates the fluid leakoff by creating wormholes into the reservoir along the fracture. Therefore, retarding the acid-rock reaction rate is one of the most important tasks in designing fluids for acid fracturing in carbonates.

Viscosifying the fracturing fluid has been prevalently used to retard the acid-carbonate reaction rate. Polymers, crosslinked polymers, and viscoelastic surfactants are commonly added to HCl to increase fluid viscosity. This paper focuses on the effect of a viscoelastic surfactant on the retardation of HCl-calcite reaction rate.

There are several ways that viscoelastic surfactant can affect the reaction of the acid with the rock. The viscoelastic surfactant will increase the viscosity of the acid which will reduce the rate of transfer of H+ from the bulk solution to the surface of the rock. Viscoelastic surfactant molecules can adsorb on the rock surface and form a barrier that reduces acid reaction with the rock. Finally, viscoelastic surfactant solutions are non-Newtonian fluids and can change the flow pattern close to surface of the rock, and therefore, affect the way the acid reacts with the rock.

The present study uses the rotating disk instrument to examine the reaction of viscoelastic surfactant-based acids with calcite. This instrument has been extensively used to investigate the reaction of acids and chelating agents with carbonate rocks.1–17 It has also been used to study mass and heat transfer into non-Newtonian fluids.13,15–17

The objectives of this study are to:

  1. determine the effect of surfactant on the dissolution rate of calcite,

  2. examine the impact of the surfactant on the diffusion coefficient of H+, and

  3. assess the influence of temperature on the diffusion coefficient.

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