Oil well Stimulation con't 4(Sandstone Acidizing)

Fluids Available

Hydrochloric acid

Preflush

Overflush

Hydrofluoric acid systems

Mud Acid

Organic Mud Acid

Clay Acid

Organic Acids

Formic

Acetic

Citric (L1)

Selection Criteria

Formation mineralogy

Sensitivity

Deconsolidation

Precipitation

Fines release

Reactivity

Chemical composition

Surface area

Rock Structure

HCl solubility / carbonates

Clay distribution

Preflush - Brine

Ammonium chloride

Minimum spacing at any moment between formation brine and HCl

      3 wt% (<5% Clay)

      4 wt% (5-10%)

      5 wt% (10-15%)

      6 wt% (>15%)

Main Acid - Volume

1      Target: skin reduction

2      Never zero

3      Minimum skin achievable: pseudoskin value

4      Maximum reduction: 90%

Systems Available

Mud Acid

Organic Mud Acid

Clay Acid

Organic Clay Acid

Mud Acid

1      Nine HCI-HF formulations in MA selection Guide

2      Dissolves siliceous minerals

3      Dowell offered the first commercial Mud Acid Service in 1940 in the (U.S. Gulf Coast)

Mud Acid Reaction Simplified Version

Organic Mud Acid

1      Formic acid (9% L036 replaces 12% HCl)

2      Less corrosive than comparable Mud Acid formulations

3      Reaction rate ~ 1/4 that of Mud Acid

4      Reduces sludged tendency

Organic Mud Acid Preparation

Dissolving Y-1 (Ammonium Bifluoride) in HCl solution

Add enough HCl to completely react with Y-1

Add L36 last

Blending 20% HF (H200) and L36 into fresh water

Clay      Acid:
a retarded Mud Acid

Improved Penetration with Clay Acid

Clay Acid & Mud Acid

1      Preflush to Mud Acid: sensitivity

2      Main Acid: carbonate-cemented, long MA treatments

3      Overflush to Mud Acid: enhanced clay control

4      Shut-in and bring production back slowly

Primary Reaction – What to do ?

Alkali compounds

              1.     Push away formation brine

              2.     Brine with no Na, K, Ca

                      Ammonium Chloride Brine

CaF2

                      Dissolve CaCO3

                      HCl Preflush

Secondary Reaction – What to do ?

AlFx

                      Maintain a low-pH environment

                     

Silica Gel

                      Maintain a low-pH environment

                       Include HCl in HF treatment (Mud Acid)

Tertiary Reaction – What to do?

AlFy

              1.     Maintain a low-pH environment

                      pH > 2.5 a problem

May be localized in the formation

              2.     Overdisplace fluids         

                              HCl overflush

Matrix Stimulation: Carbonate

Kinetics of HCl Reaction

Mass-Transfer-Limited

Key Factors in Carbonate Acidizing

1.  Penetration

2.   Acid reactivity

3.   Injection rates

4.   Diversion

Injection Rates: dissolution patterns

Patterns change depending on:

Temperature

Injection velocity

Surface reaction rate

Impact of Pump Rate and Temperature

Reaction rate can be too high even with Organic Acids at high temperature.

Core entrance after 15%  HCl flow

Wormhole Pattern from Radial Flow

Pore Level Model