Optimization of integrated geological-engineering design of volume fracturing with fan-shaped well pattern

According to the variable toe-to-heel well spacing, combined with the dislocation theory, discrete lattice method, and finite-element-method(FEM) based fluid-solid coupling, an integrated geological-engineering method of volume fracturing for fan-shaped well pattern is proposed considering the geomechanical modeling, induced stress calculation, hydraulic fracturing simulation, and post-frac productivity evaluation. Besides, we propose the differential fracturing design for the conventional productivity-area and the potential production area for fan-shaped horizontal wells. After the fracturing of the conventional production area for H1 fan-shaped well platform, the research shows that the maximum reduction of the horizontal principal stress difference in the potential productivity-area is 0.2 MPa, which cannot cause the stress reversal, but this reduction is still conducive to the lateral propagation of hydraulic fractures. According to the optimized fracturing design, in zone-Ⅰ of the potential production area, only Well 2 is fractured, with a cluster spacing of 30 m and an injection rate of 12 m^(3)/min per stage;in zone-Ⅱ, Well 2 is fractured before Well 3, with a cluster spacing of 30 m and an injection rate of 12 m^(3)/min per stage. The swept area of the pore pressure drop in the potential production area is small, showing that the reservoir is not well developed. The hydraulic fracturing in the toe area can be improved by, for example, properly densifying the fractures and adjusting the fracture distribution, in order to enhance the swept volume and increase the reservoir utilization.

Diagnostic Method and Adaptability Analysis of Multiple Water Breakthroughs in Horizontal Well in Combined Well Pattern

The combined well pattern has been widely used in reservoir development, with a common pattern being a horizontal well in the center for oil production and multiple vertical wells around for water injection. For this type of well pattern, determining the horizontal well is affected by which injection wells, especially when the injecting water breaks through, accurately determining the direction of water inflow will provide an important basis for targeted water well measures. Based on the production performance data of horizontal wells, the semi logarithmic relationship curves of water-oil ratio, derivative water-oil ratio, and cumulative production were used for the first time to determine the breakthrough problem of water injection in the surrounding water injection wells of horizontal wells based on their response characteristics. The adaptability of this method under different influencing factors was analyzed. Introducing the parameter of cumulative production not only preserves the variation trend of the derivative of water-oil ratio with time, but also facilitates the processing of actual production data.

Deployment and Exploration of a Gas Storage Well Pattern Based on the Threshold Radius

To tackle the problem that wells that are deployed in a specific pattern based on the requirements of gas reservoir development are not suitable for gas storage,we have conducted concentrically circular injection and production simulation experiments for gas storage,discovered the existence of a threshold radius,denoted by Rt,and derived the expression for Rt.Based on the analysis and discussion results,we propose a strategy for deploying gas storage wells in specific patterns.The expression for Rt shows that it is affected by factors such as the gas storage gas production/injection time,the upper pressure limit,the lower pressure limit,the bottomhole flow pressure at the ends of injection and production,the and permeability.The analysis and discussion results show that the Rt of a gas storage facility is much smaller than the Rt for gas reservoir development.In the gas storage facilities in China,the Rt for gas production is less than the Rt for the gas injection,and Rt increases with the difference in the operating pressure and with permeability K.Based on the characteristics of Rt,we propose three suggestions for gas storage well pattern deployment:(1)calculate Rt according to the designed functions of the gas storage facility and deploy the well pattern according to Rt;(2)deploy sparser,large-wellbore patterns in high-permeability areas and denser,small-wellbore patterns in high-permeability areas;and(3)achieve the gas injection well pattern by new drilling,and the gas production well pattern through a combination of the gas injection well pattern and old wells.By assessing a gas storage facility with a perfect well pattern after a number of adjustments,we found that the Rt of the 12 wells calculated in this paper is basically close to the corresponding actual radius,which validates our method.The results of this study provide a methodological basis for well pattern deployment in new gas storage construction.

