API PUBL 4711-2001 pdf download.Methods for Determining Inputs to Environmental Petroleum Hydrocarbon Mobility and Recovery Models.
4.1 Sample Collection Sediments – Analysis of soil properties at a laboratory scale begins with obtaining representative soil samples. Ideally, soil samples should be undisturbed. This reflects the fact that parameters of interest are dependent on the geometry of pore bodies and throats. Preservation of these features or reproducing these features represents a significant challenge. At one extreme, soil samples can be recovered using hollow stem auger and split spoon sampling equipment [see ASTM D1586-99 or ASTM D1452-80(1995)e1]. After extraction of product, samples can be dried and sieved to remove larger materials (e.g., gravel) that would block flow in a standard core holder (diameters of a few centimeters). Lastly, samples can be placed in coreholders with confining pressure similar to the in situ overburden pressure. The assumption associated with this approach is that the procedure will result in pore bodies and throats representative of the in situ conditions. In general, this approach should be avoided when possible due to potential biases associated with disturbing the architecture of the sediments. Alternatively, soil samples can be collected in liners. This can be accomplished using hollow stem auger drilling systems equipped with continuous sampling systems or using direct push drilling systems (see ASTM D6282-98) or thin-walled sampling tubes (see ASTM D1587-94). The liners serve as core preservation systems. Immediately after recovery the core should be frozen on site. This has been accomplished using liquid nitrogen or dry ice. Assuming the pore space is fully saturated with water (worst case), this will effect an approximate 5 percent expansion of the pore space. This may fracture the plastic sleeve. Even if this occurs, it is hoped that the general geometry of the particles will be preserved. Note: further complication is added by the fact that petroleum liquids shrink at reduced temperatures.
Beyond the above, options are limited. Within the crude oil production industry, a number of special coring systems have been developed, including pressure retaining cores, sponge-lined coring systems, full-closure coring systems, rubber sleeve core barrels, and side wall coring systems [see API (1998)]. Unfortunately, these systems are generally not available for collection of shallow unconsolidated soils. Notable exceptions are described by: · Durnford et al., (1991) Ð A prototype sampler is described that cools soil at the bottom end of a sampler to near freezing. Cooling is achieved by allowing compressed carbon dioxide to expand to atmospheric pressure in situ at the bottom end of the sampler. The principle benefit is retention of liquids in the soil core. Samples are frozen upon retrieval to the surface. · McElwee et al., (1991) Ð A prototype sampler that closes in situ is described. Nitrogen gas is used to inflate a bladder located in the drive shoe of the sampler. The primary advantage noted is improved recovery of sands and gravels.
Residual Liquids – Often it is desirable to obtain measurements of product and water saturation from cores. In such instances it is necessary to address the issue of whether the residual fluids present in samples after collection are representative of liquid saturations present in situ. Using conventional split spoon or direct push techniques, significant biases can be introduced, including: · Driving sampling equipment through sediments may compress the samples and displace fluids. · Fluids that accumulate in sampling devices above the sample (e.g., water in portions of the sampler above the sample) will drain through the sample as it is brought to the surface. As this occurs, water and product saturations within the sample may be altered significantly. · As the sample is brought to the surface, air can invade the pore space. If this occurs, product and/or water will drain out. The severity of the above problems will increase with the hydraulic conductivity of the sediments and the length of the sample collected.