FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
THE PATENTS RULES, 2003
Provisional/ Complete specification
[See section 10 and rule 13]
1. Title of invention:
"VISCOELASTIC SURFACTANT BASED SELF-DIVERTING ACID (VSDA) FOR STIMULATION OF CARBONATE RESERVOIRS."
2. Applicant(s):
Name Nationality Address
Oil and Natural Gas India IOGPT, Phase -If, Panvel -
Corporation Ltd. 410221, Navi Mumbai,
Maharashtra, India.
2.Preamble to the description: The following specification particularly describes the invention and the manner in which it is to be performed.

VISC0ELAST1C SURFACTANT BASED SELF-DIVERTING ACID (VSDA) FOR STIMULATION OF CARBONATE RESERVOIRS-BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to an acid formulation that acts as a self-diverting acid, particularly used for stimulation of oil and gas wells in heterogeneous carbonate reservoirs. The formulation improves the acid coverage in multilayered wells with long perforation intervals. It also enhances the acid exposure in long horizontal and multilateral wells and drain holes.
2. Description of the Prior Art:
Matrix acidization is a common stimulation practice in carbonate reservoirs for maintaining / improving productivity. The goal of matrix acidization in carbonate reservoirs is to reduce or bypass near welibore damages by generating a network of macroscopic channels called wormholes. In naturally fractured carbonates, the wormholes connect the existing fractures, thereby create long and deeply penetrating flow paths for reservoir fluids inflow. Wells completed in heterogeneous formations or in multiple layers / sub-layers need uniform acid exposure in entire welibore or among the layers / sublayers. Achieving adequate zonal coverage of acids is essential for effective treatment.
A. Acid diversion techniques:
Diversion in oil and gas well acidization is the process by which the original flow distribution is altered to provide adequate acid distribution over the low permeable or more damaged productive intervals. The

diversion technique best suited for a particular condition depends on many factors including the petrophysical properties of the reservoir, type of fluid that is produced or inject after the treatment, type of well completion, casing and cement sheath integrity, bottom hole temperature and pressure etc.
There are various acid diversion techniques available in the industry viz. mechanical diversion, ball sealers, particulate diversion and chemical diversion techniques. Chemical diversion techniques can be further sub-divided into foam acid diversion and viscous acid diversion based on polymer or some special surfactants.
Each technique has its own advantages and disadvantages with respect to the diversion efficiency, operational flexibility, requirement of hardware and cost consideration.
I) Self-diverting Acids:
Self-diverting acid (SDA) is the preferred acid diversion technique in the industry for its adequate diversion efficiency, operational flexibility and cost effectiveness. The formulation of self-diverting acid is prepared by modifying the acid solution with polymer or surfactant of desired quality. Initially, the acid solution preferentially enters into high permeable or less damaged zone and gains viscosity in-situ during acid spending process. The increased viscosity of the spent acid solution elevates the injection pressure and diverts the remaining fluid into the low permeable or more damaged intervals at elevated pressure.
In-situ generation of viscosity during acid spending process of self-diverting acids can be established either by In-situ cross-linking of water soluble polymer mixed with acid or reaction of surfactant added

to the treatment fluid to the reaction products generated during acid spending process. The critical parameter of designing a self-diverting acid system is to select the viscosifier, i.e. polymer/cross linker/breaker or surfactants.
Surfactant based systems are advantageous over polymer based systems due to its negligible post treatment residual problems.
B. Viscoetastic Surfactant Based Self-Diverting Acid (VSDA):
Viscoelastic surfactants (VES) based self-diverting acid (VSDA) has been developed in IOGPT by modifying the acid solution with a suitable VES. The formulation has been developed based on the extensive literature survey and laboratory evaluations confirming excellent diversion efficiency, compatibility with reservoir rock/fluid and ample operational flexibility.
B. 1 Viscoelastic Surfactants (VES) Gel System
Viscoelastic surfactants belong to a class of compounds that form micelles in an aqueous system containing certain anions. It is a stable liquid that thickens aqueous fluids by forming a network of elongated aggregations that generates a viscoelastic gel. The micelle structures of viscoelastic surfactant in brine appear rod-shaped or worm-like as the micelle elongated. When the viscoelastic surfactants fluid breaks, the micelles become spherical, with a diameter roughly equal to the diameter of the rod-shaped micelles. The viscosity and degree of viscoelasticity obtained is dependent upon the conditions employed i.e. surfactant concentration, brine concentration and temperature. The advantages of using VES as viscosifier in self-diverting acid systems are:

* Compatible in acidic media,
* Easy to prepare and apply in the field.
* Stable at higher temperatures (up to 120°C)
* Easy to flow back.
* It contains no metal ions, hence environment friendly.

