Effectiveness Of Hydraulic Fracturing Research Articles

ИССЛЕДОВАНИЕ И АНАЛИЗ МЕТОДОВ ОПРЕДЕЛЕНИЯ ЭФФЕКТИВНОСТИ ПРИМЕНЕНИЯ ТЕХНОЛОГИИ ПРОПАНТНОГО ГИДРОРАЗРЫВА ПЛАСТА

Актуальность. Большинство месторождений ООО «ЛУКОЙЛ-ПЕРМЬ» находятся на завершающей стадии разработки, и на них выполняется большое количество операций гидроразрыва пласта (ГРП). В условиях высокой выработки запасов, обводнения скважин и снижения пластового давления необходимо планировать проведение ГРП на тех скважинах, где может быть получен максимальный прирост дебитов нефти. Цель: разработка методики прогноза дебитов нефти с использованием вероятностно-статистических методов. Объекты: данные по использованию пропантного ГРП на карбонатном В3В4 объекте Батырбайского месторождения. Методы: анализ данных по оценке эффективности ГРП с помощью использования вероятностно-статистических методов. Результаты. С использованием вероятностно-статистического метода построены многомерные математические модели оценки эффективности ГРП, которые позволили определить приросты дебитов нефти. Одним из основных преимуществ вероятностно-статистического метода является то, что все используемые для прогноза показатели, имеющие различные размерности, были переведены в безразмерные величины, что упрощает сопоставительный анализ. Кроме этого, выполнено сравнение полученных приростов дебитов нефти с методом расчета дебита жидкости через удельный коэффициент продуктивности. Выполненный анализ показал, что разработанный вероятностно-статистический метод прогноза обеспечивает более точную сходимость с фактическими данными по приростам дебитов нефти, в отличие от результатов, полученных по методу удельного коэффициента продуктивности. Значения коэффициентов парной корреляции для вероятностно-статистического метода составили по объекту разработки В3В4 – 0,85, тогда как по методу удельной продуктивности, соответственно, – 0,15, что на статистическом уровне подтверждает более высокую эффективность предлагаемого метода.

EFFECTIVENESS FROM FRACTURING AT A LATE STAGE OF DEVELOPMENT (ON THE EXAMPLE REPUBLIC OF BASHKORTOSTAN OIL FIELDS)

Background As a result of the introduction of a large number of fields with complicated geological and physical conditions - mainly with carbonate reservoirs saturated with highly viscous oil, one of the main tasks is the oil production intensification. Well productivity depends on the natural permeability of the bottom-hole formation zone and the productive formation as a whole. Changes in the reservoir properties of the near-wellbore zone and reservoir properties of the formation occur due to inorganic salts, clay swelling, formation of stable oil-water emulsions, asphalt-tar and paraffin deposits and corrosion products. In this connection, they attach great importance to methods that can increase oil production rate and restore, and sometimes improve the reservoir properties. Aims and Objectives The aim of the work is to analyze the effectiveness of acid hydraulic fracturing using proppants in wells at a late stage of development in oil fields of the Republic of Bashkortostan. Results The analysis of the feasibility of increasing the volume of injected acid during hydraulic fracturing in a number of fields of the Chekmagushnet oil and gas production department is carried out. It has been established that with an increase in the volume of injected hydrochloric acid in order to increase the current and, as a result, the final oil recovery coefficient, it is not possible to obtain a significant technical and economic effect - it was recommended to carry out acid fracturing using acid in a volume of from 20 to 40 m3 .

Research on Application of Hydraulic Fracturing Enhanced Technology in Coal Mine

Coal and gas outburst accident is one of the critical disasters restricting the safety production of coal enterprises in China. To solve the problems of low permeability and difficulty in gas extraction, a hydraulic fracturing test was carried out according to the occurrence characteristics of coal and rock gas in Pingdingshan Coal Mine No. 12; and the change law of the gas concentration and flow rate after hydraulic fracturing were obtained through long-term monitoring. Based on the test results, the effect of hydraulic fracturing on gas permeability enhancement of coal and rock was analyzed. The results have shown that the gas content decreases obviously and yet the water content increases since the hydraulic fracturing test completed, which makes the effect of gas displacement remarkably. It not only provides a reference for implementing hydraulic permeability enhancement and enhanced extraction in deep high outburst mines but also provides safe and efficient production in mining.

