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The tight sand reservoir in Dabei Area has been the main block of exploration and development of natural gas inTarimBasin. Because of low porosity and fracture development, there exist errors in calculation of reservoir saturation. According to micro-resistivity image logging and acoustic full-wave logging, the reservoir fractural effectiveness is quantitatively evaluated; the result indicates that the reservoir with Stoneley wave permeability is greater than 0.2 × 10
^{-3}μm
^{2}; the reservoir connection is good. If the FVPA is greater than 0.055%; the fractures are developed. A new matrix saturation model is established based on the conductive pore water in consideration of the influence of low porosity. After modeling and analyzing the effect of porosity and its occurrence on the cementation index, the method for saturation calculation in Kuqa Area is established: the newly established dual porosity model is for fracture developed reservoirs, and the model based on the conductive pore water is for fracture less-developed reservoirs. By comparing the results of saturation in mercury injection experiment from coring section, precision of the calculation method is proven.

The tight sandstone reservoir of the Cretaceous Bashijiqike Formation in Dabei Area of Kuqa Depression in Xinjiang Tarim Oilfield is buried below 6000 m; its porosity is generally between 3% and 8%, and its permeability is between 0.01 and 0.1 mD; it belongs to a deep tight sandstone reservoir [

Sulige Gas Field in China was used as example by Li Xia (2013), methods for identifying tight sandstone gas reservoirs were denoted based on conventional logging data, these methods included the contrast method of equivalent P-wave elasticity modulus, A-K intersection combination method, the intersection of P-wave and S-wave ratios with Poisson’s ratio and the method of Poisson’s ratio and compression coefficient ratio [

In allusion to the issues of deep burial, higher pressure, high temperature and hard for identification of gas and water layers in the tight sandstone gas reservoirs in Dabei Area, by using the logging data as the major means and on the basis of fine evaluation of reservoir fractures, a novel approach for calculating saturation is established in consideration of the influential factors of fractures and low porosity and etc., which can meet the requirements of evaluating the saturation and precision of tight sandstone gas reservoirs in Babei Area of Tarim Basin.

Whether the fractures in tight sandstone reservoirs are grown is the key indicator for determining the value of development in the reservoir, the qualitative and quantitative depiction of fractures is the prerequisite of consequent calculation of saturation in the reservoir. Micro-fractures are generally developed in the tight sandstone reservoirs in Kuqa Depression [

Micro-resistivity imaging logging was one of the effective methods of fracture identification [

porosity in Track 4 is obtained from the porosity difference calculated from the curves of density logging and sonic velocity logging [

For the evaluation of fracture development by taking the reservoir as a characteristic scale, the reservoir characteristic parameters from acoustic and electric loggings are output based on the reservoir. The tight sandstone reservoir types in Dabei Area are classified based on the data of mercury intrusion capillary curve and thin slice analysis. The reservoirs are divided into 4 types, Type I: ϕ (permeability) > 9%, K (porosity) > 1 mD; Type II: ϕ is 6% - 9%, K is 0.1 - 1 mD; Type III: ϕ is 3.5% - 6%, K is 0.055 - 0.11 mD; Dry Layer: ϕ < 3.5%, K < 0.055 mD. In

above criterion.

In a common resistivity logging, fluid type in reservoir can be qualitatively distinguished according to the characteristics of formation invasion, but this method is only suitable for the formations with conventional porosity and permeability. In the tight sandstone reservoirs in deep formation of Dabei Area, permeability is poor, at the same time, as it is influenced by fractures, the curve between the deep induction resistivity and shallow induction resistivity is not very different, the fluid is impossibly identified based on the invasion characteristics (it is shown as the curve in Track III of

For sandstone formation, it is generally believable that Archie Theory and its improved method are used for saturation calculations [

According to the Archie theory [

R I = R t R o = b S w n (1)

where in Equation (1), the exponent of saturation n is a given value, that is in the cross plot of resistivity increase coefficient and water saturation in a double logarithmic coordinate, the fitting relationship between the both is in a straight line, but there are roughly 4 relationships shown in

