Designed & Presented by Mr ĐỖ QUANG KHÁNH, HCMUT Đỗ Quang Khánh – HoChiMinh City University of Technology Email: dqkhanh@hcmut.edu.vn or doquangkhanh@yahoo.com What is Formation Damage?  Damage can be anything that obstructs the normal flow of fluids to the surface  Formation damage specifically refers to obstructions occurring in the near-wellbore region of the rock matrix.=> Concerns the formation of a volume of rock with a reduced permeability in the near wellbore zone  Ultimate economics usually favor control of formation damage rather than stimulation to overcome limited productivity Sources of Formation Damage  Damage during drilling operations  Damage during completion operations  Damage during well stimulation  Damage caused by other operations Damage during drilling operations  Mud solids may block pores, vugs, and natural or induced fractures  Mud filtrate invasion into oil and gas zones may oil-wet the formation and cause water or emulsion blocks  Pore or fractures near the wellbore may be sealed by the trowelling action of the bit, drill collars and drill pipe  Cement or mud solids may plug large pores, vugs, and natural or induced fractures  Chemical flushes used to scour hole ahead of cement may cause changes in clays in the producing formation  Filtrate from high fluid loss cement slurries may bring about changes in the producing formation Damage during completion operations  Damage during perforating   Perforations may be plugged with shaped charge debris and solids from perforating fluids Formation around perforation is crushed and compacted by perforating process  Damage while running tubing and packer   If returns are lost while running tubing, solids in the well fluid may plug any fracture system near the wellbore Perforations may be plugged if solids are forced into perforations by the hydrostatic differential pressure into the formation  Damage during production initiation     Damage may be caused by incompatible circulation fluids and by loss of clays or another fines into perforation pores, vugs Damage may result from depositing of mill scale, clay, or excess thread dope from tubing collars in perforation when circulating to clean a well Completion fluids containing blown asphalt may cause damage by oil-wetting the formation and by plugging perforations and formation Clean-up of a well at high rates can result in severe plugging within the formation by particles which, for one reason or another, are free to move Damage during well stimulation  Perforations, formation pores, and fractures may be plugged with solids while killing or circulating a well with mud or with unfiltered oil or water  Damage may be caused by filtrate from circulating fluids  Breaking down or fracturing the formation with acid may shrink the mud cake between the sand face and cement or may affect mud channel in the annulus allowing vertical communication of unwanted fluids  Acidizing sandstone with hydrofluoric acid may leave insoluble precipitates in formation Properly designed treatment minimizes effect  Damage may be caused by hydraulic fracturing fluids  Damage may be caused by incompatible fluid in fracture acidizing of carbonates Damage caused by other operations  Damage caused by cleaning of paraffin solids from the tubing, casing, or wellbore  Damage during well servicing or workover  Damage during producing phase  Damage during water injection  Damage during gas injection Common Formation Damage Mechanisms  Fines invasion and migration (particles, etc.)  Rock-fluid incompatibility (clay swelling, etc.)  Fluid-fluid incompatibility (emulsion generation, etc.)  Phase trapping and blocking (water entrapment in gas reservoirs)  Adsorption and wettability alteration  Biological activity (bacteria, slime production) Particle Plugging within the Formation  The pore system provides a tortuous path to the wellbore  Particles can move through the pore system  Particle movement is affected by wettability and by the fluid phases in the pore system Particulate Capture Mechanisms Straining Bridging DEPOSITION ENTRAINMENT FLOW Solid particles TYPICAL HYDRAULIC TUBE Example: Permeability Impairment Versus Damage Penetration A well with radius rw equal to 0.328 ft and damage penetration ft beyond well (rs= 3.328 ft) 1) What is skin effect if permeability impairment results in k/ks = and 10, respectively? 2) What would be the required damage depth to give same skin as with k/ks=10 but the actual permeability impairment being k/ks = 5? Solution 1) From Hawkins formula, calculate skin for each permeability impairment: For k/ks = For k/ks = 10 2) Since skin is 20.9 for k/ks = 10 and using k/ks = 5, re-arrange Hawkins’ formula for damage penetration: Rate Dependent Pseudo Skins ΣSpseudo  The pseudo-skins include all phase and rate dependent effects  Turbulence in high-rate gas procedures ( affect very high-rate oil wells)  This skin effect is equal to Dq s’ = s + Dq  s'– apparent Skin  s – actual Skin  D – Non-Darcy coefficient  q – well test rate Phase Dependent Skin ΣSpseudo  Near wellbore phase changes • Flowing bottomhole pressure is below the bubble point pressure, in the case of oil wells • Liquid formation around the well, in the case of gas retrograde condensate reservoirs  Relative Permeability effects Partial Penetration Skin Sc+ ϴ Sc+ ϴ = Sc + Sϴ  Skin due to partial completion Sc  • Bending of flowlines  • Positive Skin  Skin due to well deviation Sϴ  • High angle, large negative Skin  Perforated height < reservoir thickness  Effect becomes negligible when completion height > 75% of reservoir thickness Partial Penetration Skin Sc+ ϴ  z w : elevation of the perforation midpoint from the base of the reservoir  hw : perforated height  h:reservoir height  r w : well radius  θ: angle of well deviation Cinco-Ley et al – 1975: Tables and Partial Penetration Skin Sc+ ϴ Partial Penetration Skin Sc+ ϴ Ex: Partial Penetration Skin Sc+ ϴ  A well with a radius r w =0.328 ft is completed in a 33 ft reservoir In order to avoid severe water coning problems, only ft are completed and the midpoint of the perforation is 29 ft above the base of the reservoir Calculate the skin effect due to partial completion for a vertical well What would be the composite skin effect if θ =45deg ?  Solution: a)r w =0.328 ft; h=33 ft; h w =8 ft; z w =29 ft; θ =0 hD =100; zw /h=0.875; hw/h=0.25 sc =8.6 & sθ =0 ‐> s c+θ =8.6 b) θ =45deg =>s c =8.6 & s θ =‐2.7 ‐> s c+θ ≈6 Skin effect due to Partial Completion Skin effect due to Slant wells Perforation Skin Sp the the the the horizontal skin (Sh ), wellbore skin (Swb ), vertical skin (Sv ) & crushed zone skin (Sc ) Perforation Skin Sp  the horizontal skin (Sh ):  the wellbore skin (Swb ):  the vertical skin (Sv ):  the crushed zone skin (Sc )  =>This allows the calculation of the overall skin for the combination of damage and perforation (Sdp) Perforation Skin Sp  The method varies depending on whether the perforation terminates inside the damaged zone or not  For perforations terminating inside the damaged zone (lp < ld )  For the (hopefully) more relevant case of perforations that extend beyond the damage zone (lp>ld), the perforation length and wellbore radius are modified: Ex of Perforation Skin Sp  Ex:  A sensitivity analysis: ... productivity Sources of Formation Damage  Damage during drilling operations  Damage during completion operations  Damage during well stimulation  Damage caused by other operations Damage during drilling...What is Formation Damage?  Damage can be anything that obstructs the normal flow of fluids to the surface  Formation damage specifically refers to obstructions... clays in the producing formation  Filtrate from high fluid loss cement slurries may bring about changes in the producing formation Damage during completion operations  Damage during perforating