Study of the effects of hydraulic fractures on gas and water flow in shale gas reservoirs

ABSTRACT Horizontal drilling and multi-stage hydraulic fracturing are the two key technologies for the successful development of shale gas reservoirs. However, there exist large uncertainties in optimizing hydraulic fracturing which affects water retention into shale matrix and gas productivity. In this work, a basic reservoir model with six-stage hydraulic-fracturing treatment was constructed to understand water retention and gas production in shale gas reservoirs. Gas diffusion, gas desorption, Darcy flow as well as non-Darcy flow were considered in this model. Then, a sensitivity study was performed to investigate the effects of hydraulic fractures on water retention and gas conductivity. The results indicate that only 34% of the fracturing water could be recovered back to the surface, and most of them remain in shale formations to interfere with gas production. The increase of fracture half-length, fracture width, fracture permeability, and fracture conductivity will reduce water retention while water retention in shale matrix will increase with the increasing of fracture spacing and fracture number. This work can provide a better understanding of the effects of hydraulic fractures on gas and water flow so as to optimize the hydraulic-fracturing treatment in shale gas reservoirs.

Evaluating the technological effect of a horizontal hydraulic fracturing

The paper analyzes the technological effect of horizontal hydraulic fracturing. Hydraulic fracturing is understood as a mechanical stimulation method where rock is fractured along the planes of minimal strength as a result of action from fluid injected into the formation.

Investigation of the notch angle in hydraulic fracturing using XFEM

This paper analyzes the initiation and propagation of hydraulic fracturing by nonlinear numerical modeling. A notch located at the wellbore stimulates hydraulic fracturing. The notch has a pivotal role in hydraulic fracturing. There is evidence that the notch can affect the required fluid pressure and the fracture propagation and velocity. Therefore, it has been associated with the effectiveness of hydraulic fracturing in tight gas reservoirs. The objective of this study is to explore the effect of the notch angle on hydraulic fracturing. Models in two- and three dimensions are presented and analyzed. These models are used to find the breakdown pressure and stress intensity factors, respectively. The numerical approximation of hydraulically driven fracture propagation is based on the extended finite element method (XFEM). XFEM enriches finite elements and nodes at elements intersected by the crack during the crack propagation. XFEM reproduces the discontinuity in the displacement domain without the discretization of this property directly in the mesh. Therefore, the generated mesh is independent of the crack propagation. The accuracy of the numerical approach is validated by reproducing laboratory-scale hydraulic fracturing tests and comparing results.

Brittleness Evaluation of Glutenite Based On Energy Balance and Damage Evolution

Tight glutenite reservoirs are typically characterized by highly variable lithology and permeability, low and complex porosity, and strong heterogeneity. Glutenite brittleness is an essential indicator for screening fracture targets, selecting technological parameters, and predicting the hydraulic fracturing effect of tight glutenite reservoir exploitation. Glutenite formations with high brittleness are more likely to be effectively fractured and form complex fractures. Accurate evaluation of glutenite brittleness facilitates the recovery of oil and gas in a tight glutenite reservoir. Accordingly, two brittleness indexes are proposed in this paper based on energy balance and damage evolution analysis of complete stress–strain curves to evaluate the brittleness of glutenite. Uniaxial and triaxial compression tests of glutenite specimens were carried out and the brittleness indexes were verified by comparison with other existing indexes. The relationships between the mechanical properties and brittleness of glutenite under confining pressure were analyzed based on experimental results and the effects of mechanical and structural parameters on glutenite brittleness are investigated with a numerical approach. The brittleness of glutenite increases with the increase of gravel size and/or volume content. During hydraulic fracturing design, attention should be paid to the brittleness of the matrix and the size and content of gravel. This paper provides a new perspective for glutenite brittleness evaluation from the perspectives of energy dissipation and damage evolution. Our results provide guidance for fracturing layer selection and may also facilitate field operations of tight glutenite fracturing.

