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Core analyses in tight gas reservoirs
Obtaining and analyzing cores is crucial to the proper understanding of any layered, complex reservoir system. To obtain the data needed to understand the fluid flow properties, the mechanical properties and the depositional environment of a specific reservoir requires that cores be cut, handled correctly, and tested in the laboratory using modern and sophisticated laboratory methods. Of primary importance is measuring the rock properties under restored reservoir conditions. The effect of net overburden pressure (NOB) must be reproduced in the laboratory to obtain the most accurate quantitative information from the cores.
To provide all the data needed to characterize the reservoir and depositional system, a core should be cut in the pay interval and in the layers of rock above and below the pay interval. Core from the shales and mudstones above and below the pay interval help the geologist determine the environment of deposition. Knowing more about the deposition allows the reservoir engineer to better estimate the morphology and size of the gas-bearing reservoir layers. Also, mechanical property tests can be run on the shales to determine estimates of Poisson’s ratio and Young’s modulus. Additional tests can be run to measure the shale density and the sonic travel time in the shale to assist in the analyses of the density- and sonic-log data.
Handling and testing cores
After cutting the cores in the field, it is important to handle the core properly:
- The core should not be hammered out of the barrel. It should be pumped out.
- Once the core is laid out on the pipe racks, it should be wiped with rags to remove the mud (do not wash with water), then described as quickly as possible. Bedding features, natural fractures, and lithology should be described foot by foot. Permanent markers should be used to label the depth of the core and clearly mark the up direction on the core.
- As quickly as feasible, the core should be wrapped in heat shrinking plastic, then sealed in paraffin for the trip to the core analysis laboratory.
- Precautions should be taken to minimize alteration of the core properties while retrieving and describing the core in the field.
Once in the laboratory, the core is unwrapped and slabbed, and plugs are cut for testing. Normally, a core plug should be cut every foot in the core, trying to properly sample all the rock—not just the cleaner pay zones. Routine core analyses can be run on these core plugs. Once the routine core analyses are completed, additional core plugs are cut for special core analyses. Sometimes samples of whole core are used for testing. Both the routine and the special core analyses are required to calibrate the openhole logging data, and to prepare the data sets required to design the optimum completion. The core plugs must also be treated with care. For example, if a core plug from a shaly sand is placed in a standard oven, it is likely that the clays in the pores will be altered as they dry out. A more accurate core analysis usually is achieved if the core plugs are dried in a humidity controlled oven in which the free water is evaporated, but the bound clay water is not affected.
Routine core analyses
Routine core analyses should be run on core plugs cut every foot along the core. Routine core analyses should consist of measurements of:
- Grain density
- Porosity and permeability to air (both unstressed and stressed)
- Cation exchange capacity
- Fluid saturations analysis
In addition, each core plug should be described in detail to understand the lithology and grain size and to note any natural fractures and other details that could be of importance to the geologist, petrophysicist, or engineer.
The porosity is used to determine values of gas in place and to develop correlations with permeability. The grain density should be used to determine how to correlate the density log values and to validate any calculation of lithology from log data. The cation exchange capacity can be used to determine how much electric current can be transmitted by the rock rather than the fluid in the pore space. The cation exchange capacity must be measured in the laboratory, using samples of rock, and is a function of the amount and type of clay in the rock. Saturation analysis measures the amount of water, oil, and gas in the core plugs in the laboratory. Saturation analysis can be misleading in rocks that are cored with water based mud because of mud filtrate invasion during the coring process and problems that occur with core retrieval and handling prior to running the laboratory tests. However, the values of water saturation from the core analysis of cores cut with an oil-based mud can be used to calibrate the log data and to estimate values of gas in place in the reservoir.
The measurements of porosity and permeability are a function of the net stress applied to the rock when the measurements are taken. For low porosity rock, it is very important to take measurements at different values of net stress to fully understand how the reservoir will behave as the gas is produced and the reservoir pressure declines. The data in Fig. 1 illustrate how the values of porosity changed in Travis Peak sandstone cores when the cores were tested both at low net stress and at simulated net overburden pressure (NOB). Notice that the measurements of porosity are one to two porosity units less when measured under net overburden pressure than when measured under minimal stress.
