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:
Deep Penetrating Retarded Acid System (DPRAS) For Stimulation Of Carbonate Reservoir.
2. Applicant(s):
Name Nationality Address
Oil and Natural Gas India IOGPT, Phase -ll, Panvel -
Corporation Ltd. 410221, 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.

DEEP PENETRATING RETARDED ACID SYSTEM (DPRAS) FOR STIMULATION OF CARBONATE RESERVOIR
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention is aimed at developing an Acid-in-Oil emulsified composition act as deep penetrating retarded acid system (DPRAS), particularly used to stimulate oil wells.
2. Description of the Prior Art:
Acidizing treatment is used to remove wellbore damage or enhance matrix permeability of carbonate formations. Acids commonly used in carbonate formations are 'Hydrochloric acid of varying concentrations. They make dissolution figures named wormholes, which increase the permeability. The wormhole growth rate depends on acid diffusion and acid injection rate. With plain acid, the acid consumption is high because of a high value of the diffusion coefficient. At low flow rate, most of the acid is spent at the formation face, as result wormholes cannot penetrate very far into the formation. The use of retarded acids generally called emulsified acids is a way to overcome low wormhole propagation rates. In emulsified acid, the diffusion rate is ten to forty times lower than straight acid. The emulsion consists of diesel (continuous phase), HCI acid (dispersed phase) and an emulsifier. The role of diesel is to act as a diffusion barrier between the acid and the rock. Decreasing the diffusion coefficient slows down the dissolution and allows live acid to penetrate deeper into the formation even at low injection rate. This makes the acid-in-diesel emulsion of potent/a! use for stimulating low permeability zones.

Historically, emulsified acids have primarily been used in fracture acidizing. By combining information from theoretical studies, experimental studies, and field testing, a better understanding has been gained of the application of emulsified acids in matrix acidizing. This invention is to be used as stimulation fluid for matrix treatments in heterogeneous carbonate reservoir formations.
Emulsified acid is diffusion retarded, which makes it an effective worm holing fluid at low injection rates, in low permeability or damaged formations. At these low rates, plain HCI acid will mostly spent on the formation face and is unable to create worm holes that penetrate deep into the formation.
Acid-in-oil emulsions are effective stimulation fluids in large intervals where streaks of high permeability can act as thief zones
The goal of matrix stimulation of carbonate formations is to decrease skin by creating wormholes and to increase the effective well bore radius while bypassing damaged areas. In naturally fractured carbonates, the wormholes will connect the existing fractures, thereby creating long and deeply penetrating flow channels. In formations without natural fractures, the worm hole length will be less, but it may still be several feet. The permeability of the near-well bore area containing the wormholes is usually several orders of magnitude larger than the original permeability of the rock and skin values of -2 to -4 after an acid treatment.
Emulsified acids contain the acid as either the internal or the external phase. The former, which is more common, normally contains 30 percent hydrocarbon as the external phase and 15% hydrochloric acid as the internal phase. Viscosity created by emulsification of the oil can

retard the rate of acid transfer to the rock surface. This reduction in mass transfer rate, and its corresponding reduction in acid reaction rate, can increase the depth of acid penetration into the rock formation before the acid reacts with the rock or damaging material.
The increased frictional resistance to flow of these fluids down well tubulars may limit use of oil external emulsified acids. The presence of surfactants in the acidizing fluid, to produce the emulsion, can affect the wetting characteristics of the rock formation, i.e., change a water wet rock surface into an oil wet surface. This can necessitate remedial post acidizing treatments to restore the rock surface to a water wet state if successful oil production is to be attained. Invented emulsified acid uses non-ionic surfactants of sorbitan esters that do not alter the wettability of rock.
The invented emulsified acid consists of mixture two no-ionic emulsifiers (Esters of sorbitan oleates) with HLB 4.3 and 15. The emulsion consists of diesel (continuous phase), HCt acid (dispersed phase) and an emulsifier. The role of diesel is to act as a diffusion barrier between the acid and the rock. Decreasing the diffusion coefficient slows down the dissolution and allows live acid to penetrate deeper into the formation even at low injection rate. This makes the acid-in-diesel emulsion of potential use for stimulating low permeability zones. Emulsified acid consists of an internal acid phase (typically HCI of varying strengths) and an external oil phase. The volume fraction of the internal phase can range between 30 to 70%. An emulsifier is necessary to generate an acid-in-oil emulsion that will be stable long enough for matrix acidization operations. Reaction of plain acid with carbonate rock is instantaneous. So the acid pumped, down the hole, during matrix treatment, may be spent near the well bore instead of penetrating deep in to the reservoir. To