A New Fracability Evaluation Approach for Shale Reservoirs Based on Multivariate Analysis: A Case Study in Zhaotong Shale Gas Demonstration Zone in Sichuan, China

Fracability characterizes the effectiveness of hydraulic fracturing. The existing assessment methods cannot reflect the actual value of the effectiveness due to a lack of comprehensive consideration and neglect of the influences of engineering factors. This study aims to solve this problem by implementing geological static data and production dynamic data in multivariate analysis in Zhaotong shale gas demonstration zone. First, the reservoir quality index (RQI) was introduced to evaluate the exploration potential by integrating the geological parameters with gray relational analysis. Moreover, the differences in fracturing fluid types and proppant sizes were considered, and the operating parameters were normalized on the basis of the equivalence principle. Finally, the general reservoir fracability index (GRFI) was proposed based on a dimensioned processing of the various parameters. A case study was conducted to verify the accuracy and feasibility of this new approach. The results demonstrate that (1) the organic carbon and gas content are adjusted to contribute the most to the calculation of the RQI, while the effective porosity contributes the least; (2) the fracturing scale is the main operating parameter determining the fracability, which has the strongest correlation with the effectiveness of fracking; and (3) the GRFI has a positive correlation with shale gas production, and the lower limit of the GRFI of 2,000 corresponds to a daily production of 50,000 m3/d; this value is defined as the threshold value of a stripper well. The GRFI is consistent with the productivity trend of shale gas wells in the research block, which suggests that the new model is accurate and practical for well candidate selection.

Numerical Modeling of Discrete Fractures in a Field-Scale FORGE EGS Reservoir

The response of the Fallon FORGE geothermal reservoir to hydraulic stimulation is modeled numerically to aid the conceptual design of the well geometries and completions. The numerical modeling approach is based on (a) a full hydro-mechanical coupling; and (b) explicit representation of a discrete fracture network (DFN). The reservoir-scale model has a core region of 1500 × 1200 × 1200 m3 volume around the positions of conceptual sub-horizontal well designs at the Fallon FORGE site. It contains relevant geological structure and a fracture network constructed and simplified using borehole data. Stimulation was modeled in six stages/zones along the horizontal well using both openhole (low injection rate at 5 kg/s) and cased borehole (combination of high, 80 kg/s, and low injection rates) completions to test the effectiveness of hydraulic fracturing combined with hydro-shearing. The reservoir stimulation in the two models was analyzed and quantitatively assessed. Results indicate that the Fallon FORGE site is well suited for an EGS by multi-staged hydraulic stimulation using sub-horizontal wells. The conclusion is based on the evaluation of potential for fracture slip and aperture increase as functions of induced pressure changes and the orientations of the natural joints relative to the regional principal stresses. Advantages and limitations of explicit representation of the DFN in the numerical models are presented and discussed.

A review of recent research and development of the effect of hydraulic fracturing on gas adsorption and desorption in coal seams

Permeability improvement of coal seams by hydraulic fracturing can promote desorption of coalbed methane. However, the water-block effect will restrain desorption of coalbed methane. Based on the e...

A Digital Speckle Experiment on the Cross-Couples Propagation of Artificial Fracture with Bedding Plane

The key to improving the hydraulic fracturing effect is to activate the weak bedding planes and form complicated fractures in an unconventional shale reservoir. In this paper, an experimental method, based on the digital speckle correlation technique, is put forward to measure the fracture tip displacement field when a type I fracture is cross-coupled with a weak bedding plane; the experimental specimen is similar to the material of a shale reservoir. Through observing the fracture tip displacement field, we can determine the location of the fracture tip and calculate the fracture tip stress intensity factor. This can reveal the changing characteristics of fracture tips after the artificial fracture intersects with the bedding plane. Experimental results show that the weak bedding plane is easily activated to form a shearing slip and fracture blunting, and then the fracture type changes from a type I to type I–II composite fracture and extends along the bedding plane. A weaker bedding plane, results in a bigger angle of the bedding plane, a bigger lateral load, larger angle of the crack turned, making it easier for the bedding to be activated and allowing more complex fracture morphology.

