Coring and Core Analysis
Core retrieval and analysis in all the petroleum industry phases are important. The only approach to attain intact, vertically-continuous samples is via the core to visually examine deposition sequences and reserve character variations.
The data available from no other source from properly examined cores can provide clear proof of the presence, quantity, distribution and delivery of hydrocarbons. The core of the pore system in the future reservoir is essential for an understanding.
Analyses of rock samples yield data basic to the evaluation of the productive potential of a hydrocarbon reservoir. Bit cuttings are, of course, rock samples; their small size, however, precludes their furnishing more than qualitative information. The desire to obtain and examine larger, unbroken pieces of reservoir rock led to the development of coring techniques, by which relatively large reservoir rock samples are obtained, either from the bottom during drilling, or from the side of the bore hole wall after drilling.
Types of cores:
Coring Analysis Methods:
For the evaluation of core samples, a number of widely accepted sampling and analysis techniques have been developed. The selection of the technique depends on the recovered core type, its lithology, and the pore system type. Core analyzes can be classified into various types: (1) conventional or plug analysis, (2) whole core analysis and (3) sidewall core analysis.
1. Conventional (Routine) or plug analysis
??????????Conventional or whole core methods can also be used for continuous core analysis, but conventional core analysis is more widely used. This technique uses a small sample to represent an interval between core processes and produces acceptable results when the pores are relatively homogenous. Conventional core plugs are usually collected once a foot or between three and four times a meter. Variations in pore system development or lithology require more frequent sampling. Sample density should be adequate to define net pay, hydrocarbon-water transition zones, contact levels, and formation boundaries. Sampling can be done statistically at the mid-point of each foot or the most representative sample in each foot can be selected.
2. Whole core analysis
Whole core analysis examines the full length and maximum sample size of the whole core in the interval being tested. In heterogeneous formations, large samples are mandatory, where the majority of the porosity and permeability are due to fractures, solution vugs or erratically formed porosity systems. In such cases, in relation to the size of conventional core analysis plug samples, the volume of the individual pores may be large. An entire core analysis variation, called an observational full diameter, utilizes selected core lengths rather than the whole core.
3.?Side-wall core analysis
?Sidewall core analysis is performed on cores recovered by any of the sidewall coring techniques. Percussion sidewall cores from hard, well-cemented formations are badly altered during the coring process and generally fail to produce suitable measurements of mechanical and petrophysical properties. The use of sidewall boring or hydraulic press for collecting of core sidewall samples will reduce alteration of samples. Sidewall coring and analysis produce acceptable results when suitable formations are accurately sampled, such as soft Miocene and Oligocene sandstones found in Gulf Coast and California Reservoirs. Quality data can be improved in wells where only sidewall cores are available via correlations between conventional and sidewall core values.
4.?Special core analysis (SCAL)
Special core analysis (SCAL) normally covers measurements like the Archie m and n factors, other resistivity parameters like Qv (used in the Waxman-Smits equation), reservoir compressibility, capillary pressure, relative permeability.
The Archie cementation exponent m and the associated Formation Resistivity Factor (F or FRF), are measured together with stressed porosity in a special pressure cell. The sample is first cleaned and made fully water wet. Then the atmospheric porosity is measured. Artificial formation brine is made which is equal in composition to the actual formation brine. The resistivity of this brine is measured in a separate cell. The rock sample is cleaned again and fully saturated with this brine. It is then put into the measurement cell and subjected to hydrostatic stress (about 70 bar), and the resistivity is measured. The amount of brine expelled gives the reduction in pore volume from which the in-situ porosity can be calculated.
Petrophysical properties Of Coring
1. Porosity (Φ)
●?????The porosity of a rock is the fraction of the volume of space between the solid particles of the rock to the total rock volume. The space includes all pores, cracks, vugs, inter- and intra-crystalline spaces. The porosity is conventionally given the symbol ? (Phi), and is expressed either as a fraction varying between 0 and 1, or a percentage varying between 0% and 100%.
