chemical wash

A chemical wash in oilfield cementing operations is a low-viscosity, chemically active fluid pumped ahead of the cement slurry in the cementing train to clean drilling mud and mud cake from the casing interior and the borehole wall, improving the mud removal efficiency and the quality of cement bonding to both the casing and the formation; the chemical wash achieves its cleaning action through a combination of surfactant activity (reducing the interfacial tension between the oil-wet mud film and the aqueous wash fluid, allowing the wash to wet and lift the mud from metal and rock surfaces), dispersant activity (destabilizing the mud cake structure by chelating or adsorbing on the clay particles and mineral weighting agents that form the cake), and turbulent flow (the low viscosity of the chemical wash, typically 1 to 5 mPas, allows it to achieve turbulent flow at practical pump rates, scouring the borehole wall mechanically at Reynolds numbers above 2,100 while the chemicals act on the mud film). In Western Canada Sedimentary Basin primary cementing programs for surface casing (406 mm to 508 mm OD), intermediate casing (244 mm to 340 mm OD), and production casing (114 mm to 178 mm OD) in Cardium, Montney, and Duvernay horizontal wells, the chemical wash is a standard stage in every cementing train because the water-base and oil-base muds used in WCSB drilling leave a mud cake of 3 to 15 mm thickness on the borehole wall and an oil-wet film on the casing interior that would prevent hydraulic bonding between the cement slurry and the casing or formation surfaces if not removed before cement placement; inadequate mud removal creates microannuli (gaps of 0.1 to 2 mm between the casing outer wall and the cement sheath) that allow gas migration along the casing-cement interface under WCSB wellbore pressures and temperatures, which is the principal cause of sustained casing pressure (SCP) detected at WCSB surface wellheads and reported under AER Directive 020. The chemical wash train in a WCSB primary cement job typically consists of a preflush volume of 0.5 to 2 bbl per 100 ft of casing (0.26 to 1.05 m3 per 30 m) pumped at 4 to 8 bbl/min ahead of the spacer and cement slurry, with contact time of 10 to 15 minutes at the top of the cement column governed by pump rate and annular velocity, as AER Directive 009 requires minimum 10-minute turbulent contact time at each point in the cemented annulus for wells within 300 m of fresh water zones.

• Chemical wash formulation: surfactant and dispersant chemistry for water-base and oil-base mud removal in WCSB cementing: Chemical wash formulations for WCSB cementing are tailored to the mud type being displaced: for water-base mud (WBM) systems (KCl-polymer, KCl-PHPA, or glycol-based inhibitive muds used in WCSB Cardium and Montney surface and intermediate holes), the chemical wash contains anionic or amphoteric surfactants (sodium dodecyl sulfate, cocamidopropyl betaine) at 1 to 3 percent concentration and an inorganic or organic dispersant (sodium hexametaphosphate, polynaphthalene sulfonate) at 0.5 to 1.5 percent that disrupts the WBM filter cake by deflocculating bentonite and barite particles. For oil-base mud (OBM) systems used in WCSB Montney and Duvernay horizontal sections (inverted emulsion with base oil, organophilic clay, and CaCl2 brine internal phase), the chemical wash must include an oil-wetting reversal agent (a mutual solvent such as ethylene glycol monobutyl ether at 3 to 8 percent) that penetrates the OBM film on the casing and formation surfaces, makes the surface water-wet, and allows the cement slurry to bond to the hydrophilically conditioned surface; without wettability reversal on OBM-contacted surfaces, cement adhesion to the casing is less than 20 percent of the adhesion achieved on water-wet surfaces, predisposing the completion to microannulus gas migration.
• Turbulent flow requirements and pump rate design for chemical wash effectiveness in WCSB casing cementing programs: The mechanical cleaning action of turbulent flow is as important as the chemical activity of the wash fluid in removing mud cake from WCSB borehole walls; turbulent flow (Reynolds number above 2,100, preferably above 4,000) exerts wall shear stresses of 0.5 to 5 Pa on the borehole surface that erode the mud cake physically while the surfactants and dispersants attack the mud chemically. The pump rate required to achieve turbulent flow in the annulus between the casing and the borehole wall depends on the annular gap, the chemical wash viscosity, and the density; for a 244 mm casing in a 311 mm borehole (67 mm radial annular gap) with a 1.03 SG chemical wash at 2 mPas viscosity, turbulent flow requires an annular velocity of at least 0.9 m/s, corresponding to a pump rate of 5.8 bbl/min (0.92 m3/min); WCSB cementing engineers verify the turbulent flow condition using a hydraulics simulator (CEMPRO+, WellCAT, or equivalent) that calculates the Reynolds number at each annular cross-section as a function of pump rate, fluid properties, and wellbore geometry. Wells with eccentric casing (casing not centered in the borehole due to missing or inadequate centralizers) present a particular challenge in WCSB horizontal sections where gravity causes the casing to lie on the low side of the borehole; the narrow gap on the low side may remain laminar even when the wide gap on the high side is turbulent, leaving a mud channel on the low side that is the most common source of WCSB cement channeling and SCP.
• Chemical wash compatibility testing with cement slurry and spacer fluids for WCSB primary cementing quality control: Chemical wash fluids must be compatible with both the drilling mud being displaced (to prevent precipitation of mud solid or formation of a viscous mixing zone at the mud-wash interface) and the cement spacer or slurry that follows (to prevent contamination of the cement with wash fluid that would retard setting, reduce compressive strength, or cause slurry segregation). Compatibility testing for WCSB cementing programs is conducted at the wellsite or by the cementing service company laboratory before the job, using spot mixing tests that combine the chemical wash with the mud (1 part wash to 1 part mud) and with the cement slurry (1 part wash to 1 part slurry) to confirm that no gelling, precipitation, or viscosity increase occurs; a compatibility test failure (gel formation at the mud-wash or wash-cement interface) requires reformulation of the wash chemistry before the cement job is pumped. AER Directive 009 requires that cementing programs for WCSB surface and intermediate casing include documented fluid compatibility testing results in the cementing program design submitted to AER at least 72 hours before a primary cement job on a well within a groundwater protection zone.
• Chemical wash volume calculation and contact time verification for AER Directive 009 compliance in WCSB primary cementing: AER Directive 009 specifies minimum contact time requirements for cement precursor fluids in the annulus of WCSB wells completed within 300 m of the base of groundwater protection: the chemical wash must achieve a minimum 10-minute turbulent contact time at the top of the planned cement column to satisfy the directive's mud removal standard. The volume of chemical wash required to achieve a 10-minute contact time is calculated as the annular volume per metre of annulus at the planned turbulent flow pump rate; for the above 244 mm casing example at 5.8 bbl/min with a 0.52 m2/m annular cross-section, a 10-minute contact time at any point in the annulus requires 5.8 min bbl pumped = 58 bbl (9.2 m3) of chemical wash total volume. WCSB operators must demonstrate Directive 009 compliance by including the chemical wash volume calculation in the cementing program, recording the actual pump rate and volume during the cement job on the cementing job report, and submitting the cementing job report to AER within 30 days of job completion under Directive 009 reporting requirements.
• Chemical wash effectiveness evaluation by cement bond log and temperature log interpretation in WCSB production casing programs: After a WCSB production casing primary cement job, the quality of mud removal and cement bonding is evaluated by cement bond log (CBL), variable density log (VDL), and in some WCSB critical wells by ultrasonic imaging tools (USIT, CAST-V). The CBL amplitude at any depth reflects the fraction of the casing circumference bonded to cement; a CBL amplitude below 5 millivolts (relative to the free-pipe amplitude of 50 to 100 mV) indicates bonded cement covering more than 80 percent of the casing circumference, while amplitudes above 20 mV indicate poor bond, often corresponding to depths where chemical wash turbulent flow was not achieved and mud channeling is present. Temperature logs run immediately after the cement job detect the exothermic heat of hydration of Portland cement as a temperature increase of 3 to 15 degrees Celsius above the geothermal gradient at cemented intervals; intervals with no temperature anomaly indicate cement absence (mud channeling) and flag the depth intervals where chemical wash efficiency was inadequate to remove the mud before cement placement, confirming the locations that may require remedial cementing or perforation squeeze treatments to restore zonal isolation in WCSB production wells.