σ = σ w ( P − P c 1 − P c ) t (2)

As P only represents the conductive formation water in the rock pores, it is simply called a conductive pore water, thus P = S w ϕ , if X w = P c , μ = t , then Equation (2) is turned into

R t = R w ⋅ ( 1 − X w S w ϕ − X w ) μ (3)

If there exists a rock saturation, there is a rock resistivity

R o = R w ⋅ ( 1 − X w ϕ − X w ) μ (4)

By combination of Equation (3) with Equation (4), a resistivity increase coefficient is obtained

R I = R t R o = ( ϕ − X w ϕ S w − X w ) μ = ( 1 − X w / ϕ S w − X w / ϕ ) μ (5)

According to Equation (2), X w = − 0.01 , μ = 2 , a tendency plot of resistivity increase coefficient changing with different porosities can be derived (

If a cementation index equation is introduced

R o R w = a ϕ m (6)

It is combined with Equation (5), a new equation for saturation calculation is obtained:

S w = ( 1 − X w ϕ ) ⋅ ( a R w ϕ m R t ) 1 μ + X w ϕ (7)

In the equation, the effect of different porosities on the resistivity increase coefficient is taken into account, which is suitable for quantitative calculation of saturation in the reservoirs with low porosity.

In the conductive pore water, only the effects of low porosity and high irreducible water are taken into account without taking the fractures into account. By using the idea of dual porosity medium theory, the pore space volume is divided into 2 parts, which include the matrix pores and fractures. For the rock per unit volume, the volume that the rock takes is ϕ f , then the volume that the matrix pores take is ϕ b = 1 − ϕ f where because the matrix pores are smaller, it can be considered that there is no mud invasion, while because of good permeability in the fractures, it is easy for mud invasion. Based on the above rock model, the rock resistivity R tb of matrix pores can be calculated through rock resistivity R t and fracture pore ϕ f , the equation for calculation is :

R tb = R t R mf ( 1 − ϕ f m f ) R mf − R t ϕ f m (8)

1) The matrix saturation: in consideration of the impact of porosity and irreducible water on the matrix saturation, the equation based on conductive pore water is used.

S wb = ( 1 − X w ϕ ) ⋅ ( a R w ϕ m b R t ) 1 μ + X w ϕ (9)

where, the cementation index m b of the matrix pores can be obtained through a litho-electric experiment of small cores sampling from the area, as shown in

Lithology | Pebbly sandstone | Mid-sandstone | Fine-sandstone |
---|---|---|---|

m_{b} | 2.017 | 1.943 | 1.966 |

2) The water saturation in fractures: according to the permeability property of dual pore types, S xof = 1.0 can be obtained from:

S wf nf = ( 1 / R t ) − ( 1 / R XO ) + ( 1 / R mf ) ϕ f m f ( 1 / R w ) ϕ f m f (10)

To examine the impact of pore space of fractures and etc in the reservoirs on the cementing index m, the rock core is simplified into a regular hexahedron, the length of its edge is a, a oblique crossing joint with the dip angle as β and its width as d f breaks through the whole hexahedron, where pores and holes are developed internally. It is also simplified as a regular hexahedron with its length of edge as b, and the pores and holes are parallel with fractures (see the model shown in

m f = − log [ a ′ ⋅ ( 1 b ′ + d f + a ′ − b ′ ⋅ cos β a ′ ⋅ d f ⋅ cos β ) ] log { [ ( a ′ 2 cos β − b ′ 2 ) ⋅ d f + b ′ 3 ] / a ′ 3 } (11)

According to Equation (11), make the edge length of rocks as 1, the relationship between the pore cementing index, width and dip angle of the fractures shown in

In consideration of fractures with narrow width in the area, but their dip angles are generally larger, the bond index of fractures m f takes 1.2 - 1.5.

