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:
DEVELOPMENT OF GELLED EMULSIFIED ACID SYSTEM FOR STIMULATION OF CARBONATE RESERVOIRS.
2. Applicant(s):
Name Nationality Address
Oil and Natural Gas India IOGPT, Phase -II, Panvel -410221,
Corporation Ltd. Navi Mumbai, Maharashtra, India.
3. Preamble to the description:
The following specification particularly describes the invention and the manner in which it is to be performed.

Gelled Emulsified Acid System For Stimulation Of Carbonate
Reservoirs
BACKGROUND OF THE INVENTION:
Field of the invention:
The present invention relates to customization of a Gelled Emulsified Acid system which has the dual properties of high viscosity and retardation of reaction for effective matrix acidization and acid fracturing of carbonate reservoirs.
Description of the Prior Art:
Hydrochloric acid (HCI) is mostly used worldwide for acidization of carbonate reservoirs. It reacts readily with carbonate minerals and is available in large quantities at a relatively low price. The objective of acidizing in carbonate reservoirs is to dissolve the rock matrix and bypass the damage. For this reason, both damage and rock characteristics must be taken into account when designing a treatment in carbonate reservoirs. Normally, most of the matrix acidization and acid fracturing treatments are carried out by using plain acid, gelled acid, retarded emulsified acid and self-diverting acid systems.
■ Plain acids are normally 15-28% HCI or organic acids of suitable concentration. Plain HCI react very fast in high temperature and leaked off rapidly due to very low viscosity.
■ Gelled acids are customized by using a suitable thickener or viscosifier to increase viscosity of the system which control acid leak-off into the reservoir, but with limited retardation of reaction.
■ Emulsified acids are homogeneous emulsion of HCI and diesel in presence of a suitable emulsifier, effective for retardation of reaction rate at reservoir conditions, but normally have nominal viscosity,
■ Self-diverting acids are customized by using some special polymers or surfactants, which increase the viscosity in-situ during acid spending process. The viscosified spent acids divert the remaining live acids to the other potential areas and improve affectivity of the treatments in heterogeneous reservoirs.

Challenges with the present formulations:
Carbonate rocks are sedimentary rocks resulting mostly from organic activity. The vast majority of these sediments are composed of marine organisms that vary in size from a few microns to several centimeters. Some carbonate sediments are generated by the chemical precipitation of calcium carbonate (CaCO3). Usually, carbonate reservoirs contain double porosity or high permeability contrasts, which are inherited from their process of formation.
The main objective matrix acidization and acid fracturing in carbonate (limestone) reservoirs is to improve productivity or injectivity by way of improving the natural connection of the wellbore with the reservoir. Generating narrow and long fissures (wormholes) or acid etched fracture is the main source of increasing connectivity of reservoirs to wellbore. The generation of long wormholes or fracture depends on the type and properties of acid formulation used in the treatments. Therefore, the success of matrix or fracture acidization treatments is governed by the quality of acid formulations and placement of treatment in the desired areas. The main challenges in acidization of carbonate reservoirs are:
■ Rapid spending of acid in high reservoir temperature.
■ Excessive leak-off of acids into the reservoirs due to lower viscosity of the systems.
■ Ineffective stimulation of low permeable layers in multilayered heterogeneous reservoirs, as most of the acids enters into the high permeable layers level.
To obtain the optimum results from a fracture acidizing treatment live acid must penetrate deep inside the reservoir as possible. Therefore, higher viscosity and retardation in a single acid formulation is the need for better etched fracture conductivity.
DETAILED DESCRIBTION OF INNOVATION:
The chemical formulation of Gelled Emulsified Acid system is based on polymer gel and emulsion technology wherein a compound containing multiple hydroxyl groups is used as emulsifier to form a stable emulsion of a gelled acid system prepared by adding a bio-polymer in 15% HCI. The system possesses high viscosity as well as excellent retardation of reaction at elevated temperature. The higher viscosity and

