Rolling development technology of subtle reservoirs in middle and deep layers —— Taking lithologic reservoirs in the second member of Shahejie Formation in Tuo block of Shengtuo Oilfield as an example

During the sedimentary period from the lower sub-member of the third member of Shahejie Formation to the lower sub-member of the second member of Shahejie Formation in Shengtuo Oilfield, the conditions of source, reservoir and cap rock are very favorable, and lithologic reservoirs are easy to form. With the continuous improvement of exploration degree, the productivity construction of the new area will also be oriented to medium-deep lithologic reservoirs or some special lithologic reservoirs. From the aspects of sedimentary model determination, rolling tracking, logging constrained inversion and multi-means scheme optimization, new requirements are put forward for the preparation of underground geology and development scheme of oil and gas fields, especially the application of seismic reservoir prediction technology in the development stage, which makes the development of medium-deep lithologic reservoirs possible. Taking the rolling development of Tuo 142 block as an example, the successful application of rolling tracking, logging constrained inversion and facies-controlled oil reservoir description is expounded in detail. In view of the complexity of subtle reservoirs in the middle and deep layers, a complete set of rolling development technology is summarized.

Rolling tracking logging; Constrained inversion; Facies-controlled reservoir; Description of subtle reservoirs in middle and deep layers

I. Overview

At present, the main body of Shengtuo Oilfield has entered the ultra-high water cut stage and is in the stage of decreasing production. Through several rounds of fine reservoir description, it is found that the remaining oil in the main oil group is dispersed, and the production technology is high, so it is very difficult to stabilize the production only by tapping the potential. Shengtuo Oilfield is located in the compound oil and gas accumulation zone, with abundant oil and gas resources and various types of oil and gas reservoirs. During the sedimentary period from the third member of Shahejie Formation to the lower member of Shahejie Formation, alluvial fans, underwater fans and turbidite fans deposited in the northern part of Chenjiazhuang uplift and the eastern part of Dongying Delta front formed various sandstone lithologic reservoirs. With the improvement of exploration degree and exploration technology, finding such subtle oil and gas reservoirs has become an important goal to increase reserves and stabilize production. The productivity construction of the new area will also be oriented to medium-deep lithologic reservoirs or some special lithologic reservoirs. In the early stage of Shengtuo Oilfield development, some wells have encountered subtle reservoirs in the middle and deep layers, but due to the constraints of exploration degree and development technology, they have not been able to form a scale. The application of seismic reservoir prediction technology in the development stage makes it possible to develop lithologic reservoirs in the middle and deep layers quickly.

Since the Eighth Five-Year Plan, the application of seismic reservoir prediction technology in China has developed rapidly and the application effect is also remarkable. Among them, logging constrained seismic inversion technology represents the development trend of seismic reservoir prediction technology. In recent years, in order to optimize the development and deployment of complex lithologic reservoirs and improve the development effect and economic benefits, the development seismic community has carried out high-precision seismic reservoir prediction research. For shallow lithologic reservoirs (depth less than 2000 meters), seismic reservoir prediction accuracy is high. Under the condition of 1 constrained well /km2, the depth error of seismic reservoir prediction is generally less than 5m, and the thickness error is generally less than 3m. For medium-deep reservoirs, the prediction accuracy of seismic reservoirs is relatively poor, but timely tracking and rolling well placement still have a good guiding role in the development of complex lithologic reservoirs.

The second member of Shahejie Formation 15 sand group in Shengsan District of Shengtuo Oilfield is a delta front subfacies estuary bar deposit. 1999, the second member of Shahejie Formation 15 sand formation was discovered when interpreting the three-dimensional data in this area. From the infill well 1 ~ 7 in the second member of Shahejie Formation, select the first key well 3-4-2 16 in the favorable position of sand body to continue drilling and drill in the second member of Shahejie Formation 15 1. After that, the first logging constrained inversion was carried out by using the data of this well, and the second batch of key wells Tuo 142- 1 (coring) and Tuo 142-2 were deployed, all of which encountered the second member of Shahejie Formation 15 sand group, obtained industrial oil flow and opened Tuo/Kloc-0.

The thickness of Tuo 142 Shahejie Formation Member 2 15 Sand Formation is about 150m, and the upper reservoir is developed. Tuo 142 well area is simple in structure, with a fault in the northeast and lithologic pinch-out in the west and south. It is a monoclinic structure inclined to the southeast, with a dip angle of about 8 and a buried depth of 2580. After rolling development, the proven oil-bearing area of this block is 2.3km2, and the geological reserve is 560× 104t.

