Research on reservoir heterogeneity of complex region

Abstract： From the context of this area， combined with the well，core and logging data， the characteristics of heterogeneity of this region can be studied. Through calculating the parameters like porosity， permeability，saturation，permeability coefficient of variation，sudden coefficient and differential， the heterogeneity in the layer and the plane has been studied. The sand groups in the Chang 6 reservoir mainly are delta front sand underwater distributary channel sand body， the heterogeneity in the layer is weak； In the plane， the heterogeneity of distributary channel sand body is weak， while the one in estuary dam， front sheet sands and underwater natural levee is strong. Through comprehensive analysis， Chang 6 reservoir in Hulangmao region is strong heterogeneity reservoir. After comparing permeability coefficient of variation，sudden coefficient and differential of 3 layer and 5 layer in Chang 6，it can conclude that the development effect of 5 layer is better than 3 layer in the process of water-flooding development. This research can be some guidance to some area in the mainland.

Key words： heterogeneity； Hulangmao region； Chang 6 reservoir

Ordos Basin， also known as the Sanganling Basin， it is north to Yin mountain，Big Qin mountain and Wolf mountain， south to Qinling； west to Helan mountain，Liupan mountain， east to Lvliang mountain. The basin is crossing Shanxi ，Gansu ，Shanxi ，Ningxia Hui Autonomous Region and Inner Mongolia autonomous Region five provinces， with the total area of 37×104km2 . The thickness of sedimentary rock in the basin is 5000-10000m， which is a whole settlement， depression migration， simple structure large polycyclic sedimentary basin， is the second medium-sized， Cenozoic sedimentary basin. The basement of basin is Archean，Proterozoic metamorphic basement， the Paleozoic， Mesozoic and Cenozoic cover sedimentary are overlying it， which has a dual structure， is a rich-prospecting oil and gas sedimentary basin.

The basin basement has a strong rigidity and stableness， the internal structure of the basin is relatively simple， except the tension and compression can still be seen clearly on the edge of basin， the rest of it are west inclined slope， the inclination is only about half a degree， the average decline of slope is about 8-10m/km， a series of smaller magnitude nose uplift develop on the large monoclinic. The geological structure of Hulangmao area in Jingan oilfield is located in Shanbei slope. As the regional tectonic is gentle， Chang 6 reservoir distribution is mainly controlled by lithology. . With the oil field development coming into the late， low well productivity and and poor water-flooding results gradually prominent. To further tap the development potential of Chang 6 reservoir of Hulangmao area， it is necessary to carry out research on reservoir heterogeneity

1 The plane heterogeneity

Reservoir plane heterogeneity refers to the plane geometry of sand， scale， connectivity， porosity and saturation and permeability changes which result in the heterogeneity， it directly affects the plane water-displacing-oil efficiency in the water-flooding development[1].

1.1 The plane distribution of sand body

The development of Chang 6 reservoir sand body is controlled by microfacies， the study area mainly developed four rivers in the plane， the provenance of the first， second， third diversion river come from the northeast， the center of river is located in line of Sai222 ～ Liu135-03， Liu119-19 ～ Liu127-13～ 133-8，Gao1～Liu124-23， the scale of sand body is large ； The fourth’s comes from east-north. In the flanks of four underwater distributary channel are developing underwater natural levee. Before arriving at slope， the four underwater divided river have meet， contiguous sand thickness， extending along the flow direction， forming the natural potential funnel-shaped ，estuary dam microfacies sand dam of sand body thickness flower-shaped distribution， where slightly larger area sheet sand microfacies deposition develop outside （Figure 1）.

1.2 variations in reservoir properties

Statistics on 29 the coring well reservoir properties data near study area， it can be found that Chang 6 has poor reservoir properties. Its average core analysis porosity is 12.06%， the average core analysis permeability is 2.91 × 10-3μm2. Differential of permeability is 2.13，the sudden coefficient is 1.39 ，core analysis permeability coefficient of variation is 0.37， reflecting a strong flat heterogeneity of the study area.

As an example that sandstone permeability flat distribution of 612 sand group ， the small layer is mainly underwater distributary channel sand body， its electric measuring permeability generally is more than 3 × 10-3μm2， porosity is between 12% -13% ， oil-bearing saturation is between 0.4% -0.5%. Sheet sand in natural levee on both sides of the river and the edge of estuary dam， its electric measuring permeability is generally 2-3 × 10-3μm2， porosity is between 10% -12%， oil-bearing saturation is between 0.3% -0.4%.