retardation of reaction in the single acid formulation considerably improves deep penetration of acids into the reservoirs, control leak-off and increases acid etched fracture length during acid fracturing.
Brief about Gelled Emulsified Acid (GEA) system: Gelled acid systems:
Gelled acids were developed primarily for acid fracturing, but have found some applications in matrix acidizing also. They are used in acid fracturing to increase the viscosity and decrease the leak-off rate. The same principle applies to matrix acidizing conditions in fissured or vugular formations with low primary porosity. In this case, gelled acids are used mainly to clean up the high-permeability channels and minimize fluid loss in the lower permeability matrix. Gelled acid is prepared by adding a suitable gelling agent to the acid formulation for improving viscosity of the system. The various gelling agents are xanthan polymer (XC), guar gum (GG), hydroxyethyle cellulose (HEC), carboxymethyl hydroxyethyle cellulose (CMHEC), polyvinyl alcohol (PVA), polyacrylamide (PAM), polyvinyl-pyrrolidone (PVP) etc.
Emulsified acid system:
Acid in oil emulsified acids is more common in well stimulation. The system normally contains 30 percent diesel in the external phase (continuous phase) and 70 percent 15% HCI in the internal phase (dispersed phase). Viscosity and the barrier created by the external oil phase on the acid droplets retard the rate of acid transfer from the bulk solution to the rock surface. This reduction in mass transfer rate and the corresponding reduction of reaction rate between acid and rock matrix provide deep penetration of acid into the rock formation before the acid reacts with the rock or damaging material.
Gelled Emulsified Acid (GEA) system:
The gelled emulsified acid was prepared by emulsifying the gelled acid (prepared with 15% HCI, gelling agent and other additives) with diesel in presence of three identified emulsifiers, Emulsifier-1, Emulfier-2 and Emulsifier-3. The acid to diesel ration has been maintained 70:30.

Laboratory studies:
Objective of these studies is to customize an effective formulation of gelled emulsified acid system for carbonate reservoirs which will have
a) Adequate viscosity for effective leak-off control and
b) Adequate retardation of reaction.
For customizing the formulation, gelled acid is prepared with different gelling agents and then formulated the emulsion of gelled acid and diesel (gelled emulsified acid) in the ration of 70:30 in presence of suitable emulsifier. LR grade HCI and additives such as acid corrosion inhibitor (ACI), surfactant and some emulsifiers collected from Nhava Base have been used in the study.
A. Evaluation of suitable gelling agent for gelled acid:
Following gelling agents were collected for evaluation of suitable gelling agent for gelled acid system:
■ Hydroxyethyle cellulose (HEC)
■ Poly anionic cellulose (PAC)
■ Xanthan polymer (XC polymer)
■ Guar gum
■ Hydroxypropyl guar (HPG) Gelled acid was prepared in 15% HCI and additives using various gelling agents as mentioned above and aged in ambient and higher temperature. The figures 1 and 2 depict the gelled acid prepared with a biopolymer gel at different concentrations. The stability of gelled acid prepared in 15% HCI, are shown in table-1.

S/N Gelling agent Concentration Observations,
1. Guar 1% ■ Does not hydrate fully in acidic medium.
■ Hydrated in water and added acid.
■ Stable at ambient temperature for 2-3 hrs.
■ Breaking starts 50°C.
2. HPG 0.75% ■ Does not hydrate fully in acidic medium.
■ Hydrated in water and added acid.
■ Stable at ambient temperature for 2-3 hrs.
■ Breaking starts at 50-60°C.
3. HEC 1% ■ Form linear gel in acidic medium.
■ Not stable even at ambient temperature.

4. XC
polymer 1%XC ■ Very stable at ambient temperature
more than 24 Hrs.
■ Breaking starts at 78°C.
■ Completely break at 90°C
5. PAC 1% ■ Does not form gei in acidic condition.
Table-1: stability of gel ed acid with different gelling agents
Stability of gelled acid prepared with different concentration of HCI and XC polymer using sea water is shown in the table 2 below.