Fig. 2 illustrates the effect of net overburden pressure on the measurement of air permeability on Travis Peak cores from a well in east Texas. For high permeability (10–100 md) core plugs, the permeability under the original overburden pressure is slightly less than the value of unstressed permeability for that same core plug. However, as the permeability of the core plugs decrease, the effect of NOB on the core plug increases substantially. For the core plugs that had values of unstressed permeability of around 0.01 md, the values of permeability under NOB were about an order of magnitude lower, or 0.001 md. The lower permeability rocks are the most stress sensitive because the lower permeability core samples have smaller pore throat diameters than the higher permeability rocks. As overburden stress increases, the diameter of the pore throat decreases. Because the permeability of a rock is roughly proportional to the square of the diameter of the pore throat, the permeability reduction in low permeability rocks is much more dramatic than in high permeability rocks. In other words, if you make a big pore throat slightly smaller by adding stress to the rock, the permeability is not reduced by much. If you make a very small pore throat even smaller by adding stress to the rock, the permeability is reduced substantially. The reduction is typically an order of magnitude or more, as illustrated in Fig. 2.
After the values of porosity and permeability under NOB conditions are measured in the laboratory, the values can be correlated. Fig. 3 illustrates a typical correlation of permeability at NOB vs. porosity at NOB for a tight gas reservoir.
The data in Fig. 3 came from the Travis Peak four-well dataset. The data in Fig. 3 can be used to estimate values of permeability from estimated values of porosity. For example, once the values of porosity are determined from openhole log data, the correlation in Fig. 3 can be used to estimate permeability for the same rock type in the Travis Peak formations. However, it should be remembered that these estimates are from routine core analyses, which means the core has been tested dry with no water in the core. If similar measurements are made at connate water saturation, the permeability in the core is further reduced, maybe by a factor of 2 or even an order of magnitude in some cases. As such, in tight gas reservoirs, it is often found that in-situ permeabilities to gas are 10 to 100 times lower than gas permeabilities measured at ambient conditions on dry core plugs cut from whole core. If cores come from a percussion sidewall device, the core plugs are typically altered, and the values of permeability under unstressed conditions can be even more optimistic.
Special core analyses
To fully understand the properties of tight gas formations, special core analyses must be run on selected core plugs to measure values of gas permeability vs. water saturation, resistivity index, formation factor, capillary pressure, acoustic velocity, and the rock mechanical properties. The values of resistivity index and formation factor are used to better analyze the porosity and resistivity logs. The acoustic velocity can be used to better estimate porosity and to determine how to estimate the mechanical properties of the rock from log data. The mechanical properties are measured and correlated to log measurements and lithology. The capillary pressure measurements and the gas permeability vs. water saturation relative permeability measurements are required to properly simulate fluid flow in the reservoir and to design hydraulic fracture treatments.
It is important to choose the correct core samples for conducting the special core analyses. Special core analysis tests are expensive and require weeks or months of special laboratory measurements. As such, the core samples must be chosen with care to provide the optimum data for designing the well completion and the well stimulation treatment and forecasting future gas recovery. A good way to select the core samples for special core analysis testing is to:
- Form a team of geologists, engineers, and petrophysicists
- Lay out the core
- Have the routine core analysis and log analysis available
- Determine how many rock types or lithology types that are contained in the core are important to the completion and stimulation process
- Pick three to six locations for each rock type or lithology where core plugs are cut for testing
In the SFE No. 3 well that was part of the Gas Research Institute (GRI) Tight Gas Sands Project, special core analyses were run and described in detail. Fig. 4 shows how the log analysis, the routine core analysis, and the special core analysis can be combined to develop a detailed description of a layered, tight gas reservoir.
- Thomas, R.D. and Ward, D.C. 1972. Effect of Overburden Pressure and Water Saturation on Gas Permeability of Tight Sandstone Cores. J Pet Technol 24 (2): 120-124. SPE-3634-PA. http://dx.doi.org/10.2118/3634-PA.
- Jones, F.O. and Owens, W.W. 1980. A Laboratory Study of Low-Permeability Gas Sands. J Pet Technol 32 (9): 1631–1640. SPE-7551-PA. http://dx.doi.org/10.2118/7551-PA.
- Soeder, D.J. and Randolph, P.L. 1987. Porosity, Permeability, and Pore Structure of the Tight Mesaverde Sandstone, Piceance Basin, Colorado. SPE Form Eval 2 (2): 129-136. SPE-13134-PA. http://dx.doi.org/10.2118/13134-PA.
- Staged Field Experiment No. 3: Application of Advanced Technologies in Tight Gas Sandstones—Travis Peak and Cotton Valley Formations, Waskom Field, Harrison County, Texas. 1991. Gas Research Inst. Report, GRI-91/0048, CER Corp. and S.A. Holditch & Assocs. Inc.
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