Zhang, Hui (PetroChina) | Wang, Lizhi (Schlumberger) | Wang, Zhimin (PetroChina) | Pan, Yuanwei (Schlumberger) | Wang, Haiying (PetroChina) | Qiu, Kaibin (Schlumberger) | Liu, Xinyu (PetroChina) | Yang, Pin (Schlumberger)
Located at the foothills of Tianshan mountains, western China, the Dibei tight gas reservoir has become one of the key exploration areas in last decade because of its large gas reserve potential. The previous exploration effort yielded mixed results with large variations of the production rates from these exploration wells and many rates are too low to be deemed as discovery wells. Petrophysical properties were excluded as controlling factors because these properties for most exploration wells are very similar. Under the large tectonic stress, heterogeneous natural fracture systems are induced and unevenly distributed in the reservoir, which might be the controlling factor for production. However, due to the limitation of the seismic data quality, quantitative fracture modeling with seismic is not possible for this field. A new method predicting the 3D occurrence of the natural fractures in the reservoir is needed.
In this study, geomechanics-based methods were used to predict the natural fracture systems in the reservoir. The methods started from classification of natural fracture systems based on borehole image and core data into either fold-related and/or fault-related fractures. Geomechanics-based structure restoration was conducted to compute the deformation and the perturbed stress field from the restoration of complex geological structures through time. A correlation was established between the fold-related perturbated stress field and the occurrence of fold-related fractures from wells to predict the 3D occurrence of this type of natural fractures. Meanwhile, the computation of the perturbed stress field around 3D discontinuities (i.e. faults) for one or more tectonic events was conducted by the Boundary Element Method (BEM) until a good match was achieved between the fault-related perturbed stresses and observed fault-related fractures from the wellbore. By using the output from the two methods, the discrete fracture network (DFN) model was constructed to explicitly represent the occurrence and geometry of the natural fracture system in the reservoir in a geological model. A geomechanical model was constructed based on an integrated workflow from 1D to 3D. The fracture stability was then calculated based on the 3D geomechnical model.
Detailed analysis was conducted among the DFN model, the geological model of the reservoir and productivity of the exploration wells, and very good correlation was revealed between the productivity of the exploration wells and the occurrence and geometry of the natural fractures and the structural position of the reservoir.
This study shows that geomechanics-based methods efficiently capture the occurrence of natural fracture systems and reveal the production-controlling factors of the tight gas reservoir. It demonstrates that geomechanics is a powerful tool to support successful exploration of the tight gas reservoir in tectonically stressed environments.
Africa (Sub-Sahara) Bowleven has started drilling operations at the Moambe exploration well on the Bomono permit in Cameroon. Moambe is the second well in a two-well program, approximately 2 km east of the first well, Zingana. It targets a previously undrilled Paleocene Tertiary three-way dip fault block containing multiple sands and will be drilled to an estimated 1620 m in measured depth. Both wells will be logged. Bowleven is the operator and holds 100% interest. Asia Pacific Murphy Oil discovered gas at its Permai exploration well in deepwater Block H in the South China Sea offshore Malaysia.
A challenge in oil-reservoir studies is evaluating the ability of geomechanical, statistical, and geophysical methods to predict discrete geological features. This problem arises frequently with fracture corridors, which are discrete, tabular subvertical fracture clusters. Fracture corridors can be inferred from well data such as horizontal-borehole-image logs. Unfortunately, well data, and especially borehole image logs, are sparse, and predictive methods are needed to fill in the gap between wells. One way to evaluate such methods is to compare predicted and inferred fracture corridors statistically, using chi-squared and contingency tables.
In this article, we propose a modified contingency table to validate fracture-corridor-prediction techniques. We introduce two important modifications to capture special aspects of fracture corridors. The first modification is the incorporation of exclusion zones where no fracture corridors can exist, and the second modification is taking into consideration the fuzzy nature of fracture-corridor indicators from wells such as circulation losses. An indicator is fuzzy when it has more than one possible interpretation. The reliability of an indicator is the probability that it correctly suggests a fracture corridor. The indicators with reliability of unity are hard indicators, and “soft” and “fuzzy” indicators are those with reliability that is less than unity.