Chemical Wash Resolving OBM Contamination and Restoring Cement Bond in WCSB Montney Casing Program

A northeast British Columbia Montney horizontal well drilled with an invert emulsion OBM (base oil: 80/20 oil-water ratio, CaCl2 brine internal phase, 1.35 SG) required production casing cementing across a 1,800 m horizontal lateral section. A previous offset well cemented with a standard WBM chemical wash showed CBL amplitudes of 35 to 65 mV across 60 percent of the lateral, indicating extensive mud channeling attributable to OBM wettability contamination on the casing. The subject well's cementing program specified an OBM-compatible chemical wash containing 6 percent ethylene glycol monobutyl ether (mutual solvent), 2 percent anionic surfactant blend, and 1 percent polynaphthalene sulfonate dispersant at 1.04 SG and 2.1 mPas viscosity, pumped at 7.2 bbl/min to achieve an annular velocity of 1.15 m/s and a Reynolds number of 3,800 (turbulent) at the 8 1/2 in lateral section. Post-cement CBL showed amplitudes below 8 mV across 78 percent of the cemented lateral and below 15 mV across 91 percent, representing a substantial improvement over the offset well; SCP at the wellhead was zero over the 18-month monitoring period following completion.

Related Terms

Primary cementing is the wellbore operation in which chemical washes are pumped as the lead stage of the cementing train; chemical wash mud removal quality determines the hydraulic bond integrity of the primary cement sheath in WCSB surface, intermediate, and production casing programs. Cement bond log (CBL) evaluates the effectiveness of chemical wash mud removal by measuring casing-to-cement acoustic coupling; CBL amplitudes below 5 mV confirm adequate mud removal and cement bonding in WCSB cased production intervals. Spacer fluid is the viscous fluid pumped between the chemical wash and the cement slurry; spacers prevent mud from contaminating the cement and provide additional mud removal capacity that complements the chemical wash in WCSB production casing cementing. Sustained casing pressure (SCP) at WCSB wellheads is the primary consequence of inadequate chemical wash performance; microannuli created by unbonded mud channels at the casing-cement interface allow gas migration that is detected as SCP and reported under AER Directive 020. Mud cake is the filter cake deposited on the borehole wall by drilling fluid filtrate invasion; chemical wash formulations target the mud cake structure by disrupting clay platelet and weighting agent packing to allow cement to bond directly to the formation surface in WCSB wells.