MODELING of Water Saturation in Shallow Sandstone Oilfield

3D geological modeling technology has achieved visualization of underground reservoirs in oil and gas fields, and has been widely applied in oil and gas field development. 3D geological modeling includes structural modeling and attribute modeling, and strata and faults can be completed based on data from oil and gas field exploration and development. The porosity and permeability in the geological attribute model can be established based on logging interpretation conclusions and other relevant data, The establishment of a saturation model has always been a difficulty in 3D geological modelling. As the water saturation, is different from porosity and permeability of an oil field. Saturation is distributed under the control of factors such as gravity and capillary force, and cannot be established through spatial simulation interpolation. Instead, the influence of factors such as gravity and capillary force should be considered. This article takes the shallow sandstone M oilfield in Kazakhstan as an example, and establishes the J function of the oilfield by combining the capillary pressure experiment of the core well, in order to establish an oilfield saturation model.


General Situation
The research area is the M oilfield located on the right bank of the Enba River in the Tiemir District of Aktobin Oblast, Kazakhstan, with a total area of approximately 50km 2 .The surface of the oil field is a gentle hilly plain, and the absolute elevation within the working area ranges from 215 to 275m.The entire oil field is covered with dense brown sandy loam soil, with very few being covered with loose brown loam soil, salt marshes, and no forest areas.The surface of the oil field presents a desert landscape landform, manifested as a thin layer of barren humus soil vegetation layer, and the surface is covered with sparse herbaceous plants that are desertified and semi desertified.Therefore, the land in this area is not suitable for agricultural cultivation and belongs to a poor pasture.The hydrological network within the oilfield belongs to the Enba River Basin.The large rivers are the Emba River and the Tiemir River, which extend to the right.The water flow in the valley is relatively stable, and surface water flow in individual blocks will be interrupted, only remaining in the alluvial layer at the mouth of the river.The smallest tributaries and canyons both have arid riverbed, and the water flow in the riverbed can only be seen during periods of snow melting and heavy rain.The area has an extreme continental climate with cold winters and hot and dry summers.The annual average temperature is+6 degrees Celsius, The lowest temperature in winter is -33.9 degrees Celsius, The highest temperature in summer is+36.7 degrees Celsius.The duration of snow cover is 130-150 days.The average annual precipitation fluctuates between 180-220mm.The oil field has developed highways, and a dirt road network has been widely developed in the area, which can be used for most of the year.

Regional Structural Characteristics
In terms of regional geological structure, the study area is located on the eastern slope of the Caspian Sea Basin, with some located on the eastern edge of the slope zone.According to geological geophysical data, the surface of the crystalline basement in this area is located at a depth of 8km underground.There are five large anticlines in a nearly north-south direction distributed on the eastern edge of the slope zone, accompanied by local salt dome structures.The salt dome structure at the edge of the basin belongs to the type of fault depression and concealed fault depression, with the top of the salt body buried at a depth of 300-600m.The target layer in the research area is located in the upper strata of the salt dome, and the rock dislocation in the stratigraphic is small, which is related to the one-time structural origin of the salt dome and the rare occurrence of fault structures.
A salt dome with the same name as the oil field developed in the research area.The salt dome is a short axis anticline embedded in the Permian to Upper Cretaceous strata.The research objective of this article is the shallow area above the salt layer, which is relatively simple and mainly controlled by the salt dome structure.It forms a large drape around the salt dome, providing a favorable structural background for the salt reservoir.Four normal faults with a northeast strike are mainly developed within the M oilfield work area, with steep dip angles.The work area is divided into three parts: western, central, and eastern, resulting in shallow reservoir burial in the eastern and western regions and deep reservoir burial in the central region (see Fig. 1 and 2).This article studies two sets of strata, R-II and R-III, on the salt layer.R-II is subdivided into three sub layers: K1a, K1b, and K1h, while R-III is subdivided into three sub layers: U1, U2, and U3.The sedimentary environment is delta front facies, with sandstone mainly composed of fine sand and silt.The reservoir is relatively loose and has good physical properties.The average porosity explained by logging is 31.1%,and the average permeability is 1560mD.It is a high porosity and high permeability reservoir.

Establishing the J-Function of Oil Field Water Saturation
The J-function method is a commonly used method that combines conventional mercury intrusion data and physical parameters to characterize reservoir pore throat characteristics [1] .Therefore, the J-function method established by Leverett is first used to determine the average capillary pressure curve of the entire sedimentary facies zone [2] .J(S w ) = P c σcosθ √k φ ⁄ Eq. 1 Among them: J (Sw) -J function; Pc -capillary pressure; σ-Interface tension; θ-Wetting contact angle; K -Air permeability; φthe porosity.
Averaging the capillary pressure curve: By using equation 1, the corresponding data points of Pc and Sw measured in the laboratory can be converted into J (Sw) function and Sw corresponding data points.
This article takes the K1b sub layer of R-II and the U2 sub layer of R-III as examples, and chooses to use a power function to fit the J function: In the formula: a is the dimensionless displacement pressure of the J function of the core; B is the Jfunction curve index of the core.
Figure 3 shows the J-function model of the two small layers in the study area (see Figure 3).  =  *   *   Eq. 7 Using equations 6, calculate the values of c and d to determine the average capillary pressure of the reservoir.The average capillary pressure curve is as follows (see Fig. 4): K1br sub layer:   = 0.0053 *  0.0649 *   Eq. 8 U-II sub layer:   = 0.0058 *  0.0677 *   Eq. 9 Eq.11

3D Geological Modeling of Water Saturation
Based on the water saturation formula established using the J function, a three-dimensional geological model of water saturation was established [3] .Firstly, based on the reservoir facies, the water saturation value of non reservoir layers is set to 1.The reservoir facies is established in two parts, with residual oil areas below the oil-water interface.This part of oil cannot be extracted and is not included in the crude oil reserve range.Therefore, the water saturation value can be set to 1; In reservoirs above the oil-water interface, the water saturation is calculated based on the oil column, and a three-dimensional water saturation model for the entire area is established [4] (see Fig. 5)

Figure 1 .
Figure 1.Top Structure Map of M Oilfield

Figure 3 .
Figure 3. J-function and standardized water saturation intersection diagram

Figure 4 .
Figure 4. Intersection diagram of mercury saturation and average capillary pressure