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The aim of this paper is to solve the problems such as complex morphology of water cone and ridge, as well as out-of-control flooding resulting from the abundant natural energy, formation heterogeneity and irregular inter-bed distribution. In this paper, the numerical simulation technique combined with the orthogonal experimental design method is applied to study the factors of engineering and geology that affect the oil-water flow regularity on Tahe sandstone reservoir with bottom water, such as reservoir permeability, liquid production rate, interlayered property and scale and location, liquid production rate, length of horizontal well etc. Results show that reservoir permeability, inter-layer and liquid production rate are key factors that influence the performance of coning and ridging water; the optimal lengths of horizontal wells change with different permeability formation. Besides, inter-layer physical properties also play an important role in the vertical well location. This study could provide theoretical and technical guidance for the early well spacing, the technical strategy of development as well as measures for water control and stability in sandstone reservoir with bottom water.

The oil recovery and water cut are high in sandstone reservoir with bottom water drive with continuously supplying energy. However, Smith C.R.’s and others show that the formation of the bottom water cone is caused by the pressure gradient during oil production [

Due to the unique sedimentation types, the porous medium and flow matrix of sandstone reservoirs with the bottom water, Yu, Aggour M. A., Khana I. S. and Jin Lu have gotten that its coning rule, mining mechanism and the development characteristics are fundamentally different from the conventional limestone reservoirs [

Therefore, based on the existed experience, the oil and water flow law and development mechanism are studied by using the method of petroleum reservoir engineering and numerical simulation. In order to guide development practice in the bottom water reservoir of Tahe oilfield, the oil and water flow pattern is further explored. And the relevant study objectives are as follows.

• Build the Mechanism Model of Oil-water Flow in Bottom Water Sandstone Reservoir by Using Related Geological Parameters and Penetration Curve of Tahe Reservoir

• Analyze the relationship between development index, such as the recovery and water cut, and geological and engineering factors, such as permeability, the length of horizontal well, liquid production rate and sandwich distribution.

• Evaluate the sensitivity of each factor to the development index by using the orthogonal design principle

Numerical simulation is a method using the computer to solve the reservoir mathematical model, simulate the underground water flow, and give a time of oil and water distribution to predict the reservoir dynamics. The parameters of reservoir physical characteristics are originated from the Tahe oilfield, and the corresponding model has been built, which is close to the actual formation. The relevant parameters are as follows: the mid-depth is 3666 m, the effective thickness is 12 m, the depth of oil/water interface is 3672 m, the porosity is 0.208, the permeability is 733.3 mD, the initial oil saturation is 0.608, the permeability is 1.33, the oil density is 0.787g/cm^{3}, the water density is 1.141 g/cm^{3}, the volume coefficient is 1.34, the oil-water viscosity is 2.78 and 0.602 MPa respectively. There is a production well in the center of dimensional coordinates of the geological model with the top surrounded by closed boundaries. A grid system is consisted of 56 × 35 × 25 grids in the X - Z - Y three directions, with the vertical layer located at 1st - 24th layer (

DX | DY | DZ | |||
---|---|---|---|---|---|

1st - 3rd layer | 4th - 53th layer | 54th - 56th layer | 1st - 24th layer | 25th layer | |

50 | 10 | 50 | 10 | 0.5 | 5 |

There are six types of permeability which is consisted of 100, 300, 500, 700, 900 and 1200 mD to simulate the influence of permeability on oil-water flow in the presence of 130 m^{3}/d.

As is shown in Figures 3-5, lower permeability is more sensitive to the recovery ratio and water cut, and the reservoir permeability is logarithmically related

to the oil recovery ratio. The lower the reservoir permeability is, the more difficult the oil and water flow will be. When the permeability is less than 100 mD, the water cut and the recovery factor of the oil well keeps in a low level. With the increase of the permeability, the water breakthrough time is prolonged and the ultimate recovery factor is enhanced. The low recovery degree and the high water content correspond to the 100 mD (permeability value). Along with the development of the reservoir, the bottom water flows into the bottom of the well, so that it could lead to water breakthrough along predominant pathway prematurely, which results in a dramatical increase of the water cut and a serious effect on the oil production. Facing the poor fluidity at the low permeability, the method of acid fracturing can be used to improve the water displacement effectively.

