20100589-FP

Petrotech-2010 31 October-3 November 2010, New Delhi, India

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Fig. 1 : Productivity Comparison

Paper ID : 20100589 Proppant fracturing in limestone reservoirs of Mumbai Offshore – A new Dawn

Rajarshi Das, M.K. Kumar, S.P. Nainwal & RK Jain

WSS, ONGC, Ahmedabad

E-mail : [email protected]

Abstract

Well stimulation for production enhancement has been a regular requirement in limestone reservoirs in fields of Mumbai Offshore, and matrix acidisation has been been practically the only stimulation technique employed for production enhancement. In recent times, fracture acidisation has been attempted in some wells, however the production enhancement achieved in such wells was shortlived. This expedited efforts for examining the feasibility of alternative stimulation technology.

Proppant based hydraulic fracturing emerged as a suitable alternative. Limestone reservoirs have presented challenges to proppant fracturing, nevertheless proppant fracturing is in practice in some major limestone formations worldwide. Similar challenges were posed by the fields in Mumbai Offshore, ranging from technical to logistical. Strategies worked out for mitigating these challenges included MDT approach for candidate selection and well completion integrity tests. Wells in Mumbai High & Heera fields were shortlisted and subjected to integrity tests for final selection. Fracturing fluid optimization and compatibility studies proved to be useful tools for ensuring smooth operations. The first quick phase of fracture campaign saw planning and execution of proppant fracturing treatments in a total of 2 wells, ranging from oil producer to water injector. The campaign was an operational success and the wells are under evaluation. It was for the first time that the limestone formations were successfully fractured in Indian offshore environment and the success has resulted in more wells being planned for fracturing campaigns in near future. This paper describes the successful fracturing jobs carried out in the wells of Mumbai High. The candidate selection, the reservoir and the well completion aspects, the treatment design, pre-fracture treatments and the execution details have been elaborated here. Introduction

Hydraulic fracturing has emerged as the most effective technique in exploitation of low permeability reservoirs worldwide. Though it is more common in sandstone formations; it is also done in carbonate reservoirs (limestone and chalk). Hydraulic fracturing in carbonates can be primarily of two types:

• Acid fracturing : fracturing fluid is acid slurry with a gel-based pad

• Proppant fracturing : fracturing fluid is gel based with proppants pumped to keep the fractures open

Acid fracturing is a common and widely practiced treatment in carbonate reservoirs, as compared to proppant fracturing, however, there are some advantages of proppant fracturing vis-à-vis acid fracturing as seen in Table 1. It has been observed that though acid fracturing leads to better productivity initially, the decline in productivity is quite steep (refer Fig. 1), while the productivity gained after proppant fracturing is better sustained. This results in a higher oil recovery/ production in case of proppant fracturing.


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Table-1 : Comparison of acid fracturing vs. proppant fracturing

Parameter Acid fracturing Proppant Fracturing

Frac length Shorter fractures (especially in high temp. formation)

Longer fractures

Frac height Limited Height growth beyond payzone, may connect to water bodies up/below

Formation compressive strength

Reduced due to etching No effect

Closure stress Etching may not support frac pressures> 5000 psi

No concern, when suitable proppant used

Post-job Productivity Initially larger, but shows steeper decline

Initially less, but greater sustenance

Fracture Closure Extent

Result of asperities embedment, asperities crushing, and viscous flow (creep)

Result of proppant embedment, proppant crushing, and proppant flowback

Emulsion & sludge problems

Maybe Significant and detrimental Not significant

Environmental concerns

Acid disposal a concern

Negligible

Acid fracturing has been tried in some wells of Mumbai High and Heera fields, where the initial post-frac productivity could not be sustained beyond a few months. With an objective to plan and implement proppant based hydraulic fracturing in Mumbai offshore fields, the producing assets of Western Offshore approached WSS, Ahmedabad. This was followed by brainstorming sessions involving the Assets, the concerned services and R&D institute on proppant fracturing in carbonate reservoirs. The challenges for proppant fracturing in multilayer heterogeneous reservoirs were deliberated and the strategies to overcome these challenges were discussed at length and finally, the methodology for fracturing job implementation was outlined.

