LABORATORY DETERMATION OF PERMEABILITY Some slides in this section are from NExT PERF Short Course Notes, 1999. Some slides appear to have been obtained from unknown primary sources that were not cited by NExT. Note that some slides have a notes section.

LABORATORY METHODS FOR DETERMINING ABSOLUTE PERMEABILITY Plug Slab Taken for Photography Description Archival Most Common Full Diameter When the core material arrives in the core laboratory, the most common practice is to cut into two pieces (one-third and two-third pieces). The one-third section is slabbed, often photographed using regular and ultraviolet light, described by geologists, and eventually archived for future study or review. The two-thirds section is the part that is used to cut core plugs for routine and special core analysis. Core plugs for routine analysis might be selected on a one per foot or one per two foot basis. Core plugs for special core analyses may not be selected until the core is sufficiently described. If the reservoir is very heterogeneous, core testing will be conducted using the whole-core or full-diameter pieces of the whole-core. Heterogeneous Whole Core Heterogeneous

WHOLE-CORE METHOD Uses selected pieces from the full or whole core Core sizes 2 1/2 to 5 1/2 inches in diameter Several inches to several feet long Most applicable approach for very heterogeneous formations. Additional expense limits the practical number of tests. Whole core method is similar to perm-plug method except for the core samples are larger. The sample size can be 4 to 9-inch pieces of the core (sometimes called full-diameter core) to a 3-foot section of the core. The use of larger samples yields more accurate values of permeability for all reservoirs but especially for more heterogeneous reservoirs. Cores from complex lithologies such as vugular carbonates (i.e., limestones and dolomites) require the use of whole-cores to get representative measurements. In addition to higher expense, the whole-core section must be selected before a detailed core description is available, making it more difficult to select the zones of interest.

CORE PLUG METHOD Most commonly applied method. Uses small cylindrical core samples 3/4 inch to 1 1/2 inch diameter 1 to a few inches long May not apply to heterogeneous formations. Since small plugs are used, the permeability determined may not be representative of the entire reservoir. In addition, extra care is usually taken to minimize any sampling bias when the core plugs are cut. Best suited for homogeneous/uniform reservoirs but the method is commonly used because it is less expensive than whole-core analysis. In addition, geologists generally prefer to leave a one-third slabbed section intact rather than taking out whole sections. Using only core plugs may be inaccurate for very heterogeneous reservoirs such as vugular carbonates. For these types of reservoirs, whole core analyses are preferable.

~1 ft kH kV Suitable Unacceptable ? Or Full- Diameter Full `4” - 9” Whole Core Analysis (2-3 ft) Fracture k and ? Matrix Only III IV V I IIa IIb Different Lithologies Require Careful Selection of Suitable Core Plugs or Require Whole-Core Analyses Examples of Recommended plugging locations Lithology Type I - Homogeneous formation Lithology Type IIa - Stratified zone with uniform layers, layers are thicker than core plug diameter. Lithology Type IIb - Same as IIa except for thinner layers. Cut as thick of core plugs as possible, parallel to bedding. Lithology Type III - Uniform, equally distributed heterogeneities. Whole-core analysis recommended. Lithology Type IV - Grossly heterogeneous. Whole-core analysis recommended. Lithology Type V - Fractured Core. Cut core plugs to determine properties of matrix. Flow properties of total system must usually be determined from welltesting or other analyses of flow performance.

WHOLE CORE Whole Core Photograph, Misoa “C” Sandstone, Venezuela Photo by W. Ayers

LAB PROCEDURE FOR MEASURING PERMEABILITY Cut core plugs from whole core or use sample from whole core Clean core and extract reservoir fluids, then dry the core Flow a fluid through core at several flow rates Record inlet and outlet pressures for each rate

PERM PLUG METHOD LIQUID FLOW Measure inlet and outlet pressures (p1 and p2) at several different flow rates Graph ratio of flow rate to area (q/A) versus the pressure function (p1 - p2)/L For laminar flow, data follow a straight line with slope of k/ At very high flow rates, turbulent flow is indicated by a deviation from straight line through origin Core must be saturated with one liquid only in the pores (SL=1). Permeability measured is absolute permeability.