Secondly, the comprehensive study of fine geology established a geological conceptual model.

1. Determination of sedimentary facies

Tuo 142 sand body with dark mudstone. Through core observation, thin section identification and regional sedimentary data, it is considered that this sand body belongs to the estuary sand bar of delta front subfacies, and its provenance comes from the northeast. The plane of sand body is finger-shaped and thickens and forks towards the center of the lake.

(1) rock characteristics

According to the grain size data of Well Tuo 142- 1, the rock type should be silty sandstone, with median average grain size of 0. 13mm, median maximum grain size of 0.245mm and median minimum grain size of 0.062mm；; The shale content is 6.2%, the sorting coefficient is 1.67, and the sorting is medium.

The thin section analysis of casting shows that the average content of sandstone is 43%, feldspar is 27% and cuttings is 25%. The rock type is feldspathic lithic sandstone, sub-angular and round, with point-and-line cementation, reflecting the medium maturity of sandstone composition and structure.

(2) Sedimentary structures and biological fossils

The horizontal bedding, wavy cross bedding and mudstone interbedding in the core of the sand body reflect the sedimentary characteristics of the far sand bar in the delta front. Carbonaceous interlayer reflects interdental bay deposition in still water, and the appearance of snail fossils confirms the existence of interdental bay in delta plain.

(3) Probability curve and C-M diagram

The S-shaped C-M diagram of sand body and the development of RS section in it show a completely suspended transport mode that does not change with depth (Figure 1). Figure 2 reflects the probability distribution of traction current.

2. Seismic facies model

Seismic facies refers to seismic reflection units with a certain distribution range. Its seismic reflection parameters such as reflection structure, geometry, amplitude, frequency, continuity and layer velocity are different from those of adjacent units. For sedimentary bodies, seismic facies can be understood as the sum of the main characteristics of sedimentary facies on seismic reflection profile. After careful study of sedimentary facies and repeated comparison with the main characteristics of seismic reflection profile, it is considered that the wedge-shaped seismic reflection characteristics of Tuo 142 block conform to the estuary sand bar of delta front subfacies.

3. Electrical phase mode

The lithology of the second member of Shahejie Formation 15 sand group 1 sand body reservoir is mainly siltstone, and the lithologic combination is anti-rhythmic. The spontaneous potential curve is mainly funnel-shaped and a few are box-shaped. The spontaneous potential at the top of the second member of Shahejie Formation 15 sand body reservoir is bell-shaped and the lithologic combination is positive rhythm. The middle and lower parts are mainly funnel-shaped, and the lithology is mainly siltstone mixed with fine sand. The lithologic combination should be a compound rhythm dominated by anti-rhythm.

Figure 1 tuo 142- 1 Member 2 of Jingsha 15 C-M diagram of sand group samples.

Fig. 2 Particle size probability distribution diagram of Tuo 142- 1 Member 2 of Jingsha 15 sand group samples.

According to the above characteristic facies marks, the sedimentary facies types can be determined; Combined with sedimentary facies model, it is applied to the interpretation process of logging constrained inversion, which ensures the rationality and credibility of the interpretation results.

3. Using logging constrained inversion technology to process 3D seismic data.

Logging constrained seismic inversion is a model-based wave impedance inversion technique. This method uses logging data, takes seismic interpretation horizon as control, extrapolates and interpolates from well points to form initial wave impedance model; Then, the initial wave impedance model is constantly updated by using the * * * yoke gradient method, so that the synthetic record of the model is closest to the actual earthquake record, and the obtained wave impedance model is the inversion result.

Feasibility analysis of 1. logging constrained inversion

Tuo 142 sand body is the reservoir deposited by delta front estuary bar. The reservoir velocity is 3550m/s, the density is 2.26g/m3, the mudstone velocity is 2990m/s, and the density is 2.42g/m3. The impedance difference between them is 787 g/(m2 s). The buried depth of Tuo 142 sand body is about 2700m, and the average thickness of single sand body is greater than12 m.. When the surrounding rock is thick enough, the minimum thickness of sand body that can be distinguished by seismic data in this area at the depth of 2700 meters is only 12 meters, so it is difficult for logging constrained inversion.

