chemical diversion

Chemical diversion in matrix acid stimulation is the practice of using a chemical agent injected into the wellbore ahead of, mixed with, or following the acid treatment stage to temporarily reduce the injectivity of high-permeability, acid-accepting zones and force subsequent acid volumes into lower-permeability, under-stimulated intervals that would otherwise receive little or no acid due to the inherent tendency of injected fluids to preferentially enter the path of least resistance in a heterogeneous perforated interval; the chemical diverter creates a temporary plugging effect in the high-permeability zones that persists long enough for the acid treatment to be redistributed across the full perforated interval or pay zone, improving the conformance of the stimulation and increasing the fraction of the reservoir contacted by live acid, before breaking down under wellbore conditions (temperature, pressure, or produced fluid exposure) and restoring the full injectivity of the treated interval for production. In Western Canada Sedimentary Basin matrix acid stimulation programs targeting heterogeneous WCSB Cardium, Viking, and Devonian carbonate intervals, chemical diversion is the primary technique for achieving uniform acid placement when perforation clusters have permeability variation of 10 to 100-fold across a 10 to 40 m gross interval, because without diversion the 70 to 90 percent of injected acid that enters the highest-permeability cluster creates a stimulated zone around that cluster while the remaining clusters receive insufficient acid to overcome skin damage or dissolve near-wellbore carbonate cement, leaving a partially stimulated wellbore that produces at a fraction of the achievable rate if all clusters had been effectively treated. The chemical diverters used in WCSB matrix acid programs include foam diverters (nitrogen foam generated in-situ by co-injecting N2 and surfactant with the acid, creating a high apparent viscosity in the high-permeability zone that blocks further acid entry), viscous gel diverters (crosslinked polymer slugs that form a temporary plug in the high-permeability interval and are placed as alternating slugs between acid stages), particulate diverters (degradable bridging particles sized to bridge across the perforations of the high-permeability cluster), and self-diverting acids (viscoelastic surfactant-based acids that increase viscosity as the acid is spent and carbonate is dissolved, creating in-situ diversion without a separate diverter injection). Chemical diversion differs from mechanical diversion (ball sealers, straddle packers, bridge plugs) in that the diverting agent is pumped as part of the fluid system rather than set as a discrete barrier, making it applicable in horizontal WCSB wells and through-tubing treatments where mechanical placement is geometrically constrained.

