Description of Fracture Network of Hydraulic Fracturing Vertical Wells in Unconventional Reservoirs

Based on the distribution of complex fractures after volume fracturing in unconventional reservoirs, the fractal theory is used to describe the distribution of volume fracture network in unconventional reservoirs. The method for calculating the fractal parameters of the fracture network is given. The box dimension method is used to analyze a fracturing core, and the fractal dimension is calculated. The fractal index of fracture network in fracturing vertical wells are also firstly calculated by introducing an analysis method. On this basis, the conventional dual-media model and the fractal dual-media model are compared, and the distribution of reservoir permeability and porosity are analyzed. The results show that the fractal porosity/permeability can be used to describe the reservoir physical properties more accurately. At the same time, the flow rate calculating by conventional dual-media model and the fractal dual-media model were calculated and compared. The comparative analysis found that the flow rate calculated by the conventional dual-media model was relatively high in the early stage, but the flow rate was not much different in the later stage. The research results provide certain guiding significance for the description of fracture network of volume fracturing vertical well in unconventional reservoirs.

A New Productivity Prediction Hybrid Model for Multi-Fractured Horizontal Wells in Tight Oil Reservoirs

Single well productivity is an important index to evaluate the effect of volume fracturing. However, there are many factors affecting the productivity of Multi-fractured horizontal wells (MFHWs) in unconventional reservoirs and the relationship is complex, which makes productivity prediction very difficult. In this paper, taking the tight oil reservoir in Songliao Basin as the research object, a new mixed model of initial cumulative oil production of MFHWs was established, which can consider the geological factors and volume fracturing factors at the same time. Firstly, based on the big data, the multi-level evaluation system was established by using the analytic hierarchy process (AHP). Then, the weight factor was calculated to uncover key factors that dominate productivity of MFHWs. Finally, the fuzzy logic method was used to calculate the Euclidean distance and quantitatively predict the production of any horizontal wells. The simulation results shown that: the order of the main factors affecting production in the study area was horizontal section sandstone length, number of stages, formation pressure, proppant amount, net pay thickness, permeability, porosity, oil saturation, fracturing fluid volume. The hybrid model has been applied to the productivity prediction of 185 MFHWs in tight oil reservoirs in China, the prediction error was less than 5%. The new model can be used to predict production for MFHWs quickly and economically.

Reservoir characteristics and effective development technology in typical low-permeability to ultralow-permeability reservoirs of China National Petroleum Corporation

Low-permeability to ultralow-permeability reservoirs of the China National Petroleum Corporation are crucial to increase the reserve volumes and the production of crude oil in the present and future times. This study aimed to address the two major technical bottlenecks faced by the low-permeability to ultralow-permeability reservoirs by a comprehensive use of technologies and methods such as rate-controlled mercury injection, nuclear magnetic resonance, conventional logging, physical simulation, numerical simulation, and field practices. The reservoir characteristics of low-permeability to ultralow-permeability reservoirs were first analyzed. The water flooding development adjustment mode in the middle and high water-cut stages for the low-permeability to ultralow-permeability reservoirs, where water is injected along the fracture zone and lateral displacement were established. The formation mechanism and distribution principles of dynamic fractures, residual oil description, and expanding sweep volume were studied. The development mode for Type II ultralow-permeability reservoirs with a combination of horizontal well and volume fracturing was determined; this led to a significant improvement in the initial stages of single-well production. The volume fracturing core theory and optimization design, horizontal well trajectory optimization adjustment, horizontal well injection-production well pattern optimization, and horizontal well staged fracturing suitable for reservoirs with different characteristics were developed. This understanding of the reservoir characteristics and the breakthrough of key technologies for effective development will substantially support the oil-gas valent weight of the Changqing Oilfield to exceed 50 million tons per year, the stable production of the Daqing Oilfield with 40 million tons per year (oil-gas valent weight), and the realization of 20 million tons per year (oil-gas valent weight) in the Xinjiang Oilfield.

