Formation Damage Effects and Overview Where is the damage? How does it affect production? 3/14/2009 1
Impact of Damage on Production Look at Effect of Damage Type of Damage Severity of Plugging Depth of Damage Ability to Prevent/Remove/By-Pass 3/14/2009 2
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Horizontal Well Formation Damage Theories Zone of Invasion - Homogeneous Case Is damage evenly spread out along the well path or does it go down the highest permeability streaks? 3/14/2009 4 50423009
Horizontal Well Formation Damage Theories Zone of Invasion - Heterogeneous Case 3/14/2009 5 50423010
Observations on Damage 1. Shallow damage is the most common and makes the biggest impact on production. 2. It may take significant damage to create large drops in production 3. The problem, however, is that the highest permeability zones are the easiest to damage, and that can have a major impact on productivity. 3/14/2009 6
Pressure Distribution Around a 200 md Oil Well 2800 2700 Pressure, psi 2600 2500 2400 2300 2200 2100 The pressure drop around an unfractured wellbore shows the importance of the near wellbore on inflow. If this area is significant damaged, the effect is immediately noticeable. 2000 0 200 400 600 800 1000 1200 Radial Distance, ft World Oil, Modern Sandface Completion Practices, 2003 3/14/2009 7
1. The most noticeable damage impacts the the well in the first 30 cm or 12 inches of the formation. Effect of Depth and Extent of Damage on Production % of original Flow 100 90 80 70 60 50 40 30 20 10 80% Damage 90% Damage 95% Damage 0 0 1 2 3 3/14/2009 Radial Extent George of E. Damage, King Engineering m 8
2. The effect of the damage thickness compared with the amount of damage (as a percent of initial permeability). Only the most severe damage has a significant effect in a thin layer. (Darcy law beds-in-series calculation) Effect of Damage Layer Thickness Productivity Ratio 1 0.1 0.01 0.001 0.01 0.1 1 10 Damaged Perm, % of initial 0.01" thick 0.05" thick 0.1" thick 1" thick 3/14/2009 9
2. (continued) As the damage layer thickens and becomes more severe, the impact on production builds quickly. Productivity Loss From Formation Damage Productivity as a Fraction of Undamaged Prod. 1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0 2 4 6 8 10 Depth of Damaged Zone, inches Kd/Ki = 0.5 Kd/Ki = 0.2 Kd/Ki = 0.1 Kd/Ki = 0.05 Kd/Ki = 0.02 World Oil, Modern 3/14/2009 Sandface Completion 10
3. Highest permeability zones are easiest to damage: Pore Size vs. Permeability permeability (m 350 300 250 200 150 100 50 SDA-01 Balakhany X (SP2) Hg permeability md SDA-01 Fasila B (SP3) Hg permeability md SDA-02 Balakhany VIIIC Hg permeability md SDA-02 Fasila D (SP4) Hg permeability md K md Power (K md) Large connected pores and natural fractures dominate the permeability of a formation. y = 1.3661x 2.4865 R 2 = 0.982 The size of particles that can cause damage in these larger pores is larger than 1/3 rd and smaller than 1/7 th. For a 8 micron pore, the small size would be 1 micron and less. 0 0 1 2 3 4 5 6 7 8 9 10 mean hydraulic radius (µm) 3/14/2009 11 MI for BP SD field
The Effect of Damage on Production Rate = ( P x k x h) / (141.2 µ o β o s) Where: P = differential pressure (drawdown due to skin) k = reservoir permeability, md h = height of zone, ft µ o = viscosity, cp β o = reservoir vol factor s = skin factor What are the variables that can be improved, modified or impacted in a positive way? 3/14/2009 12
Productivity and Skin Factor Q 1 /Q o = 7/(7+s) Just an estimation, but not too far off between skin numbers of zero and about 15. Where: Q 1 = productivity of zone w/ skin, bpd Q o = initial productivity of zone, bpd s = skin factor, dimensionless 3/14/2009 13
A better presentation of the damage from increasing skin factor. Skin only has an impact if the well can really produce the higher rate and the facilities can process it. Productivity Factor, % 250 200 150 100 50 0 Productivity Ratio vs. Skin Factor Range of Skin Factors Associated with Frac Pack -5 5 15 25 35 45 55 Skin Factor Range of Skin Factors Associated with Cased Hole Gravel Pack Completions 3/14/2009 14
