Description:Field of the invention

[0001] The present invention generally relates to compositions for the treatment of subterranean formations. More particularly, the present invention relates to a composition for acid diversion treatment of subterranean formations.

Background of the invention

[0002] Subterranean formations are underground hydrocarbon bearing zones from where hydrocarbons migrate into a production reservoir. It is often necessary to perform stimulation treatment of the subterranean formations to boost migration of hydrocarbon into production reservoir. Stimulation treatments are performed by channeling treatment fluids through an injection well into subterranean formations to increase the number of pathways for hydrocarbon to migrate out of the formations. The subterranean formations in general are heterogenous in terms of permeability or porosity, which may affect the migration of hydrocarbons, which in turn reduces productivity.

[0003] Due to variations in permeability of the subterranean formations, the treatment fluids used during stimulation may travel in an uneven manner within the formations. As a result, stimulation treatment in the highly permeable zones of the formation would be higher as compared to low permeable zones. In certain formations, low permeable zones may also be left untreated during stimulation due to the uneven permeability.

[0004] Conventionally, diversion treatments are performed in subterranean formations to combat the heterogenous porosity of the formation, to divert treatment fluids towards low permeable zones, for an even treatment of subterranean formations. Diversion treatments dissolve portions of the low permeable zones to increase permeability, which in turn increases the migration of hydrocarbons to production reservoir. It is well known that the subterranean formation comprises acid-soluble components, and therefore diversion is often achieved by contacting the acid-soluble components of the formation with an acid solution. These treatments using acid solution are referred to as "acidizing" or “acid diversion”. During acid diversion, the acid solution is placed in a desired region within the subterranean formation, thereby creating an optimal condition to dissolve the acid-soluble components of the formation to increase the porosity. This results in the creation of additional pathways for hydrocarbons to migrate to a production reservoir.

[0005] Traditionally, polymer-based fluids are used in acid diversion. However, these polymeric solutions exhibit increased viscosity due to the large size of molecules and may also block subterranean formations. Further, existing polymer fluids for performing acid diversion may not function at ultra-low temperature. Typically, known acid diversion composition attains optimal function at temperature above 60oC. But at lower temperatures below 60oC, the polymer does not undergo effective cross-linking for acid diversion treatment.
[0006] In light of the aforementioned drawbacks, there is a need for a composition of treatment fluid for effective acid diversion treatment of subterranean formations. There is a need for a treatment fluid composition for effective acid diversion treatment in subterranean formations where temperatures are below 60oC.

Summary of the Invention

[0007] In various embodiments of the present invention, a composition for in-situ acid diversion is provided. The composition comprises a mixture of an aqueous base, a viscosifier, a breaker, and a cross-linker system comprising a combination of a cross-linking agent and an activator. In an embodiment of the present invention, the composition comprises a viscosifier in a range from 0.40 to 0.60% (w/v), a breaker in a range from 0.40 to 0.80% (w/v), a cross-linking agent is in a range from 0.08 to 0.20% (w/v) and an activator is in a range from 0.02 to 0.08% (w/v).

[0008] In various embodiments of the present invention the aqueous base is selected from a group comprising of fresh water, salt water, brine, formation water, produced water, sea water, or combinations thereof.

[0009] In various embodiments of the present invention, the viscosifier is selected from a group comprising of xanthun gum, guar gum.

[0010] In various embodiments of the present invention, the cross-linking agent is chromium acetate.
[0011] In various embodiments of the present invention the activator is potassium permanganate.

[0012] In various embodiments of the present invention the breaker is selected from a group comprising of ammonium salts of peroxidisulphate, peroxidisulphate salts of ammonium, potassium and sodium, sodium bromate, enzyme based breaker

Brief description of the drawings

[0013] The present invention is described by way of embodiments illustrated in the accompanying drawings herein:

[0014] Figs. 1 to 3 demonstrate the consistency of X-linked gelled mass formed after 15 minutes at 40, 50 & 55±2oC at different concentrations of APS [with fixed concentration of XC polymer (0.5 wt%), cross-linking agent Cr2OAc3 (0.1 wt%), & activator (0.05 wt%)].