At present, the application of deep seismic reservoir prediction technology in oilfield development is still in the exploration stage. In order to improve the prediction accuracy and effectively guide the development and deployment in this area, especially to confirm the feasibility of logging constrained inversion, the forward model verifies that the reflection of seismic event axis is the reflection of sand body and the existence of Tuo 142 sand body.

2. Reasonable selection and constrained inversion of rolling tracking logging parameters.

Tuo 142 sand body is divided into five independent sand bodies * * * from top to bottom, that is, Es2 15 sand group 1 ~ 5 sand body. 15 152 sand body has a large thickness and distribution scale, and has a good response to seismic profile, while 152 sand body has a large distribution scale. Only the boundary, shape and thickness changes of 15 1 and 152 sand bodies are predicted by inversion processing.

In the process of prediction, based on the actual data, the synthetic seismic records are carefully made, and the parameters are optimized by real well encryption, so that the inversion results can reflect the underground geological conditions more truly.

Due to the poor seismic geological conditions of reservoir prediction in this block, the prediction accuracy is not high. In order to effectively guide the well location deployment, six rolling tracking inversions were carried out in Tuo 142 block. According to the results of the first inversion, the first batch of key wells were deployed. After drilling, it was found that the thickness of sand body was different from the predicted result, so new wells were added for the second inversion. For the same reason, the third inversion added new wells. According to the results of the third reservoir prediction, combined with geology and reservoir work, the scheme of annual production capacity 10.2× 104t was compiled and adopted.

During the implementation of the scheme, the well is tracked while drilling, and not only the inversion profile is used to guide drilling, but also the inversion situation is adjusted by drilling data, which makes the next inversion more practical. After the implementation of the scheme well, the drilling success rate is 100%. Among them, 9 wells drilled sand bodies of Es2 15 sand group with an average effective thickness of 4m, and 6 wells drilled sand bodies of Es2 15 sand group with an average effective thickness of12.7m..

The fourth inversion of Tuo 142 block was carried out by using the drilling data after the implementation of the scheme, which expanded the sand body area and redrawn the map. Combined with the new understanding of geology and reservoir engineering, the petroleum geological reserve 193× 104t is increased.

After the fifth and sixth inversions, the sand body area is expanded again, and the petroleum geological reserve is increased by 442× 104t, and the edge expansion scheme with annual production capacity of 14. 1× 104t is worked out.

3. Accuracy analysis of logging constrained inversion

The application of the fourth constrained inversion in middle and deep reservoirs has achieved good results, but there are also some problems. For reservoirs with vertical upper layer thickness, rapid phase change and obvious lateral velocity change, they are not displayed after inversion, but both upper and lower reservoirs are displayed. How to correctly establish the initial wave impedance model reflecting the stratum change and how to give appropriate constraints need further research work. The reservoir near Shengbei fault is blank in the inversion profile, which shows that the signal-to-noise ratio of seismic data is greatly affected by the fault.

Table 1 logging constrained inversion prediction accuracy table

It can be seen from Table 1 that the inversion results of reservoirs with thicknesses less than 12m and more than 8m show that the top interface is clear, but the thickness determination is not ideal, indicating that the longitudinal resolution of the middle and deep layers is above 12m. Practice has proved that the application of logging constrained inversion in reservoir prediction in Shengtuo middle-deep layer is successful, but there are also problems of signal-to-noise ratio and resolution reduction of middle-deep seismic data.

Fourthly, the application of facies-controlled reservoir interpretation technology in middle and deep reservoir prediction.

Interpretation technology of facies-controlled reservoirs in subtle reservoirs in middle and deep layers is the most important link in seismic reservoir prediction technology. Its purpose is to use the established reservoir sedimentary model and seismic facies model to complete the mutual transformation and mutual verification of seismic facies and sedimentary facies, and then describe and predict the reservoir by using the seismic profile after logging constraint inversion. Because the process of mutual transformation and mutual confirmation between seismic facies and sedimentary facies is an approximately abstract process, its success depends on the experience of geologists and the reliability of various data generated due to the multiplicity of seismic facies, especially the limitation of seismic resolution, structural illusion and seismic velocity trap. Therefore, based on the data of drilling, logging, stratigraphy and geochemistry, and the sedimentary facies of subtle reservoirs, the corresponding relationship among core facies, logging facies and seismic facies is established to overcome the multiplicity of seismic facies and improve the accuracy of prediction and description of subtle reservoirs. Following this principle, facies-controlled reservoir interpretation technology should combine earthquake with geology, macro with micro, and qualitative with quantitative. Using the sedimentary facies model established by predecessors, the same (similar) seismic facies model is established for oil and gas reservoirs with the same (similar) genesis, and the profile after inversion is described and predicted by logging constraints.