Example Productivity for skins of -1, 5, 10 and 50 in a well with a undamaged (s=0) production capacity of 1000 bpd s = -1, Q 1 = 1166 bpd s = 5, Q 1 = 583 bpd s = 10, Q1 = 412 bpd The best stimulation results are usually for damage removal and damage by-pass. 3/14/2009 15
Another way of stating damage - Damage in a horizontal well increasing skin makes the well behave like the drilled lateral was much shorter. 3/14/2009 16
Damage Causes Obstructions in the natural flow path in the reservoir. Pseudo damage such as turbulence - very real effect, but no visible obstructions Structural damage from depletion matrix compression, etc. 3/14/2009 17
Flow Path Obstructions Scale, paraffin, asphaltenes, salt, etc., Perforations 12 spf w/ 0.75 / 1.9 cm holes only opens 2% of the casing wall. Tubing too small - too much friction Any fluid column in the well, even a flowing fluid column holds a backpressure on the well: Salt water = 0.46 psi/ft Dead oil = 0.36 psi/ft Gas lifted oil = 0.26 psi/ft Gas = 0.1 psi/ft (highly variable with pressure) 3/14/2009 18
Pseudo Damage Turbulence high rate wells gas zones most affected Affected areas: perfs (too few, too small) Near wellbore (tortuosity) fracture (conductivity too low) tubing (tubing too small, too rough) surface (debottle necking needed) 3/14/2009 19
Structural Damage Tubular Deposits scale paraffin asphaltenes salt solids (fill) corrosion products 3/14/2009 20
Perforation Damage debris from perforating sand in perf tunnel - mixing? mud particles particles in injected fluids pressure drop induced deposits scales asphaltenes paraffins 3/14/2009 21
Near Well Damage in-depth plugging by injected particles migrating fines water swellable clays water blocks, water sat. re-establishment polymer damage wetting by surfactants relative permeability problems matrix structure collapse 3/14/2009 22
Deeper Damage water blocks formation matrix structure collapse natural fracture closing 3/14/2009 23
Other Common Damages fines migration (increasing skin) water blocks scale emulsions paraffin and asphaltenes turbulence rate dependent skin perf debris Initial damage from mud and DIF s 3/14/2009 24
Skin Components and Determination Total Skins (s ) = S o + S tp + D Q Where: So = laminar skin S tp = 2-phase skin D = rate dependent skin Q = rate 3/14/2009 25
Skin Components and Determination Multi-rate tests => S o, S tp, D B/U Test => total skin, k 2 B/U Tests => S o, D, k 3/14/2009 26
Cleaning Damage Most damage is removed by simple clean-up flow. How much drawdown? How long to flow it? How soon to flow it? To do it? 3/14/2009 27
Root Causes of Residual Damage After Clean-up Flow. High perm formations less affected? Major damage removers: Flow Rate per unit area, Flow Volume per unit area, Pressure pulse? Drawdown per unit area a control? 3/14/2009 28
First Problem We don t understand cleanup by flow It s a matter of flow rate and volume through a given area. 3/14/2009 29
Clean-up flow is diluted by the length of interval open at once for cleanout. A 10 ft pay in a vertical well w/ 6 diameter yields contact area of 16 ft 2 For the same pay thickness, a horizontal well or a fractured well may contact 100 s of times more pay zone area than a vertical well. A 1000 ft pay in a horizontal well w/ 6 diam. yields contact area of 1600 ft 2 Now, think about the set drawdown say 500 psi - per unit area, the velocity generated, and the total volume per area. 3/14/2009 30
Which has the potential of cleaning up faster and more completely? 5000 bpd 1000 bpd Horiz. Well assume 5x more than vertical flow (typical) would generate 5000 bpd, but spread over 1600 ft 2 and release a clean-up flow of only 3 gal per hour per ft. Vertical well - 500 psi dd and an inflow of 100 bbl/day/ft spread out over just 16 ft 2 will generate a clean-up flow of 110 gal per hour per ft. 3/14/2009 31
Second Problem We don t understand damage. How it got there How it is removed. How to prevent it. What operations put the well s productivity at risk. 3/14/2009 32
