The Bazhenov Formation In Search of Big Shale Oil in Upper Salym, Part 2
Aleksey D. Alekseev Senior Petrophysicist of Salym Petroleum Development N.V., Ph.D. (Geoscience)
This article looks at the work performed by Salym Petroleum Development N.V. (SPD) for the pilot development of the Bazhenov Formation. The research program has helped to identify key geological features of the Bazhenov Formation of the Upper Salym field, which have helped define ways of drilling, completion, field development and НС reserves estimation.
In Search of the “Middle Bakken” in the Upper Salym
As noted in the first part of this article (ROGTEC #34), there are two ways to develop shale oil formations. The first method is to search for “sweet spots” using remote methods (seismic, etc.) followed by the drilling of exploration wells in areas that appear promising. This is way the Soviet and Russian oil companies have been approaching it for the last 45 years, and they have produced no more than 5 million tonnes of oil in that time. This is an extremely low yield when you bear in mind the potential of the Bazhenov Formation. The other way to attack the formation is along the lines of the Bakken. Successful development of this formation became to possible due to dolomites and sandstones within Middle Bakken. Initially, these layers have very low permeability, but with the use of hydraulic fracturing, fractures are created which allow the hydrocarbons to migrate to the wellbore. As an example, this approach allowed a 412% increase in North Dakota over a 4 year period [31]. At the Bakken formation, the main reason for economical production is through the drilling of horizontal wellbores in pure dolomite and sandstone of Middle Bakken. Drilling also takes place under extreme high reservoir pressure, and is carried out in areas that are favourable to the development of artificial fractures [33].
Because of the lack of other ways the reservoir pressure support, the availability of ABFP is one of the key factors that determines the “life” of a well in terms of economic development.
Upper Salym is located about 150 km south of the famous Salym field, which has the highest number of natural flows in the Bazhenov formation. However, since 1966, the largest flow rates have not exceeded more than 10-15 m3/day, but even they are now in serious doubt. SPD have conducted extensive testing, and as an example, a well with starting production rates of 20-40 m3/day, reduced to a production rate of only 1-2 m3/day. With this lack of significant natural flow rates the strategy selection is obvious – to find something similar to the Middle Bakken at the Bazhenov Formation, drill horizontal bores, and perform multi-stage hydraulic fracturing in accordance with the North American experience, only adjusted for the Bazhenov Formation.
Figure 1 shows the geological and geophysical characteristics of the Bazhenov Formation at Upper Salym. These include four sets of data: standard and special open hole logs (I-IX); special core analysis (SCAL) data (X-XIII); mud logging (XIV-XV); and thermometry log (T-log) during water injection test (XVI). The coincidence of GR curves (II) in various studies ensures that all data are interrelated.
When analysing the petrophysical evaluation plot of Figure1, no specialist will have any problems defining the mineral composition, reservoir properties and the permeability. This indicates that there is no “mystery” to the Bazhenov, but only a lack of necessary evidence and data. As can be seen from the graph, mud cake, which is a direct indication of a carrier bed (reservoir), is found at the Bazhenov Formation of Upper Salym. The fact that this interval is a reservoir has been confirmed by nuclear magnetic resonance logs (NMR) with the presence of free fluids via the distribution of T2 (VIII) and by highly-precise T-logs during the injection test (XVI). The low Poisson’s ratio, and large values of the Young’s modulus (IV) indicate that this interval contains both brittle and hard variations. A relatively low horizontal stress of the surrounding rocks (V) indicates that this interval has partly reached its fracturing capacity, which is confirmed by the drilling induced fracture according to the FMI (IX). This carrier bed has a thickness of about 8 meters, and its mineral composition includes silicite with minor admixtures of shale materials (X). Its porosity reaches 12-13% (XI), and the permeability is either 1mD (XII) or higher.
Due to the contrasting characteristics we can expect to accurately predict the properties of this body using seismic data. According to the results of geochemical core analyses, this interval is characterized by a clear positive anomaly per the geochemical productivity index (PI) (XII). Based on this, It is clear that this layer is similar in make up to the Middle Bakken.