retard the activity of the acid, it need to be dispersed as droplets in oil phase so that the oil acts as barrier between the rock and acid thereby controlling reaction rate. The stability of the dispersed acid henceforth called, as 'emulsified acid', is a function of type and quantity of emulsifier used.
Retardation Mechanisms of Emulsified Acid:
The chemical reaction of HCI with dolomite is a heterogeneous
reaction where liquid HCI reacts with solid dolomite at the surface of
the rock according to the following reaction:
Ca Mg (C03) (S)+4H+ Ca2+ + Mg2+ 2C02 (aq) +2H20
This reaction can be viewed as occurring in three steps:
■ Mass transfer of H+ from the bulk of the fluid to the surface of the rock by diffusion
■ Reaction of H+ with dolomite at the rock surface
■ Mass transfer of the reaction products (Ca2+ and Mg2+) to the bulk of the fluid by diffusion.
These steps occur in a series. The slowest step controls the overall reaction rate. Temperature determines which of these steps is the more dominant. At low temperatures, the chemical reaction becomes slower than the diffusion of reactants and products to and from the surface thereby controlling the overall reaction rate. Above 150°F, the reaction is mostly controlled by the rate of diffusion of H+ to the rock surface. In high-temperature formations, the effective diffusion coefficient of H+ determines the rate of the reaction.
The most accepted mechanism proposed to explain these low diffusivities is the dense packing of the acid droplets, which reduces the mobility of the acid in the emulsion. The H+ contained in the droplets faces an additional transport resistance to molecular diffusion brought

about by the Brownian diffusion of the acid droplets, which are much larger particles than hydrogen tons, therefore, yielding much lower effective diffusivities. Stability of the emulsion at the desired operational temperature and separation in to two distinct phases after completion of operations is depended on the nature of the surfactants and ratio in which they are mixed. HLB
Emulsions at elevated temperatures require, evaluation of number of parameters, such as suitable HLB number of emulsifier, acid to diesel ratio and acid concentration in the dispersed phase for assessing stability. It is commonly accepted that the HLB number of emulsifier or combination of emulsifiers, in the range 3 to 6 is suitable for generating stable acid-in-oil emulsions. In the present study 15% HCI is used for all the experiments and the acid to diesel ratio is fixed at 30: 70 or 50: 50 or 70:30. Combinations of non-ionic emulsifiers are selected as they are pH insensitive and can be safely used at low pH range. Two non-ionic emulsifiers, Sorbiton Mono-oleate (SPAN 80) and Polyoxyethylene Sorbiton Mono-oleate (Tween 80) with HLB numbers 4.3 and 15 respectively were chosen for initial studies. Phase behavior and emulsion stability tests were carried out up to a temperature of 120°C, varying the effective HLB and emulsifier dose to correlate operational time requirements.
3. DETAILED DESCRIPTION OF THE INVENTION:
Laboratory experiments were conducted using AR grade Hydrochloric Acid and High Speed Diesel. Emulsifiers used for preparing acid -in-oil emulsion are mixture of nonionic type: SPAN 80 ( Sorbiton Monooleat) of HLB 4.3 and TWEEN 80 (Polyoxyethylene Sorbitan monooleate) of HLB 15.Varied the concentration of two emulsifiers to