Some examples of GOM skins versus md-ft Note the increasing skin in higher conductivity wells why? It should be easier to remove damage in higher perm formations. The key here is that turbulent (non-darcy or non mechanical) skins are nearly always higher in high capacity wells especially gas. Industry Frac Pack Oil & Gas Well Performance (permeability-thickness vs. skin) Skin 40 35 30 25 20 15 Heidrun Gp There are many Harding Appraisal GP damage MC109 Frac-Enzyme - Oil mechanisms, Pompano - Oil but few are Mars FP - Oil permanent if Mars HRWP we learn how Marathon Data to remove Oryx HRWF- HI 379 - Oil & Gas them. Elf Hylia X/L + HEC Tail - Oil DD FF EE 10 Ew ing Bank FP- Oil 5 0-5 100 1,000 10,000 100,000 1,000,000 3/14/2009 Permeability Thickness (md-ft) 33
Cleanup examples from Alaska wells high losses into high PI wells, but. 2500 Fluid Loss Rate from Pre Workover PI - Alaska Daily Loss, bbls 2000 1500 1000 500 0 0 2 4 6 8 10 PI, bbls/day/psi 3/14/2009 SPE 26042 34
there was actually little correlation with amount lost. % change in PI on Workover % change in PI 60 40 20 0-20 -40-60 -80-100 0 2000 4000 6000 8000 10000 12000 Total George E. King Fluid Engineering Loss, bbls 3/14/2009 35
Of higher importance was whether the perfs were protected or not. PI Change, Perfs Not Protected 40 20 PI of Wells 0.3 0.5 1.1 2.1 3.1 6 18.9 % Change in PI 0-20 -40-60 1 3 5 7 9 11 13 15 Short Term PI Change Long Term PI Change -80-100 SPE 26042 3/14/2009 36
When perfs were protected, that was little risk of long term damage. PI Change After Workover - Perfs Protected 50 40 30 PI of wells 0.3 1.2 3.1 4.6 8.4 % Change in PI 20 10 0-10 1 2 3 4 5 6 7 8 9 10 11 Short Term PI Change Long Term PI Change -20-30 -40 SPE 26042 3/14/2009 37
When the perfs were not protected, the well was damaged. Damage in Fractured Wells with Unprotected Perforations % Damage 0-10 -20-30 -40-50 -60-70 -80 1 2 3 4 5 PI i = 7.55 PI i = 6.21 PI i = 1.92 PI i = 7.55 PI i = 18.1 3/14/2009 38
One very detrimental action was running a scraper prior to packer setting. The scraping and surging drives debris into unprotected perfs. Effect of Scraping or Milling Adjacent to Open Perforations % Change in PI 20 10 0-10 -20-30 -40-50 -60 Perfs not protected by LCM prior to scraping 1 Perfs protected 2 by LCM Short Term PI Change Long Term PI Change SPE 26042 3/14/2009 39
Sized particulates, particularly those that can be removed, are much less damaging than most polymers, even the so-called clean polymers. Kill Pills: Summary of Overall Effectiveness in Non Fractured Wells 10 5 Sized Borate Salts (4) Cellulose Fibers (3) HEC Pills No Pills % Change in PI 0-5 -10-15 Sized Sodium Chloride (12) 1 2 3 4 5 6 7 No Near Perf Milling (6) No Near Perf Milling (8) -20-25 SPE 26042 Near Perf Milling or Scraping (3) Near Perf Milling or Scraping (10) 3/14/2009 40
Third Problem We don t know enough about timing of damage removal. Variety of causes Polymer dehydration Decomposition of materials Adsorption, absorption and capillary effects Field data from Troika (100,000 md-ft) show initial flow improves PI, but later flow does not. 3/14/2009 41
Deepwater Well Cleanup Lessons On initial cleanup, PI erratically increased as choke opened. Typical response was a decrease, as if well / flow path were loading up, then sharp PI increase, seemingly when the obstruction was unloaded. Little partly broken polymer recovered, but early load water recovery matched PI incr. Lower skins were linked to both sand flow before completion (sand surge removed damage), increased cleanup flow volumes (and drawdown) on initial cleanup, and more effective frac stimulations. 3/14/2009 42
Deepwater Cleanup Lessons Wells cleaned up with increasing drawdown immediately following backflow start. Cleanup was increasing, measured by increasing PI, at the end of the first short cleanup periods prior to shut-in of the well. After initiation of production operations - after first cleanup flow, no further cleanup of damage was seen, regardless of drawdown. The reason is not known, but may be due to polymer adhering or cooking out? 3/14/2009 43
Fourth Problem We don t understand how damage impacts economic return. 3/14/2009 44
Example Economics - Skin Sensitivity 170 40 160 35 PV-10, $ mm 150 140 130 120 30 25 20 PV10 ROR 110 15 100 10 0 5 10 15 20 25 Skin 3/14/2009 45