1. Suggestions on facies-controlled reservoir interpretation technology

The reservoir near Shengbei fault is blank in the inversion profile. First, it is interpreted as an isolated wedge-shaped sand body with a gap of 200 ~ 300 m between it and the fault, which is different from the delta front subfacies estuary dam facies model in geologists' mind. After determining the sedimentary facies, a rolling well was deployed in the margin direction of the estuary dam and the blank area near the fault. Drilling more than 90 meters in Shahejie Formation Member 2 15 sand formation.

Fig. 3 Technical process of facies-controlled reservoir prediction

2. Technical process of facies-controlled reservoir interpretation

Based on the data of drilling, logging, stratigraphy and geochemistry, and on the basis of determining the sedimentary facies of subtle oil and gas reservoirs, the corresponding relationship among core facies, logging facies and seismic facies is established to overcome the ambiguity of using seismic facies only and improve the accuracy of prediction and description of subtle oil and gas reservoirs. After that, the inversion results can only be applied to the quantitative description of reservoirs after forward verification (Figure 3).

Fifth, choose the parameter optimization scheme design.

The optimization of reservoir engineering parameters is directly related to the reasonable compilation and implementation of development plan. Therefore, various methods should be adopted for optimization, and the engineering parameters of the same reservoir should be reasonably determined by analogy method, empirical formula method and similar reservoir numerical simulation method to ensure the rational development and utilization of the reservoir.

1. Dynamic analysis and understanding development

At present, 37 wells have been drilled and 29 wells have been put into production in Tuo 142 block. At the beginning, the average single well oil production was 3 1.3t/d, and by May 2000 1, the average single well oil production was 25.7t/d, the average dynamic liquid level was 1049.0m, and the cumulative oil production was 22. Through the analysis of the dynamic data of 29 trial production wells, we have the following understandings of this reservoir: ① The initial production of oil wells is high, but it decreases rapidly; ② The energy of edge and bottom water in the reservoir is insufficient; ③ The waterless oil production period in this block is long.

2. Development mode and mining mode

Due to the lack of water energy at the edge and bottom of the reservoir, natural energy is mainly elastic energy of the reservoir. The elastic recovery ratio of Tuo 142 block calculated by the mathematical model is only 6. 1%, which is low due to natural gas energy mining. Compared with water injection development, the oil recovery rate can be increased by 30%. Therefore, from the perspective of enhancing oil recovery and economic benefits, water injection development is a more suitable development method for this block.

3. Optimization of water injection timing

In order to make maximum use of formation energy, it is very important to choose a reasonable water injection time. Using mathematical model optimization, combined with production pressure difference and adjacent area analysis, it is concluded that stable production should be maintained after water cut, production pressure difference should be enlarged, and formation pressure should not be kept too low. Drawing on the experience that the total formation pressure of Tuo 7 block 1 1 and 12 sand bodies decreased to 7MPa before water injection, Tuo 142 block was initially selected for water injection when the total formation pressure decreased to 7MPa, that is, the formation pressure remained at about 20MPa.

At present, the formation pressure in Tuo 142 block is close to the level it should maintain, and the oil well production has declined to a great extent. The average dynamic liquid level of oil wells in this block is lower than 1000m, indicating that the edge water energy supply is insufficient, and the formation energy should be injected in time according to the original design.

4. Development series

According to the general principle of stratigraphic division and combination, it is necessary to ensure that the stratum has a certain oil layer thickness, geological reserves and productivity, and there is little difference between the physical properties and oil-bearing properties of the oil layer in the stratum, and there are stable interlayers and interlayers, with the main stratum as the main object and the secondary stratum as the consideration. The main sand bodies in Tuo 142 block are Sha-2 member 152 sand bodies, and Sha-2 member 15 1, 153, 154+5 sand bodies have certain reserves, but the scale is small. According to the comprehensive study of reservoir geology and reservoir engineering, the reservoir has the following characteristics: ① there are stable barriers between sand bodies; ② The productivity of each sand body is high; ③ Each sand body has an independent oil-water system; ④ There are some differences in crude oil properties among sand bodies; ⑤ There are certain material conditions. Shahejie Formation 15 1 sand body reserves are 49× 104t, Shahejie Formation 152 sand body reserves are 584× 104t and Shahejie Formation 153 sand body reserves are/kloc-0.