Performance analysis and optimal design for well patterns in anisotropic formations

The mechanism of the effects of anisotropic permeability on well patterns and reservoir development are investigated by coordinate transformation, fluid flow analysis, and reservoir development concepts. Anisotropy of permeability has reconstructive effects on well patterns. The originally designed flooding units are broken up, and new pattern units are made up of the wells that belong to different original units. The behavior possesses strong randomness, and leads to a complicated relationship among the injection and production wells, and unpredictable productivity of the formations. To prevent the break-up of well patterns, well lines should be either parallel or perpendicular to the maximum principal direction of the anisotropic permeability （i.e. the fracture direction）. To optimize the development effects ofanisotropic formations, the latitudinal and longitudinal well spacing of the well network are calculated from the principal values of the anisotropic permeability.

Analysis of the co-seismic responses of the fluid well pattern system in Jiangsu Province to the Wenchuan and Tohoku earthquakes

This thesis discusses the earthquake reflecting ability of the observation well pattern system of Jiangsu Province, China, which has been digitally renovated, and probes into the cause of the major differences in the earthquake reflecting abilities of well holes at different measurement points. This is achieved through the analysis of the co-seismic responses to the Wenchuan (2008; Ms8.0, China) and Tohoku (2011; Ms9.0, Japan) earthquakes. We found that the co-seismic response of water level from regional well holes in Jiangsu Province was stronger than that of water temperature. The water-level co-seismic response follows a consistent law and is closely related to the earthquake magnitude. The co-seismic response of water temperature strongly varied among well points, and was more often manifested as a slow restorative change. The co-seismic responses also varied based on tectonic elements. The response in central and northern Jiangsu was weaker than that of southern Jiangsu, possibly due to the thicker loess cover layer in central Jiangsu which makes it less effective at capturing the micro-changes of stress-strain states relative to the hilly land in the south. The more complicated geological structure in southern Jiangsu makes it contribute to greater changes in the state of underground water after a minor disturbance.

Experimental evaluation on well pattern adaptability of ultra-low permeability reservoir using sandstone flat model

As for ultra-low permeability reservoir,the adaptability of common nine-spot well pattern is studied through large-scale flat models made by micro-fractured natural sandstone outcrops.Combined with non-linear porous flow characteristics,the concept of dimensionless pressure sweep efficiency and deliverability index are put forward to evaluate the physical models' well pattern adaptability.Through experiments,the models' pressure distribution is measured and on which basis,the pressure gradient fields are drawn and the porous flow regions of these models are divided into dead oil region,non-linear porous flow region,and quasi-linear porous flow region with the help of twin-core non-linear porous flow curve.The results indicate that rectangular well pattern in fracture reservoirs has the best adaptability,while the worst is inverted nine-spot equilateral well pattern.With the increase of drawdown pressure,dead oil region decreases,pressure sweep efficiency and deliverability index increase; meantime,the deliverability index of rectangular well pattern has much more rational increase.Under the same drawdown pressure,the rectangular well pattern has the largest pressure sweep efficiency.

Using cyclic alternating water injection to enhance oil recovery for carbonate reservoirs developed by linear horizontal well pattern

In view of high water cut and low oil recovery caused by the unidirectional flow in linear pattern of horizontal wells for the carbonate reservoirs in the Middle East,this paper provides a novel approach to improve oil recovery by converting linear water injection to cyclic alternating water injection patterns including cyclic alternating water injection with apparent inverted seven-spot pattern or apparent five-spot pattern and cyclic differential alternating water injection.The main advantage of using this strategy is that the swept efficiency is improved by changing injection-production streamlines and displacement directions,which means displacement from two different direction for the same region during a complete cycle.This technology is effective in increasing the swept efficiency and tapping the remaining oil,thus resulting in higher oil recovery.Field application with three new patterns in a carbonate reservoir in the Middle East is successful.By optimizing injection and production parameters based on the cyclic alternating well pattern,the test well group had a maximum increase of daily oil production per well of 23.84 m^(3) and maximum water cut drop of 18%.By further optimizing the distance(keep a long distance)between the heels of injection and production wells,the waterflooding performance could be better with water cut decreasing and oil production increasing.