A new method for evaluating the effectiveness of hydraulic fracturing in tight reservoirs

The effectiveness of hydraulic fracturing in tight reservoirs is considered to be influenced by three main factors, namely, stimulated reservoir volume (SRV), fracture density, and fracture conductivity. No conclusive method is available to predict the effectiveness of hydraulic fracturing; there is often a failure to determine whether production differences among wells are caused by the effectiveness of the hydraulic fracturing or by the geological conditions, fluid properties, and other factors. Based on the principles of energy conservation and material balance, a mathematical model for calculating the average length of fractures and hypothetical number of fractures was established by the calculus of variations to obtain the key parameters for describing fracture density and conductivity. The degree of influence of the individual parameters on the effectiveness of hydraulic fracturing is obtained from Grey correlation analysis, and two calculation models for the comprehensive evaluation parameter are proposed. The comprehensive evaluation parameters produced from the two models have a preferable correlation with the well production. This paper presents a rigorous method for evaluating the effectiveness of hydraulic fracturing, which can explain the reasons for the production differences between wells.

Advanced Hydraulic Fracture Characterization Using Pulse Testing Analysis

Effective hydraulic fracture characterization is important for successful reservoir stimulation. This represents one of the primary challenges for completion engineers in the oil and gas industry, especially for unconventional tight gas reservoirs which are important sources of gas production in the US. Multi-stage hydraulic fracturing in horizontal wells is the most popular completion method for these types of reservoirs. An advanced methodology has been developed to characterize hydraulic fractures via pulse testing analysis. This technique combines analytical solutions and numerical modeling based on the pressure pulse response recorded at offset wells before, during, and after horizontal well fracture operations to characterize fracture orientation, height, and length. Different conceptual models were tested for random well pair spacing and orientations to generate correlations and families of type curves for a single-stage vertical fracture characterization. The methodology was then expanded to be implemented in more complex cases of multi-stage fractures. For validation and calibration purposes, a case study for a single fracture characterization was presented to match the numerical results with the field data. A good agreement in fracture orientation, height, and length was obtained with this reasonably cost-effective technique.

Impact of the Hydraulic Fracturing on Indoor Radon Concentrations in Ohio: A Multilevel Modeling Approach.

According to the United States Environmental Protection Agency (U.S. EPA), exposure to radon gas is the second leading cause of lung cancer after smoking. Extant research that has reported that fracking activity increases the radon levels. “Fracking” also known as hydraulic fracturing, which is a technology that is used to extract naturally occurring shale gas from the Marcellus and the Utica shales. Based on the data from the Ohio Radon Information System (ORIS) from 2007 to 2014 in Ohio, this research uses multilevel modeling (MLM) to examine the association between the incidences of hydraulic fracturing and elevated airborne radon levels. The ORIS data include information on 118,421 individual records of households geocoded to zip code areas. Individual records include radon concentrations, device types of the test, and seasons. Euclidean distances between zip code centroid to the 1,162 fracking wells are measured at the zip code level. Two additional zip code variables, namely the population density and urbanicity, are also included as control variables. Multilevel modeling results show that at the zip code level, distance to fracking wells and population density are significant and negative covariate of the radon concentration. By comparing with urban areas, urban clusters, and rural areas are significant which linked to higher radon concentrations. These findings lend support to the effect of hydraulic fracturing in influencing radon concentrations, and promote public policies that need to be geographically adaptable.

Effect of compressive stresses on permeability of coal with fracture filled with lightweight proppant

The paper presents laboratory testing results on effect of hydraulic fracturing on permeability of coal with and without wedging of created fractures by proppant. The tests were carried out with hollow aluminosilicate microspheres ASPM-500 without and with additional polymeric thermoreactive coating, specimens of dense long flame (grade D) coal with permeability of less than 3·10−3 μm2 and fractured fat coking coal (grade ZH) with permeability over 60·10−3 μm2. It is found that creation of fractures with and without propping considerably improves gas permeability of dense coal with weakly developed natural jointing. Created fractures are less effective in highly jointed highly permeable coal. It is experimentally proved that filling of created fractures with small opening of 0.4 mm with lightweight proppant under uniform compression of 1–5 MPa improves permeability of dense coal by 7–19.5 times. In jointed coal, application of lightweight proppant under the same conditions improves permeability less considerably—by 1.2–2.6 times. It is revealed that efficiency gas drainage in coal by hydraulic fractures without propping lowers as compressive stresses decrease. Reduction in permeability of coal with propped fracture under growing compression is probably connected with redistribution and compaction of proppant particles in the fracture. The obtained results show that application of lightweight propping materials is a promising way of advance in the technology of hydraulic fracturing for stimulation of coalbed methane drainage. The long-range research, in the authors’ opinion, should be focused on the development of lightweight proppants with improved strength characteristics.