S/N Acid Conc. Conc. of XC and
additives Observations
1. 15% 1%XC ■ Stable at ambient temperature.
■ Breaking starts at 75°C.
■ Completely break at 80°C
2. 15% 1%XC + 10%IPA ■ Stable at ambient temperature.
* breaking starts at 75°C.
■ Completely break at 85°C
3. 15% 1%XC +0.5% Thio-urea ■ Stable at ambient temperature.
■ Breaking start at 80°C.
■ Breaks within 5 min at 90°C.
4. 5% 1%XC ■ Stable upto 80°C.
" Breaking start at 85°C.
■ Sustained for 30 min at 85°C.
5. 2% 1% XC ■ Stable upto 80UC.
• Breaking start at 90°C.
1 Sustained for 45 min at 90°C.
Tabl e-2: stability of gelled acid prepared in sea water
B. Study of viscosity of XC polymer gel and gelled acid:
The viscosity of the XC polymer gel and that of gelled acid has been evaluated using Brookfield model DV-III viscometer. The viscosity of 1% XC polymer gel in different temperature and shear rate id shown in fig-3 and the viscosity at 90°C with time (gel stability at 90°C) is shown in fig-4.
The gelled acid was prepared in 15% HCI acid and additives. The viscosity of gelled acid with 1% XC polymer is shown in the fig-5:

From the above studies it is observed that,
■ XC polymer forms stable gelled acid at temperature upto 80°C. The
formulation starts breaking from 80°C and completely breaks at 90°C. ■ The gel becomes more stable as concentration of acid decreases.
C. Evaluation of emulsifier for Gelled Emulsified Acid (GEA):
The gelled emulsified acid was prepared by emulsifying the gelled acid with diesel in presence of three identified emulsifiers, Emulsifier-1, Emulfier-2 and Emulsifier-3 (a compound containing multiple hydroxy! groups). The acid to diesel ration has been maintained as 70:30. The stability of Gelled Emulsified Acid formulation prepared in sea water at ambient and higher temperature is shown in the table-3 below:

S/N Emulsifiers Observations
1. 2% Emulsifier-1 ■ Stable at ambient temperature.
■ Breaking starts at 80°C.
■ Completely break at 90°C
2. 2% Emulsifier-2 ■ Phase separation at ambient temperature within 2.0 hrs.
3. 2% Emulsifier-3 ■ Stable at ambient temperature for
more than 24 hrs.
■ Breaking start at 80°C.
• Phase separation in 5 min at 90°C.
4. 2% Emulsifier-3 +2% Emulsifier-1 ■ Stable at ambient temperature.
■ Breaking start at 85°C.
■ Phase separation in 5 min at
90°C.
5. 3% Emulsifier-3 +2% Emulsifier-1 ■ Stable at ambient temperature.
■ Breaking started at 85°C.
■ Breaks in 5-6 min at 90°C.
6. 4% Emulsifier-3 +2% Emulsifier-1 ■ Stable at ambient temperature.
■ Breaking started at 85°C.
■ Breaks in 10 min at 90°C.
7. 4% Emulsifier-3 +2% Emulsifier-1 +0.5% Thiourea ■ Stable at ambient temperature.
■ Breaking started at 87bC.
■ Breaks in 10minat90°C.
Table-3: stability of Gelled Emulsified Acid with different additives