B.2. Rheological Properties
Viscoelastic surfactants behave more like non-Newtonian fluid, with a flatter apparent viscosity profile across the shear rate spectrum. The VES gel is shear thinning, but its rheology is completely reversible and has no permanent degradation of viscosity when exposed to high share conditions.
VSDA is a polymer free surfactant based fluid system applicable in temperature greater than 120°C. In this system, viscosity of the reactive fluid increases in-situ, during acid spending process.
The gain of viscosity is due to the transformation of spherical micelles to the rod like or worm like micelles created by the VES gel during spending of acid into the rock matrix. During pumping acid into carbonate reservoirs, HCI reacts quickly to the calcium carbonate and/or dolomite and form chlorides of Ca or Mg.
Virtually, the formation of worm like or rod shaped micelles, and hence, gaining of viscosity is due to the reaction of viscoelastic
The chemical reactions of lime stones (calcite and dolomite) with HCI are as follows:


surfactant and CaCI2/MgCI2 generated in the reactions during acid spending process. The viscosity of VSDA at various temperature and shear rate is as shown Fig-1.
B.3 Flow Back Of Ves Gel:
Breaking of the thick gel takes place by contact of formation hydrocarbon during flow back or dilution with formation/injection fluid.
After breaking, the Theological behavior become nearly Newtonian and flow back on application of normal draw down.
3. DETAILED DESCRIPTION OF THE INVENTION:
Extensive laboratory experiments were conducted using AR grade Hydrochloric Acid, VES samples collected from various sources and other compatible additives.
The laboratory experiments carried for development of VSDA includes:
* Compatibility of viscoelastic surfactant (VES) with acid and other additives.
* Flow behavior studies of VES gel in formation core.
* Evaluation of acid diversion efficiency.
3.1. Compatibility Study:
Compatibility study was carried out on three different VES samples as
follows:
Procedure:
Weighed the required quantity VES in a beaker and added required
100 ml of 15% HCI. Stirred the contents for 15 minutes at 1000 rpm.
During stirring, added required quantity of acid corrosion inhibitor (ACI),

EDTA and KCI. The resultant viscous formulation is transferred to a graduated cylinder for ageing at desired temperature.
* Formulation was prepared as per the following concentrations: 15% HCI+ +0.5% EDTA + 3% VES + 3% KCI +1% ACI
* Kept for 30 minutes at 90°C and 120°C.
Equipments:
* Weighing Balance, High-speed stirrer, Beakers, Graduated cylinder,
hot air oven.
Observations:
VES sample-1 I VES sample -2 I VES Sample-3
* No appreciable change * The gel breaks at * Not compatible in the formulation. 120°C. with acid.
* Slight dilution is * White color fine observed at high precipitation after temperature. keeping 24 hrs.at
90°C |
Flow test on core sample: Procedure:
* Prepared 3% (w/v) VES gel in 3% KCI brine using VES sample-!
* Prepared a sandstone core plug of 1.5" dia. and 3" length as shown in fig-2.
* Cleaned the core in a core cleaning apparatus with solvents.
* Mounted the core plug in a high pressure high temperature core holder in the core flow set up as shown in fig-3.
* Applied confining pressure of 2000 psi on the core.
* Initially, 3% KCI brine is injected to flush the core as well as to calculate the permeability of the core maintaining a back pressure of 580 psi.