[0015] Fig 4 demonstrates the effect of treatment of gelled mass with mud acid (12 wt% hydrochloric acid (HCl) + 3 wt% hydrofluoric (HF) acid) and 15 wt% HCl at room temperature.

[0016] Fig. 5 demonstrates the breaking tendency of linear gel after 3 hours.

[0017] Fig. 6 demonstrates the breaking tendency of broken gel after 3 hours.

Detailed description of the invention

[0018] The present invention discloses a composition of a treatment fluid for acid diversion, in accordance with an embodiment of the present invention. The invention provides a composition for acid diversion that exhibits acid diversion at ultra-low temperatures. In an embodiment of the present invention, the composition achieves accelerated cross-linking at ultra-low temperatures below 60°C. In an embodiment, the composition of present invention is stable during acid diversion treatment under various conditions. By way of the composition, the treatment fluid is diverted from high permeable zones to low permeable regions of the subterranean formation. In an embodiment of the present invention the composition for acid diversion is environmentally friendly.

[0019] The composition for acid diversion, in accordance with an embodiment of the present invention, enables diversion of the treatment fluid during stimulation treatment of a subterranean formation to effectively increase the permeability of the subterranean formation and increase the production of hydrocarbons. The constituents of the composition interact with each other to increase the viscosity of the treatment fluid. This increase in viscosity facilitates diversion of subsequent portions of the treatment fluid away from high permeable zones of the subterranean formation that may have already been treated and to less permeable zones of the subterranean formation. Stimulation treatment with the composition of present invention results in a greater increase in the permeability of the subterranean formation when compared to conventional acid stimulation treatment fluids. In an embodiment of the present invention, the composition reacts with various elements of the subterranean formation and the pH of the composition increases. As a result, interaction between the constituents of composition decreases. This causes a reduction in viscosity of the composition which enables the composition fluid to flow back to the injection well without blocking pores of the subterranean formation or causing damage to the subterranean formation compared to existing acid diversion treatments.

[0020] The disclosure is provided to enable a person having ordinary skill in the art to practice the invention. Exemplary embodiments herein are provided only for illustrative purposes and various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. The terminology and phraseology used herein is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications, and equivalents consistent with the principles and features disclosed herein. For purposes of clarity, details relating to technical material that is known in the technical fields related to the invention have been briefly described or omitted so as not to unnecessarily obscure the present invention.

[0021] In various embodiments of the present invention, the composition for acid diversion comprises an aqueous base, a viscosifier, a cross-linking system and a breaker. In an embodiment of the present invention, the cross-linking system comprises of a cross-linking agent and an activator. The composition effectively cross-links the viscosifier with the cross-linking agent during acid diversion treatment in subterranean formations under ultra-low temperature conditions. The composition facilitates cross-linking between polymer and cross-linking agent thereby increasing viscosity of the fluid to create a barrier to divert subsequently injected treatment fluid to other low permeable regions of the subterranean formation. The composition for acid diversion exhibits superior properties to enhance the porosity of low permeable zones of the subterranean formations, thereby enhancing the productivity of hydrocarbons.

[0022] In an embodiment of the present invention, the aqueous base is selected from a group comprising of fresh water, salt water, brine, formation water, produced water, sea water, or combinations thereof. In an embodiment of the present invention the composition for acid diversion comprises an aqueous base in an amount sufficient to maintain the pH of the composition in a range of from 6.5 to 7.5.