According to the mud logging results (XIV), in the cross section of the Bazhenov Formation of Upper Salym there are three separate reservoirs: A – in the Upper Bazhenov, B – in the Middle Bazhenov, and reservoir, with C is on the border with the underlying Abalak formation. The best of them is reservoir B in the Middle Bazhenov, which is also characterized by a significant increase of the rate of penetration while drilling (XV). Reservoirs A and C in this case are not sufficiently developed.
Incidentally, only the presence of several reservoirs having different formation pressure can explain the specific Bazhenov Formation behaviour when in the process of drilling, during loss of circulation, a spontaneous oil natural flow or blowout occurs. This loss of drilling mud circulation, due to intense absorption, after pulling the bit out of the Bazhenov Formation which caused the majority of the accidents involving uncontrolled oil blowouts. The mechanism of spontaneous natural flow from the Bazhenov Formation can be described as follows. As is well known, it is characterized by a significant AHFP, and enclosing deposits have a pressure similar to hydrostatic deposits. On the border of the Bazhenov Formation and the underlying Abalak formation there is a 1-4-meters carbonate interlayer, which was marked by V.I. Belkin at the Salym field as KS1 [17]. This interlayer is represented in some areas by cavity fractured dolomite, the permeability of which may be equal to several Darcies. Overbalanced drilling is used for the well, so the penetration of this formation is accompanied by a significant amount of lost circulation, which results in a significant reduction in bottomhole pressure causing drawdown and stimulating the inflow in permeable intervals of the Bazhenov formation. Similar things were seen while drilling the well #153 of the Palyanovsk field, which almost led to an accident [38].
The abnormally high formation pressure (AHFP) determines another specific feature of the Bazhenov Formation, which has a critical impact on productivity. Fig. 2 shows a graph of the formation pressure and stresses with depths built according to the geomechanical modelling using the results of actual tests performed directly in the wells. As can be seen from Fig. 3 the formation pressure in the best reservoir is almost identical in value to the minimum horizontal stress, i.e. with hydraulic fracturing pressure. Thus, if the Bazhenov is penetrated with at least a minimum overburden on the formation, it is very likely that it has undergone hydraulic fracturing with drilling mud, where the mud solids acted as the propping agent. Currently, most drilling fluids are shale-based, so such hydraulic fracturing leads to a deep clogging of even the most permeable zones. The greater the overburden, the more we see of this effect. For this reason, the true potential of this section of the Bazhenov Formation could possibly be much better than the results that we have seen from well testing. To estimate it, a balanced hole should be drilled, although this carries a higher risk of a blowout. In fact, it is important to pay attention to the equivalent circulating density (ECD) while drilling. ECD is much easier to control when there is a large gap between the drilling tool and the borehole wall. For this reason, the most commonly used well completion technique at the Bazhenov Formation is to use small diameter liners. However, this is initially non-optimal, as it increases the risk of creating deep clogging of the bottom-hole zone due to induced fracturing.
The proximity of the values of the formation pressure to the hydraulic pressure may be one of the reasons why a well, once shut in due to the blowout, cannot then be brought to a high flow rate. Apparently, the same phenomenon may explain why wells drilled nearby in the same geological conditions with similar logs, often produce quite different results.
The Bazhenov Formation is shrouded by many questions and uncertainties, which make its development very risky and does not allow investors to prepare anything close to a sound investment plan. Perhaps one of the first questions to ask is regarding its reserves of light oil. The fact that the resource potential of these deposits is very large is not in any doubt. Oil deposits do not contain bottom and aquifer waters, and the available content of bound water is very low, so we can assume that the deposits of the Bazhenov Formation are saturated with hydrocarbons from the top to bottom.
Figure 3 shows the geological data, together with the lab data of the cores. As mentioned, within Upper Salym in the middle of the Bazhenov Formation there is carrier bed (5), which is almost identical to the sandy Middle Bakken. It has been well determined through standard logging, and according to the caliper data, that there is mud cake present. It has also been characterized by a local negative anomaly by gamma-ray (GR) and a positive anomaly by the neutron log (NEU) ( II). Highly-precise thermometry data during production testing (XI) indicates that this interval will produce an inflow of formation fluids into the wellbore. According to the laboratory testing results from unextracted cores, this layer has an average porosity of 12-13%, and permeability of about 1 mD (IX, X). It is also characterized by almost a constant value of the grain density of the rock (V). Determination of the organic content (VI) with the Rock-Eval 6 shows that this interval has a reduced content of organic matter, sometimes close to zero. That is, in the aggregate characteristics, it is almost indistinguishable from a conventional reservoir with low permeability and reservoir properties, so the geological reserves of oil in it can be estimated by the traditional volumetric method using a common set of logging data where the porosity is calculated from density logs, and the NET is determined by the combination of three methods: gamma ray, neutron logging and caliper.