From the above studies it is observed that,
■ Emulsifier-1 forms emulsion, but stability is not adequate.
■ Emulsifier-2 is not effective in this system.
■ Emulsifier-3 forms stable emulsion of gelled acid with diesel.
■ The gelled emulsified acid is stable upto 90°C in different concentration of emulsifiers and additives. The fig-6 and 7 depicts the Gelled Emulsified Acid (GEA) with Emulsifier-3 and Emulsifier-2 respectively.
D. Viscosity of Gelled Emulsified Acid (GEA) system:
The viscosity of the Gelled Emulsified Acid has been evaluated using Brookfield viscometer. Sea water has been used for preparation of gelled acid. The viscosities of gelled emulsified acid systems prepared by using XC Polymer, acid corrosion inhibitor, Surfactant and Emulsifier-3 by using 70:30 acid to diesel ratio are shown in the figures 8, 9 and 10:
The viscosities of Gelled Emulsified Acid with different emulsifiers and stabilizers at 170sec'-1 shear rate is shown in the fig-10:
E. Behavior of Gelled Emulsified Acid with CaC03:
During the gelled acid stability studies, it was observed that the stability of gelled acid is better with lower concentration of HCI acid. The gelled acid prepared in sea water was stable upto 30 minutes with 5% HCI and upto 45 minutes with 2% HCI at 90°C. The gelled emulsified acid prepared by using Emulsifier-3 and Thiurea has been spent by adding calculated quantity of CaC03 at higher temperature. During spending, lot of foam was generated and the spent acid had considerable viscosity, shown in the fig-11.
The properties of the spent acid has been observed and shown in the table-4 below:

S/N Emulsifiers Observations on spent acid
1. 2% Emulsifier-3
+1%Thiourea+2%
Surfactant ■ Spent acid maintained high viscosity. ■Stable at 90°C for more than 03 hours.

2. 2% Emulsifier-3 +2% Surfactant ■ Spent acid maintained good viscosity.
■ Stable at 90°C for more than 01 hour.
3. 2% Emulsifier-3 +1% surfactant ■ Spent acid maintained good viscosity. ■ Stable at 90°C for more than 01 hour.
4. 2% Emulsifier-3 +0.5% surfactant ■ Spent acid maintained good viscosity.
■ Stable at 90°C for about 01 hour.
Table-4: Properties of spent of Gelled Emulsified Acid
F. Acid Retardation study:
Retardation of reaction of gelled acid and Gelled Emulsified Acid system has been carried out by putting sufficient quantity of calcium carbonate into the acid solution and let the acids react. The sample of acid has been decanted at regular interval and the concentration of acid has been evaluated by titration method. The concentration of acid in gelled acid and gelled emulsified acid system vs. time plot is shown in the fig-12:
G. Optimization of XC polymer concentration:
The concentration of XC polymer of Gelled Emulsified Acid was optimized based on the viscosity at low shear rate. The viscosity of Gelled Emulsified Acid prepared by using sea water was evaluated by using a Fann Model-35A viscometer. The viscosities with 2% Emulsifier-3 and 0.5%, 0.75% and 1% XC at 10.2 sec"1 shear rate (6 rpm at FANN viscometer) are shown in fig-13.
H. Study of viscosity of spent acid:
The viscosity of the spent acid has been studied by preparing the acid solution and spending the acids by adding CaC03 using FANN Model-35A viscometer. Fig-14 depicts the viscosity of spent Gelled Emulsified Acid at different temperatures.
The stability of spent acid at temperature 80°C and 90°C have been studied by evaluating the viscosity vs. time at two shear rates. Figure-15 and 16 depicts the viscosities of spent Gelled Emulsified Acid with time at 80°C and 90°C respectively.