arrive at an optimum HLB Value ( preferred below six for oil continuous phase) for obtaining W/O emulsion stable up to 120°C.
3.1 Preparation of Acid: Prepared 15% Hydrochloric Acid AR grade of Sp.Gr 1.18, by diluting it to 1:1 using distilled water for conducting all the emulsion tests.
3.2 Preparation of emulsion: Weighed the required quantity of emulsifiers, in a beaker and added required amount of diesel. Stirred the contents for two minutes at 1000 rpm, and slowly added 15% HCL (prepared by diluting 1.18 sp.gr to 1:1) continued stirring for 10 minutes. The resultant viscous emulsion is transferred to a graduated cylinder for heating to desired temperature.
3.3 Optimization of HLB: Objective:
For generating stable O/W or W/O emulsions, the blend of emulsifier system requires a specific Hydophile Lipophile Balance number. HLB number is specific to a specific emulsifier molecule. The resultant HLB of blend is derived by algebraically adding the HLB of individual emulsifiers or mixture of which is derived from its chemical structure. For W/O type of emulsions where acid is dispersed phase and oil continuous phase, an HLB range of 3-6 is desired. Procedure:
HLB number, specific to acid diesel system in the range 3-6, can be obtained by mixing different wt% of sorbitan monooleate (Span 80), HLB 4.3 and polyoxyethylene sorbitan monooeate, HLB 15 to arrive at an optimum ratio of combination of emulsifiers. Stability tests were conducted at varied temperatures; Varied the ratio of Tween 80 to Span 80 from 0.025:975 to 0.25: 0.75, keeping total weight of emulsifier at 5 grams. Composition of the formulation is:

- 15% Hydrochloric acid
- Acid to diesel ratio 30:70, 50: 50, 70:30
- Emulsifier (w/v) 1-5% (ratio of the two varied from 0.05:0.95 to 0.25:0.75 in case of Span and Tween).
Mixture of emulsifiers were weighed in beaker and dissolved in diesel as is explained in Figs. 1A and Figs.lB. Added Slowly 15% acid, stirring at 1000 rpm over 10 minutes. Transferred the contents to graduated cylinder and measured the separation of acid phase as a function of time.
Equipment:
Weighing Balance, High-speed stirrer, Beakers, Graduated cylinder,
hot air oven.
Observation:
Optimum HLB for 30:70 formulations appears to be at 5.37 and for
50:50 and 70:30 at 5.1.
Inference:
Based on the stability, the effective HLB equivalent to 5.37 is selected
to carry out rest of the experiments for 30:70 formulations, and 5.10
for 50:50 or 70:30 formulations
3.4 PHASE BEHAVIOR STUDIES AT ELEVATED TEMPERATURES: Objective:
To determine the variation of emulsion stability with temperature and emulsifier concentration at affixed HLB of 5.37 for 30:70 and 5.10 for 50:50 or 70:30 formulations. Procedure:
Weighed graded amounts of mixture of SPAN 80 and TWEEN 80 emulsifiers corresponding to required HLB, ranging the total weight of the emulsifier from 1% to 5% (w/v); dissolved in 30-70 ml of diesel and stirred for few minutes. Slowly added 70- 30 ml of 15% HCI

stirring at 1000 rpm for 10 minutes. Transferred the resultant emulsion
to graduated cylinder. Heated the emulsion to the required
temperatures ranging from 90°C to 120°C, and observed the breaking
of emulsion as a function of time as is set out in Figs 2A, 2B and 2C.
Equipment:
Weighing Balance, beakers, High-speed stirrer, graduated cylinder,
hot air oven.
Observation:
With change in emulsifier concentration from 2% to 5% at different
temperatures, stability of the emulsion varies from 28 minutes to 75
minutes. Stability of emulsion increases with increase in emulsifier
concentration and decreases with increase in temperature
Inference:
> An emulsifier dose of minimum 3% (w/v), HLB 5.10 is sufficient to achieve stability of more than 38 minutes at 120°C, for 15% HCI with 50:50 formulation.
> An emulsifier dose of min 3%, HLB 5.10, is required for 70:30 formulation for more than 40 min stability at 120°C
3.5 BREAKING PROFILE OF EMULSION AT FIXED TEMPERATURE AND DOSE: Objective:
Breaking profile of emulsion formulation at a fixed temperature and dose.
Procedure:
Weighed graded amounts of mixture of SPAN 80 and TWEEN 80 in the ratiol: 9, corresponding to HLB 5.37, and 0.075:0.925 corresponding to HLB 5.10 for different formulations i.e. 30:70, 50:50, 70:30 at a fixed total wt% of emulsifier; observed the phase separation as a function of time as set out in Figs. 3A, 3B and 3C.