* VES gel is then injected through the core and recorded the injection
rate and pressure online.
A plot of injection rate, differential pressure was created by the gel vs. time at different flow rate as given in fig-4.
Observations:
* The permeability of the core was found 17.4 mD.
* VES gel easily penetrates through the core.
* No filter cake is deposited on the face of the core.
* VES gel is stable in high-pressure condition.
3.3 ACID DIVERSION STUDY IN PARALLEL PACK FLOW SET UP:
Objective:
Linear flow experiment was carried out in parallel core flow set up for evaluating the acid diversion efficiency with VSDA formulation.
Procedure:
* Prepared two sand packs of different permeability (K1 & K2) in (3 1/2' X 1 Y/) API cells.
* Packs samples were mounted in separate core holders as shown in fig-5.
* Applied a confining pressure of 2000 psi on both the packs.
* The core holders then put in a water bath maintained at 90°C.
* Connected the core holders through the back pressure regulator for applying back pressure during the experiment.
* Initially, the cores were flushed with KCI brine. Then determined permeability if each pack separately.

* Prepared 400 ml VSDA with following formulation using VES
sample-1.
15% HCI+ 1% ACI + 0.5% EDTA + 3% VES
* Injected VSDA through both the packs in parallel.
* Stopped injection for about 15 minutes for elevation of viscosity of VSDA in the pack during acid spending.
* Collected the fluid returned from each pack.
* Recorded volume of fluid returned/unit time interval.
* Recorded injection rate and pressure with time on-line as shown in Fig-6A and Fig.6B.
Observations on acid diversion study:
* Initially, almost 80% of injected VSDA has been passed through the high permeable pack, i.e. the least resistance flow path.
* The injection pressure increased from 600 to about 950 psi as the VES transformed into a viscous gel during acid spending and the gel occupies the pore spaces of high permeable pack.
* On continuous injection at elevated pressure, the flow through the low permeable pack increased gradually.
* Finally, the flow path has completely altered and almost 80% of the injected acid passed through low permeable pack.
* On injection of the over-flush, the injection pressure gradually decreased and all the gels flushed out.
* The post treatment permeability were evaluated and found improvement of 50% in low perm pack and 64% in high perm pack respectively.
4. FIELD APPLICATION:
* The invented VSDA formulation has been implemented in six wells
of Heera field located in Mumbai Offshore.

* The reservoir of Heera field consists of heterogeneous carbonate.
* All the wells were completed either in multiple layers or in long perforation intervals.
* The bottom hole temperature is ~101°C.
* Real time treatment data of all the wells clearly indicates the effective diversion of acids from high perm to the low perm streaks.
* The real time data of well A and B are given in the figures-7A and 7B respectively.
* The treatments yielded encouraging production gain in all the wells.
* Pre and post treatment production performance of the six wells are given in figure-8 below.
5. SUMMARY OF THE INVENTION:
A viscoelastic surfactant based self-diverting acid (VSDA) is
formulated by modifying the 15% plain HCI with viscoelastic surfactant (VES) for effective stimulation of heterogeneous carbonate reservoirs. Compatible additives like acid corrosion inhibitor (ACI) and EDTA are used in the formulation. VES presence in VSDA increases the viscosity of spent acid in-situ and diverts the acids to the low permeable or more damaged areas.
Laboratory evaluations and real time job data indicates effective diversion of acids towards tow permeable layer and easy flow-back of spent acids. Field implementation of VSDA yielded encouraging production gain.

BRIEF DESCRIPTION OF DRAWINGS
The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
1) Fig-1 is an illustration of the viscosity of VSDA at various temperature and shear rate.
2) Fig -2 is a schematic illustration of a sandstone core.
3) Fig.3 is an illustration of the Linear Core Flow Set up.
4) Fig.4 is a diagram of the pressure and injection rate vs. time plot.
5) Fig.5 is a diagram of the Parallel core flow set up.
6) Fig.6A is a diagram of pressure and injection rate vs. time plot in parallel flow set up.
7) Fig 6B is a diagram of flow distribution vs. injection time in parallel flow set up.
8) Fig 7A is an illustration of the real time treatment plot of well A.
9) Fig 7B is an illustration of the real time treatment plot of Real time treatment plot of well B.
10) Fig. 8 is an illustration of Pre and post treatment production
performance of the six wells.

We claim:
1. Viscoelastic surfactant (VES) is identified for the formulation of VSDA.
2. Complete formulation of VSDA is designed by modifying the 15% HCI solution with VES and other additives based on formation characteristics.
3. The formulation elevates considerable viscosity in-situ and diverts the remaining acids towards the low permeable streaks.
4. 15% HCI with 3% (w/v) of VES is sufficient to obtain adequate diversion of acids.