Therefore, this block has the conditions of slicing mining. According to the above principles and the actual situation, this block is divided into three sets of strata for mining: Shahejie Formation 15 1-2 sand body, Shahejie Formation 153 sand body and Shahejie Formation 154+5 sand body.

5. Well pattern and well spacing

With the infilling of well pattern, the ultimate recovery and recoverable reserves of the oilfield increase. However, after the well pattern is encrypted to a certain extent, the new output value brought by the increase of recoverable reserves will be less than the increased investment. Therefore, the reasonable well pattern density is the well pattern density when the new output value of the infilled well pattern is equal to the investment amount.

According to the above ideas, the calculation results of reasonable well pattern density of three layers in Tuo 142 block are as follows: Shahejie Formation 15 1 ~ 2 sand body, well pattern density is 7.68 wells /m2, total number of wells is 44, and well spacing is 315m; ; Shahejie Formation Member 2 153 sand body, well pattern density of 5.65 wells /m2, total number of wells 12, and well spacing of 360m; Member 2 of Shahejie Formation 154+5, with a well pattern density of 3.27 wells /m2, with a total of 8 wells with a spacing of 490m m. ..

6. Determination of production capacity

(1) determination of vertical well productivity

According to the data of production test wells, the production pressure difference of this well is 3.7 ~ 7.7 MPa, and the actual production pressure difference of the adjacent Tuo 7 block is 2.7 ~ 2.8 MPa. Because the permeability and other physical properties of this block are worse than those of Tuo 7 block, the production pressure difference of Tuo 142 block is 4MPa. According to the analysis of production test data, the oil recovery index is1.97 ~18.34t/(d MPa), the oil recovery index per meter should be 0.179 ~1.22t/(d MPa), and the average oil recovery index is 6.447t.

Considering that the reservoir needs to be mined by layers, the oil recovery index per meter should be calculated by layers. The oil recovery indexes of sand bodies in the second member of Shahejie Formation 15 1, 152 and 153 are 0.75, 0.52, 0.47 and 0.56 t/(d MPa) respectively.

According to the determined production pressure difference and rice production index, the initial single well productivity of four sand bodies is calculated. Es2 15 1 sand body 10t/d, es2 152 sand body 20t/d, es2 153 sand body 20t/d, es2154+.

(2) Determination of horizontal well productivity

Because the sand bodies of the second member of Shahejie Formation 153 and 155 in this block have bottom water, if horizontal wells are used for mining, bottom water coning can be effectively suppressed, water cut rising can be controlled, and economic benefits can be improved. According to the current horizontal well screening standards, the sand bodies of Shahejie Formation II 15 1, 152, 153 and 154+5 in this block all meet the production standards of horizontal wells. Because the well pattern of Shahejie Formation 152 sand body is basically perfect, the sand body of Shahejie Formation 152.

The initial productivity of horizontal wells should be determined according to static parameters, and the empirical calculation formula of single well productivity of horizontal wells should be adopted. After calculation, the oil production index is 33t/(d MPa). Considering the influence of edge and bottom water, the production pressure difference should not be too high. If the value is 1.5MPa, the horizontal well productivity is 49.5t/d, and the value is 40.0 t/d. ..

7. Programme deployment and indicator summary

According to the actual situation, the overall deployment and development of this block adopts the seven-point well pattern commonly used in Shengtuo Oilfield. There are 65 wells and 47 oil wells in total, including 9 old wells/kloc-0 and 28 new wells (4 horizontal wells). The annual production capacity in the first three years was 23.5× 104t. After 10 years, the recovery ratio is 20. 12% and the water cut is 85.2%.

Ending of intransitive verbs

The rolling development technology of subtle oil and gas reservoirs is the author's summary of the development scheme of medium and deep subtle oil and gas reservoirs in Shengtuo Oilfield. Through the research on the rolling development of the second member of Shahejie Formation 15 sandstone lithologic reservoir in Tuo 142 block, it provides successful experience for the rolling development of similar reservoirs. For the rolling development of subtle reservoirs in the middle and deep layers, the prediction accuracy of logging constrained inversion is relatively poor. Only by tracking the rolling well arrangement in time can it play a good guiding role in the development of complex lithologic reservoirs.