●?????Whole core porosity is usually less than conventional plug porosity because there is a strong tendency to sample the more porous zones preferentially. Whole core samples incorporate tighter parts of the pore system that are frequently excluded from conventional samples. However, whole core porosity may be higher than that determined from conventional analysis when large solution voids are present or when the core is badly invaded by mud solids.
●?????In samples having a porosity greater than 30%, sidewall core porosity is 1 to 2% lower than conventional analysis porosity. This is due to a slightly compacted coring effect. Medium and low porosity percussion sidewall samples show porosity that is too high due to fracturing and cereal light scattering, particularly from highly cemented rocks.
●?????As the real porosity decreases, the deviation between measured porosity and true porosity increases. Uncertainty caused by the systemic variance of the core porosity of the sidewall relative to the plug analysis can be reduced by creating correlations between the core of the sidewall and conventional values.
●?????After retrieving representative core samples and delivering them to the laboratory, there are several methods that can be used to
?determine porosity. To determine porosity, we need to determine two of the three volumes, Vb, Vp, or Vg. Once these are determined, then the porosity and the third volume are known.
领英推荐
2. Permeability (K)
●?????It is the same as effective porosity. Permeability is the property of rocks that is an indication of the ability for fluids (gas or liquid) to flow through rocks. High permeability will allow fluids to move rapidly through rocks. Permeability is affected by the pressure in a rock. The unit of measure is called the Darcy, named after Henry Darcy (1803–1858). Sandstones may vary in permeability from less than one to over 50,000 millidarcies (md). Permeabilities are more commonly in the range of tens to hundreds of millidarcies. A rock with 25% porosity and a permeability of 1 md will not yield a significant flow of water. Such “tight” rocks are usually artificially stimulated (fractured or acidized) to create permeability and yield a flow.
●?????Whole core samples which contain vugs and fractures, which are completely removed from core analysis plugs. The whole core permeability is measured in two horizontal directions in order to offset this effect, particularly in fractured samples. Parallel to the major fracturing plans is a calculation (k or kmax) that represents the impact of the fractures as the flow pathways. The first is perpendicular to the second measurement.
●?????Whole core permeability can be reduced by as much as 50 to 80% by the invasion of drilling mud solids into the pore system or the build-up of powdered rock on the core surface.
●?????As the real value decreases, the relative reduction of permeability seems to be decreasing. Whole core samples can involve sandblasting to deal with the accumulation of powdered rock before permeability measurement. There is no way to fix the reduction in the permeability caused by the penetration of the pores by drilling mud fines. These fines may result in a significantly lower permeability than conventional permeability for the whole core.
3. Water saturation
●?????Water saturation (Sw) is the measure of pore volume filled with water; the water may be mobile or capillary bound.
●?????Water saturation can be defined as effective or total depending on the porosity terminology being used. There are also a number of different terms to describe water saturation: initial (Swi), connate (Swc) or irreducible (Swirr).
●?????Initial water saturation is the proportion of water present in the pores at time of discovery; this may range from 100% to a value equivalent to the irreducible water saturation. Irreducible water saturation is the proportion of capillary-bound water that is immoveable by normal production processes. The irreducible water saturation?varies with rock quality and will be higher in low-permeability rocks.
●?????Water saturation (Sw) equals the fraction of pore volume (Vp) that is filled with water:
Sw = Vw / Vp × 100
?Or it can be calculated from well logs using Archie’s Equation:
Where:
●?????Sw: Water saturation
●?????a: Tortuosity factor(.6-1)
●?????m: Cementation exponent
●??????: porosity
●?????Rw: Water resistivity(lab)
●?????Rt: True resistivity or formation resistivity (log)
●?????n: The saturation exponent=2
?
??Hydrocarbon saturation
?It is the volume of pores that is filled with hydrocarbons; it is derived from the relationship: