Study of the Law about Water-Cut Variation for the Fractured Metamorphic Reservoir of Buried Hill with Bottom Water — — A Case study at Budate Reservoir in Beir Depression , Hailar Basin

Aiming at the complex flowing environment including the buried hill of Metamorphite, the active bottom water and the fracture at Budate Reservoir within Beir Depression of the Hailar Basin, combining the laboratory studies and based on analysis of its drive mechanism, field wells’ parameters were used to analyze the effects of different conditions of the fractured metamorphic reservoir with bottom water on its law of water-cut variation and the waterflooding efficiency. The results show that for the Budate buried hill reservoir with bottom water, the gravity should be taken into consideration to determine reasonable perforation ratio and production pressure difference. And because of the acid sensitivity of the buried hill reservoir, application of proper clay stabilizer will enhance the field oil recovery to a satisfactory extent.


Introduction
Currently majority of the discovered buried hill reservoirs home and abroad belong to the type of carbonate reservoir [1] [2], in which there are complex types of pore canals always including solution crevice, fracture and so on.Thereby the fluid flowing inside shows the unique features.Many scholars home and abroad have made progressive advance in this area with the reservoir engineering method, numerical simulation and others [3][4][5].However, Budate Reservoir in the Beir depression of the Hailar Basin is the buried hill reservoir with bottom water, where it is very complicated of the fluid flowing laws that have not been reported academically.
So it is a new topic to perform the study of the law of fluid flowing in this kind of reservoir and the fruits acquired will have positive reference value for the same kind of reservoirs.

Reservoir Features of Budate Buried Hill
Budate Reservoir lies in the bottom of the Hailar sedi-mentary basin.It developed from the Trias and was composed mainly of the carbon siltpelite, a slightly metamorphite and the unequigranular feldspar rock-fragment sandstone.Due to the dissection of many faults, each faulted-block is a buried hill reservoir with its independent oil-water interface.
Budate Reservoir has geological reserve about 1811×10 4 t, but very poor physical properties such as the average effective porosity of 5.3％, the average gas permeability of 0.14×10 -3 μm 2 and water-sensitivity coefficient of 0.64 which shows a bit strong water sensitivity.
The fractures developed plus the pores make the formation a fracture-pore reservoir.Because the displacement energy source underlies the oil reservoir, at the oil-water interface below, the bottom water should firstly overcome the gravity itself then to displace crude oil bottom-up.In this process the action of gravity should be taken into consideration.

Mathematical Model of the Water-Cut Variation
In the development program of bottom water reservoir, perforation is always done at the top of the reservoir to avoid early water breakthrough.In contrast with the total reservoir thickness, the fluid flowing in the porous media could be presumed as the combination of the horizontal flowing at the top layers perforated and the vertical flowing in the sub-layers [7][8].Hen the equation about the water-cut variation could be derived as following:

Analysis of Law on Water-Cut Variation at
Budate Buried Hill Reservoir The Water-cut Variation curves under several production pressure differences are shown in Figure 3  2) If gravity considered, the water-cut increases along the rise of water saturation slowly.This phenomenon could be explained that the gravity of the bottom water itself that are displacing oil upwards decreases the water breakthrough or fingering, so as to slow the rising velocity of water-cut.3) If gravity considered, the sizes of production pressure differences have obvious influence on the law of Water-cut Variation.The less production pressure difference, the slower rising velocity of water-cut along with the increase of water saturation, so is its reduced extent.This phenomenon could be explained that the action of gravity becomes less along with the increase of production pressure differences and at a certain big value of production pressure differences the action of gravity could be neglected.1) Along with the increase of perforation ratio, the effect of gravity on reducing the rising velocity of water-cut becomes less.This case will be the nearly the same as that while gravity unconsidered.
2) Along with the increase of perforation ratio, the effect of production pressure difference on the increase of water-cut becomes less, even disappears.
When the oil layers are completely perforated, that's x=1, the fluid in the whole reservoir flows in the radial direction.Then the bottom water will drive the oil at the least efficiency and its energy will make the oil well drought.
Such laws discussed above are in accordance with the actual development cases for block-faulted reservoir with bottom water.