A structural grid is overlaid on the reservoir top in an oil field. Each cell of the grid is examined for the presence and reliability of inferred fracture corridors and exclusion zones and the confidence level of predicted fracture corridors. The results are summarized in a contingency table and are used to calculate chi-squared and conditional probability of having an actual fracture corridor given a predicted fracture corridor.
Three actual case studies are included to demonstrate how single or joint predictive methods can be statistically evaluated and how conditional probabilities are calculated using the modified contingency tables. The first example tests seismic faults as indicators of fracture corridors. The other examples test fracture corridors predicted by a simple geomechanical method.
Lv, Zuobin (Tianjin Branch of CNOOC Ltd.) | Huo, Chunliang (Tianjin Branch of CNOOC Ltd.) | Ge, Lizhen (Tianjin Branch of CNOOC Ltd.) | Xu, Jing (Tianjin Branch of CNOOC Ltd.) | Zhu, Zhiqiang (Tianjin Branch of CNOOC Ltd.)
JZS oilfield is an offshore metamorphic rock fractured buried hill oilfield. It was put into development in July 2010. The overall production situation of the oilfield is good, but some problems have been exposed. The main performance is as follows: It is difficult to accurately characterize the heterogeneity of fracture space distribution; In the numerical simulation of fractured reservoir, it is impossible to accurately describe and predict the fracture flow of fluid channeling in corner point grid system.
In order to solve the above problems, this study presents a new integrated fractured reservoir geological modeling and numerical simulation research method based on unstructured grid. There are three key aspects to this method. (1) The multi-scale (large, middle and small) discrete fracture system is established by combining outcrop measurement data with well point information and seismic attributes. On the basis of post-stack 3D seismic data, ants attributes are extracted, then the ant body results are transformed into large scale fractures; Using azimuth anisotropy attribute based on pre-stack inversion and combining the distribution orientation of large-scale fractures, the middle-scale fractures are established; According to the power law distribution relation between the cumulative frequency and the fracture length of large scale and small scale which based on outcrop observation, the imaging logging data and pre-stack inversion azimuth anisotropy attribute, small scale fractures are constructed by DFN technology.(2) For multi-scale fractures, the unstructured grid division technique is used to build a 3D model that conforms to the heterogeneity of dual media. In this study, a layered triangular prism grid generation technique is proposed. It is used to establish model of multi-scale fractures based on unstructured grid. Using large-scale fractures as a constraint, full 3D unstructured grid model is set up, and the discrete fracture model can accurately describe the fracture system and the coupling relationship between matrix and the fracture;(3)The triple-medium numerical simulation of the reservoir in the study area is carried out by using the automatic history fitting technology of ensemble kalman filter (EnKF). After several parameter adjustments, both the coincidence rate of the index and the fitting precision are higher than before.
Multi-scale discrete fracture model based on the large-scale fractures discretization processing, equivalent medium processing to middle and small scale fractures, keeps the seepage characteristic of the large-scale discrete fractures model and ensures the calculation efficiency. The results show that the new method has obvious advantages in computing speed and that the fitting effect is closer to the actual production performance.
Wu, Kunyu (Research Institute of Exploration & Development of Qinghai Oil Field, CNPC) | Zhang, Yongshu (Research Institute of Exploration & Development of Qinghai Oil Field, CNPC) | Zhang, Shenqin (Research Institute of Exploration & Development of Qinghai Oil Field, CNPC)
The Qaidam Basin is a big intermountain Mesozoic-Cenozoic petroliferous basin in western China. The huge thickness Cenozoic strata and tectonic deformation formed a good combination of hydrocarbon sources, reservoirs and caps, leading to huge hydrocarbon potential of the basin. The Western Yingxiongling Area locates in the western part of the Qaidam basin, during the Cenozoic era a special tectonic dynamics background was formed by the joint control of the sinistral strike-slip fault of the East Kunlun and the sinistral strike-slip fault of the Altun (
The vast majority of grids for reservoir modeling and simulation workflows are based on pillar gridding or stairstep grid technologies. The grids are part of a feature-rich and well-established modeling workflow provided by many commercial software packages. Undesirable and significant simplifications to the gridding often arise when employing such approaches in structurally complex areas, and this will clearly lead to poor predictions from the downstream modeling.