Hydraulic fracturing in high-temperature granite characterized by acoustic emission

Given the limited successes of hydraulic fracturing in enhanced geothermal systems (EGS), understanding of the hydraulic fracturing characteristics in high-temperature granites remains challenging and crucial. In this study, four groups of cube granite specimens (dimensions: 300/300/300mm) were tested to investigate the characteristics of the hydraulic fracture, for two confining pressures (0.1/0.1/0.1MPa and 10/25/30MPa) and temperatures (20°C and 120°C). Acoustic emission (AE) was employed to characterize the hydraulic fracturing processes. The experimental results show that for different temperatures the fracture geometry is almost unchanged, whereas the injection pressure curves, spatial distribution of AE energy, and AE-based source mechanisms are significantly changed. (1) At 120°C, The fracture pressures are increased by 3.6~4.9MPa and two remarkable pressure peaks appear at the injection pressure curves, implying the hydraulic fracture propagates intermittently with increasing resistances. (2) The spatial distributions of AE energy delineate a microcrack-band distributing along the hydraulic fracture. At the high temperature (120°C), the effective width of the microcrack-band is reduced by 40~56.4%, and the fracture energy is reduced by about 75% adjacent to the wellbore (about 40% of the fracture length) in the microcrack-band. (3) Based on the AE source analysis, the fracture mechanisms in the microcrack-band indicate the high-temperature (120°C) reduces the proportion of shear microcracks by 6~12%. The characteristics of high-temperature reducing the effectiveness of EGS hydraulic fracturing are due to the change in granite microstructures from temperature induction and the transient temperature differential (ΔT) between granite and fracturing fluid. In the EGS hydraulic fracturing, the net pressure should be enhanced in real-time with hydraulic fracture propagation to avoid fracturing arrest, ΔT between high-temperature granite and fracturing fluid should be lowered to enlarge the stimulated reservoir volume, and the proppant is suggested to be appropriately placed to prevent the further reduction of the fracturing effectiveness from fracture closures.

Laboratory experiments on blockage removing and stimulation of CBM reservoirs by composite pulsating hydraulic fracturing of radial horizontal wells

It is a common phenomenon during CBM drilling and production that reservoir damage is not eliminated completely. In view of this, a technical idea of composite pulsating hydraulic fracturing of radial horizontal wells which is conducive to blockage removing and stimulation was put forward in this paper. Speaking of the hydraulic jetting in a multi-branch radial well, it is to conduct pulsating hydraulic fracturing moderately through a high-diversion radial hole, so as to crush and break coal beds near the main hole to the uttermost. Thus, an extensive pressure relief and permeability increase area where high-diversion pathways are combined with fracture networks is formed. Then, to verify its technical principles, laboratory tests on pulsating hydraulic fracturing of radial wells were designed and carried out. Besides, the relationships of the features of acoustic emission (AE) response during the formation of fractures by composite fracturing of radial horizontal wells vs. coal breaking degree and macro fracture morphology were experimentally studied by using a pulse servo fatigue testing machine and an acoustic emission detector. And the following research results were obtained. First, under experimental conditions, fractures initially occur when the pressure of composite pulsating hydraulic fracturing of radial horizontal wells is 1/3–1/4 of the peak pressure of conventional fracturing, and the amount of its AE events is 1.38–7.07 times that of conventional fracturing. Second, when composite pulsating hydraulic fracturing of radial horizontal wells is conducted, AE emission signals respond strongly, the peak pressure for macro fracturing is lower and a larger fracture network can be generated more easily under the same condition. Third, radial laterals amount, borehole length, dynamic loading frequency and amplitude are the important factors affecting the effect of composite pulsating hydraulic fracturing of radial horizontal wells. In conclusion, composite pulsating hydraulic fracturing of radial horizontal wells provides a new idea of removing the blockages in CBM reservoirs and developing CBM efficiently, realizing effective blockage removing and stimulation of CBM wells.