Effect of the Clay Stabilizers
The water sensitivity index of the core samples from Budate Reservoir fall into the range of 0.60～0.67(a bit strong water sensitivity).Besides the oil and water relative permeability curve measured, in the lab other two relative permeability curves were also measured with clay stabilizer CS-5 and CS-7 (see in Table 2).Then the effects of clay stabilizers on relative permeability curves and on the law of Water-cut Variation are analyzed to provide reference for the optimization of clay stabilizers at Budate Reservoir.
Based on the parameters from well D108-229 (see in Table 1), three curves of Water-cut Variation of water, CS-5 and CS-7 are shown in Figure 5 for gravity unconsidered and in Figure 6 for gravity considered in Figure 7.  Conclusions could be drawn as: 1) Using clay stabilizers can reduce the rising velocity of water-cut at the stage of low water saturation, that's oilfield can recovery more oil at the stage of low water saturation than that in the case of water flooding.
2) Using clay stabilizer CS-5 makes the point of water breakthrough later than that the case of water flooding.ter-cut along with the increase of water saturation, so is its reduced extent.At a certain big value of production pressure differences the action of gravity could be neglected.
2) Both the perforation ratio and the production pressure difference have great influence on the law of Wa-ter-cut Variation for the Block-faulted reservoir of buried hill with bottom water.Along with the increase of perforation ratio, the effect of gravity on reducing the rising velocity of water-cut becomes less.This water-cut curve will be the nearly the same as that while gravity unconsidered.Along with the increase of perforation ratio, the effect of production pressure difference on the increase of water-cut becomes less, even disappears.K g -gas permeability, 10 -3 μm 2 ; φ -porosity, dimensionless; S wi -irreducible water saturation, dimensionless; S or -residual oil saturation, dimensionless.

Figure 1 .
Figure 1.Schematic of the water core for well D112-227.We could recognize that at the Block-faulted reservoir of buried hill with bottom water, the gravity would have great influence on the law of Water-cut Variation as:Considering strong water sensitivity of the Beir Depression, in the lab three groups of relative permeability curves were measured as in Figure2, one displacing fluid is water, the others are two kinds of clay stabilizer solutions(CS-05 and CS-07).1)If gravity unconsidered (β=1), the calculated water-cut will increase more quickly than that in the case of gravity considered along with the change of water saturation.That's if gravity considered, during early period of low water saturation, the curves are steeper with slower rate of oil production and less oil recovery; during the later period of high water saturation, more oil can be produced and the residual oil retained by water displacement should be developed by the tertiary oil recov-

Figure 2 .
Figure 2. Relative permeability curves measured by different displacing agent

3 ) 4 ) 5 Conclusions 1 )
Using clay stabilizer CS-7 makes the rising velocity of water-cut smooth and makes the oil recovery at the low water saturation the biggest number but with earlier point of water breakthrough.Using clay stabilizers makes the flowing range of two phases wider.The anhydrous oil recovery efficiency and the ultimate number both obviously larger than that of the case of water flooding.Gravity has great influence on the law of Water-cut Variation for the Block-faulted reservoir of buried hill with bottom water: if gravity considered, the water-cut increases along with the rise of water saturation slowly at the stage of low water saturation.The less production pressure difference, the slower rising velocity of wa-

3 )
Clay stabilizers can reduce the rising velocity of water-cut at the stage of low water saturation and make the anhydrous oil recovery efficiency and the ultimate number both obviously larger than that of the case of water flooding.6 Nomenclature K rw -relative permeability water, dimensionless; K ro -relative permeability oil, dimensionless; μ w -water viscosity, mPa•s; μ o -oil viscosity, mPa•s, mPa•s; γ w -relative density of water, dimensionless; γ o -relative density of oil, dimensionless; H -thickness to avoid water in a oil well, m; α' -Reservoir pressure coefficient, dimensionless; R e -well spacing, m; R w -wellbore radius, m; S w -water saturation;