• Foam diversion mechanics and nitrogen foam design for WCSB Cardium and Viking matrix acid jobs: Foam diversion uses the high apparent viscosity of nitrogen foam (a dispersion of nitrogen bubbles in surfactant-stabilized liquid at foam quality of 50 to 80 percent N2 by volume) to reduce the mobility of the injected acid in high-permeability zones, diverting subsequent acid into lower-permeability intervals. When nitrogen foam is co-injected with HCl acid into a WCSB Cardium interval where one perforation cluster has 150 mD permeability and adjacent clusters have 20 to 40 mD, the foam preferentially enters the 150 mD cluster (because initial injection follows the same permeability contrast), but the high apparent viscosity of the foam (typically 50 to 200 mPas at 70 percent foam quality) rapidly increases the flow resistance in the high-permeability cluster by a factor of 10 to 50 compared to the acid alone, equalizing the injection profile across the permeability contrast within 3 to 8 minutes of foam injection. Foam diverter design for WCSB conditions specifies the surfactant type (anionic or amphoteric for HCl compatibility and foam stability at 50 to 90 degrees Celsius bottomhole temperature), the foam quality (60 to 75 percent N2 for most WCSB Cretaceous treatments), and the foam slug volume (2 to 5 m3 of foam per diversion stage) based on the permeability contrast and perforated interval length derived from production logs and core analysis.
• Viscoelastic surfactant self-diverting acid mechanism in WCSB Devonian carbonate matrix stimulation: Self-diverting acid (SDA) is a hydrochloric acid formulation containing a viscoelastic surfactant (VES) that remains a low-viscosity fluid (5 to 10 mPas) at low pH while the acid is active, allowing easy injection into the formation, but increases dramatically in viscosity (200 to 2,000 mPas) as the acid is spent and pH rises above 4 and the carbonate dissolution products (calcium and magnesium ions) trigger the VES to form rod-like micelles that create a gel-like structure in the spent acid zone. In WCSB Devonian carbonate matrix acid jobs at Nisku and Wabamun reef flank targets, the SDA mechanism diverts acid away from the most-dissolved, highest-permeability wormhole channels (where spent acid accumulates and the VES gel forms) and redirects fresh acid into less-dissolved, lower-permeability carbonate zones, creating a more uniform wormhole distribution across the treated interval. The VES gel breaks when contacted by formation hydrocarbons (oil or gas dissolve the surfactant micelles) or by produced water (dilution reduces the ion concentration below the micelle-forming threshold), making SDA cleanup in WCSB producers self-activating without a separate breaker stage; cleanup times of 2 to 8 hours after shut-in are typical for WCSB Devonian carbonate SDA treatments at bottomhole temperatures of 70 to 100 degrees Celsius.
• Particulate chemical diverters and degradable bridging agents for WCSB perforated interval acid conformance: Particulate diverters use degradable solid particles (polylactic acid (PLA) beads, benzoic acid flakes, or rock salt granules) sized to bridge across the perforation tunnel entrance of the high-permeability clusters, creating a temporary partial seal that increases the flow resistance of that cluster and diverts subsequent acid to adjacent lower-permeability clusters without a nitrogen injection system. In WCSB Viking Formation matrix acid programs where the perforated interval spans 15 to 30 m with 3 to 6 perforation clusters of variable injectivity, particulate diverter slugs of 50 to 150 kg of 3 to 6 mm PLA beads are pumped between acid stages; the beads bridge across the 10 to 15 mm perforation tunnel entrance of the dominant cluster, increasing its flow resistance by 3 to 10-fold for the duration of the subsequent acid stage. PLA beads degrade by hydrolysis at WCSB bottomhole temperatures of 50 to 80 degrees Celsius over 12 to 48 hours, fully restoring perforation open area for production without mechanical removal; benzoic acid flakes dissolve in produced oil over 4 to 12 hours and in produced water over 24 to 72 hours, providing a similar degradation timeline. Particulate diverter selection between PLA and benzoic acid is based on the produced fluid type (oil versus gas/water), bottomhole temperature, and the required diversion persistence time relative to the acid contact time.
• Chemical diversion effectiveness evaluation using step-rate tests and injection pressure response analysis in WCSB acid jobs: Confirming that chemical diversion is working during a WCSB matrix acid job requires monitoring the wellhead injection pressure and rate response as the diverter slug is pumped; effective diversion produces a characteristic pressure increase of 1 to 4 MPa above the baseline injection pressure as the diverter builds resistance in the high-permeability cluster, followed by a pressure decrease as the diverted acid opens new permeability in the lower-permeability clusters. In WCSB real-time acid job monitoring, the diversion ratio (injection pressure with diverter divided by pressure without diverter) is tracked for each cycle; a ratio greater than 1.3 indicates effective plugging of the dominant cluster, while less than 1.1 suggests inadequate resistance and continued acid entry into the high-permeability zone. Post-job temperature logs run 12 to 24 hours after the job confirm diversion effectiveness; a well-diverted WCSB Cardium job shows uniform temperature anomalies across all perforation clusters rather than the single dominant-cluster anomaly of an undiverted treatment.
• Chemical diversion design workflow and WCSB regulatory submission requirements under AER Directive 065: Designing a chemical diversion program for a WCSB matrix acid job begins with characterization of the perforated interval heterogeneity from production logs, spinner surveys, or core permeability data to quantify the permeability contrast between clusters; this data drives the diverter type selection, diverter slug volume, and the number of acid-diverter cycles required to achieve uniform stimulation. AER Directive 065 (Resources Applications for Conventional Oil and Gas Reservoirs) requires that operators submit a well stimulation report for matrix acid jobs with surface treating pressure above 70 percent of the fracture gradient (to confirm the job remained in matrix rather than fracturing the formation), and the chemical diverter program is documented in the stimulation report as part of the injection fluid schedule; WCSB operators must retain stimulation records for 5 years after well completion and produce them for AER inspection on request. Post-job skin analysis using pressure buildup testing is the definitive indicator of diversion success; a post-job skin factor more negative than minus 2 across the perforated interval confirms adequate conformance, while a positive skin indicates incomplete diversion and may trigger a repeat stimulation on the WCSB Cardium or Viking completion.

Foam Diversion Improving WCSB Viking Matrix Acid Conformance Across Four Perforation Clusters

A central Alberta Viking Formation oil producer with four perforation clusters over a 28 m gross interval underwent a 15 percent HCl matrix acid job; a pre-job production log confirmed that 78 percent of production was coming from the top cluster (permeability 185 mD) while the three lower clusters (42, 28, and 19 mD) were contributing 22 percent combined. An initial 5 m3 acid stage without diversion increased the top cluster injectivity further (pressure decrease of 1.4 MPa at constant rate), confirming the dominant cluster would absorb all subsequent acid without diversion. Three nitrogen foam diverter stages (3 m3 each at 70 percent foam quality, co-injected at 0.4 m3/min) were alternated with three 6 m3 acid stages; each foam stage produced a 2.1 to 2.8 MPa pressure increase indicating successful temporary resistance in the top cluster. Post-job production log showed the top cluster contributing 41 percent and the three lower clusters contributing 59 percent collectively, a dramatic improvement in conformance. Total well production increased from 18 m3/d pre-job to 54 m3/d post-job, with the majority of the incremental production attributable to the three previously under-stimulated clusters now effectively acidized by the diverted treatment.

Related Terms

Matrix acidizing is the stimulation method in which chemical diversion is applied; diversion improves acid placement conformance across heterogeneous WCSB perforated intervals where permeability contrast concentrates acid in the dominant cluster. Acid stimulation is the broader category; chemical diversion specifically addresses the placement efficiency problem in multi-cluster WCSB Cardium, Viking, and Devonian carbonate completions. Foam diverter is the most common chemical diversion agent in WCSB matrix acid programs; nitrogen foam co-injected with surfactant creates a high-apparent-viscosity temporary plug in the dominant cluster, forcing acid into lower-permeability zones. Viscoelastic surfactant (VES) is the active component of self-diverting acid formulations; VES micelle formation as the acid spends creates in-situ diversion without a separate diverter injection in WCSB Devonian carbonate programs. Production log is the primary evaluation tool for chemical diversion; spinner survey and temperature log comparison of pre- and post-job cluster contributions confirms whether the diversion program achieved uniform acid placement conformance across the WCSB perforated interval.