Conclusions on laboratory results:
♦ XC polymer forms stable gelled acid at temperature upto 80°C. The formulation starts breaking from 80°C and completely breaks at 90°C.
♦ The gel becomes more stable as concentration of acid decreases. The stability is found 30-45 minutes with HCI concentrate 2-5% in sea water.
♦ Emulsifier-3 (a compound containing multiple hydroxyl groups) forms stable emulsion of gelled acid with diesel.
♦ The retardation studies indicate that the acid spends with CaC03 at about 25 minutes in comparison to 08 minutes in case of gelled acid.
♦ The stability of spent acid of Gelled Emulsified Acid system customized with Emulsifier-3 shows good viscosity at 80-90°C,
♦ The spent GEA prepared by using 0.75% Xc polymer and Emulsifier-3 shows adequate viscosity at low shear conditions at 80°C and 90°C.
FIELD APPLICABILITY:
The developed formulation is applicable for matrix acidization and acid fracturing of carbonate reservoirs in the following areas:
■ Matrix acidization of multilayered heterogeneous reservoirs.
■ Matrix acidization of horizontal and multilateral wells in carbonate reservoirs.
■ Acid fracturing of tight carbonate reservoirs
Matrix acidization of horizontal and multilateral wells
During acidization of multilayered wells, major Portion of the acids preferably enter into more permeable layers bypassing the low permeable layers. Therefore, stimulation of low permeable zones is more important for achieving desired production from low permeable zones. However, during acidization of multilayered wells, most of the acid take the path of most permeable zone (least resistant path), leaving the low permeable zones untreated. With improved viscosity and adequate retardation of reaction rate, GEA formulation is a novel solution for uniform stimulation multilayered reservoirs.

Matrix acidization of horizontal and multilateral wells:
In acidization of horizontal wells, most of the injected acids normally consumed in nearby area, leaving far end of the lateral untreated. The problem is further magnified with increased reservoir temperature and formation heterogeneity. The customized GEA formulation is a solution for improved placement of acids in horizontal laterals.
Acid fracturing of tight carbonate reservoirs:
The effective length of an acidized fracture and its conductivity is limited by the distance traveled by live acid along the fracture. This limit is a greater problem especially in high temperature carbonate reservoirs. Fracture length and conductivity is governed by:
■ Acid convection to the rock surface.
■ Acid reaction rate.
■ Acid leak-off rate into the formation.
To obtain the optimum results from a fracture acidizing, higher viscosity and retardation in a single acid formulation is the need for better etched fracture conductivity, which is the novel property of developed GEA system.
TECHNICAL ADVANTAGES:
The Gelled Emulsified Acid (GEA) customized with 15% HCI, acid corrosion inhibitor, surfactant, EDTA, XC polymer, Emulsifier-3 and Diesel provides adequate stability, considerable viscosity and retardation of reaction at temperature upto 90°C. For preparation of GEA, 70:30 acid to diesel ratio is to be maintained. The innovative formulation is capable of improving near wellbore wormhole network, zonal coverage and etched fracture conductivity on matrix acidization of multilayered and horizontal/multilateral wells and acid fracturing in carbonate reservoirs.

WE CLAIM
1. It is claimed that the Gelled Emulsified Acid (GEA) is a stable acid system formulated by emulsifying the Gelled Acid and Diesel in presence of an emulsifier, keeping the Acid to Diesel ratio 70:30.
2. It is claimed that the stability of the system depends on the acid gelling agent, type of emulsifier and acid concentration. XC polymer is the effective gelling agent for temperature upto 90°C.
3. It is claimed that the gel becomes more stable as concentration of acid decreases. The stability is found 30-45 minutes with HCI concentration 2-5% in sea water.
4. It is claimed that the Emulsifier-3, which is a compound containing multiple hydroxyl groups gives the best result in forming the stable emulsion of XC polymer based Gelled Acid and Diesel.
5. It is claimed that the retardation of reaction of GEA with CaCO3 at about 25 minutes, whereas the retardation is about 08 minutes in case of gelled acid.
6. It is claimed that the optimized concentration of 0.75% XC polymer and 2% Emulsifier-3 gives best results for temperature upto 90°C.
7. It is claimed that the spent GEA possesses viscosity up to 50cp at 80°C and 40cp at 90°C for 30 minutes in 10.2 sec'-1 shear condition.
8. It is claimed that the GEA system can be used for effective matrix acidization of multilayered and horizontal wells and acid fracturing in carbonate reservoirs