There are six types of liquid production rate that are consisted of 30, 50, 70, 100, 130, 150 m^{3}/d to simulate the influence of liquid production rate on oil-water flow at 700 mD homogeneous reservoir to simulate the influence of daily fluid production rate.

As is shown in Figures 6-8 , the trend line is made up of several points in

pressure’s increase resulted from the high daily liquid production rate, combined with smooth flow channel, which contributes to the quick coning of the bottom water and the reservoir were seriously flooded before the oil well product water [

There are four velocities of the length of the well stage that consist of 200, 300, 400, 500 m horizontal section to simulate the influence of length of the well stage on oil-water flow in the 700 mD homogeneous reservoir under 130 m^{3}/d liquid production rate .

As is shown in

There are three factors which consist of six sandwich properties 0, 0.1, 1, 10 and 15 mD, four sandwich scales 1/100, 1/15, 1/4, 9/16 of the model area, four locations at 1/4, 1/2, 2/3, 1 of the height to simulate the influence of interlayer on oil-water flow in a 700 mD homogeneous reservoir under 130 m^{3}/d production rate.

As is shown in Figures 12-14, the trend line is made up of several points in

with the increase of interlayer permeability in different development years. When the interlayered permeability is higher than 15 mD, the recovery is almost unaffected by the interlayered permeability. Based on the fact that the bottom water plays double vertical and plane displacement role in oil-water flow when the interlayered permeability is 0.1 mD, the interlayer has an optimal permeability, which could contribute to the improvement of oil recovery factor and swept volume; In

According to the advantage that the orthogonal test could take the result of multi-factor interaction into account and avoid the impact of comprehensive experimental program and the cumbersome date, we could adapt the orthogonal test to reduce the number of tests and ensure the test results reliable. The interrelated influence of the eight factors such as the reservoir permeability, the length of horizontal wells, the property of an impervious break etc, are studied by applying this method in L_{27}, and the sensitivity on the oil recovery is studied at the end of 30 years (

The larger the range of the factor is, the more distinguished the influence on the recovery factor becomes. The order of the sensitivity is shown as follows: the reservoir permeability > the property of sandwich > the daily fluid production

factor Leve | K /mD | Z | Q /(m^{3}∙d) | L /m | H /m | Interlayer property | scale /(m × m) | location /m | FOE /% |
---|---|---|---|---|---|---|---|---|---|

