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Acid stimulation in carbonate reservoirs involves using acidic fluids to enhance oil or gas production by increasing the reservoir's permeability. This is achieved by dissolving the carbonate rock and creating or widening flow paths, such as wormholes, that connect the wellbore to the reservoir. The process can be categorized into matrix acidizing (at sub-fracturing pressures) and acid fracturing (creating fractures).
Here's a more detailed breakdown:
1. What is Stimulation?
Purpose:
To improve the flow of hydrocarbons from the reservoir to the wellbore by increasing the permeability of the rock.
Why in Carbonates?
Carbonate reservoirs often have low permeability due to their complex pore structure and mineralogy.
How it Works:
Acid reacts with the carbonate rock, dissolving it and creating channels or widening existing ones.
2. Matrix Acidizing:
Process:
Acid is injected into the wellbore at pressures below the fracture pressure of the rock.
Goal:
To remove near-wellbore damage (e.g., drilling mud, scale buildup) and improve the permeability of the formation around the wellbore.
Suitable for:
High-permeability reservoirs where the acid can flow through the rock and dissolve material.
3. Acid Fracturing:
Process:
Acid is injected at higher pressures, fracturing the rock and creating new pathways.
Goal:
To create longer, more conductive fractures that extend further into the reservoir.
Suitable for:
Low-permeability reservoirs where fracturing is necessary to create significant flow paths.
4. Key Aspects of Carbonate Acid Stimulation:
Wormhole Formation:
In both matrix acidizing and acid fracturing, the acid creates wormholes (highly conductive channels) that can significantly improve flow.
Acid Selection:
The choice of acid (e.g., hydrochloric acid (HCl), EDTA) depends on the reservoir conditions, rock type, and desired outcome.
Additives:
Additives can be used to modify the acid's properties (e.g., viscosity, reactivity) to optimize the stimulation process.
Rock Properties:
Mineralogy, pore structure, and heterogeneity of the carbonate rock all influence the effectiveness of acid stimulation.
5. Factors Affecting Success:
Injection Rate:
The rate at which acid is injected affects wormhole morphology and stimulation efficiency.
Acid Concentration and Volume:
Proper selection of acid type and concentration is crucial for achieving desired results.
Reservoir Heterogeneity:
The presence of different rock types or variations in pore structure can affect how the acid flows and reacts.
Temperature and Pressure:
These factors can influence the reaction rate and the overall effectiveness of the stimulation treatment.

GOR can be lower, equal, or higher than Rsi depending on the reservoir type and drive mechanism

1. Depletion-Drive (Solution-Gas Drive) Reservoirs
When pressure drops below Pb → gas evolves → flows with oil.

GOR increases above Rsi.

Eventually, GOR can peak, then decline as oil rate drops faster.

✅ GOR > Rsi (typical behavior)

2. Water-Drive Reservoirs
Pressure may stay near bubble point longer.

Less gas liberation.

GOR may remain close to Rsi or even below Rsi if gas is trapped or segregated.

✅ GOR ≈ Rsi or GOR < Rsi

3. Gas-Cap Drive Reservoirs
If gas cap is produced → very high GOR.

If only oil zone is produced → GOR may increase moderately.

GOR can be much higher than Rsi depending on coning or breakthrough.

✅ GOR > Rsi (if gas cap coning occurs)

4. Heavy Oil Reservoirs
Low Rsi to begin with.

Gas mobility is low.

GOR can remain low or increase slowly.

✅ GOR may stay ≤ Rsi for long time

In Carbonate, there are two types of acid to be used.<br><img src='https://www.esimtech.com/wp-content/uploads/2024/05/Well-acidizing-process.jpg' style='max-width:100%'>

Stimulation:

Wax - Solvvent

RMP

Reserve Calculation

WSO

FD

GP

Sand Screen

The Beggs & Brill correlation is widely used in PROSPER to estimate pressure drop in multiphase flow. The total pressure gradient is:

\[
\frac{dP}{dL} = \left( \frac{dP}{dL} \right)_{\text{elevation}} + \left( \frac{dP}{dL} \right)_{\text{friction}} + \left( \frac{dP}{dL} \right)_{\text{acceleration}}
\]

Where:

- \(\left( \frac{dP}{dL} \right)_{\text{elevation}} = \rho_m \cdot g \cdot \sin(\theta)\)
- \(\left( \frac{dP}{dL} \right)_{\text{friction}} = \frac{f \cdot \rho_m \cdot v_m^2}{2D}\)
- \(\left( \frac{dP}{dL} \right)_{\text{acceleration}} = \frac{d(\rho_m v_m)}{dt}\)

**Image example below shows a typical VLP curve from PROSPER:**

<img src="https://wiki.pengtools.com/images/thumb/1/12/Vertical_Lift_Performance_curve.png/400px-Vertical_Lift_Performance_curve.png" style="max-width:100%">

The Beggs & Brill correlation is widely used in PROSPER to estimate pressure drop in multiphase flow. The total pressure gradient is:

\[
\frac{dP}{dL} = \left( \frac{dP}{dL} \right)_{\text{elevation}} + \left( \frac{dP}{dL} \right)_{\text{friction}} + \left( \frac{dP}{dL} \right)_{\text{acceleration}}
\]

Where:

- \(\left( \frac{dP}{dL} \right)_{\text{elevation}} = \rho_m \cdot g \cdot \sin(\theta)\)
- \(\left( \frac{dP}{dL} \right)_{\text{friction}} = \frac{f \cdot \rho_m \cdot v_m^2}{2D}\)
- \(\left( \frac{dP}{dL} \right)_{\text{acceleration}} = \frac{d(\rho_m v_m)}{dt}\)

**Image example below shows a typical VLP curve from PROSPER:**

<img src="https://wiki.pengtools.com/images/thumb/1/12/Vertical_Lift_Performance_curve.png/400px-Vertical_Lift_Performance_curve.png" style="max-width:100%">