Microsoft Word C040531e doc Reference number ISO 13503 5 2006(E) © ISO 2006 INTERNATIONAL STANDARD ISO 13503 5 First edition 2006 07 01 Petroleum and natural gas industries — Completion fluids and mat[.]
Objective
This article discusses a standardized test procedure designed to evaluate the long-term conductivity of proppants under laboratory conditions using a standard apparatus and test setup The primary goal is to establish a consistent method for assessing proppant conductivity over time, ensuring reliable comparisons across samples It is important to note that this procedure is intended solely for laboratory evaluation and not for determining absolute proppant pack conductivities in downhole reservoir conditions Factors such as fines, formation hardness, resident fluids, and other variables are beyond the scope of this standardized testing method, recognizing its limitations in simulating complex reservoir environments.
Discussion
During the ISO 13503 procedure, a closure stress is applied across the test unit for approximately 50 hours to allow the proppant sample bed to reach a semi-steady state Throughout this process, measurements including proppant pack width, differential pressure, temperature, and flow rates are recorded at each stress level These data are then used to calculate the proppant pack’s permeability and conductivity, ensuring accurate assessment of its performance under stress conditions.
Multiple flow rates are employed to verify transducer performance and identify the Darcy flow regime at each stress level, with average data reported A minimum pressure drop of 0.01 kPa (0.002 psi) is recommended to ensure accurate measurements; if not achieved, flow rates should be increased Under specified flow rates and temperature conditions, no significant non-Darcy flow or inertial effects are observed After completing flow rate tests at each closure stress in all cells, the stress is incrementally increased, allowing approximately 50 hours for the proppant bed to reach a semi-steady state before applying additional flow rate tests to determine proppant pack conductivity This procedure continues across all targeted closure stresses and flow rates Ensuring single-phase flow is critical for obtaining precise conductivity measurements during the testing process.
This article emphasizes the importance of reporting test condition parameters, including test fluid, temperature, loading, sandstone type, and testing duration, at each stress level It highlights that documenting long-term conductivity and permeability data is essential for accurate evaluation Additionally, varying test conditions can be employed to assess different proppant characteristics, which may lead to diverse results, thereby providing a comprehensive understanding of proppant performance under different scenarios.
Test fluid
The test fluid consists of a 2% by mass potassium chloride (KCl) solution in deionized or distilled water filtered to a minimum of 7 micrometers The potassium chloride used must be at least 99.0% pure by mass to ensure the accuracy and reliability of testing results.
Sandstone
Ohio sandstone cores should measure between 17.70 cm and 17.78 cm (6.96 in to 7.00 in) in length, with widths ranging from 3.71 cm to 3.81 cm (1.46 in to 1.50 in), and a minimum thickness of 0.9 cm (0.35 in) The ends of the cores must be rounded to ensure proper fit into the test unit, and the parallel thickness should be maintained within ± 0.008 cm (± 0.003 in) to meet quality standards.
7 Long-term conductivity test apparatus
Test unit
The test unit features a linear flow design with a proppant bed area of 64.5 cm² (10 in²), ensuring precise flow measurement Detailed illustrations, such as Figure C.1, demonstrate the test unit's construction and how multiple cells can be stacked for scalable testing Materials used for the pistons and test chambers include corrosion-resistant options like 316 stainless steel (ISO 3506-1, Grade A4), Monel, or Hastelloy, ensuring durability under testing conditions Filters within the test unit are typically constructed from Monel wire cloth with a 150 µm (100 US mesh) opening, capable of retaining particles larger than 114 µm to prevent debris passage and ensure accurate results.
Hydraulic load frame
The hydraulic load frame must have a minimum capacity of 667 kN (150,000 lbf) to effectively support testing requirements To ensure uniform stress distribution, the platens should be parallel, promoting accurate and consistent results A four-post design is recommended for the hydraulic load frame, as it minimizes warping that could affect the test cell's integrity Additionally, each post should have a minimum diameter of 6.35 cm (2.5 inches) to enhance stability and load capacity.