Changing ??range in permeability values and distribution of sand body in the channel has higher connectivity， along the river ， change in permeability is little； in the direction perpendicular to the river， penetration in the river flank gradually becomes smaller. Sand body in underwater distributary channel is thicker which has better properties and strong property of reservoir， in water-flooding process， it is easy to form water channels to obtain high yield. Both sides of the river natural levee have many sand interlayer， sand connectivity and reservoir properties are poor， so the development of production is not high.

2 heterogeneity in the layer

The heterogeneity in the layer refers to changes of reservoir properties in the vertical in a single sand layer， is the key geological parameters which directly effects and controls the flooding-thickness and sweep coefficient of the single sand layer. The heterogeneity within the layer is mainly about the analysis of rhythm within the layer， bedding structure and discontinuous sandwich within layer and so on[2].

2.1 rhythms in the layer

A large number of laboratory experiments and water-flooding practice around world show that in the process of the reservoir water-flooding development， the longitudinal spreading thickness is better on anti- rhythm， followed by complex rhythm， positive rhythm is the worst.

The Chang 6 mainly develops delta front subfacies deposition controlled by sedimentary cycles， mostly in positive rhythm， supplied by anti- rhythms and complex rhythms. Positive rhythm often is seen in the delta front underwater distributary channel and underwater natural levee ， the underwater distributary channel mainly has middle-thick fine-grained sandstone ， natural potential curve is bell-shaped or box-shaped， natural gamma curve is low ； underwater natural levee mainly has thin fine-grained sandstone and siltstone ， spontaneous potential curve is tooth-shaped ， natural gamma value is low - meddle . Anti rhythm is often seen in the estuary dam and front sheet sands ， the estuary dam is mainly middle-thick layer of siltstone ， shale-siltstone ， natural potential is funnel-shaped ， natural gamma curve is middle value ??； front sheet sands are mainly middle-thin siltstone， shale-siltstone ， natural potential curve is funnel-shaped and tooth-shaped， natural gamma curve values ??moderate. Complex rhythm occurred mainly in shunt bay which consists of mudstone， siltstone and laminated sand shale rocks， natural potential curve is low-flat， the clip has slightly negative anomaly value， value of natural gamma curves is high ，tooth-shaped. Because the Chang 6 reservoir granularity rhythm is mainly positive， therefore effects the longitudinal spreading thickness of the water-flooding and the development are poor.

2.2 characteristics of bedding

The Chang6 reservoirs mainly develop bedding type like parallel bedding，，ripple bedding， cross-bedding，convolute bedding and deformed bedding. The underwater distributary channel develops cross-bedding， parallel bedding and sand grain bedding （Figure 2）. The estuary dam mainly has parallel bedding and cross-bedding. Underwater natural levee and sheet sands develop horizontal bedding， ripple bedding and convolute bedding （Figure 3）. The shunt bay develops parallel bedding， ripple bedding and deformation bedding. Parallel bedding is common in argillaceous siltstone and silt-mudstone. On one hand， the parallel bedding in process of water-flooding is beneficial to the balance of water-displacing-oil and improves sweep coeffcient ， but it also reduces the permeability of the reservoir， particularly has great impact on the vertical permeability[3].

2.3 interbedded stratum in the layer

Interbedded stratum in the layer has the ability to isolate or block the effects on underground fluid in the process of water-flooding and mining and thus have a significant impact on water-displacing-oil process[4].

The interbedded stratum in the layer of Chang 6 reservoir is developed， mainly consist of mud and calcareous interbedded stratum. Lithology of muddy intercalation is mudstone and silty mudstone， which showed high tooth-shaped values ??in natural potential and natural gamma curve. Lithology of calcareous interbedded stratum is calcareous sand layer ， which showed a high spike value on electrical resistivity and acoustic-time curve.

Statistics show that the interbedded stratum occurs in the small layer of Chang 6 reservoir is average 1.9 ， which the one in Chang 612 ， 631 is average 2.2 ， the number of interbedded stratum is least one ， up to six ， heterogeneity is serious.