[0023] In an embodiment of the present invention, the composition for acid diversion comprises a viscosifier imparting viscoelastic behavior to the composition. The viscosifier increases elastic modulus of the composition in part, due to the ability of the viscosifier to form viscosifying micelles. In an exemplary embodiment of the present invention, the viscosifier is selected from a group comprising of xanthum gum, guar gum, preferably xanthum gum (XC Polymer). In an embodiment of the present invention, the xanthum gum imparts desired viscosity to the composition for enhanced cross-linking and remains stable in acidic medium. In an embodiment of the present invention, the amount of viscosifier in the concentration is in a range from 0.40 to 0.60% (w/v).

[0024] Cross-linking of the viscosifier component of the composition increases the viscosity and facilitates sealing of the high permeable reservoir zone temporarily. During acid treatment, the composition is expected to remain in a viscous state inside a formation. During acid diversion treatment, the viscosifier undergoes crosslinking with a cross-linking system to increase the viscosity of the composition. In an embodiment of the present invention, the cross-linking system comprises a mixture of a cross-linking agent and an activator. During acid diversion treatment, the activator accelerates the crosslinking reaction between the viscosifier and the cross-linking agent, thereby increasing the viscosity of the treatment fluid. In an exemplary embodiment of the present invention the cross-linking agent is chromium acetate. In an exemplary embodiment of the present invention the activator is potassium permanganate. In an embodiment of the present invention, the amount of cross-linking agent in the composition is in a range from 0.08 to 0.20% (w/v), preferably 0.1% (w/v). In another embodiment of the present invention, the amount of activator in the composition is in a range from 0.02 to 0.08% (w/v), preferably 0.05% (w/v). The cross-linking system comprising a mixture of chromium acetate and potassium permanganate is compatible with the aqueous base without adverse reaction. Advantageously, the composition for acid diversion of the present invention comprising cross-linking system surprisingly exhibits excellent rheology and is capable of initiating cross-linking reaction in subterranean formations having low temperature conditions in the range of 45oC to 60oC.

[0025] In an embodiment of the present invention, the composition for acid diversion comprises of a breaker. The breaker facilitates breakdown of the molecular backbone of the cross-linker to reduce its molecular weight. This results in reduction of the viscosity of the treatment fluid so that the treatment fluid is more easily recovered from the subterranean formation. In an exemplary embodiment of the present invention, the amount of breaker in the composition is in a range from 0.40 to 0.80% (w/v). In another exemplary embodiment of the present invention, the amount of breaker in the composition is 0.4% (w/v) at 55°C, 0.6% (w/v) at 50°C & 0.8% (w/v) at 40°C. In an exemplary embodiment of the present invention, the breaker is selected from a group comprising of ammonium per sulphate (APS), sodium bromate, per-oxidisulphate salts of ammonium, potassium and sodium. In an exemplary embodiment of the present invention the amount of breaker in the composition varies with different operating temperature.

[0026] Advantageously, the composition for acid diversion of the present invention is environmentally friendly and prevents corrosion of the wellbore equipment, since the composition is devoid of any corrosive material acid.

[0027] Advantageously, the composition for acid diversion in accordance with an embodiment of the present invention exhibits surprising accelerates cross-linking at a low temperature condition below 60oC. Conventionally, at low temperatures, chromium (Cr3+) cannot cross link with XC polymer. However, in an advantageous embodiment of the present invention, in the presence of potassium permanganate, chromium (Cr3+) oxidizes into chromium (Cr6+). Cr6+ facilitates the cross linking with XC polymer at ultra-low temperature below 60oC. The diversion of acid into target area (low permeable zone) is only possible when the high permeable area needs to be plugged temporarily by using this innovative method. The composition of the present invention facilitates diversion of acid to a low permeable zone after sealing a high permeable zone prior to execute acid job in a secondary zone.

[0028] The composition for acid diversion aids in efficiently diverting the treatment fluid to homogenize the porosity of the subterranean formations. Therefore, the treatment fluid composition of the present invention is capable of being applied in high pressure even at low temperature conditions for acid diversion treatments.

