The primary difference between acid fracturing and hydraulic fracturing lies in the fluid used to create and maintain the fractures and the type of rock formation targeted. Acid fracturing uses acid to dissolve the formation rock, while conventional hydraulic fracturing uses a pressurized fluid containing proppant to physically hold fractures open.
As stated in the reference, acid fracturing, also referred to as “fracture acidizing,” is a special case of hydraulic fracturing. This means acid fracturing is a specific technique within the broader category of hydraulic fracturing.
Understanding Hydraulic Fracturing
Hydraulic fracturing, often called "fracking," is a well stimulation technique used to increase the production of oil and natural gas from reservoirs. It involves injecting a high-pressure fluid (typically water mixed with sand, ceramic beads, or other materials called "proppant") into a wellbore to create or enlarge fractures in the target rock formation. The proppant stays in these fractures, keeping them open after the injection pressure is released, creating pathways for hydrocarbons to flow to the well.
Understanding Acid Fracturing (Fracture Acidizing)
Acid fracturing is a variation of hydraulic fracturing where the injected fluid contains acid, typically hydrochloric acid (HCl), rather than proppant-laden fluid. The acid reacts with and dissolves certain types of rock, most commonly carbonate rocks like limestone and dolomite. This dissolution process etches uneven surfaces on the fracture faces. These etched surfaces can act as conduits for fluid flow even if the fracture partially closes after pressure is released, potentially reducing or eliminating the need for proppant. The reference notes that "An acid fracturing treatment typically requires a much larger volume of acids than a matrix acidizing treatment does," distinguishing it from simple acid matrix stimulation which targets pore spaces rather than creating fractures.
Key Differences Summarized
Here's a breakdown of the main distinctions:
| Feature | Hydraulic Fracturing (General) | Acid Fracturing (Fracture Acidizing) |
|---|---|---|
| Method | Creates fractures using pressure and holds them open with proppant. | Creates and etches fractures by dissolving rock using acid. |
| Fracturing Fluid | Typically water-based fluid with proppant. | Acid (e.g., HCl). May or may not use proppant. |
| Primary Mechanism | Physical propping of fractures. | Chemical dissolution and etching of fracture faces. |
| Applicability | Wide range of lithologies (shale, sandstone, carbonates, etc.). | Primarily applicable in low- to moderate-permeability carbonate reservoirs. |
| Goal | Create conductive pathways held open by proppant. | Create conductive pathways through dissolution and etching. |
| Relationship | Broader technique. | Special case or variation of hydraulic fracturing. |
Practical Implications
- Formation Type: Acid fracturing is specifically effective in carbonate rocks because these rocks react readily with common acids. It is generally not used in sandstone or shale formations, where conventional hydraulic fracturing with proppant is the standard. The reference confirms that acid fracturing is "generally applicable in low- to moderate-permeability carbonate reservoirs."
- Proppant Usage: While some acid fracturing treatments might use a small amount of proppant, the primary method of maintaining conductivity is the etched fracture surface created by the acid dissolving the rock. Conventional hydraulic fracturing heavily relies on proppant to keep the fracture open.
- Mechanism: The process of fracture creation differs. In hydraulic fracturing, fluid pressure exceeds the rock's tensile strength to crack it, and proppant prevents it from closing. In acid fracturing, pressure also initiates the fracture, but the acid's chemical action alters the fracture surfaces, making them more conductive.
In essence, acid fracturing leverages a chemical reaction (dissolution) to enhance fracture conductivity within specific rock types, distinguishing it from the purely mechanical propping used in most hydraulic fracturing operations.