The hydraulic pressurization source shall be capable of holding any desired closure stress [± 1,0 % or
The hydraulic load frame must be capable of applying a pressure of at least 345 kPa (50 psi) for 50 hours, with the greater of the two values being maintained It should accommodate a loading rate change of 4,448 N/min (1,000 lbf/min) or 690 kPa/min (100 psi/min) on a 64.5 cm² (10 in²) test cell Additionally, a calibrated electronic load cell is essential for accurately calibrating the stress between the hydraulic ram and the opposing platen of the load frame, ensuring precise load application.
Pack width measurement device(s)
Pack width measurements must be taken at both ends of the test unit using a measuring device with an accuracy of 0.0025 cm (0.001 inches) or better An example of width slats for measuring pack widths is illustrated in Figure C.4 Accurate measurement of pack width ensures quality control and compliance with packaging standards Using precise measuring tools is essential for reliable results in packaging assessments.
Test fluid drive system
Certain constant-flow-rate pumps, such as chromatographic pumps, are suitable for this application, but pulsation dampening may be necessary using devices like piston or bladder accumulators To ensure accurate measurements, pressure fluctuations during differential pressure and flow rate assessments—crucial for conductivity calculations—must be kept below 1% Each laboratory should identify the most effective pulsation dampening technique for their systems Additionally, large pressure spikes may indicate pump issues or trapped gas, requiring correction prior to data recording to ensure reliable results.
1) Monel and Hastalloy are examples of suitable products available commercially This information is given for the convenience of users of this part of ISO 13503 and does not constitute an endorsement by ISO of this product
Differential pressure transducers
Differential pressure transducers with a range of 0 kPa to 7 kPa (0 psi to 1,0 psi) are satisfactory The transducer shall be capable of measuring the differential pressure to ± 0,1 % of full scale.
Back-pressure regulators
The back-pressure regulator shall be capable of maintaining a pressure of 2,07 MPa to 3,45 MPa (300 psi to
When applying stress to the cells, it is essential to account for back-pressure For instance, if the back-pressure is 3.45 MPa (500 psi), the applied stress should be increased by this amount, totaling 3.45 MPa (500 psi), to compensate for the outward pressure exerted from the pistons Properly considering back-pressure ensures accurate stress measurements and reliable test results.
Balance
The balance shall be capable of accommodating a minimum capacity of 100 g with a precision greater than
Oxygen removal
To ensure accurate conductivity testing and minimize equipment corrosion, the test fluid’s oxygen content should be reduced to mimic reservoir fluids De-oxygenation is effectively achieved using a two-reservoir system, which helps maintain the fluid’s quality and integrity during testing.
The first reservoir contains fluid used for oxygen removal It is connected to a nitrogen gas supply that bubbles nitrogen through the fluid at low pressure, below 103 kPa (15 psi), and at a controlled, low rate The nitrogen is first filtered through an oxygen and moisture trap, such as the Agilent Model OT3-4, which effectively removes oxygen to achieve a low oxygen concentration, ensuring the purity of the process.
An equivalent system operates by passing nitrogen through heated copper shavings at 370°C (698°F), where copper reacts with trace oxygen to form copper oxide, effectively removing oxygen from the gas An indicating trap, such as the Chrom Tech, Inc oxygen trap (part #10T-4-HP), is used after the oxygen-removal process to provide visual confirmation of oxygen removal; when the trap becomes oxygen-saturated, both the trap and the system’s filters should be replaced to ensure continued efficiency.
The second reservoir holds the oxygen-free fluid; this is the supply reservoir for the pumping system
All fluids in each reservoir are held in sealed, inert-gas pressurized containers to eliminate oxygen contamination from the air.
Temperature control
Ensure that the test cell and proppant pack are maintained at the specified temperature within ± 1 °C (± 3 °F) to ensure accurate results The temperature for test conditions should be measured precisely at the temperature port of the conductivity cell, as shown in Figure C.1.