Candidate selection Candidate selection is a cornerstone for the success of proppant fracturing operations. It assumed greater significance in the situation, where it was to be a maiden campaign in limestone formation, with the right mix of equipment and expertise. Both Heera and MH fields have wells completed in limestone reservoirs. Multi-disciplinary approach was adopted with expertise from the producing assets, well services, WSS, Ahmedabad and IOGPT, the R&D institute with the responsibility of data collection and analysis for candidate well selection. A total of 30 wells were screened initially from the two fields. Detailed data was assimilated for all the wells and the wells were thoroughly discussed in order to arrive at the suitable candidate wells. The candidature of the wells was analyzed from the reservoir angle, which is very significant, e.g. if there are any water/gas zones in proximity to the target zone and resultant possibilities of post-frac water/ gas production. The wells likely to produce with high gas/water production after fracturing were not selected, as any oil production enhancement would be wiped out due to increased gas/ water production. After a thorough analysis of the wells on the basis of reservoir characteristics, production behaviour, etc. the list was pruned down to 9 wells. It was the decided to further review these wells on the basis of well completion and safety related aspects.


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Planning Well completion often is a deciding factor in the operational viability of fracturing treatments. The typical existing well completion in the wells tentatively selected for fracturing had completion strings of 2 7/8” or 3 1/2”, with sidepocket mandrels housing gaslift valves, packers, surface controlled subsurface safety valves, landing nipple and pump out plug. This posed the following limitations :

• Existing completion (X-mas tree, well head) might not support the fracturing pressures.

• The selected wells were 10-20 years old and integrity of the tubing was a concern.

• Usage of 2 7/8” tubing as frac string would increase surface pumping pressures due to increased frictional losses.

• There was a possibility of the proppant laden frac fluid getting stuck in the gaslift side pocket mandrel with dummy valve in place. In such an eventuality, it would have been difficult to retrieve the dummy valves after fracturing.

• Coiled tubing application would have been required in each well for pre-frac treatments as well as post-frac well cleaning / activation. Surface Controlled Subsurface Safety Valves (SCSSV) in some wells might have not allowed conveyance of coiled tubing.

• Packers of 5000 psi rated might fail at higher fracturing pressures

• In some wells, there might be pressures in the outer annular section, exceeding the safe limits. There could be a possibility of communication of the tubing-casing annulus with the outer annulus. Fracturing in such wells may lead to communication of the high pressure to the outer annulus, causing grave safety concern.

To ensure the feasibility of fracturing under such a scenario, it was decided to adopt the following precautionary measures before minifrac jobs :

• Tubing Integrity : All wells needed to be checked for tubing corrosion/ pitting by Multi-finger Caliper tool (MIT). The tubing was also to be pressure tested upto maximum allowable limit (about 5000 psi) by putting a blanking plug in the landing nipple and keeping the annulus filled.

• Packer : The installed packer should be pressure tested by pressurizing both tubing and annulus to max. allowable pressure and keeping a positive differential across the gas lift valve.

• Gas lift mandrel and valves: Gas lift mandrel integrity will be checked during the tubing integrity check.

• Annular section pressure recording : Detailed study of the selected wells should be taken up to rule out the communication to the outer annulus.

The completion integrity testing was needed to be carried out in order to confirm the final candidates’ list for Minifrac/ fracturing. The crew and equipment mobilization was planned only after getting a confirmation of the well completion integrity. Further, the completion integrity would be tested finally during the minifrac. Only after the minifrac, the feasibility of carrying out the fracturing job with the existing completion could be ascertained. On the basis of the well completion integrity studies, two wells MWAE and HRXI were taken up for fracturing. All necessary mobilization of chemicals, equipment and expertise was made as per plan.

Job Design Job design is one of the significant factors in ensuring the success of a fracturing job. The initial job designs were made with the 3-D Frac simulator Fracpro. Tentative job designs were prepared for all 9 wells, with detailed information regarding expected surface pressures and frac dimensions. Sensitivity analysis has been done on the job size, pumping rates, tubing size etc. Tentative job sizes varied from 20 - 30MT in Mumbai High and 30 - 40 MT in Heera fields. However, the job designs were to be validated through Minifrac job, prior to the main frac job, and the design finalized. Laboratory studies play a significant role in ensuring a better quality stimulation treatment. Oil, surfactant, drill water and sea water samples were collected for carrying out the following tests to ensure the fracturing fluid is compatible to the formation and to ensure QA/QC of the fracturing fluid :


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• Gellation & hydration : Optimization of gellation / hydration time was to be done in order to achieve the required base gel viscosity.

