This project was purposed to locate sweet spots using a certain cut off values for hydraulic and perforation fracture. To be able to achieve the desired results of the project, it was necessary that an eight-step procedure to be followed. The procedure involved Techlog and Excel programs. The activities that had to be performed included geochemical, geomechanical, and petrophysical properties that elaborate the lithology and stratigraphy of the formation of Bakken.
The Braaflat 11-11H geochemical properties, which were established through appropriate logs on Techlog, were useful in calculating the ratio of the oxygen index, the hydrogen index, the normalized oil ratio, and the production index. The produce of the hydrocarbon was calculated according to specific assumptions that have been mentioned in the report. During the project, the Larionov’s older rocks method was used to calculate the volume of the shale. And the total and effective properties were calculated using the density log. At this point, the saturation level of water was required and it was acquired through the calculations that were based on the Simandoux method. The calculations that were used to acquire the Lithology were based on six parameters that have been mentioned in the report. The permeability of the water was evaluated through the Next NMR log. A complete follow up of the procedure allowed the project to establish the sweet spots in the vertical section of the area. Correlations to the spots for the lateral part were made based on the lithology and the geochemical properties of the vertical part, creating the sweet spots.
According to the findings, the sweet spots were located at 9901.5 to 9904.5ft and at 9907.5 to 9908.5ft in the middle of Bakken formation. The cutoff values that were used to determine the sweet spots were BI ;0.4, FI ;0.4, Sw ;0.7. The permeability of the water was determined using ; 0.01 mD as the cut off value for the vertical section, and the cut-off values of S2 ;20, PI ; 0.0039. The cut off value for the normalized oil ratio were ; 1.7, and (Quartz + Dolomite) ; 0.33 for the lateral part. The type of kerogen in the upper and lower parts of the formations was found to be type II, while the middle part had the type III kerogen.
This study has provided datasets of the Braaflat 11-11H and poecks 1-14-23 wells found in the Bakken formation that have been targeted. In order to come up with the desired characteristics of the Bakken formation, all the logs, approximated to be one hundred, had to be used. The project intends to achieve its goal of identifying and recommending the sweet spots for the perforation and hydraulic fracturing by following a specific procedure which involves a series of tasks. To be able to demonstrate a complete understanding of the formation, a gamma-ray log will be employed on the Braaflat 11-11H well found in the lateral part for hydraulic fracturing after applying the first seven steps of the procedure. Besides, the use of Techlog and Excel programs will be helpful during the course of the project.
Being one of the largest oil plays, the Bakken play is located in three areas, the Western North Dakota, Eastern Montana, and parts of Saskatchewan within the Williston Basin. Bakken play is one of the main oil plays and oil development formations in North America. Through examining lithology and stratigraphy, the deposition of the Bakken shale is estimated to have occurred during the late Devonian and the early Mississippian ages. Initially, oil was discovered in Bakken in the year 1951. However, it started being processed on large scale for commercial purposes only seven years ago. The first well was discovered by Henry Bakken, who was a farmer and the owner of the land where the well that later became part of the larger Bakken play was discovered in North Dakota.
According to Liu et al. (2017), unconventional reservoirs are becoming important contributors to the production of oil and gas because of the depletion of conventional resources and the increasing demands of energy around the world. With three divisions, the Bakken play is among the main unconventional reservoirs. The Bakken reservoir is divided into the upper, middle, and lower divisions. The upper and lower divisions have a high organic content and are, therefore, the source rocks where oil is generated. The oil is then transferred to the middle Bakken formation which has a low organic content.
The first task entailed determining the type of kerogen of each division of the Bakken oil reservoir. The geochemical properties of the Braaflat 11-11H well were acquired from the data that was provided. The excel sheet that was made available (Pyrolysis Data) displayed the parameters that were to be used. The parameters that were given were S1, S2, S3, TOC, and Tmax. We used these parameters to calculate the hydrogen index (HI), oxygen index (OI), the normalized oil ratio (NOR), and the production index (PI). We were able to determine the kerogen types by plotting OI vs. HI, and the kerogen quality by plotting TOC vs. S2.