[0029] In an exemplary embodiment of the present invention, average viscosity of the base gel prepared using the composition of the present invention is in a range from 50 Cp to 55 Cp at 511 S-1. In an embodiment of the present invention, the viscous nature enhances the stability of the composition under high pressure conditions. In an embodiment of the present invention, the composition has a density in a range from 1.01 to 1.05 g/cm3.

[0030] In an embodiment of the present invention, the composition for acid diversion exhibits significant cross-linking with minimum dosage of activator and cross-linking agent. In an embodiment of the present invention, the cross-linking system comprises a mixture of a cross-linking agent chromium acetate and an activator potassium permanganate facilitates acceleration of cross-linking agent for cross-linking with XC polymer at ultra-low temperatures below 60°C. The composition for acid diversion, by way of selection of additives in the specified amount in accordance with various embodiments of the present invention attains superior properties to satisfy the requirements for well applications at low temperature conditions.

[0031] The properties of the composition for acid diversion are provided in table 1, in accordance with an embodiment of the present invention.

Composition Density (g/cm3) Base gel viscosity Corrosion Rate
Viscosifier 1.01 to 1.05 50 cP to 55 cP Negligible Corrosion
Cross-Linker System
Breaker
Table 1

[0032] The disclosure herein provides for examples illustrating the process for preparing the composition for acid diversion in accordance with an embodiment of the present invention. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments may be practiced and to fu1ther enable those of skill in the art to practice the embodiments. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein.

Working Examples

[0033] The composition for acid diversion in accordance with an embodiment of the present invention is provided in table 2 below.

Composition for Acid Diversion Amount
XC polymer 0.5% (w/v)
Cross Linking System
a) Chromium acetate
b) Potassium permanganate a) 0.1% (w/v)

b) 0.05% (w/v)
Ammonium per sulphate 0.8 % (w/v) at 40°C,
0.6 % (w/v) at 50°C
0.4 % (w/v) at 55°C.

Table 2

[0034] The composition of table 2 was subjected to various studies provided herein below:

Gelation & hydration Studies

[0035] Gelation and hydration study was conducted to test the stability, by preparing a linear gel with 0.5 wt% XC Polymer in technical water using magnetic stirrer. The gelation/hydration time of the linear gel was observed. Stability of the X-linked polymer plug plays vital role to isolate the high permeable zone and provides less permeable zone for acid treatment. The test for stability of the linear gel prepared with composition of the present invention was carried out at 55°c for 4 hrs using water bath. The linear gel was observed to become viscous and stable.

Optimization Of Cross-Linking Agent

[0036] At a fixed concentration of 0.5 wt% XC polymer (gel viscosity 52 cP at 511 s-1), studies were carried out at different concentrations of the cross-linker Cr2OAc3 with and without the addition of activator. The consistency of thermogel at different temperature over varying duration is provided in table 3.

Thermogel Composition
Consistency of Thermogel at
40±2oC
Observed Consistency after Time (Min)
15 30 45 60 75 90 105
0.5 wt% XC polymer +
0.1 wt% Cr2OAc3 No
X-linking No
X-linking No
X-linking No
X-linking No
X-linking No
X-linking No
X-linking
0.5 wt% XC polymer +
0.1 wt% Cr2OAc3 + 0.05 wt% ULT Activator Excellent polymer plug Excellent polymer plug Excellent polymer
plug Excellent polymer plug Excellent polymer plug Excellent polymer plug Excellent polymer
plug
50±2oC
0.5 wt% XC polymer +
0.1 wt%Cr2OAc3 No
X-linking No
X-linking No
X-linking No
X-linking No
X-linking No
X-linking No
X-linking
0.5 wt% XC polymer +
0.1 wt% Cr2OAc3 + 0.05 wt% Activator Excellent polymer plug Excellent polymer plug Excellent polymer
plug Excellent polymer plug Excellent polymer plug Excellent polymer plug Excellent polymer plug
55±2oC
0.5 wt% XC polymer + 0.1 wt% Cr2OAc3 No
X-linking No
X-linking No
X-linking No
X-linking No
X-linking No
X-linking No
X-linking
0.5 wt% XC polymer +
0.1 wt% Cr2OAc3 + 0.05 wt% Activator Excellent polymer plug Excellent polymer plug Excellent polymer
plug Excellent polymer plug Excellent polymer plug Excellent polymer plug Excellent polymer plug
Table 3

