Re: Petrophysics MSc Course Notes The Neutron Log Dr Paul Glover Page Pdf
Rapheal Charlton
The search or economic accumulations of oil and gas starts with the recognition of likely geologicalprovinces, progresses to seismic surveying, and the drilling of one or more wild-cat wells. If one islucky, these wells may encounter oil, and if that is the case, measurements made down the hole withwireline tools are used to assess whether sufficient oil is present, and whether it can be produced.Clearly, the evaluation of sub-surface formations requires the combined efforts of geologists,petrophysicists, drilling engineers and even geophysicists. However, it is the geologist andpetrophysicist that has the most influence.
The geologist is interested in the lithology, stratigraphy and depositional environment of the sub-surface strata penetrated by the drilling bit. The exploration geologist uses wireline tool responses in anumber of wells to create a large scale image of the sub-surface geology by correlating wirelineresponses that are characteristic of a given formation or horizon between formations. This picture isvery useful when carrying out initial reservoir modelling and in the decision where to drill new wells.Later the production geologist carries out much the same process with much more well information,and adds any extra information that has been gathered to produce a detailed geological model of thereservoir and related sub-surface formations. This model will be the basis of reservoir modelling, andall major reservoir management decisions from primary drainage through to enhanced oil recovery andshut-down.
Petrophysics MSc Course Notes The Neutron Log Dr Paul Glover Page Pdf
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There is a large database of information available to both the geologist and the petrophysicist, and astime passes the amount and variety of information increases. Table 1.1 summarizes a few of the mainmeasurement that a geologist or petrophysicist will have access to, arranged in approximatechronological order. It is the responsibility of the wellsite geologist or engineer to ensure that all thisdata is properly collected and recorded.
This course is designed to introduce you to wireline logs and the simple analysis of the data that theyprovide. The following sections cover the theory behind the operation of each tool, how each tooloperates, its advantages and limitations, and its main applications. There is a huge number of toolsavailable in the industry, many too many to be covered individually. However, all are variations on asmall number of generic tool types, which will be described.
It should be remembered at all times that the main job of the petrophysicist is to evaluate the amountof hydrocarbons in place in the reservoir. Hence, the evaluation sequence for a straightforwardreservoir will be as follows:
2. For the reservoir intervals only, distinguish between hydrocarbons and water filling thepores, hence calculate water saturation in reservoir rocks(Hydrocarbons are electrical insulators, while water conducts.)
Oil and gas reservoirs have come into being over large periods of time as the result of geologicalprocesses. The gasses and oils have been formed from organic remains, have migrated into thereservoir rock, and then have been trapped there by overlying rock formations with very lowpermeability. Hence, for a hydrocarbon reservoir to exist we need the following to be available at thesame location:
1. A source rock containing the original organic remains.2. Pressure and temperature conditions suitable to convert the organic remains into oil and gas.3. A porous, permeable reservoir rock where the hydrocarbon can accumulate.4. A migration pathway from the source rock to the reservoir rock for the hydrocarbons.5. A suitable trap to keep the hydrocarbons in the reservoir rock until we wish to exploit it.
These processes can take extremely long periods of time. Most formations that contain reservoirs aresedimentary rocks, where the deposition of organically rich material has been followed by cleansandstones that form high porosity well connected pore systems, and are subsequently capped byshales with very low permeabilities. Here the burial of the initial deposition provides the pressures andtemperatures to produce hydrocarbons. The hydrocarbons are less dense than water, so migrateupwards into the sandstones, replacing the water that originally occupied the reservoir sandstone,where the hydrocarbons are constrained from rising further by the shale cap. The hydrocarbon thenwaits until we decide to produce it.
The depositional and post-depositional history of the reservoir rock, and particularly its diagenetichistory (compaction, cementation and dissolution), all contribute to the mineralogical composition ofthe rock, and hence its grain size distribution, porosity, pore size distribution and the connectivity ofits pores. Note that in the process of migration the hydrocarbon replaces water in the reservoir rockbecause it is less dense than water. In practice, the replacement is almost never complete, with somewater associated with even the best oil accumulations. The reason for the remaining water is that thegrains comprising the reservoir rock are usually water-wet, i.e., having a chemical preference to becovered in water rather than hydrocarbon, hence they retain a thin film of water when the hydrocarbonreplaces most of the water in the pores. Oil-wet rocks do exist, and the ability to distinguish betweenoil and water wet rocks is extremely important in reservoir management, especially in the final stagesof reservoir production. In general, any given reservoir rock the pore space will be occupied by awater saturation Sw, a gas saturation Sg, and an oil saturation So.