OI=S3TOCx100 , HI=S2TOCx100PI=S1S1+S2 , NOR=S1TOCx100Task 2
The task entailed calculating the hydrocarbon yield of the upper division of the Bakken reservoir. We estimated the original HI value to be 600 (HIo=600), the original production index to be 0.02 (PIo=0.02), and the dead carbon to be 0 (CR=0).
Task 3 entailed the establishment of the geochemical properties of the Braaflat 11-11H well. The properties were calculated using Techlog. Evaluating the mechanics of a source rock is essential in determining how a rock can be easily produced. The dynamic properties of a rock define its brittleness and fracability. We selected the dynamic properties under the geomechanics which is found under the elastic properties in Techlog in order to assign the data type appropriately in the right family selection for the compressional slowness, bulk density, and shear slowness. These classifications provided us with four dynamic properties: Bulk modulus, Young’s modulus, Poisson’s ratio, and Shear modulus. The Young’s modulus and the Poisson’s ratio have been saved in Techlog because they will be used in the upcoming tasks. The Young’s Modulus denotes the ratio of stress to the corresponding strain which determines the capacity that is needed to maintain a fracture open. The Poisson’s ratio, on the other hand, denotes the ratio of the longitudinal to the transverse strains which amount to a measure of the geometric change of the shape under stress. The four properties are identified in the figure below.
Task 4 called for the calculations of the shale volume, formation lithology, water saturation, and the total and effective porosities with the use of Techlog under the Petro Physics tab. The shale volume was calculated using the Larionov’s older rocks method. It was then followed by the calculations of the effective porosity which was done using bulk density and the shale volume. The next step of the task involved the calculation of the level of water saturation, which was done using the Simandoux method. The formation water resistivity log is essential when it comes to calculating water saturation. But since it was not available in the Braaflat 11-11H main well, it was copied directly from the Poeckes 1-14-23H well to the Braaflat well because both wells target the same formation. Besides, the value of shale resistivity needed to be corrected by plotting gammaray log (HCGR) and the formation resistivity log (RT) next to each other using the data editor. The new the resistivity of shale value was found to be 23.9789 ohms. And it corresponded with the highest gammaray value which was 131.398 gAPI. The final step of the task involved the calculation of the lithology using is the six parameters: compressional slowness, effective porosity, water saturation, shale volume, water saturation, bulk density, and the photoelectric factor. These calculations helped to determine the volumes of anhydrite, quartz, calcite, and dolomite. The figure below shows a log view of the calculated parameters.
Task 5 involved establishing the permeability of the Braaflat 11-11H using the NMR log. Permeability is evaluated for the Braaflat 11-11H well. To obtain permeability (K), TCMR and T2LM are used to calculate In the equation: K = a ?4 T22gm
In this case,
K stands for permeability in darcy
a is a constant and has a default value of 4.6
? is porosity provided in decimal units and
T2gm stands for the geometric mean which is given in ms
The task was aimed at identifying the sweet spots, which would be achieved by calculating the brittleness index (BI) and fracability index (FI). To necessitate the calculations, several parameters had to be exported from Techlog to Excel. The Young’s modulus and Poisson’s ratio, which have to be first normalized, are part of these calculations. The variables have to be normalized in order to avoid the use of calibration instruments and enable the conversation into rations and values that can be easily compared to the rocks that are found in the surrounding environment. This will enable the calculation of FI and BI and, in turn, sweet points can be located by use of the available cut-off values. To obtain the sweet spots, the equation below is employed.
XNormalized=x-xminEmax-EminFI=En+µn2BI=Quartz+CarbonatesQuartz+Carbonates+Clay+TOCIn this case,
En is the Normalized Young’s modulus
µn is the Normalized Poisson’s ratio and
FI > 0.4
BI > 0.4
K > 0.01 mD and
Sw < 0.7 are the cut-off values.
Here, the task involves creating and naming the zones where the sweet spots are found in numbers using Techlog. The variables that had been acquired in task 6 had to be imported from the Excel program to Techlog to enable in marking the sweet spots. Log views are used to create the sweet-spots are created in Techlog using four parameters: fracability index, brittleness index, water saturation, permeability.