This temperature is essential for determining fluid viscosity using Table C.1 The thermocouple assembly consists of a temperature-control device and a data-acquisition system or an equivalent setup The temperature control should feature programmable PID controllers with self-tuning capabilities to accommodate various temperature conditions and flow rates, ensuring accurate and efficient viscosity measurements.
A temperature of 121 °C (250 °F) is employed in the test for ceramics and resin-coated proppants and 66 °C
(150 °F) for naturally occurring sands The temperature for the silica-saturation vessel (see Annex B) should be 11 °C (20 °F) above testing temperature of 66 °C (150 °F) for naturally occurring sands Sand 20 °C
Resin-coated and ceramic proppants are heated to approximately 121 °C (250 °F) to ensure that the fluid is saturated with silica before reaching the cell It is crucial to maintain the proper temperature of the fluid arriving at the cell for accurate testing Conducting tests with different fluids or temperatures can provide valuable insights into proppant pack conductivity.
The Agilent Model OT3-4 is a commercially available product that serves as a suitable example for users referencing ISO 13503 Please note that this information is provided solely for convenience and does not constitute an endorsement by ISO of this specific product.
Chrom Tech, Inc part # 10T-4-HP is an example of a commercially available, suitable product that meets the requirements of ISO 13503 This information is provided for user convenience and does not constitute an endorsement by ISO of this specific product.
Silica saturation and monitoring
Maintaining a silica-saturated solution flowing through the proppant pack is essential to prevent the dissolution of Ohio sandstone and proppant To ensure this, a high-pressure cylinder with a minimum volume of 300 ml is required, facilitating the continuous circulation of the saturated solution and preserving the integrity of the formation.
10 ml/min flow rate capacity, such as a Whitey sample cylinder 316L-HDF4 4) , or equivalent equipped with 0,635 cm (0,25 in) female pipe ends is needed For equipment setup, see Annex B
Pressure indicators and flow rates
Pressure indicators in the test fluid-flow stream with back-pressure must be calibrated initially and rechecked before each test to ensure accuracy Constant-flow-rate pumps should be tested at various flow rates using appropriate flow meters or precise measurement tools such as balances, containers, and stopwatches, with back-pressure applied High- and low-pressure transducers must be zeroed prior to each test run, and only the linear, repeatable portion of the transducer's range should be used for reliable measurements.
Zero pack width measurement
To accurately measure the width of the proppant pack, it is essential to consider variations in sandstone thickness, the compressibility of sandstone, as well as the compression and thermal expansion of the metal These factors ensure precise assessments in hydraulic fracturing and reservoir stimulation Proper accounting for these variables enhances measurement accuracy and improves overall operational efficiency.
Using callipers, measure and record the thickness of the cores and metal shims, then mark the core width on its face with a pencil Place two cores in each cell and ensure they are matched so the combined thickness at their ends is consistent Cores exceeding 0.008 cm (0.003 inches) from parallel should be rejected If the bottom core varies from end to end, the top core must be offset to ensure the total core thickness at each end remains equal.
To accurately assess the width adjustment factor or zero pack width for Ohio sandstone and square rings, measure the vertical dimension of the complete test unit—including pistons, shims, and sandstone cores—without proppant, at each closure stress and testing temperature These measurements should be precise within ± 0.0025 cm (± 0.001 in) Begin by determining the initial zero width by measuring the vertical dimension of the pistons, shims, and sandstone cores alone This baseline measurement is then subtracted from the total measured dimensions during testing to calculate the actual width of the proppant pack at each closure stress and temperature, ensuring accurate assessment for each lot and cell.
Pistons for the baseline cell(s) should be labeled sequentially according to their stacking order Place the two matched sandstone cores within the cell, and if necessary, proceed with stacking additional cells following the configuration illustrated in Figure C.1.
8.2.2.2.4 Heat the cells to the temperature at which the test will be run Ramp closure stress at a rate of
To accurately measure the piston, use telescoping gauges and digital callipers or equivalents to determine the distance from the width slat to the bottom plate and from the width slat to the top press plate or the other width slat All measurements should be taken twice, with both readings falling within a ±0.0050 cm tolerance, ensuring precision Additionally, perform a second measurement 30 minutes after the initial reading to monitor stability Continue taking measurements until the system reaches a steady-state, indicating consistent and reliable results for quality assessment.
The Whitey sample cylinder made of 316L-HDF4 is a commercially available example of a suitable product This information is provided to assist users of ISO 13503 specifications and does not imply any endorsement by ISO of this particular product.
Six measurements should be taken, with at least three recorded, ensuring consistency within 1% of each other The final measurement must be documented, accounting for sandstone core compression and metal expansion due to pressure and temperature changes These values are essential for calculating proppant pack widths, as outlined in Clause 12 Continue taking measurements at specified stress intervals until reaching the maximum stress, following the procedures outlined in Clause 12.
Determination of cell width
To accurately measure the inside of the cell, take readings at three points—two beside the high and low pressure ports and one near the middle port—using telescoping gauges and digital calipers Average these three measurements to obtain a precise cell width To calculate the required amount of proppant, multiply the average cell width by the desired proppant load and divide by 38.1 mm, as demonstrated in the example for a 9.76 kg/m² (2.00 lb/ft²) loading.
EXAMPLE (38,35 mm + 38,40 mm + 38,37 mm)/3 = 38,37 mm
A two pound per square foot loading requires (63,00 g/38,10) × 38,37 = 63,44 g of proppant.
Hydraulic load frame
Regular calibration of the load cell is essential and should be performed at least once annually or sooner if long-term conductivity measurements become unreliable Using a load cell is preferred over hydraulic pressure gauges for accurately determining the closure stress applied to the test cell In certain systems, the load cell integrates into the setup and requires calibration by an external calibration source to ensure precise measurements.
Hydraulic load frame
Regular testing of the hydraulic system, including lines, fittings, and pumps, is essential to detect leaks and ensure proper operation This involves placing a high-strength block with at least 64.5 cm² (10.0 in²) surface area between the platens at maximum load, shutting in the system, and monitoring pressure or load changes over 30 minutes A variation greater than ±2% of the maximum reading indicates potential issues; if so, all lines and fittings should be inspected for leaks If no external leaks are found, internal leaks in the control valve or hydraulic ram may be the cause.
Test fluid system
The initial complete test fluid system, including the pump, lines, fittings, and conductivity test unit, must be inspected for leaks before use During the leak test, the conductivity test unit should contain at least a monolayer of proppant material to ensure accurate results Proper leak testing is essential to maintain system integrity and performance.
NOTE With no proppant between the platens, neither the square seal rings nor the downstream equipment can be tested
Apply a closure stress exceeding 3.45 MPa (500 psi) to the conductivity unit and flow fluid through the system under a back-pressure ranging from 2.07 MPa to 3.45 MPa (300 psi to 500 psi) Once the system is closed, ensure that the pressure remains stable, with a change of no more than 0.1 kPa (0.01 psi) over a 5-minute period Additionally, inspect all lines and fittings thoroughly to verify proper sealing and integrity.
10 Procedure for loading the cells
Preparation of the test unit
After selecting the cores, apply transparent tape to both the top and bottom to prevent sealant from sticking, then trim excess tape with a sharp knife Ensure all portholes inside the cells and the top of the bottom piston are covered with transparent tape Finally, record the average width of the sandstone core for accurate measurement and analysis.
10.1.2.2 With a spatula, apply a thin layer of high-temperature room temperature vulcanizing (RTV) silicone adhesive sealer around the sides of the cores Allow the RTV to cure
An alternative method for preparing cores involves placing them in the test unit, ensuring the bottom piston is leveled within 0.13 mm (0.005 inches) and securing it with set screws The inside of the cell should be lightly sprayed with silicone lubricant to facilitate easy removal Marked and taped cores can be stacked up to four at a time within the cell, provided RTV or an equivalent molding compound is applied around each core’s edges before placement Allow sufficient time for the RTV to cure, ensuring proper adhesion and stability during testing.