A quantitative evaluation for well pattern adaptability in ultra-low permeability oil reservoirs:A case study of Triassic Chang 6 and Chang 8 reservoirs in Ordos Basin

Based on the previous studies and development practice in recent 10 years, a quantitative evaluation method for the adaptability of well patterns to ultra-low permeability reservoirs was established using cluster analysis and gray correlation method, and it includes 10 evaluation parameters in the four aspects of optimal evaluation parameters, determination of weights for evaluation parameters, development stage division, and determination of classification coefficients. This evaluation method was used to evaluate the well pattern adaptability of 13 main ultra-low permeability reservoirs in Triassic Chang 6 and Chang 8 of Ordos Basin. Three basic understandings were obtained: Firstly, the well pattern for ultra-low permeability type-I reservoirs has generally good adaptability, with proper well pattern forms and well pattern parameters. Secondly, square inverted nine-spot well pattern is suitable for reservoirs with no fractures; rhombic inverted nine-spot injection pattern is suitable for reservoirs with some fractures; and rectangular well pattern is suitable for reservoirs with rich fractures. Thirdly, for the ultra-low permeability type-Ⅱ and type-Ⅲ reservoirs, with the principles of well pattern form determination, the row spacing needs to be optimized further to improve the level of development of such reservoirs.

Remaining Oil Distribution Law and Potential Tapping Strategy of Horizontal Well Pattern in Narrow Oil Rim Reservoir with Gas Cap and Edge Water

For thin oil rim reservoir with gas cap and edge water, it is helpful to improve the development effect to find out the distribution law of remaining oil in this kind of reservoirs. For this reason, taking the narrow oil rim reservoir with gas cap and edge water of Oilfield A in Bohai Sea as a case, the main controlling factors, including reservoir structure, fault, gas cap energy, edge water energy and well pattern, affecting the distribution of residual oil in this kind of reservoir were analyzed by using the data of core, logging, paleogeomorphology and production. Then, the distribution law of remaining oil was summarized. Generally, the remaining oil distribution is mainly potato-shaped or strip-shaped in plane. Vertically, it depends on the energy of gas cap and edge water. For the reservoir with big gas gap and weak edge water, the remaining oil mainly lies in the bottom of oil column. And for the reservoir with small gas gap and strong edge water, the remaining oil mainly locates at the top of oil column. Aiming at different distribution modes of remaining oil, the corresponding potential tapping strategies of horizontal wells are put forward: in the late stage of development, for the reservoir with big gas gap and weak edge water, the remaining oil concentrates at the bottom of the oil column, and the position of horizontal well should be placed at the lower 1/3 to the lower 1/5 of the oil column;for the reservoir with small gas cap and strong edge water, the remaining oil locates at the top of the oil column, and the position of horizontal well should be put at the upper 1/5 to the upper 1/3 of the oil column height, vertically. Based on the study on remaining oil of Oilfield A, a potential tapping strategy of well pattern thickening and vertical position optimization of horizontal well was proposed. This strategy guided the efficient implementation of the comprehensive adjustment plan of the oilfield. Moreover, 18 infill development wells were implemented in Oilfield A, and the average production of the infill wells is 2.1 times that of the surrounding old wells. It is estimated that the ultimate recovery factor of the oilfield will reach 33.9%, which is 2.3% higher than that before infilling wells. This study can be used for reference in the development of similar reservoirs.