An example of a completion method that minimizes damage. Perforating, but with enough underbalance to create the flow necessary to clean the perforations. 3/14/2009 46
Look at some of the most common damage mechanisms. In some formations, a specific damage is very detrimental, while in another formation, the damage is insignificant. Type of Damage Most Probable Location Impact on Productivity if not Removed Moderate to Severe Surge Pressure Needed for Removal? Yes Flow Vol. Or Time Needed for Removal? Spurt Volume Mud Cake in Pay Zone Formation Face Mud Filt. <12 Minor - mod. No No Whole Mud Severe Loss Cement Filtrate Perforation Crush Zone Formation sand in perfs Fractures and large vugs <12 into pay ½ around perf perf tunnels Only if clay damage Moderate to Severe Most severe Possible, Can help in few cases. No Depend on Cond. few cases similar Yes 4 to 12 gal/perf Cleanup and reperf Removal Hampered by Limits on Cleanup Vol? Yes Yes, flow combined with solvent treatment No Yes Depends on initial &later actions Alternate Methods of Removal or Prevention Acids, Soaps, Enzymes Few successful whole mud removals when vol. > 200 bbls. Surge small zones into chamber, acids, pulses, fracs Develop good perf and prepack actions 3/14/2009 47
When looking at the range of skins, it is useful to know that some wells have high skins but are really restricted by other factors such as tubing flow limits, facility limits, etc., and are not really limited by the skins. Type of Damage Skin range Comments Mud Cake in Pay +5 to +300, Zone +15 is typical Mud skin is usually shallow and has more impact when turbulence and non-darcy skin problems are most severe. Mud cake is usually by-passed by perforating. Mud Filtrate +3 to +30 Filtrate usually recovered by steady flow and time. Related to relative perm effects. This is usually a short lived problem (1 to 3 weeks) Whole Mud Loss (in pay zone) >+50 Options depend on mud volume lost. Enzymes, solvents and acids for small volumes (<10 bbls). Sidetrack if over 1000 bbls. Low solids mud can be removed by concentrating on viscosifier destruction or dispersment. Cement Filtrate +10 to +20 Very shallow clay problems. Perforate with deep penetrating Perforation Crush Zone Formation sand in perfs charges to get beyond. Use leakoff control on cement. +10 to +20 Perf small intervals underbalanced. Isolation packer breaksown, explosive sleeve breakdown (very simple) - must be accomplished prior to gravel packing. >+50 Most severe typical damage - cleanout and recompletion required 3/14/2009 48
Table 3: Completion Efficiency Factors (Stracke, SPE 16212) Shown as a percent of wells in each bracketed completion efficiency range. Factor 100-75% 75-50% 50-25% 25-0% Underbalance perf w/ surge chamber 26 26 26 23 Perforate and wash perfs 25 40 19 15 Underbalance perf w/ surge chamber + wash perfs 71 14 14 0 Surge Vol < 4 gal/ft 23 19 35 23 Surge Vol > 4 gal/ft 43 29 14 14 Surge differential < 1000 psi 27 24 24 24 Surge differential >1000 psi 19 27 31 23 Drilling overbalance <300 psi 15 48 19 19 Drilling overbalance >300 psi 31 31 23 16 Drilling overbalance >1000 psi 27 31 23 19 Completion overbalance <300 psi 25 39 25 12 Completion overbalance >300 psi < 600 psi 21 36 21 21 Completion overbalance >600 psi 47 21 5 26 This data is difficult to understand unless the attention is focused at those factors which deliver the most wells in the 100% to 75% efficiency range, e.g., underbalance perf w/surge chamber and wash perfs. 3/14/2009 49
Notice that underbalance perforating at high underbalance (which causes flow) delivers a perforation that is from 40% to nearly 300% larger than the other methods of perforating. Table 4: Perforation Tunnel Volume Following Perforating at Different Conditions (Regalbuto and Riggs, SPE PE, Feb 1988) Pressure Differential Hole Volume, perforated Hole Volume, perforated/surged Overbalanced, 18 cc 500 psi, no surge Balanced perforated, no surge 31 cc Underbalanced perforated, 500 20 cc 42 cc psi Balanced perforated, delayed 31 cc 48 cc 1000 psi surge Underbalanced perforated, 1000 psi 50 cc 75 cc 3/14/2009 50