Hydraulic Fracturing Effect on Filtration Resistance in Gas Drainage Hole Area in Coal

Features of stress state and jointing as well as their effect on hydraulic fracture propagation direction in coal seams are considered. The flow resistance in drainage areas is analyzed depending on hydrofracture orientation, thickness of seams and spacing of holes. The comparison of one-stage and multi-stage hydrofractures created in-plane and orthogonally to hole axes is given. In simulated reservoir conditions, permeability of dense coal is studied without a fracture and with a through propped fracture subjected to confining pressure. The recommendations are developed for improving efficiency of gas drainage in coal seams based on in-mine hydraulic fracturing.

Evaluation of underground hydraulic fracturing using transient electromagnetic method.

The effective area of hydraulic fracturing is the core index to evaluate its effects. Through conducting transient electromagnetic tests, this paper deals with the influential range of the underground hydraulic fracturing as well as water-cut detection and gas extraction in the fracturing area. The resistivity response law of the coal seam in hydraulic fracturing process is explored, and the water-bearing area is determined. The obtained results from the tests show that the water-cut areas of the coal seam, measured by anti-interference transient electromagnetic instrument after fracturing, are commonly placed in the low-resistance area of the transient test. Further, the variations of amplitude of the low-resistance area in various directions of the test line are different. According to the variation law of the apparent resistivity of the coal seam before and after fracturing, the effective influential area of the hydraulic fracturing is defined, and the influence range is evaluated to be 35m. The water cut and the gas extraction tests of the coal seam before and after fracturing are performed. The results reveal that the growth of water content in the coal seam is inversely proportional to the distance from the hydraulic fracturing borehole. The effective fracturing zone with the increment of the water content reaching 0.2% is the effective fracturing zone, and the effective fracturing zone of #9 and #10 is 38m. After hydraulic fracturing, the gas extraction concentration would be in the range of 25.4-75.4%, with the average of 70.22%, which is 21.22% higher than that of the original coal body. The net amount of the gas extraction after fracturing is about eight times of that before fracturing. The effective fracturing range, which is determined by transient electromagnetic, is verified successfully. Exploring the effective fracturing regions of the hydraulic fracturing process would be very helpful in improving the evaluation system of the hydraulic fracturing effect.

Study on the Permeability of Weakly Cemented Sandstones

Fractured rocks are a type of complex media that widely exist in various projects including energy, hydraulic, and underground space engineering, whose permeability properties are a hotspot in current rock mechanics domain. Aiming at investigating the seepage characteristics of the fracture surfaces in different rock strata, uniaxial compressive test and permeability test were performed on single-fracture homogenous and heterogeneous rocks. Specifically, rock’s physical and mechanical parameters were measured in uniaxial tests while the initial width of the single fracture was determined through CT scanning. In combination with test results and the calculation model of the displacement of single-fracture heterogeneous rock under triaxial stress condition, the calculation formula of the permeability coefficient of single-fracture heterogeneous rock was derived. Results show that hydraulic pressure in the fracture can affect the permeability coefficient of the fractured rock. Hydraulic fracturing effect occurred with the increase of hydraulic pressure in the fracture, which then generates slight normal deformations of the rock masses on both two sides of the fracture surface, decreases the contact area in the fracture, and leads to the increases of both fracture width and permeability coefficient. For single-fracture rock, the lithological properties of the rock masses on both two sides of the fracture surface impose significant effects on the permeability coefficient. Under same hydraulic pressure and confining pressure, the permeability coefficient of single-fracture coarse sandstone is greatest, followed by that of single-fracture heterogeneous rock, and finally by single-fracture fine sandstone. Theoretical calculation results agree well with the test results, suggesting that the derived theoretical formula can adequately describe the variation tendencies of permeability coefficient with confining pressure and hydraulic pressure in the fracture.