Experiments on the influences of well pattern on water flooding characteristics of dissolution vug-cave reservoir

Based on the similarity criterion, volcanic rock samples were taken from outcrops to make experimental models. Water flooding experiments of five-spot well pattern, nine-spot well pattern, five-spot to nine-spot well pattern, the relationship between relative well and fracture positions, and injection rate in dissolution vug-cave reservoirs with/without fractures were carried out to analyze variation regularities of development indexes, find out development characteristics of water flooding with different well patterns and sort out the optimal water flooding development mode. For dissolution vug-cave reservoirs without fractures, five-spot well pattern waterflooding has very small sweeping area, serious water channeling and low oil recovery. When the well pattern was adjusted from five-spot to nine-spot well pattern, oil recovery could be largely improved, but the corner well far from the injector is little affected. In dissolution vug-cave reservoirs with fractures, when the injector and producer are not connected by fractures, the fractures could effectively connect the poorly linked vugs to improve the development effect of water flooding. Whether there are fractures or not in dissolution vug-cave reservoirs, the development effect of nine-spot well-pattern is much better than that of five-spot well pattern and five-spot to nine-spot well pattern, this is more evident when there are fractures, and the edge well has better development indexes than corner well. At the high-water cut stage of water flooding with nine-spot well pattern, the oil recovery can be further improved with staggered line-drive pattern by converting the corner well into injection well. It is helpful to increase the oil production of corner well of nine-spot well pattern by increasing the injection rate, and improve ultimate oil recovery, but the water-free production period would be greatly shortened and water-free recovery would decrease.

Evaluation of infill well pattern based on the dynamic change of reservoirs during coalbed methane development

With the deepening of coalbed methane(CBM)exploration and development,the problem of low gas production has gradually become one of the main factors restricting the development of the CBM industry in China.Reasonable well pattern deployment can improve the productivity of CBM wells and reduce the cost of production,while the reservoir changes of CBM wells play a important role for well pattern infilling.In this study,the dynamic characteristics of the average reservoir pressure(ARP),permeability,and drainage radius during the development process of CBM wells are systematically analyzed,and predicted the production changes of well groups before and after infilling wells in combination with the characteristics of reservoir changes.The results show that the high gas production wells have a larger pressure drop,long drainage radius,and a large increase in permeability.On the contrary,low gas production wells are characterized by small drainage radius,damaged permeability and difficult to recover.The productivity of infilled horizontal wells is predicted for two well groups with different productivity and reservoir dynamic characteristics.After infilling wells,the production of current wells has increased at different degrees.It is predicted that the average gas production of low gas production well group H1 and middle gas production well group H2 will increase 1.64 and 2.09 times respectively after 3000 days production simulation.In addition,the pressure interference between wells has increased significantly,and the overall gas production of the well group has greatly increased.Infill wells can achieve better development results in areas with superior CBM resources,recoverable reservoir permeability,and small drainage radius during the early production process.The research results provide a reference for the later infill adjustment of CBM well patterns in the study area.

THE PRODUCTIVITY OF WELL PATTERN WITH HORIZONTAL WELLS AND VERTICAL WELLS

The calculation of productivity and the choice of well pattern of mixedhorizontal and vertical wells were discussed in this paper. Because the naturall drainage areas ofhorizontal wells are oval rather than circular, rectangular drainage well patterns should beadopted. Using pseudo 3-di-mensional method and conformal transformation, the potential function ofa row of horizontal wells was deduced. Using this function and the superposition principle, ananalytical solution of the productivity for the rectangular mixed well pattern was given.Quantitative means for defining the shapes of the optimum pattern were developed in this paper byintroducing two dimensionless parameters, the shape factor and dimensionless length of horizontalwell. The formula for productvity was used to optimize the mixed well patterns. The curves of theoptimized shape factor against dimensionless length of horizontal well were drawn. The results showthat the pattern area and formation thickness have no effect on the optimal shape factors and can beapplied to the design of development for oil fields.

Optimization of horizontal well staggered patterns

Staggered line-drive patterns are widely used in oilfields. In this paper, to optimize a staggered pattern of horizontal wells, a 3D problem was divided into two 2D （x-y plane andy-z plane） problems with the pseudo-3D method, conformal transformation and superposition principle. A productivity equation for a horizontal well was deduced, which can be used to optimize the well pattern. A relationship between the length of horizontal wells and the shape factor of well patterns was established. The result shows that optimized well patterns can improve oil production from horizontal wells. This provides a theoretical basis for horizontal well applications to the development of oilfields, especially for overall development of oilfields by horizontal wells.