[0037] Figs. 1 to 3 the consistency of X-linked gelled mass formed after 15 minutes at 40, 50 & 55±2oC at different concentrations of Cr2OAc3 [concentration of XC polymer (0.5 wt%) & activator (0.05 wt%) fixed. The study revealed that at fixed concentrations of 0.5 wt% XC polymer and 0.05 wt% of activator, 0.1 wt% of Cr2OAc3, respectively are required for the preparation of excellent X-linked gelled mass (thermogel). It was observed that the rheological property of thermogel can be utilized for the preparation of in situ cross-linked gel, for acid diversion system. The thermogel structure was formed within 15 minutes of its placement at temperatures ranging between 40 to 55±2oC and remains stable up to at least 2 to 4 hours with no self-breaking tendency. Figs. 1 to 3 show the consistency of X-linked gelled (0.5%-XC polymer, 0.1%-Cr2OAc3 and 0.05%-ULT additive) at 40, 50 & 55?2oC.

Effect of HCL:

[0038] The thermogel developed for reservoir treated with 15 wt% HCl and mud acid (12 wt% HCl + 3 wt% HF) and observed that the cross-linked gelled mass remains unaffected. Fig. 4 shows the thermogel treated with mud acid (12 wt% HCl + 3 wt% HF) and 15 wt% HCl at room temperature. It was observed that the gel remained unaffected upon contact with the strong acid even after 24 hours, suggesting that an internal oxidative breaker is required to break the gel.

Breaking Tendency

[0039] X-linked polymer plug prepared by the composition of the present invention was kept in a water bath to observe the breaking at different time interval. The breaking tendency of the composition at different temperatures with varying concentrations of breaker was evaluated and demonstrated herein below at table 4 and figs. 5 and 6.

Thermogel Composition Test Temp. (oC) Consistency of Thermogel after Time
(Min)
60 120 180 240 300
0.5 wt% XC polymer + 0.1 wt% Cr2OAc3 + 0.05 wt% ULT Activator + 0.6 wt% APS 40 ± 2 Good
X-linked gel Weak
X-linked gel Weak
X-linked gel Linear
gel Broken gel
0.5 wt% XC polymer + 0.1 wt% Cr2OAc3 + 0.05 wt% ULT Activator + 0.8 wt% APS Good
X-linked gel Weak
X-linked gel Linear gel Broken
gel Broken gel
0.5 wt% XC polymer + 0.1 wt% Cr2OAc3 + 0.05 wt% ULT Activator + 1.0 wt% APS Weak
X-linked gel Linear gel Broken gel Broken gel Broken gel
0.5 wt% XC polymer
+ 0.4 wt% APS +0.1 wt% Cr2OAc3
+ 0.05 wt% ULT Activator 50 ± 2 Good
X-linked gel Thickened linear gel Thickened linear gel Broken gel Broken gel
0.5 wt% XC polymer
+ 0.6 wt% APS +0.1 wt% Cr2OAc3
+ 0.05 wt% ULT Activator Good X- linked gel Thickened linear gel Broken gel Broken gel Broken gel
0.5 wt% XC polymer
+ 0.8 wt% APS +0.1 wt% Cr2OAc3
+ 0.05 wt% ULT Activator Thickened linear gel Broken gel Broken gel Broken gel Broken gel
0.5 wt% XC polymer
+ 0.4 wt% APS +0.1 wt% Cr2OAc3
+ 0.05 wt% ULT Activator 55 ± 2 Good X- linked gel Thickened linear gel Broken gel Broken gel Broken gel
0.5 wt% XC polymer
+ 0.6 wt% APS +0.1 wt% Cr2OAc3
+ 0.05 wt% ULT Activator Thickened linear gel Broken gel Broken gel Broken gel Broken gel
0.5 wt% XC polymer
+ 0.8 wt% APS +0.1 wt% Cr2OAc3
+ 0.05 wt% ULT Activator Broken gel Broken gel Broken gel Broken gel Broken gel
Table 4