Influence of Volume Fracturing on Casing Stress in Horizontal Wells

In horizontal wells, the casing string is affected by the gravity effect, temperature effect, swelling effect, bending effect, friction effect and other mechanical effects. In view of this situation, the mathematical models of casing swelling effect and temperature effect caused by volume fracturing are established. The case analysis shows that the length of the unsealed section in the vertical section has a great influence on the axial shortening of the casing during fracturing. With the increase of the unsealed section length, the axial shortening of the casing increases gradually under the same wellhead pressure. In the process of fracturing, repeated squeezing and pressurization lead to periodic changes of the wellhead pressure, casing deformation and load, which leads to fatigue damage and even fracture of casing. At the same time, a large amount of fracturing fluid is continuously injected through the casing during the fracturing process, which makes the wellbore temperature change greatly. The additional stress caused by the temperature change reduces the casing strength, which has an important impact on the wellbore integrity. The mathematical model of temperature stress and its effect on the casing strength during volume fracturing is established. With the increase of the temperature stress acting on the casing, the casing collapse strength decreases gradually. When the temperature stress reaches 200 MPa, the casing collapse strength decreases to 84% of the original. The research results can provide a reference for the casing integrity design and control in the horizontal well fracturing process.

Study on the optimal design of volume fracturing for shale gas based on evaluating the fracturing effect—A case study on the Zhao Tong shale gas demonstration zone in Sichuan, China

Hydraulic fracturing is the key technology in the development of shale gas reservoirs, and it mainly adopts volume fracturing technology to communicate hydraulic fractures with natural fractures to increase the drainage area. In view of the difficulty in characterizing the complex fractures created by multistaged fracturing in horizontal shale gas wells and the immaturity of fracturing optimization design methods, this study first evaluated the stimulation effect of fracturing technology based on treatment data and microseismic data. Then, the fracture characteristics after frac were considered, and a post-frac simulation was studied based on the discrete fracture network (DFN) model and the microseismic monitoring data as constraints. Finally, from the simulation results, an optimal design method of volume fracturing for shale gas was proposed based on the evaluation of the frac effects. The National Shale Gas Demonstration Zone in Zhaotong, Sichuan Basin was used as an example to study the optimal frac design of shale gas wells. The results show that (1) after optimizing the design, the optimal interval range is 50–70 m, the liquid volume of a single stage is 1800–2200 m3, the amount of sand is 80 m~120 t, and the slurry rate is 10–12 m3/min. (2) Two different frac design schemes were implemented in two wells on the same platform, and the production of the optimized design scheme was 14.7% greater than the original scheme. Therefore, the frac optimization design based on evaluating the fracturing effect can better guide the development of subsequent shale gas wells in this area.

A Solution for Mechanical Isolation Volume Refracturing in Ultra-Low Permeability Horizontal Wells: A Case Study in Ordos Basin

With the rise of volume fracturing concept and its application in new wells, it has been shown that increasing fracturing volume is an effective measure to improve production and achieve long-term stable production of ultra-low permeability reservoirs in Ordos Basin. However, refracturing requires running liner, which makes the flow passage smaller and can't reach the volume of new wells. The challenge is how to design the tubing combination and develop matching tools to further improve refracturing volume in 5.5″ casing. The objectives of this study are to design a combined mechanical isolation liner and carry out fine-staged large-volume refracturing with a rate of 8 m3/min in 5.5″ casing. In this study, researchers have designed the liner, optimized refracturing tools and wellhead equipment by comprehensively considering reservoir pressure, wellbore, available tubing combinations and friction of fracturing fluid. Downhole microseismic technology is used to monitor refracturing microseismic events, and high-precision 3D scanning technology is used to detect and analyze the wall wear of liners after refracturing. Field tests have been carried out in Ordos ultra-low permeability horizontal wells and achieved perfect effect. This research provides a reliable solution and technical reserve for volume refracturing of ultra-low permeability reservoir in horizontal wells, and verifies that large-volume plays an important role in improving refracturing effect in the test area.