According to statistics of the number and drilling rate of sand bodies of 614 wells in the study area， it can draw that when the number of sand layer is 1-4 in Chang 611，631 ，its drilling rate is 1.1% -15.6 %， indicating that the number of sand layer in Chang 611，631 is less， rarely develop interbedded stratums ； The number of sand layer in Chang 612， Chang 621 and Chang 622 is up to seven， when the number of sand layer distribution ranges from 3-4， its drilling rate is 37.1 %.，30.0% ，31.9% ；Seen from the drilling rate of five layers ， the Chang 6 has more sand layers ， which illustrate the number of interbedded stratum is more， and then heterogeneity is more serious （Table1，Figure 4 ） .

3 The difference of heterogeneity on different waterflooding programs

In waterflooding development oilfield， reservoir permeability is an important factor on affecting the development of oil fields. Under normal circumstances， the division and correlation of a small layer are based on fully understanding microfacies. In waterflooding process， the accuracy and precision of small layer division can be verified by reservoir heterogeneity （poor permeability， mutation coefficient， coefficient of variation）. On the contrary， it also can be a reference to the small layer division in the study area.

Most of the 614 wells in this area meet the standard that the effective thickness of reservoir are less than 15， differential of permeability is less than 8， the sudden coefficient is less than 2.5， the coefficient of variation is less than 1.

For 614 wells of Chang 6 reservoir， every 6-9 wells consist of a well group， which it can be divided into 157 well groups. by mathematical statistics method， permeability contrast， sudden coefficient， the coefficient of variation of the 157 well groups have been counted to establish histograms of Layer 3 and Layer 5.

From the following differential histograms of Layer 3 and Layer 5， you can see the differential value of Chang 61 and Chang 62 in Layer 3 are mostly concentrated in the 4-6； while the differential value of Chang 611，Chang 612，Chang 621 and Chang 622 in Layer 5 are mostly concentrated in 2-4 （Figure 5）.

From the following sudden coefficient histograms of Layer 3 and Layer 5， you can see the sudden coefficient value of Chang 61 and Chang 62 in Layer 3 are mostly concentrated in the 1.5-2.5； while the sudden coefficient value of Chang 611，Chang 612，Chang 621 and Chang 622 in Layer 5 are mostly concentrated in 1-2 （Figure 6）.

From the following coefficient of variation histograms of Layer 3 and Layer 5， you can see the coefficient of variation value of Chang 61 and Chang 62 in Layer 3 are mostly concentrated in the 0.3-0.7； while the coefficient of variation value of Chang 611，Chang 612，Chang 621 and Chang 622 in Layer 5 are mostly concentrated in 0.2-0.6 （Figure 7 ）.

Conclusion

1， Among sand bodies of every microfacies in the Chang 6 reservoir of Hulangmou area， the heterogeneity of underwater distributary channel sand body is weak， but the one in estuary dam， front sheet sands and underwater natural levee is stronger.

2，The Chang 6 segment in study area is mainly developing delta front facies， mostly in positive rhythm， supplied by anti- rhythms and complex rhythms.. The positive rhythm often appears in delta front facies and underwater distributary channel， the oil and gas mainly distribute in the area which has relatively weak heterogeneity， good permeability and high saturation.

3，After analyzing permeability， porosity， oil-bearing saturation， the differential of core， the sudden coefficient and the developmental situation of interlayer and interbeded stratum of each small player in Chang 6 reservoir， it can conclude that， either in the plane or in the longitudinal direction， the heterogeneity are very serious. Seen from the differential of permeability， sudden coefficient and coefficient of variation of 157 well groups in the study area， those in the Layer 3 are always more than the Layer 5， which indicating that the heterogeneity of Layer 3 is stronger than the Layer 5.The division scheme of the Layer 5 comply with the standard that the effective thickness is less than 15， the permeability is less than 8，the onrushing coefficient is less than 2.5， the coefficient of variation is less than 1 .

References

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[2] XIA Wei-rong， ZHANG Zhan-feng， CHEN Shi-qin. “Geology of development of oil and gas field” [M] .Beijing： Petroleum Industry Press， 1999： 10- 12.

[3] DAI Ya-quan，ZHAO Jun-yin，LUO Jing-lan，ect.“Research on reservoir heterogeneity of Chang 2 segment of Pinqiao area in Ansai oilfield”[J]. Journal of Northwest University： Natural Science Edition，2010，40（2）：287-292.

[4] LI Yuan-hao，ZHANG Ming-ji，WANG Xiu-juan，ect. “Composite reservoir accumulation mechanism of Triassic Yanchang formation in Northwestern part of Ordos Basin”[J]. Lithologic reservoirs，2010，22（2）：32-36.