• Crosslink dosing : Evaluation of X-linking dosage was necessary to ensure the sufficient rheological strength of the fracturing fluid to carry the proppant deep into the targeted formation zone.

• Non-emulsification : to be done to study the extent of demulsification.

• Gel stability in formation condition : to optimize the dosage rates of the chemical constituents.

• Breaker dosing : Optimization of breaker dosing was necessary for smooth flow back.

• Formation Precipitation : Reactivity of the fracturing fluid constituents with the carbonate (limestone) to be evaluated for any precipitation.

It was observed that sea water was not conducive to a good gel formulation. Instead, drill water was found suitable in this regard and it was decided to use drill water for treatment. MV Vestfonn, the stimulation vessel (in Fig. 2), was found adequate for the execution of fracturing jobs.

Job Execution A sound job execution is as significant as a good job design in ensuring the operational success of the treatment. MiniFrac diagnostic tests (in Fig. 3) were meticulously planned with KCl water and linear gels to provide the requisite information on the formation. The rigorous testing of each component before the final job was absolute necessity to avoid premature job termination. Though logistics presented occasional constraints for operations, the treatments could be conducted as per schedule on account of well coordinated efforts. Safety was a paramount concern in the offshore environment. Since the wellheads were of 5000 psi pressure rating, wellhead isolation tool was used in one of the wells with 2 7/8” tubing. All precautionary measures were taken, including use of a pair of mechanical pressure safety valves and electronic emergency shutdown, to ensure safety. Proper

Fig. 4 : Main Frac Plot of MWAE

Fig. 3 : Mini Frac Plot of MWAE

Fig. 2: The Stimulation vessel


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coordination with the crew members at wellhead platform, those in the blending and pumping sections and sampling laboratory and the control room was essential to ensure operational smoothness. Safety meetings conducted before minifrac and the fracturing helped in enabling accident-free operations. The treatments in both the wells (refer Fig. 4 and Table 2) were successful with 30 MT of proppants pumped in each, as per design. The customized fracturing fluid was found suitable for the reservoir conditions and real-time treatment monitoring was useful.

After the frac job, the wells were cleaned upto perforation bottom with coiled tubing and activated through gas lift. Water injection was restarted in the injector and the wells kept for monitoring. There was an immediate improvement in water injection rate and the well is under observation for a long term post-frac injection trend. The oil producer however did not show a clear cut improvement and on analysis, it appears that the previous attempts at acid fracturing may have created high extent of vugginess and there is a possibility of the natural fractures getting connected. This might have led to loss of fracturing fluid/ slurry and loss of conductivity near the wellbore. There is a plan for taking up some more wells for proppant based fracturing and the lessons learnt in the first phase would be conveniently utilized for improvements in candidate selection, fracture treatment design and operations.

Table 2 : Frac parameters

Parameter MWAE HRXI

Proppant pumped (MT) 30 30

Breakdown pressure (psi) 3050 2730

Avg. pump pressure (psi) 3650 3400

Max. pump pressure (psi) 5300 3650

Pad (bbl) 180 140

Slurry vol. (bbl) 410 425

Pump rate (bpm) 19 12.5

Avg. proppant conc. (ppg) 3.8 3.64

Conclusion

The first ever proppant based fracturing treatments in limestone reservoirs of Mumbai High and Heera fields were operational a success. Candidate selection and well completion played a vital role in the fracturing campaign and proper coordination was helpful in view of the logistical constraints. The wells are under long term evaluation. This success has resulted in more wells being planned for fracturing campaigns in near future.

Acknowledgement

The authors acknowledge the Mumbai High and Neelam-Heera MDTs and all personnel of ONGC who lent direct/ indirect support towards making the fracturing campaign successful. References 1. M. P. Cleary et. al.: “Experimental and modeling evidence for major changes in hydraulic

fracturing design and field procedures.” 2. Inhouse Report on “Pre-job report on Hydraulic fracturing in wells of Mumbai High & Neelam-

Heera Assets”

20100589-FP

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