The aim of task 8 was to use the gammaray log to establish the sweet spots for the hydraulic fracturing of the Braaflat 11-11H’s lateral part. The correlations are made based on the lithology and geochemical properties of the vertical part of the well. To acquire the correlations, the geochemical properties (TOC, S1 and S2), which are provided in an Excel sheet, dolomite and quartz volumes are calculated using Techlog. They are then correlated with the gammaray log at the same depth, and the process is repeated for all the three divisions of the reservoir: upper, middle, and lower. We used excel to create various spots to compare the correlations of the lateral part of the well. During this time, we used several cut-off values and correlation. It is also important to calculate the S1, S2, TOC, the volume of quartz, and volume of dolomite values of the lateral part. The cut-off values, equations, and correlations below were used to establish the sweet points.
PI=S1S1+S2 , NOR=S1TOCx100Y= -0.0959x + 8.0904 (HCGR vs. S1)
Y= -0.0264x + 2.3472 (HCGR vs. S2)
Y= 0.33320x + 0.2051 (S2 vs. TOC)
Y= 0.0096x – 0.233 (HCGR vs. VDOL)
Y= -0.0076x + 0.5641 (HCGR vs. VQTZ)
Normalized oil ratio ; 1.7
Quartz + Dolomite ; 0.33
In order to get the best fit, several points had to be eliminated for each correlation.
Result and Discussion
left0Table 2: Geochemical Properties00Table SEQ Table * ARABIC 2: Geochemical Properties
Table SEQ Table * ARABIC 3: Quality of Geochemical Properties
Figure SEQ Figure * ARABIC 5: Kerogen quality
Figure SEQ Figure * ARABIC 6: Normalized oil ratio vs. DepthFigure 4 shows the kerogen type of the entire Bakken reservoir. The y-axis shows the hydrogen index and the x-axis shows the oxygen index. Table 1 shows the kerogen type of each division. According to the data, the upper section was found to have a HI of 300 to 600 suggesting that oil-prone organic matter. It has low oxygen of ; 30mg giving it a marine setting. Though such reservoirs are good oil and gas sources, they contain fewer amounts of oil than Type 1 kerogen sources. The lower division has type II free and also contains some type I kerogen. With an HI of 300 to 750 mg and a low OI of ;20mg, the division has highly prone organic matter. The section has the highest TOC value. This means that it has both oil and gas. The middle division mostly has type III kerogen, which is a gas-prone organic matter as it has an HI of 50 to 200 mg. It is formed by terrestrial plant matter. It is the least favorable for oil generation as it has a lot of oxygen and little hydrogen.
Figure 5 shows the kerogen quality of the reservoir. Table 2 shows the number of geochemical properties and table 3 shows the quality scale of the geochemical properties. The upper and lower divisions have high S2 and TOC values with a good kerogen quality while the middle division has a low S2 and TOC values with a poor kerogen quality. The upper and lower divisions, having a low porosity and permeability, are the source rock. The middle division is considered as the reservoir rock because of the low porosity and permeability which forces the oil to migrate from the upper and the lower sections.
Two sweet spots were found in the vertical part of the reservoir. They were along the points 9901.5 to 9904.5ft and 9907.5 to 9908.5ft. Permeability was found to be a bit high in the middle section, which is important for locating sweet spots. Results also show that there is a potential sweet spot in the upper section at point 9865ft.
Due to the fact the oil migrates from the middle section to the upper and lower sections, lateral drilling will be helpful in recovering the oil. It was discovered that there were many sweet points in the lateral part after the correlations were established from the cut off values. A further exploration is needed to establish which of them has the most accumulated hydrocarbon. The vast sweet spots can be explained by the length that goes laterally through the middle formation. No sweet spots were found in the upper and lower sections of the lateral part. The S2 ;20 was only used for the upper and lower sections since they are the source rocks.
Finally, the objective of the project was realized and sweet spots were located at points 9901.5 to 9904.5ft and at 9907.5 to 9908.5ft. It was discovered that sweet spots were only located in the middle section of the reservoir due to its low S2 and TOC values indicating that it is a potential reservoir rock. Due to this, oil migrates from the upper and the lower sections to the middle section. This justifies the company’s decision to use lateral drilling in the middle section of the Bakken formation.
Kouqi Liu, Mehdi Ostadahassan, Bailey Bubach, Hadi Jabbari, “Bakken Formation shales Nano-Scale Analysis Understand Mechanical Parameters,” p. 7, 2016.