It is well known that the fracturing capacity of voids has no significant impacts on the volume of reserves, so the only significant barrier for using the traditional volumetric method is the oil saturation coefficient. The Bazhenov rocks are oil-wet rock and characterized by very high electric formation resistivity, so it is definitely high, but how exactly how high is still to be determined. For now one can apply some value from amongst the range most frequently used in the literature from 0.85 to 0.95 [18].
On this basis, it is possible to estimate the oil reserves of the Middle Bazhenov by conventional methods. The oil is also quite conventional, but is contained in a section with very low permeability and reservoir properties.
To estimate the rest of the oil resources of this section is no mean feat. In addition to the mineral component of the Bazhenov Formation there are abundant organic substances which can be in a solid state (kerogen), and also light oil, bitumen, and asphaltenes. Due to the extremely low permeability in a bigger part of the section, the definition of oil saturation by direct laboratory methods is difficult. For these purposes, some other countries use the method of retort steps (SPE 147456), in Russia the most popular are geochemical methods.
In the Bazhenov Formation, there are two types of light oil, which are quite different in terms of recovery, and they, therefore, definitely need to be separated when calculating reserves. The difference between the oils can be most easily explained by the cross-plot shown in Fig. 4 [8].
This cross-plot shows a comparison of the total organic content (TOC, %) and the Rock-Eval parameter S1, characterizing the availability of oil-like hydrocarbons.
The Bazhenov Formation is an oil source rock, in which the process of transformation of organic matter is not yet complete. The hydrocarbons have not lost a genetic link to the original organic matter and is contained in sealed pores, which were created due to the transition of a solid organic matter into a liquid. This oil is also called proto-(micro-) oil, and in the organic geochemistry these hydrocarbons are called autochthonous. It is because of them an AHFP has developed in the Bazhenov Formation because the volume of generated bitumen is higher than the volume of the original organic matter. In addition to the autochthonous (“bound” – associated to the source rock) hydrocarbons there are mobile (“mobile” – not associated to the source rock) hydrocarbons, which have lost their association with the original organic matter, but they have not left the oil source stratum. They are called paraautochthonous, and the corresponding oil is sometimes called macro-oil. Fig. 4 shows exactly how one can separate parautochthonous hydrocarbons from autochtonous. In other words, how to distinguish «mobile» oil from “bound” oil using methods of geochemical analysis of the core. Thus, if there are more hydrocarbons than the organics can produce under these circumstances in principle, it means that they have migrated from other parts of the oil source stratum, so they are mobile. In itself, this method is an independent geochemical method of determining reservoirs in shale formations. It is developed in the VNIGNI Geochemical Centre, managed by M.V. Dakhnova [39].
Going back to Figure 3 you can now understand how the oil saturation is distributed throughout the section of the Bazhenov Formation of Upper Salym and what oil («bound» or «mobile») is present in each particular area. As noted above, the geochemical index S1 (VII) is responsible for the availability of oil-like hydrocarbons, and by its physical meaning is something like a saturation coefficient. The index S2 (VII) characterizes the oil-generating potential and represents hydrocarbon products of kerogen pyrolysis and bitumen-asphaltene materials. It helps understand how oil, bitumens and asphaltenes are distributed within the formation, because of which a large part of the Bazhenov Formation has almost no permeability.
As can be seen from Figure 3, track VII, the carrier bed in the Middle Bazhen has minimum oil content, apart from in its bottom part, but these hydrocarbons are mobile and available for recovery. Under this bed there are much more concentrated oil resources, but this oil is bound and cannot be recovered without a special stimulation. As per the parameter S2, these rocks have a high concentration of bitumen and asphaltenes, which gives them plasticity; the same is confirmed by the increase of Poisson’s ratio and Young’s modulus decrease (III). To create and maintain a fracture in such rocks is very problematic. This is the apparent reason that there were serious problems with implementing two stages of Bazhenov multifrac of a horizontal well at the Pravdinskoye field, – “the formation did not want to get fractured,” even at very high pressures.