A streamline approach for identification of the flowing and stagnant zones for five-spot well patterns in low permeability reservoirs

The mechanism of the fluid flow in low permeability reservoirs is different from that in middle-high permeability reservoirs because of the existence of the Threshold Pressure Gradient （TPG）. When the pressure gradient at some location is greater than the TPG, the fluid in porous media begins to flow. By applying the mirror image method and the principle of potential superposition, the steady-state pressure distribution and the stream function for infinite five-spot well patterns can be obtained for a low permeability reservoir with the TPG effect. Based on the streamlines distribution, the flowing and stagnant zones in five-spot well patterns can be clearly seen. By the definition of the effective startup coefficient （SUC）, the ratio of the flowing and stagnant zones can be calculated accurately. It is shown that the SUC for five-spot well patterns is not constant, but decreases with the increase of the di- mensionless TPG. By increasing the effective permeability of the formation （such as by the acid treatment and the hydraulic fracture）, in increasing the injection-production differential pressure or shortening the well space （such as by infilling well）, the SUC can be improved. The results of the sensitivity analysis show that a better choice for the SUC enhancement is to shorten the well spacing for small permeability reservoirs and to increase the pressure difference for large permeability reservoirs. This streamline approach can be used to determine the distribution of remaining oil and provide guidance for infilling well.

Water-out performance and pattern of horizontal wells for marine sandstone reservoirs in Tarim Basin, NW China

Based on geological analysis,reservoir numerical simulation and production performance analysis,water-out performance and pattern of horizontal wells in Tarim marine sandstone reservoir were studied.Compared with continental sandstone reservoirs,the marine sandstone reservoirs in Tarim Basin were characterized by low oil viscosity,good reservoir continuity,and development of interbeds,which together with the large amount of horizontal wells,resulted in fast production rate and high recovery degree of the reservoirs.The main controlling factors of uneven water-out in horizontal wells were reservoir seepage barrier,injection-production well pattern,and dominant seepage channel.Thus 9 types in 4 categories of typical water-out pattern of horizontal wells in Tarim marine sandstone reservoirs were identified,and water-out management measures were proposed for them respectively according to their water-out mechanism and remaining oil distribution characteristics.Finally,the water-out pattern can be identified based on the inflection characteristics of derivative curve of water-oil ratio.This study of the water-out pattern can provide guidance for the adjustment policy of water injection in horizontal wells in marine sandstone reservoirs of Tarim Oilfield.

断控缝洞型碳酸盐岩立式板状油藏注采井网构建

断控缝洞型碳酸盐岩油藏缝洞结构和连通关系复杂,传统井网部署易产生低产能井,需构建立体注采井网以提高产能。通过油藏数值模拟研究了不同因素对塔里木富满油田满深3和满深4井区立体注采井网波及系数的影响,并设计了先导试验方案预测产能。结果表明,满深3井区注气井与采油井单个交错排状分布时波及系数最大;采油井纵向深度越大,波及系数越大;平面注采井距在500~1000m波及系数相对稳定,井距增至1500m时,波及系数大幅度减小。满深4井区注气井、注水井与采油井单个交错排状分布时波及系数最大。先导试验中,满深3和满深4井区分别采取“浅部注N_(2),深部采油”和“顶部注N_(2),底部注水,中部采油”的井网构建方案,阶段末采出程度分别达到21.3%与22.4%,相比于基础衰竭式开发和注水开发方案有了显著提升。

