Matrix Acid Stimulation Studies Yield New Results With a Multitap Long-Core Permeameter
Authors: Cheung S.K. , Van Arsdale Howard 
Published: December 2007

Summary
Most past acidizing studies involved monitoring only overall permeabilitiesacross short cores generally less than 3 in. long. This has been a limitingcondition on understanding the acidizing process. Flow tests with longer coreswere needed to understand deep matrix acid stimulation. This paper shows thatunder certain conditions acids containing HF can penetrate beyond 12 in. fromthe wellbore, some retarded acid systems can cause deep damage, and HF acid maynot prevent water-sensitivity damage. prevent water-sensitivity damage. Resultsshow that conclusions based on short-core studies could be misleading and thatmultitap long-core studies are needed in acid stimulation research. Our resultswere obtained with a new generation of acid permeameter to measure permeabilitypermeameter to measure permeability changes under downhole conditions atseveral locations along a single piece of core up to 3 ft long. piece of coreup to 3 ft long.

Introduction
Acids are often pumped downhole in oil fields to increase productivity orinjectivity by removing skin damage around the wellbore. The type, concentration, and volume of acid are important in acid job design. The wrongacid may not remove the damage and could cause precipitation or unconsolidationdamage in some cases. For example, pumping hydrofluoric (HF) acid can causedamaging calcium fluoride precipitates in formations containing highconcentrations of calcium carbonate. Pumping large volumes of concentrated HFacids can cause near-wellbore unconsolidation in some sandstone formations. Frequently, flow tests are run in laboratories on reservoir core material underdownhole conditions to determine acid treatment design. Most of these acidizingstudies involve monitoring only overall Permeabilities across short cores, 1 to3 in. Permeabilities across short cores, 1 to 3 in. long. Observations andresults from these short-core studies have often been extrapolated to predictcore permeability response beyond 3 in. A few long-core acid stimulationstudies have been reported. Kunze and Shaughnessy used 30-in. long sandstonecores to evaluate fluoboric acid stimulation. They measured the permeabilitiesof individual core segments after acid treatments. Gdanski joined cores in fourseparate core holders in series to simulate a core 24 in. long. He evaluated HFand tetrafluoro-aluminic acids. King and Lee wrapped a 72-in. long core with afiberglass mat. They collected fluid samples from taps along the core duringtests to determine the absorption of mutual solvents in the presence of acids. Each of these studies presence of acids. Each of these studies has limitations. To overcome these limitations and to study acidizing in long samples, we havedesigned and built a permeameter able to measure real-time permeability changesunder downhole conditions at several locations along a single piece of core upto 3 ft in length during acid stimulation.

Acid Parmeameter
Schematics of the acid permeameter and core holder are shown in Figs. 1 and2. Details of the acid permeameter are described in the Appendix. The acidpermeameter can handle all mixtures of hydrochloric (HCl) and HF acids attemperatures up to 400 degrees F and net overburden pressures up to 10,000psig. Fluid can be injected accurately at rates psig. Fluid can be injectedaccurately at rates between 0.1 and 50 mL/min with unlimited throughput. Fluidpermeabilities at different locations along a core up to 3 ft in length can bemonitored continuously. All pumping, valve switching, fluid sampling, andpumping, valve switching, fluid sampling, and data acquisition operations arecomputer controlled.

Core Test Procedures
A 1 × 1 × 3 ft Berea sandstone block, with about 100 md permeability, wasobtained from the Cleveland Quarry in Ohio. The material contains about 85%quartz, 3% carbonate, 3% potassium feldspar, 4% kaolinite, 2% illite, andtraces of chlorite, smectite, and sodium feldspar. Cylindrical cores measuring1 in. in diameter and 3 ft long were cut from this sandstone block in filtered1% CaCl2 solution. The cores were cut parallel to the stratigraphic beddingplanes to minimize permeability heterogeneity along the core's permeabilityheterogeneity along the core's length. Pressure-tap connections were placedthrough a thick-walled Teflon sleeve at locations 4, 8, 15, 21, and 27 in. fromthe inlet end in this study. The long core was then sealed inside this Teflonsleeve. The sleeved multitap 3-ft core was placed in the specially designed Hassler cell of the acid permeameter. The cell overburden pressure wasmaintained at 3,000 psi, and pressure was maintained at 3,000 psi, anddischarge pressure was kept at 1,000 psig to prevent the evolution of gaseousreaction products. For most tests, the temperature products. For most tests, the temperature was maintained at 150 degrees F and the flow rate held at 10mL/min. This flow rate is considered typical in the near-wellbore region of aperforated completion. For acid tests, brine, HCl, HF-containing acids, andbrine were injected sequentially. Temperature, pressures, and flow rates werecontinuously pressures, and flow rates were continuously recorded and were usedto calculate permeabilities at various locations along the core. permeabilitiesat various locations along the core. The results of the acid tests wereinterpreted on the basis of liquid permeability changes. The cores werepreacidized with 15% HCl to remove all carbonates.
P. 98