To ensure proper setup, place the top piston into the conductivity cell and position it within the press, applying a closure pressure between 0.3 MPa (50 psi) and 1 MPa (150 psi) Attach the heat strips and heat the assembly to 66 °C (150 °F) for one hour to facilitate testing After the heating process, remove the core slabs and carefully trim any excess RTV from the face of the core Verify that there are no chips or cracks in the core to maintain test integrity.
NOTE If no heat is applied, the RTV cures in about 24 h.
Cell setup
The cells are arranged in the same order as the measured zero pack widths, as described in section 8.2.2 To secure the sandstone core, blocks can be used to hold the cell in place, ensuring the core is approximately 0.02 mm (0.0008 in) or just below the differential pressure ports This is achieved by placing one metal shim and the sandstone core without RTV on the piston within the cell Once the piston is positioned correctly, the set screws should be tightened to lock the cell in place The shim and sandstone core are then removed to install the square ring, taking care to protect its integrity.
Measure and record the thickness of the metal shim, accounting for any differences between shim thicknesses as specified in section 8.2.2.1 Place the shim at the bottom of the cell before applying a thin, even layer of RTV around the edge of the selected core, ensuring no RTV gets on the core faces Smooth the RTV with a spatula to create a uniform surface Remove the bottom tape from the core and insert it into the cell until it contacts the shim Apply RTV around the core-to-cell interface, using a cotton swab to push RTV into any cracks, then remove excess RTV and apply the top tape.
Screens are essential for preventing solids from escaping the proppant pack or clogging the ports during operation Use 150-mesh (100 US mesh) Monel or equivalent screens in all ports, including entry, exit, and differential-pressure ports, to ensure effective filtration It is important to replace these screens after each run, as they can become plugged with crushed proppant, compromising their functionality and the overall integrity of the operation.
10.2.4 Calculating the quantity of proppant
Conductivity may be tested on a volume equivalent to 0,64 cm (0,25 in) unstressed pack width or on a mass per unit cell surface area, such as 9,76 kg/m 2 (2,00 lbm/ft 2 )
Calculate the desired amount of proppant material using one of the example calculations described below: a) Mass per unit area, expressed in kilograms per square metre:
Load the desired amount of proppant, which can be calculated as given in Equation (1):
M p is the proppant mass, expressed in grams;
C is the proppant loading, expressed in kilograms per square metre
6,452 = 0,006 452 m 2 × 1 000 g/kg, where 0,006 452 m 2 is the surface area of the cell The exact amount of proppant varies depending upon your cell width (see 8.3)
The unstressed proppant pack width can be approximated as given in Equation (2):
W f is the proppant pack width, expressed in centimetres;
C is the proppant loading, expressed in kilograms per square metre; ρ is the proppant bulk density, expressed in grams per cubic centimetre [16] b) Unstressed proppant pack width equal to 6,35 mm (0,25 in)
Load the test cell with 41,0 ± 0,1 cm 3 of proppant material The approximate mass of the required proppant material can be calculated as given in Equation (3):
M P is the proppant mass, expressed in grams; ρ is the proppant bulk density, expressed in grams per cubic centimetre [16]
41,0 is 64,52 cm 2 (10,0 in 2 ) times 0,635 cm (0,25 in) pack width The exact amount of proppant varies depending upon your cell width (see 8.3)
10.2.5 Loading proppant in the cell(s)
10.2.5.1 Weigh a representative sample based on one of the above calculations
10.2.5.2 Split the sample into four units Pour one-forth of the sample as evenly as possible into the cell Continue this procedure until all four split samples are added
To ensure an even proppant layer in the test unit, use a levelling device (see Figure C.6) and make progressively deeper passes to level the proppant without packing it by vibration or tamping, which can cause material segregation Confirm that the proppant is level against the walls of the cell for accurate test results.