1 | 100 | 10 | 30 | 200 | 4 | lithologic | 200 × 100 | top | 18.6 |

2 | 100 | 10 | 30 | 200 | 6 | physico | 300 × 200 | middle | 24.1 |

∙∙∙ | ∙∙∙ | ∙∙∙ | ∙∙∙ | ∙∙∙ | ∙∙∙ | ∙∙∙ | ∙∙∙ | ∙∙∙ | ∙∙∙ |

27 | 800 | 1 | 50 | 200 | 8 | physico | 200 × 100 | bottom | 35.8 |

K_{1} | 0.245 | 0.326 | 0.291 | 0.30 | 0.307 | 0.336 | 0.301 | 0.321 | |

K_{2} | 0.351 | 0.319 | 0.318 | 0.32 | 0.328 | 0.352 | 0.321 | 0.322 | |

K_{3} | 0.367 | 0.317 | 0.353 | 0.34 | 0.327 | 0.274 | 0.34 | 0.32 | |

range | 0.122 | 0.009 | 0.062 | 0.041 | 0.021 | 0.078 | 0.039 | 0.002 | |

optimal | 800 | 10 | 80 | 300 | 6 | physico | 400 × 300 | middle | |

sorting | 1 | 7 | 3 | 4 | 6 | 2 | 5 | 8 |

rate > the length of the horizontal well > the scale of the sandwich > the vertical permeability-to-horizontal permeability ratio > the position of the sandwich (_{1} - K_{3}): the reservoir permeability is 800 mD, the interlayer is permeable, the liquid production rate is 80 m^{3}, and the optimum level of the eight factors is the highest value of the horizontal average (K_{1} - K_{3}), the optimal combination is that the length of the horizontal well is 300 m, the sandwich scale is the drainage area of 1/2, the height of water avoidance is 6 m, the vertical permeability-to-horizontal permeability ratio is 10, the sandwich is located in the middle of the reservoir, which is almost the same result as the single variable method determining the reasonable parameters (

factor | K/mD | Q/(m^{3}∙d) | L/m | H/m | K_{Bnd}/mD | S_{Bnd}/mD | location | deviation | |
---|---|---|---|---|---|---|---|---|---|

sum of square of deviations | 794.907 | 3.62 | 173.787 | 76.247 | 24.02 | 307.407 | 70.847 | 0.187 | 1451.02 |

degrees of freedom | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 16 |

F ratio | 2.383 | 0.02 | 0.958 | 0.42 | 0.132 | 1.695 | 0.391 | 0.001 | |

F critical-value | 3.63 | 3.63 | 3.63 | 3.63 | 3.63 | 3.63 | 3.63 | 3.63 | |

significant | Extremely | not | highly | little | little | Extremely | little | not |

In order to study the influence of factors on the development effect, we need further analysis variance. Based on the significance of the factors, the reservoir permeability and the sandwich property are more sensitive than others in the oil recovery. And the daily liquid production rate, the scale of sandwich, the length of horizontal well and the height of water avoidance are little sensitive to the oil recovery; the effect of the sandwich position is not significant. Therefore, based on the formation physical property and the distribution of sandwich, the reasonable wells pattern and a reasonable working system are adopted to control the bottom of the water crest and cone implementing scheme in the field effectively.

By analyzing the mechanism of the water cone and ridge, a simplified numerical model has been built combined with the engineering and geology parameters. 8 factors in the model are simulated which affect oil-water flow regularity in sandstone reservoir with bottom water drive, evaluating the significance to development index. From this study it is clear that:

• The mechanism model can present the phenomenon of water coning and cresting in bottom water reservoir in some extent.

• The permeability of the reservoir and intermediate layer is more sensitive to the oil-water flowing than other factors, such as the daily fluid production rate, the scale and location of sandwich and so on.

• The water content will be more sensitive to the reservoir permeability with the decrease of the daily liquid production rate.

• As the permeability of the reservoir increases, the sensitivity curve of water-cut to the length of horizontal well climbs up and then declines and then increases, which means the optimum horizontal length.

• The various combination patterns of sandwich property and locations could contribute significantly different impacts on the oil water flow. The perforated section will immediately be adjacent to the bottom of the low-permeability or ultra-low permeability sandwich, and it should be kept at the same distance from the conventional impermeable interlayer as the interlayer is far away from oil/water contact to inhibit the water cone and crest.

This study was supported by national science and technology major projects of China (2016ZX05031004-001-002). Thanks to my supervisor Professor Yu Gaoming for his careful guidance.

The authors declare no conflicts of interest regarding the publication of this paper.

Zhai, M.K., Yu, G.M., Zhang, Y.Z. and Wang, C.C. (2018) Effects of Engineering and Geology Factors on Oil-Water Flow Regularity in Sandstone Reservoir with Bottom Water Drive. Open Journal of Yangtze Gas and Oil, 3, 240-252. https://doi.org/10.4236/ojogas.2018.34021

FOR is the oil recovery of field %;

K is the permeability of the reservoir mD;

K_{h}/K_{b} is the ratio of vertical permeability to horizontal permeability;

Q is the daily fluid of production rate m^{3}/d;

L is the length of horizontal well m;

H is the height of water avoidance m;

K_{bnd} is the permeability of interlayer mD;

S_{bnd} is the scale of interlayer m^{2}.