[0040] From above table it was observed that the crossed link solid mass prepared as above (figs. 1, 2, and 3) transformed to linear gel with different concentration of breaker at different temperature. It was also observed that the crossed link solid mass was stable up to 60 minutes.

Field Implementation

[0041] The acid diversion composition of table 5 was implemented in an acid diversion treatment in western onshore fields of India having a temperature in the range of 40 to 550C.

Sl. No. Expected BHT
(0C) XC Polymer
ppm / (kg/M3) Cross-linker Breaker
Cr2OAc3 ULT Activator APS
ppm / (kg/M3) ppm / (kg/M3)
1 40 5000 (5) 1000 / (1) 500 / (0.5) 6000 - 8000 / (6 – 8)
2 50 5000 (5) 1000 / (1) 500 / (0.5) 4000 - 6000 / (4 – 6)
3 55 5000 (5) 1000 / (1) 500 / (0.5) 4000 - 5000 / (4-5 )
Table 5

[0042] The results shown in table 6 below, indicates successful execution of acid diversion in multiple preformation interval (high and low permeable zone) using the composition.

Description Well-A Well-B
Rate (lpm) Pumping pressure (psi) Rate (lpm) Pumping pressure (psi)
Stage I
Pre-job 300 350 250 2300
Post-job 300 150 250 1850
Stage II
Pre-job 300 500 150 2400
Post-job 300 200 250 2250
Post job result
Table 6

[0043] While the exemplary embodiments of the present invention are described and illustrated herein, it will be appreciated that they are merely illustrative. It will be understood by those skilled in the art that various modifications in form and detail may be made therein without departing from the scope of the invention.
, Claims:We Claim:

1) A composition for in-situ cross-linked acid diversion, comprising:
an aqueous base;
a viscosifier in a range from 0.40 to 0.60% (w/v);
a breaker in a range from 0.40 to 0.80% (w/v); and
a cross-linker system comprising a combination of a cross-linking agent and an activator, wherein the amount of cross-linking agent is in a range from 0.08 to 0.20% (w/v) and the amount of activator is in a range from 0.02 to 0.08% (w/v).

2) The composition as claimed in claim 1, wherein the viscosifier is selected from a group comprising of xanthun gum, guar gum.

3) The composition as claimed in claim 1, wherein the cross-linking agent is chromium acetate.

4) The composition as claimed in claim 1, wherein the activator is potassium permanganate.

5) The composition as claimed in claim 1, wherein the breaker is selected from a group comprising of ammonium salts of peroxidisulphate, peroxidisulphate salts of ammonium, potassium and sodium, sodium bromate, enzyme based breaker

6) The composition as claimed in claim 1, wherein the viscosity of viscosifier is in a range from 50-55 cP at 511 S-1.

7) The composition as claimed in claim 1, wherein the aqueous base is selected from a group comprising of fresh water, salt water, brine, formation water, produced water, sea water, or combinations thereof.

8) The composition as claimed in claim 1, wherein pH of the composition is maintained in a range from 6.5 to 7.5.

9) The composition as claimed in claim 1, wherein the composition is capable of acid diversion at an ultra-low temperature in a range of 40-55oC.

10) The composition as claimed in claim 1, wherein the composition has a density in a range from 1.01 to 1.05 g/cm3.