Equipment:
High-speed stirrer, graduated cylinder, hot air oven.
Observation:
Emulsion with 3% emulsifier dose with HLB 5.37 starts breaking after
30 minutes at 90° and 20 minutes at 120°C. With 50.50 formulations
and 5.10 HLB it starts breaking after 23 minutes and completes after
38 minutes, for 70:30 formulation it is 21 and 40 minutes.
Inference:
Formulation with HLB 5.1 and 70:30 acid to diesel ratio is suitable for
stimulating carbonate reservoir of bottom hole temperature 1200C.
Emulsion is stable for 23-25 minutes and breaks completely in 38-40
minutes.
3.6 ROCK DISSOLUTION STUDIES: Objective:
To determine the retardation of reactivity of acid in emulsified for with carbonate rock. Procedure:
Weighed known quantity of carbonate rock/ chips and added to 100 ml of plain, and emulsified acid. The contents were transferred to a glass bottle and heated to desired temperature. Samples were with drawn after regular intervals and the remaining weight of the samples were determined. From the weight difference, percentage of rock dissolved against stoicheometric equivalent acid spent versus time is determined as set out in Fig. 4. Observation:
- Reactivity of rock with plain acid is faster compared to the reactivity with emulsified acid. Retardation is significant at lower temperatures. Inference: Reactivity of acid gets reduced significantly

4. FIELD APPLICABILITY:
1. Successfully implemented invented emulsified acid formulation in #NO-1 of Mumbai High Oil well. Substantial oil gain observed as shown in Fig 5.
2. Implemented in #HD-3, #HD-5, #HD-7 wells of Heera Field As shown in Fig.6.
3. Envisaged expected oil gain per well is in the range of 150 bpd-200 bpd.
5. SUMMARY OF THE INVENTION:
Acid in oil emulsion with 15 % HCI and High speed diesel using two no-ionic emulsifiers- SPAN 80 and TWEEN 80 is formulated for stimulation of oil well. The invented emulsified acid has significant retardation effect on reactivity of acid to facilitate deep penetration in to matrix. Emulsifiers were mixed the in the ratio 0.075: 0.925 to obtain an effective HLB (Hydrophile Lipophile Balance) of 5.1 and 5.3. Acid to diesel ratio is varied to obtain emulsion stability that suits to operational needs. Emulsion stability is a function of emulsifier concentration, acid to diesel ratio and temperature. Data, on emulsion breaking times of different formulations, at varied temperature conditions is useful to design oil well stimulation jobs.

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-1A is a graphical representation showing the variation in breaking times with emulsifier ratio for 30:70 formulations.
2) Fig-1B is a graphical representation showing the variation in breaking times with emulsifier ratio for 50:50 or 70:30 formulations.
3) Fig.2A is a graphical representation showing the breaking time Vs emulsifier concentration at different temperatures for 30:70 formulations with HLB 5.37, for Span / Tween.
4) Fig. 2B is a graphical representation showing the breaking time Vs emulsifier concentration at different temperatures for 50:50 formulations at HLB 5.10, for Span/Tween.
5) Fig. 2C is a graphical representation showing the Breaking time Vs emulsifier concentration at different temperatures for 70:30 formulations at HLB 5.10 , for Span/Tween.
6) Fig. 3A is a graphical representation showing the Emulsion-breaking Profile of 3%Emulsifier for 30:70 formulations with HLB 5.37, for Span/ Tween.
7) Fig. 3B is a graphical representation showing the Breaking profile of 50:50 formulation with 3% Emulsifier at 5.10 HLB.

8) Fig. 3C is a graphical representation showing the Breaking profile of 70:30 formulation with 3% Emulsifier at 5.10 HLB.
9) Fig. 4 is a table showing the Rock dissolution rates at 1200C elevated temperatures for plain and emulsified acid.
10) Fig. Sis a table showing the Oil Gain on treatment with DPRAS
11)Fig. is a graphical representation of the Real time treatment plot of well ZA-10H with DPRAS

We claim:
1. Two non-ionic emulsifiers- SPAN 80 (Sorbitan Monooleate) and
TWEEN 80 (Polyoxyethylene Sorbitan monooeiate) are selected for
emulsifying Acid in diesel.
2. Effective HLB for generating stable acid in oil emulsion for 30: 70 acid /diesel is 5.37 and for 50:50, 70:30 it is 5.1.
3. Ratio of the emulsifiers (wt/wt) Tween 80 and Span 80 to be maintained at 0.075:0.925 for 70:30 Acid/diesel
4. 15% HCI with 70:30 acid to diesel ratio, and 2-3% emulsifier dose is sufficient to obtain emulsion stability of 40-50 minutes at 100-120°C.
5. Rock dissolution with invented emulsified acid indicate retardation effect over 40 minutes compared to plane acid.