On this basis, we know that Bazhenov formation is an oil source, and that it contains both the “mobile” oil and oil “bound” as described earlier in this piece. In terms of development costs and oil recovery methods they are completely different. In Upper Salym the intervals with mobile oil are determined by logging results, even in older wells. This is largely possible due to the availability of mud cake, which is reliable direct evidence of a carrier beds. But in other fields, this evidence often does not work. In this case, geochemical studies of cores using the Rock-Eval methods may be very valuable. It is possible to select the most interesting intervals for development, for example, by a local positive anomaly of the geochemical productivity index PI = S1 / (S1 + S2) (Fig. 3, IIX), or by the method of prof. M. Dakhnova of the definition of “mobile” hydrocarbons (Fig. 4). Geochemical core analysis data can also play an invaluable role in designing and planning methods of oil flow stimulations.
The oil resources of the Bazhenov Formation of Upper Salym were evaluated in two categories: “mobile” oil reserve of the Middle Bazhen – this is the minimum upon which one can definitely be involved into development; resources of the “bound” oil of source rocks – this is the maximum that one can be involved into development only in case of using specific recovery technologies. One of the solutions is to create a comprehensive system of fractures, but it is very difficult to do in plastic like source rocks. To maintain a fracture in such rocks would require a lot of propping agent, which would lead to the growth of fractures in the enclosing rock, and possibly out of the AHFP Bazhenov zone, and eventually result in a loss of the formation pressure. Currently, SPD experts focus their work on the study of the physical and chemical properties of the rocks of the Bazhenov Formation in the context of interaction with propping and fracturing agents. This work, judging by the lack of corresponding materials in the press, no one has ever performed yet. The result of this work will be a promising technique of hydraulic fracturing, which will be trialled later this year in a vertical well.
In conclusion, the Bazhenov Formation of Upper Salym is not abnormal. It has no sandstone and silt lenses. The analogue of the Middle Bakken in the Bazhenov Formation is represented by secondary transformed radiolarian chert. This layer almost fully consists of remains of plankton organisms – radiolarians, which have siliceous skeletons. Under the influence of the secondary transformations radiolarian remains partially dissolved and re-deposited and so formed Middle Bazhenov layer [40,41]. To be quite formal, the Bazhenov Formation itself cannot be regarded as a shale rock, because its main rock-forming material is silicite – a biogenic silica, and clay minerals have a secondary function.
Summary
From 2009-2011, SPD carried out additional exploration of the deep horizons in the Upper Salym field. According to the results of this work, the most promising horizon for further development is the Bazhenov Formation. Due to the in-depth analysis of domestic and international experience in the development of shale formations it is thought that the shale oil formations could be developed in two ways. The first method is to search for “sweet spots” by remote methods (seismic etc.) followed by drilling of vertical wells in pay zones. This is the way Soviet and Russian oil companies have followed and have continued to follow in the last 45 years producing no more than 5 million tonnes of oil. The second method – this is the way of the Bakken formation’s success, when shale oil source rocks are developed due to the availability within the shale oil formation of thick carbonate or sand layers (carrier beds). Initially, these beds have very low permeability and other reservoir properties, but when hydraulic fracturing creates artificial fractures, production is increased.
In the absence of significant natural flow rates from the Bazhenov Formation in Salym oil fields, the company chose the strategy to search for a middle Bakken analogue in the Bazhenov Formation. The results were positive, and such an analogue was found. This is an 8m bed occurring in the middle of the Bazhenov Formation, represented by radiolarite (silicite) with minor shale admixtures. Under natural conditions, it has low reservoir properties with a porosity of 12-13% and permeability of about 1mD. It is distinguished in all wells by aggregate radioactive and caliper log data. Due to its contrasting characteristics there are good preconditions to forecast its properties using seismic data. Thus, in Upper Salym, there are favourable conditions for the development of the Bazhenov Formation in accordance with the North American experience with adjustments to the specific features of the Bazhenov Formation. Currently, SPD experts together with its contractors are working on the optimal fracturing technique to be trialled in a vertical well this year.