In the classical gridding and modeling workflow, the grid is built in geological space from input horizon and fault interpretations, and the property modeling occurs in an approximated ‘depositional’ space generated from the geological space grid cells. The unstructured grids that we consider here are based on a very different workflow: a volume-based structural model is first constructed from the fault/horizon input data; a flattening (‘depositional’) mapping deforms the mesh of the structural model under mechanical and geometric constraints; the property modeling occurs in this depositional space on a regular cuboidal grid; after ‘cutting’ this grid by the geological discontinuities, the inverse depositional mapping recovers the final unstructured grid in geological space. A critical part of the depositional transformation is the improved preservation of geodetic distances and the layer-orthogonality of the grid cells.
The final grid is an accurate representation of the input structural model, and therefore the quality checking of the modeling workflow must be focused on the input data and structural model creation. We describe a variety of basic quality checking and structurally-focused tools that should be applied at this stage; these tools aim to ensure the accuracy of the depositional transformation, and consequently ensure both the quality of the generated grid and the consistent representation of the property models. A variety of quality assurance metrics applied to the depositional/geological grid geometries provide spatial measures of the ‘quality’ of the gridding and modeling workflow, and the ultimate validation of the structural quality of the input data.
Two case studies will be used to demonstrate this novel workflow for creating high-quality unstructured grids in structurally complex areas. The improved quality is validated by monitoring downstream impacts on property prediction and reservoir simulation; these improved prediction scenarios are a more accurate basis for history matching approaches.
Lai, Jie (Southwest Petroleum University) | Guo, Jianchun (Southwest Petroleum University) | Chen, Chi (Southwest Petroleum University) | Wu, Kaidi (Southwest Petroleum University) | Ma, Huiyun (Petro China Southwest Oil & Gasfield Company) | Zhou, Changlin (Petro China Southwest Oil & Gasfield Company) | Wang, Shibin (Southwest Petroleum University) | Ren, Jichuan (Southwest Petroleum University) | Wang, Zhi (Southwest Petroleum University)
As the most commonly used technology to exploit tight dolomite reservoirs, acid fracturing usually begins with injecting pad fluid to create rough-surface fractures, followed by pumping acid to form non-uniform etching on fracture surfaces. Thus, the etching pattern and acid fracture conductivity depend largely on initial character of rough-surface fractures. In this work, experiments were conducted to examine the effects of initial roughness and mechanical property of fracture surface on acid fracture conductivity.
Eight artificially split core samples were collected from tight dolomite outcrops and classified into three categories based on the surface topography and splitting force curve. Rough fracture surfaces were scanned utilizing the 3D laser scanner. Then, dynamic acid etching tests were conducted, varying the acid flow rate and acid-rock contact time. Besides, the roughness of fracture surfaces were measured utilizing the 3D laser scanner again. After that, acid fracture conductivity was determined. The effects of acid flow rate, acid-rock contact time, fracture surface topography and mechanical property on acid etching and acid fracture conductivity were discussed.
The experimental results demonstrated that the initial fracture surface topography and acid flow rate jointly controlled the acid etching pattern and the resulting acid fracture surface topography. The orientation of the fractures distributed on the fracture surface had significant effects on the acid fracture conductivity. Dissolved mass increased with longer acid-rock contact time. Longer acid-rock contact time brought higher acid fracture conductivity under low closure stress, while shorter contact time sustained higher acid fracture conductivity under high closure stress. Higher maximum splitting force referred to higher mechanical property, and more breaking stages referred to more microfractures developed. Rock samples with higher maximum splitting force and only one breaking stage exhibited higher acid fracture conductivity.
This paper provides a systematic method to study the effects of initial roughness and mechanical property of fracture surfaces on acid fracture conductivity. Compared with the results based on smooth-surface fracture, the experimental results based on rough-surface fracture can guide acid fracturing design and optimization in a more accurate way. Accordingly, a cost-effective stimulation outcome can be expected.