3) The total water saturation: the total water saturation S w in the formation is calculated with the following equation:

S w = ϕ b S wb + ϕ f S wf ϕ b + ϕ f (12)

The result of well data process in a wellblock of Dabei Area indicates that when the apparent fractural pore is bigger than 0.055%, the saturation based on dual pores is 1.5% bigger than that of the one calculated with conductive pore water. Therefore the method for saturation calculation in the well block is: when the apparent fractural pore is smaller than 0.055%, the equation based on conductive pore water is used for saturation calculation. When the apparent fractural pore is bigger than 0.055%, the dual pore equation for saturation calculation is used (

Depth/m | Porosity/% | Permeability /mD | Core saturation/% | Calculation saturation/% | Relative error/% |
---|---|---|---|---|---|

5319.887 | 6.65 | 0.114 | 45.66 | 48.08 | 5.30 |

5320.237 | 8.27 | 0.071 | 45.67 | 47.09 | 3.10 |

5320.307 | 8.77 | 0.085 | 45.68 | 46.58 | 1.98 |

5320.437 | 7.40 | 0.064 | 45.68 | 45.69 | 0.01 |

5320.497 | 8.40 | 0.095 | 45.68 | 45.34 | 0.75 |

5320.606 | 7.75 | 0.103 | 45.68 | 45.54 | 0.32 |

5320.737 | 5.56 | 0.064 | 45.69 | 46.53 | 1.84 |

5320.917 | 4.04 | 0.062 | 45.69 | 49.32 | 7.94 |

5321.097 | 5.41 | 0.054 | 45.70 | 47.98 | 4.99 |

5321.387 | 7.54 | 0.064 | 45.71 | 41.28 | 9.69 |

5321.457 | 7.92 | 0.002 | 45.71 | 38.93 | 14.83 |

5321.557 | 7.82 | 0.034 | 38.71 | 36.25 | 6.36 |

5321.657 | 6.60 | 0.054 | 35.72 | 34.35 | 3.83 |

5321.827 | 5.67 | 0.073 | 36.72 | 39.44 | 7.40 |

5322.137 | 3.34 | 0.002 | 55.73 | 71.33 | 27.99 |

5322.396 | 3.46 | 0.057 | 53.74 | 58.01 | 7.94 |

5322.517 | 5.19 | 0.039 | 45.75 | 45.74 | 0.01 |

5322.637 | 6.35 | 0.065 | 45.48 | 47.92 | 5.36 |

5322.757 | 8.74 | 0.141 | 45.52 | 45.73 | 0.46 |

5322.896 | 8.56 | 0.195 | 45.57 | 44.00 | 3.45 |

5323.046 | 8.92 | 0.106 | 45.61 | 42.03 | 7.85 |

5323.247 | 8.90 | 0.126 | 45.68 | 38.98 | 14.67 |

5323.347 | 8.49 | 0.039 | 45.71 | 37.29 | 18.42 |

5323.527 | 7.82 | 0.004 | 45.77 | 33.85 | 26.04 |

5323.637 | 6.04 | 0.186 | 35.78 | 30.15 | 15.74 |

5323.777 | 5.20 | 0.070 | 37.79 | 33.03 | 12.58 |

5323.937 | 5.64 | 0.061 | 45.79 | 40.32 | 11.95 |

5324.177 | 3.56 | 0.057 | 45.80 | 53.27 | 16.32 |

5324.437 | 4.37 | 0.063 | 45.81 | 55.33 | 20.79 |

5324.537 | 3.36 | 0.050 | 55.81 | 54.98 | 1.49 |

5324.836 | 5.77 | 0.061 | 57.82 | 51.90 | 10.24 |

5324.987 | 6.12 | 0.066 | 45.82 | 53.40 | 16.54 |

5325.197 | 4.69 | 0.057 | 45.83 | 52.18 | 13.86 |

5325.336 | 6.00 | 0.059 | 45.83 | 49.88 | 8.83 |

5325.467 | 6.91 | 0.067 | 45.84 | 44.60 | 2.70 |

5325.637 | 6.27 | 0.080 | 45.84 | 41.69 | 9.06 |

5325.787 | 8.11 | 0.063 | 45.85 | 42.20 | 7.96 |

5325.887 | 7.27 | 0.064 | 45.85 | 43.90 | 4.26 |

5326.017 | 6.79 | 0.068 | 45.86 | 50.32 | 9.73 |

5326.197 | 7.40 | 0.070 | 62.86 | 58.61 | 6.76 |

5326.327 | 5.12 | 0.086 | 65.87 | 57.02 | 13.43 |

5326.467 | 3.01 | 0.087 | 45.87 | 50.69 | 10.51 |

5326.637 | 4.08 | 0.070 | 45.88 | 48.19 | 5.04 |

Average | 8.80 |