Laboratory Determination of Absolute Permeability, Liquid Flow Slope = k m (p1 - p2) L q A Darcy Flow Non-Darcy Flow Determining permeability using a gas is similar to the method using a liquid. However, some extra terms are required to account for the changes in gas properties. Since gas has very low viscosities relative to liquids, it is often preferable because it helps speed up the core testing. For low-permeability rocks, the time reduction can be very significant.

ISSUES AFFECTING LABORATORY MEASUREMENTS OF PERMEABILITY Core Handling, Cleaning, and Sampling Fluid-Rock Interactions Pressure Changes Rock Heterogeneities (Fractures) Gas Velocity Effects (Klinkenberg)

CORE HANDLING PROCESSES AFFECT PERMEABILITY MEASUREMENTS Cleaning Drying (Clay Damage) Storage (Freezing) Sampling In addition to the coring processes, there are many processes that take place between the time the core is recovered at the well surface and the time it arrives in the laboratory, particularly in some remote international locations. All of these processes can affect the core properties. Good planning and precautions are required to minimize these changes. The concern about these handling processes usually increases as the importance of the measurements increase such as in lawsuits, equity hearings, or in fields where enhanced recovery methods are being considered. Some drying processes can cause irreversible changes in clay morphology. “Lay-down” illite can cause a factor of 10 or more change in permeability.

FLUID-ROCK INTERACTIONS AFFECT MEASUREMENTS OF PERMEABILITY Fresh water may cause clay swelling, reducing permeability Tests may cause fines migration, plugging pore throats and reducing permeability Reservoir or synthetic reservoir fluids are generally preferred Fresh water can alter the reservoir clay properties, affecting the measurement of permeability. The use of fresh water should be avoided at the rig site and in the laboratory. Sometimes, core tests are conducted with gradually less saline water to determine a formation’s sensitivity to fresh or brackish completion fluids. Permeability tests are often monitored while several core pore volumes are flowed through the core. This test procedure will show whether “fines” migration is occurring, indicating a reduction in permeability. By reversing the flow direction, the permeability may return to near its original value before plugging occurs again. Reservoir or synthetic reservoir fluids are generally preferred to minimize rock-fluid interactions. However, even these fluids can lead to problems with salt precipitation, paraffin deposition, and emulsions.

PRESSURES AFFECT LABORATORY MEASUREMENTS OF PERMEABILITY Core alterations resulting from loss of Confining Pressure during core recovery Core testing may be conducted by applying a range of net overburden pressures As the core is brought to surface, the confining pressure decreases. As this occurs, the gas and fluids expand and may be expelled. This expansion process can alter the core’s properties and stress relief fractures can develop (porosity is generally affected less than permeability). In the laboratory, after the core is cleaned and dried or resaturated with a control fluid, permeability testing can be conducted. Measuring permeabilities at net overburden pressures that simulate reservoir conditions are more expensive but may be required to get accurate measurements. However, even results after restoring net overburden pressure may include hysteresis.

CORE HETEROGENEITIES AFFECT MEASUREMENTS OF PERMEABILITY Naturally-fractured reservoirs Core plugs represent matrix permeability Total system permeability (matrix + fractures) is higher Core Mineralogy problems (Salts, Gypsum) Rocks which contain natural fractures in situ (i.e., at reservoir pressure) frequently separate along the planes of natural weakness when cored. Laboratory measurements give “matrix” permeability which is lower than in-situ permeability.

EXAMPLE CORE REPORT The next three pages are a sample core report from a full core in Texas. Core plugs were obtained, about one per foot and sent to Core Laboratories for a routine analysis that included porosity, horizontal and vertical permeability, fluid saturations, grain density, and a rough description of the core mineralogy.

Laboratory Analysis of Gas Flow Mean Pressure Method Beginning from (zg )=Constant Equation for Linear Flow From Real Gas Law, we can evaluate q at any pressure where, and, L qg A 1 2 Remember, zsc=1, (x+y)(x-y)=(x2-y2)

Laboratory Analysis of Gas Flow Mean Pressure Method canceling terms and substituting mean pressure The Mean Pressure Method is commonly used to analyze laboratory flow (low pressure) flowing temperature is isothermal Mean flow rate is volumetric rate at point in core where pressure is mean pressure value