Gao, Pengyu (Tianjin Branch of CNOOC Ltd) | Cao, Long (Tianjin Branch of CNOOC Ltd) | Jiang, Cong (Tianjin Branch of CNOOC Ltd) | Qin, Runsen (Tianjin Branch of CNOOC Ltd) | Cui, Longtao (Tianjin Branch of CNOOC Ltd) | Meng, Zhonghua (Chuanqing Drilling Engineering Co.,Ltd,CNPC)
It's difficult to fully discover all the geological reserves during exploration stage, because the fracture system of complex fault block oilfield is very complicated. As the reserve scale in single block is limited, the decline rate of the oilfield is usually very fast. As a result, finding new replacement reserves inside the oilfield is an important method to ensure stable production of complex fault block oilfields.
Base on the improvement of the Vogel method and material balance method to calculate the reservoir dynamic reserves under the degassing conditions of A14 well area. Using Allan profiling to construct lithologic docking relationship between A14 well area and adjacent fault block. Calculate SGR(Shale Gouge Ratio) for different docking areas. According to the statistics of shale content and porosity in the oilfield area, core experiment results with porosity and displacement pressure, the displacement pressure on both sides of the fault docking area can be used to predict the oil column height of adjacent block.
To ensure the initially high-speed production of A14 well area, it's necessary to reduce the times of shut-in static pressure measurement. The continuous reservoir pressure under the degassing conditions is calculated by the improvement of the Vogel method. Avoid the error of dynamic geological reserve calculation caused by too little reservoir pressure data. Result shows that the geological reserves of A14 well area is much smaller than its dynamic reserves. Study on the sealing property of faults around the A14 well area shows that the fault on the east side of the A14 well area is a non-closed fault, and the adjacent fault block is an oil-bearing fault block. Well A20 confirmed the oil-bearing properties for the fault block on the east side of the A14 well area. The result of pressure testing while drilling also shows that pressure drop in the east block of the A14 well area. All of that verify the reliability of previous research.
Aiming at the development of complex fault block oilfield, a method based on dynamic reserves research result to study the sealing property of peripheral faults to predict the height of oil columns in adjacent blocks is proposed. Achieved the purpose of finding new replacement reserves inside the oilfield. The reliability of the research is verified by the pressure testing while drilling. It provides a valuable experience for the development in similar oilfield.
Registration for the 2019 SPE Western Regional Meeting is now open. Please use the links above to register for the conference and make your hotel reservation. The 2019 SPE Western Regional Meeting promises to present an excellent opportunity to learn about the latest technical developments in areas of technology of significant interest to petroleum engineers at this time. Of course, our traditional areas of interest drilling and completion, formation evaluation, production, reservoir, facilities, health safety and the environment will provide the core of the program. Special areas of technology like heavy oil, thermal recovery, geothermal operations, and innovations will be part of the highlights of the program.
Cherivirala, Yaswanth K. (University of California, Los Angeles) | Lyu, Hongming (University of California, Los Angeles) | Alhowri, Hanni A. (University of California, Los Angeles) | Babakhani, Aydin (University of California, Los Angeles)
The onset of the era of internet of things and artificial intelligence comes with the ever-growing demand for self-sustaining and efficient sensors. Sensors based on complementary metal oxide semiconductors (CMOSs) have attracted significant attention in the implementation of distributed sensor systems for a vast number of applications because of their economical and complex integration benefits. In this work, we report CMOS-based energy-harvesting chips as wireless nodes for mapping hydraulic fractures during the shale gas extraction process. The CMOS chips are tested in a custom benchtop core-holder chamber that emulates a downhole environment. An induction coil, sized at 5×5 mm, connected to a custom CMOS chip, is used as a receiver inside the core holder to harvest electromagnetic (EM) energy transmitted by an external antenna. On the basis of the custom core-holder experiment, it is shown that encapsulated CMOS chips are able to harvest EM energy and thereby operate wirelessly. The receiver has a resonance frequency of 198 MHz. The CMOS chip is equipped with an integrated power management unit (PMU), energy-harvesting unit, and a signal-generation block. The CMOS chip inside the chamber produces an output signal with a frequency proportional to the harvested power. By measuring the frequency of the output signal produced by the chip, we are able to localize the chips within the rock inside the custom core holder.