密井网区叠前高分辨率反演及其在薄储层预测的应用

针对大庆长垣油田密井网区SⅡ油层组陆相河流-三角洲相沉积微相类型变化快、井间厚度2 m以下的窄小河道砂体预测误差大、河道走向预测多解性强、砂体间连通关系判别差异性大等问题,以长垣油田A区块为例,开展叠前井控道集处理技术、基于改进的Xu-White岩石物理模拟技术、基于“波形指示马尔科夫链-蒙特卡洛”随机模拟算法的叠前波形指示反演技术研究。结果表明:研究区砂岩储层具有低纵波阻抗、低纵横波速度比值的特征,纵波阻抗为3.8~8.2 Gg/(m^(2)·s),纵横波速度比为1.7~2.1;随时砂岩厚度的增加,其纵波阻抗及纵横波速度比有减小的趋势。通过70口后验井的定量分析及评价,在密井网区叠前波形指示反演对厚度为[1,2)m的薄互层砂体识别精度为80.7%,对厚度大于等于2 m的砂体识别精度为85.1%。研究成果对河流―三角洲相薄储层开发方案的合理制定及措施调整具有指导意义。

中深层水热型地热资源集群式开发井位部署参数研究与应用——以HTC地热田为例

在中国“双碳”战略背景下,清洁能源的高效开发利用逐渐成为各行业关注的焦点,而中深层水热型地热资源是一种储量丰富、运行稳定、绿色环保的清洁能源。近年来,随着地热资源开发利用程度逐渐增加,开发模式逐渐向集群式发展,相比以往的分布式开发模式,集群式开发具有经济、稳定、抗风险能力高、改扩建能力强等优点。但开发模式、采灌井网、采灌井距等关键参数仍在探索阶段,这些参数对集群式开发影响明显,亟须对此开展机理研究、优化关键参数。以HTC地热田作为研究对象,利用数值模拟技术,耦合地下温度场、压力场、水流场建立数学模型,分析不同开发模式、采灌井网、采灌井距条件下地下温度场、压力场、水流场变化规律,确定最优参数,指导矿场生产。实践证实,该方法可有效保障地热开发项目稳定运行,实现地热开发项目经济效益最优化。

基于密井网和井间示踪剂资料的浅水三角洲单砂体沉积构型研究——以东营凹陷胜坨油田二区沙二段1-2砂组为例

浅水三角洲分流河道和河口坝砂体平面上常呈连片状,难以准确划分其单砂体,限制了剩余油分布的精准预测,制约了油田采收率的进一步提高。基于岩心和测井等资料,借助密井网和井间示踪剂数据,系统表征东营凹陷胜坨油田二区沙二段1-2砂组浅水三角洲单砂体沉积构型特征。研究结果表明:胜坨油田二区沙二段1-2砂组发育浅水三角洲沉积,包括浅水三角洲平原亚相和浅水三角洲前缘亚相,发育分流河道、溢岸、河口坝、滩坝等微相。浅水三角洲砂体划分为4个构型级次(5~8级),本次研究的单砂体属于8级构型单元。综合利用密井网和井间示踪剂数据,识别了单砂体的6种侧向接触样式:河道-溢岸-河道拼接式、河道-河道切叠式、滩坝-滩坝拼接式、河口坝-河口坝拼接式、坝上河道切割河口坝式和泥岩分隔式。井间示踪剂资料显示,坝上河道切割河口坝式侧向接触样式砂体连通性最好,其次为河道-河道切叠式、河口坝-河口坝拼接式和滩坝-滩坝拼接式,最差为河道-溢岸-河道拼接式。建立了研究区浅水三角洲单砂体沉积构型模式,其中浅水三角洲平原微相类型主要为分流河道和溢岸,单一分流河道砂体间接触样式主要有侧向切叠、侧向与溢岸砂体相连、垂向叠加和斜列叠加4类;浅水三角洲前缘微相类型主要为分流河道、河口坝和滩坝,单一河口坝砂体间接触样式有垂向叠加、侧向拼接、斜列叠加和泥岩分隔4类,单一滩坝砂体间有侧向拼接和垂向叠加2类样式。