19_SPE-154947 Fracture Assisted Sandstone Acidizing,

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SPE 154947

Fracture Assisted Sandstone Acidizing, Alternative Approach To Increase Production in Tight Sandstone Reservoir

Haris Gunawan, Hermawan Susanto, Barkah Widyantoro (Pertamina Hulu Energi ONWJ); Jose Noguera (Halliburton)

Copyright 2012, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Oil and Gas India Conference and Exhibition held in Mumbai, India, 28–30 March 2012.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

The proven and most suitable stimulation technology to increase production in a tight sandstone formation is hydraulic fracturing. The hydraulic fracturing treatment will create a fracture and then keep it open by proppant, hence a conductive path allowing more oil and gas to be produced. Hydraulic fracturing operations in an offshore environment are sometimes considered as a costly solution and non-economic. An alternative economic solution, Fracture Assisted Sandstone Acidizing (FASSA), was evaluated. Sandstone acidizing is a method to increase oil production, by injection of compatible acid system at matrix rate, below the formation fracture gradient. This stimulation method is most suitable to remove near wellbore damage in medium-high formation permeability. To improve the effectiveness of the acid treatment in tight sandstone reservoir, a hydraulic fracture was created prior to the sandstone acidizing. Besides removing the near wellbore damage, the acid is expected to react with fracture face in sandstone and create a conductive path for oil to flow. Four successful jobs have been performed to date. This paper will describe planning, process, acid treatment selection, operation summary, and evaluation of success or failure of the treatment. In the future we also propose to perform similar treatments for a tight reservoir without having to pull out the existing completion.

Introduction

Generally development of low to medium permeability reservoir is using hydraulic fracturing. However in case of well already completed and work over cost is high, then an alternative approach has to be made. This alternative approach was tested using 4 sample wells from 3 different reservoirs type. Reservoirs are sand-stone reservoir with tight and shaly sand reservoir characteristic.

First reservoir has permeability range from 20-55 mD with porosity 25-28%. In this case we tested FASSA in two wells located in nearby reservoir. Second reservoir has 60 mD permeability and 24-30% porosity. Third reservoir has permeability 1 - 13.4 mD and porosity 19-26%. All data is taken from side wall core sample.

First reservoir consists of 2 wells. Both well prior treatment productions are 23 BFPD with 0% water cut and 3 BFPD and 85% water cut. Second reservoir prior treatment had no flow. Third reservoir prior treatment also had no flow.

Background

Sample wells are completed as single selective completion with treated reservoir on below end of tubing. Reservoir pressure is about 600-700 psi, depth of reservoir 2,800-3,300 ft-TVD. All wells uses gas lift as artificial lift method.

Fracture Assisted SandStone Acidizing (FASSA) being chosen considering two things:

1. Well already inplace, work over cost is high

2. Regular sand stone acid was not effective as it proves in offset well

The idea is to create a fracture and keep small portion of it open by washing sand stone using HF acid. The advantage of using this technique is by-pass any near wellbore damage may occured due to drilling fluid and enhances well production by creating channel after fracture closing.

2 SPE 154947 Prior job conducted, X-Ray Diffraction (XRD) data was required to find best compatible acid composition. Table 1 show

second reservoir XRD data.

Table 1 - XRD data

Also needed to consider by creating fracture in the reservoir was water oil contact. First reservoir zone has low resistivity

characteristic as shown in the log on first and second figure.

Fig 1 – Log Well #1

Perforation interval for well #1 is 25 ft (5287’ – 5312’ MD). While for well#2 is 48 ft (5620’ – 5668’ MD).

Fig 2 – Log Well#2

It’s not so clear where oil water contact since cut off used to determine water is 1 ohm. However based on petrophysic

judgement there is no oil water contact in both reservoirs.

On second reservoir, well#3, oil water contact is not shown. Perforation interval for this well is 30 ft (5,000’ – 5,030’ MD)

Fig 3 – Log well#3

On third reservoir, well #4, oil water contact not seen. Perforation interval for this well is 12 ft (4352’-4364’ MD).

Fig 4 – Log well#4

Operation Summary

Well #1 & Well #2

Well #1 and well #2 were newly side tracked well with oil production way below the approved AFE target. Evaluation done

on this well indicated that stimulation was required to produce more oil. Fracturing was preferred but because no more work

over was scheduled on this well, a fracture assisted sandstone acidizing was done. Core sample analysis of the target zone

SPE 154947 3 indicated the need of organic acid; hence the main acid was designed to be Acetic-HF acid. Sufficient amount of 5% HCl to flush the carbonate content not to contact main HF acid and non acid 5% NH4Cl preflush to stabilize the clay materials were pumped before the main acid treatment that over-flushed with 5% HCl.

Well #3

Well #3 was a newly sidetracked well. This well was not producing any fluid. The target zone was indentified as a thight sandstone reservoir with carbonate content of 16.2% and 4% of Kaolinite. This mineral content directs acid selection criteria. Preflushes were consisted of non acid 5% NH4Cl and 15% HCl acid to dissolve the carbonate content. Main acid consisted of 13.5% HCl – 1.5% HF and 15% HCl – 1% HF. The treatment over-flushed with 15% HCl acid.

Well #4

Well #4 was treated using 6.5% HCl – 1.5% HF in main treatment, preflush and overflushed using 5% HCl.

Operation Squence

The stimulation operations were performed on a jack up well services barge using coiled tubing as medium to divert acid across the reservoirs, prior to the main acid treatment, formation were bullheaded using a slick HEC-10 gel to maximum pressure of 80% rating of X-tree. Detail of the operation was as follow:

? Tubing integrity test

? Step rate test to determine the fracture extension rate and fracture extension pressure.

? Breakdown using slick gel to maximum pressure of 80% rating of x-tree.

? Main acid treatment using coiled tubing and fluidic ossication tool to divert the acid across reservoirs.

Result

As result of FASSA treatment all well was back online with good result. In overall well production was increase from 26 BOPD to 654 BOPD with associated gas. Well no#4 shows the lowest result, suspected due to permeability for this zone is low, only 1 – 13.4 mD. Below table shows pre vs post job.

FASSA treatment not applied yet to reservoir permeability higher than 100 mD. Due to precipitation along time as seen in well#3 after one year, re-treatment is needed. However in well #1 and #2 no significant decline found yet.

Conclusion

1. Fracture Assisted Sand Stone Aciding (FASSA) is suitable for permeability reservoir range from 20-60 mD so far.

2. FASSA technique improve well productivity by bypassing near wellbore damage and micro fracture created

3. Low cost treatment due to no need to pull out tubing string

4. XRD test require to find suitable Sand Stone Acid composition

5. For well with permeability < 10, FASSA treatment not so effective

6. Re-treatment possible after a year or more production