Apply a thin layer of RTV silicone around the edge of the preset core, ensuring proper coverage as per section 10.1.2 Use a spatula or similar tool to smooth and evenly distribute the RTV, taking care not to get any on the faces of the core, to ensure a clean and effective seal.
To assemble the core and cell, remove the bottom tape from the core and slide it into the cell evenly Seal the interface by applying RTV around the edge of the core-to-cell connection, then use a cotton swab to ensure the RTV penetrates any cracks, ensuring a secure and airtight fit.
Using digital callipers, measure the depth of the core at all four corners to ensure it is level within 0.1 mm (0.004 in) Confirm accuracy and make any necessary adjustments Finally, remove the tape from the top of the core to complete the process.
Multiple cells can be placed in a press, with blocks and shims used to precisely adjust the cell height within 0.1 mm (0.004 in) Follow the same loading procedure for the cells as outlined in section 10.2.5, ensuring set screws are securely tightened for optimal performance.
11 Loading cell(s) in the press
11.1 Place the test unit between the platens of the load frame
Apply 345 kPa (50 psi) pressure to the cell stack and use a square angle to verify its perpendicularity to the platens, making any necessary adjustments for precise alignment Then, gradually increase the pressure from 345 kPa (50 psi) to 3.45 MPa (500 psi) at a controlled rate of approximately 690 kPa/min to ensure proper testing conditions.
Begin the flow through the cells while inspecting for leaks around the pistons and connections, ensuring the integrity of the test If any leakage is observed around the pistons, immediately terminate the test and reload the unit with new material Verify the uniformity of the proppant pack by measuring the bed width at both ends; if there is a discrepancy of 5% or more, the test must be halted and the cell reloaded with fresh material to ensure accurate results.
To ensure accurate readings, remove air from the cells and transducer lines by thorough flushing and bleeding Flush the lines for at least one minute until no air bubbles are visible Zero the transducers according to the manufacturer's specifications with no flow to calibrate the system properly.
Initial absolute stress of 6.89 MPa (1 kpsi) should be applied for at least 12 hours and up to 24 hours at the specified temperature, with back-pressure maintained between 2.07 MPa (300 psi) and 3.45 MPa (500 psi) Once this initial stress and duration are achieved, the stress should be increased to 13.79 MPa (2 kpsi) The applied stress to the proppant pack must be held for 50 hours ± 2 hours, as holding for less than 48 hours does not qualify as long-term conductivity Subsequent stress increments should be in 13.79 MPa (2 kpsi) steps, with ramp rates of 689 kPa/min ± 34 kPa/min (100 psi/min ± 5 psi/min) to ensure accurate stress application.
Conductivity for naturally occurring sands shall be measured at pressures of 13.79 MPa (2 kpsi), 27.58 MPa (4 kpsi), and 41.37 MPa (6 kpsi) For ceramic and resin-coated proppants, measurements should be taken at multiple pressures, including 13.79 MPa, 27.58 MPa, 41.37 MPa, 55.16 MPa (8 kpsi), and 68.95 MPa (10 kpsi) Additional stress levels are optional but can be included for comprehensive testing.
Test flow rates are established based on the pressure drop between the pressure ports, with the initial flow rate set at 2 ml/min or a minimum of 0.01 kPa (0.002 psi) To ensure accurate and statistically reliable data, a minimum of five data points should be collected, and an average permeability (as described in Clause 13) must be reported over the specified measurement range.
2 ml/min to 4 ml/min or at least 0,01 kPa to 0,03 kPa (0,002 psi to 0,004 psi) Report stresses
Pack widths must be measured at each stress level and calculated to account for sandstone core compression and metal expansion, as outlined in section 8.2 Prior to each measurement, differential pressure transducers should be zeroed to ensure accurate readings.
13 Calculation of permeability and conductivity
13.1 Equations (4) to (8) shall be used to calculate the permeability of proppant packs to liquid under laminar
/ 100 (∆ ) k= àQL ⎡⎣ A P ⎤⎦ (expressed in SI units) (4)