The accurate evaluation of fractures is an essential and important step in tight sandstone reservoir evaluation. The micro-resistivity logging and full wave acoustic logging can be used for a better fracture evaluation. The micro-resisti- vity logging imaging cab be used for obtaining the intersection of apparent fractural porosity and Stoneley wave permeability; the difference between porosities calculated by density and acoustic loggings can be used for obtaining the apparent fractural porosity for evaluation. If there are no data of micro-resistivity logging, the evaluation should be carried out by using the difference between porosities calculated by density and acoustic loggings, but the result is poorer. The tight sand reservoirs in Dabei Area can be divided into 4 types by stoneley permeability and apparent fractural porosity.

Influenced by physical property, the resistivity increase index is not a given value. The effects of low porosity and high irreducible water on the saturation of tight sandstone reservoirs are taken into account in the saturation model established by using conductive pore water. The RI-S_{w} relationship established in the study is consistent with the result of rock core testing, which indicates that the method is suitable for the evaluation of tight sandstone reservoirs.

According to the result of evaluation from Dabei Area, it indicates that when the apparent fractural porosity is less than 0.055%; the equation based on conductive pore water is used for calculation; when the apparent fractural porosity is more than 0.055%, the equation based on the theory of dual porous media is deployed. And the concrete operation flow can be seen in

Supported by PetroChina Innovation Foundation (2015D-5006-0305).

Tang, J., Xin, Y., Cai, D.Y. and Zhang, C.G. (2018) A Method of Calculating Saturation for Tight Sandstone Reservoirs: A Case of Tight Sandstone Reservoir in Dabei Area of Kuqa Depression in Tarim Basin of NW China. Open Journal of Yangtze Gas and Oil, 3, 21-35. https://doi.org/10.4236/ojogas.2018.31003

R I is the resistivity increasing index, dimensionless;

R t is the formation true resistivity, Ω∙m;

R o is the full water saturated formation resistivity, Ω∙m;

S w is the water saturaton,%;

n is the exponent of saturation, dimensionless;

b is coefficient of saturation, dimensionless;

σ is the formation conductivity, 1/(Ω∙m);

σ w is the formation water conductivity, 1/(Ω∙m);

P is the volumetric ratio of unit rock fluid in the whole rock, %;

P c is the lower limit value of conduction in pore fluid in the unit rock, %;

t is a proportional coefficient, dimensionless;

R w is formation water resistivity, Ω∙m;

X w is the volume of conductive pore water , %;

ϕ is the formation porosity, %;

μ is a proportional coefficient, same as t , dimensionless;

a is the coefficient in the formaiton resistivity Factor-Porosity relationship, dimensionless;

m is the exponent in relationship: F = a ϕ − m , dimensionless;

R tb is the resistivity of matrix pores, Ω∙m;

ϕ b is the porosity of matrix pores, %;

ϕ f is the porosity of fracture, %;

R mf is the mud filtrate resistivity, Ω∙m;

m f is the exponent in relationship: F = a ϕ − m only for fracture, dimensionless;

m b is the exponent in relationship: F = a ϕ − m only for matrix pores, dimensionless;

S wb is the water saturaton in matrix pores,%;

S wf is the water saturaton in fracture,%;

n f is the exponent of saturation for fracture, dimensionless;

R XO is the resistivity of flushed zone, Ω∙m;

a ′ is the length of edge in the analytical model, m;

b ′ is the length of small pore in the analytical model, m;

β is the incliation of fracture, radian;

d f is the width of fracture, m.