Back to over-all damage What is it? Divide the well into three parts: Inflow: area from reservoir to the wellbore Completion potential: flow to surface Surface restrictions: chokes, lines, separators. Basically, anything that causes a restriction in the flow path decreases the rate and acts as damage. 3/14/2009 51
Differential pressure, P, is actually a pressure balance 100 psi 10 psi Press. Drop 25 psi 10,000 ft 4600 psi 15 psi 1000 psi Column Densities: Gas = 1.9 lb/gal = 0.1 psi/ft = 1000 1900 psi in a 10,000 ft well Dead oil = 7 lb/gal = 0.364 psi/ft = 3640 psi in a 10,000 ft well Fresh water = 8.33 lb/gal = 0.433 psi/ft = 4330 psi in a 10,000 ft well Salt water = 10 lb/gal = 0.52 psi/ft = 5200 psi in a 10,000 ft well Gas cut flowing oil = 5 lb/gal = 0.26 psi/ft = 2600 psi in a 10,000 ft well Where does the P come from? P = 4600 psi reservoir pressure -2600 psi flowing gradient for oil - 150 psi press drop - 100 psi through the choke - 25 psi through the flow line - 10 psi through the separator - 15 psi through downstream flow line -1000 psi sales line entry pressure ---------------------- 3/14/2009 52 700 psi drawdown pressure
The first step.. For the purposes of this work, consider the flow connection between the reservoir and the wellbore as the primary but not the only area of damage. Now, is it formation damage or something else that causes the restriction? 3/14/2009 53
Some sources of the damage in the reservoir-wellbore connection Wetting phases (from injected or lost fluids) Debris plugging the pores of the rock Polymer waste from frac and drilling fluids Compacted particles from perforating Limited entry (too few perforations) Converging radial flow wellbore too small Reservoir clay interactions with injected fluids Precipitation deposits (scale, paraffins, asphaltenes, salt, etc) Note that not all are really formation damage How do you identify the difference? 3/14/2009 54
And some restrictions in very high perm wells is the casing and the very limited amount of open area that perforations create. Data comparing cased and perforated skin with skins from open hole completed wells from Algeria. ALGERIAN GAS WELL COMPLETION EFFICIENCIES 20 18 16 Cased Hole - Vertical - 5" Tbg Open Hole - Vertical - 7" Tbg Turbulent Skin Mechanical Skin 14 Open Hole - Slanted - 7" Tbg Total Skin 12 10 8 65-75 degree 15-20 degree 50-65 degree 6 4 2 65-75 degree 65-80 degree 60-70 degree Turbulent Skins Normalized to Rates of 100 mmscfd 0 Tg 308 Tg 312bis Tg 315bis Teg 15y tg 307ter Tg 305bisz Teg 14z Teg 11z Teg 12z Teg 10z 3/14/2009 55 2001 2002
Identification of Damage. How good are you at deductive reasoning? Identifying the cause and source of damage is detective work. Look at the well performance before the problem Look at the flow path for potential restrictions Look to the players: Flow path ways Fluids Pressures Flow rate 3/14/2009 56
What is Completion Efficiency? A measure of the effectiveness of a completion as measured against an ideal completion with no pressure drops. Pressure drops? these are the restrictions, damage, heads, back-pressures, etc. that restrict the well s production. 3/14/2009 57
Look again - The Effect of Damage on Production Rate = ( P x k x h) / (141.2 µ o β o s) Where: P = differential pressure (drawdown due to skin) k = reservoir permeability, md h = height of zone, ft µ o = viscosity, cp β o = reservoir vol factor s = skin factor 3/14/2009 58
What changeable factors control production rate? Pressure drop need maximum drawdown and minimum backpressures. Permeability - enhance or restore k? - yes Viscosity can it be changed? yes Skin can it be made negative? These factors are where we start our stimulation design. 3/14/2009 59
Formation Damage Impact Causes Diagnosis Removal/Prevention? Basically, the severity of damage on production depends on the location, extent and type of the damage. A well can have significant deposits, fill and other problems that do not affect production. 3/14/2009 60
Conclusions Damage is usually shallow. Remove it or by-pass damage if it really causes a problem. Not every damage is in the formation. Not every drop in production is caused by damage. First, remove the pressure drops, everything else will take care of itself. 3/14/2009 61