Since gas is less dense than oil, which is less dense than water, the fluids separate in hydrocarbonreservoirs with the gas occurring just below the trapping lithology, oil a little deeper, and water at thebottom. The fluids are commonly immiscible and so we can define a gas-oil contact (GOC) and an oil-water contact (OWC). Since, gravity is the force that separates the fluids into these layers, the GOCand OWC are horizontal providing that horizontal and vertical permeability is good in the reservoirand there are no complicating structures or fractures. Note that it is not compulsory to have all threefluids occurring together. Hence in gas reservoirs the oil is missing and there is a gas-water contact(GWC). Similarly, oil reservoirs can exist without a gas cap.
We can define a reservoir rock as one that has a porosity and permeability that allows it to contain asignificant amount of extractable hydrocarbon, AND contains hydrocarbons. A non-reservoir rockmay have a porosity that is too low, a permeability that is too low, or a low or zero hydrocarbonsaturation. The major control is often the basic lithology. For example, shales often containhydrocarbon with high saturations, but have porosities and permeabilities that are much too low for thehydrocarbon to be extractable. Shales are considered to be non-reservoir rock. In contrast a highporosity, high permeability sandstone could be a reservoir rock providing that the hydrocarbonsaturations are sufficiently high, i.e., above the oil water contact.
The calculation of hydrocarbon volume requires us to know the volume of the formations containingthe hydrocarbons, the porosity of each formation, and the hydrocarbon saturation in each formation. Inpractice each reservoir will be made up of a number of zones each with its own thickness, areal extent,porosity and hydrocarbon saturation. For example, reservoir sandstones may alternate with non-reservoir shales, such that each zone is partitioned. Such zonation is mainly controlled by lithology.Hence, it is an early requirement to identify the lithologies in a particular well, identify whichformations have the required porosity to enable it to be a reservoir rock, and determine whether theformation contains hydrocarbons. Reservoir rocks containing hydrocarbons are allocated a zone code.
The volume of reservoir rock in a single zone depends upon the area of the zone A, and the thicknessof reservoir rock in the zone h. The area is obtained usually from seismic data (from the reservoirgeologist), and is the only data used in the calculation of hydrocarbon volumes in place that is notderived from petrophysical techniques. The thickness of reservoir rock is derived from the zonation ofthe reservoir based upon an initial lithological interpretation and zonation of the reservoir from thewireline logs. The bulk volume of the reservoir Vbulk=A h.
The majority of this volume is occupies by the solid rock matrix, and the remainder is made up of thepore space between the minerals. The relative amount of pore space to the bulk volume is denoted bythe porosity φ, where the porosity is the fraction of the bulk volume occupied by pore volume, and isexpressed as a fraction or as a percentage; φ=Vpore/Vbulk. However, note that the fractional form is usedin ALL calculations. The pore volume in any given zone is therefore Vpore=φ A h.
The determination of this value is the primary job of the petrophysicist, and requires a lithologicalassessment and zonation of the reservoir. At a later stage the petrophysicist may also be called upon toassess the permeability of the reservoir under various conditions. However, the primary function of thepetrophysicist is to assess the amount of hydrocarbons initially in place.
All the parameters in Eq. (1.1) except the area are derived from measurements made in the boreholeusing wireline tools or increasingly using data obtained from tools that measure the rock formationsduring drilling (measurement/logging while drilling: MWD/LWD).
Equation (1.1) gives the volume of hydrocarbons in m3, and the calculated values are often unwieldy.The oil industry uses a range of industry standard units, in which all calculations should be carried out.The volume of oil is measured in barrels, the amount of gas in cubic feet, thickness in feet, and area inacres. Conversions for these are given in Table 1.2 below.
If the area is not known, the amounts of oil or gas are quoted in bbl/acre or cu.ft/acre respectively, andif the reservoir zone thickness is also not known, the values are given in bbl/acre.foot and cu.ft/acre.ftrespectively.
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