Multi Fracture Research Articles

Study on Flow Line Characteristics and Well Test Interpretation Methods of Multi Branch Fractured Wells in Dual Porous Media

Fractured reservoirs have developed fractures, strong heterogeneity, and complex seepage patterns. The analysis of core and imaging logging data in Bohai BZ Oilfield shows that there are a large number of fractures developed in the formation, with some wells having up to 6-8 fractures per meter. To study the impact of fractures on seepage, a multi fracture system seepage field simulation method was established, and the characteristic curves of flow lines under different fracture shapes were drawn. The fracture flow line morphology shows that there is a significant difference in the distribution of flow lines between the presence and absence of fractures, and the location of the fractures The size and shape of the reservoir have varying degrees of influence on the distribution of the streamline. The streamline around the well is distributed radially around it. When the fluid encounters an area with developed fractures, it first flows into the fractures and then flows from the fractures towards the wellbore. At the same time, based on the characteristics of fracture development in BZ oilfield and the theory of vertical fracture well testing, a dual medium multi fracture system oil well testing model was established. The model was numerically solved using Laplace transform, and a dual logarithmic curve chart for dual pore medium multi branch fracture well testing was obtained. The effects of fracture branching number, fracture length, conductivity coefficient ratio, and fracture conductivity on the shape of the testing curve were analyzed. The established well testing interpretation model can analyze oilfield stratigraphic information, reservoir fracture development, and boundary conditions. The research results have important guiding significance for evaluating the dynamic performance of oil wells in a dual pore medium multi fracture system and designing oilfield development plans.

Complex fracture propagation model and plugging timing optimization for temporary plugging fracturing in naturally fractured shale

In this paper, a viscoelasticity-plastic damage constitutive equation for naturally fractured shale is deduced, coupling nonlinear tensile-shear mixed fracture mode. Dynamic perforation-erosion on fluid re-distribution among multi-clusters are considered as well. DFN-FEM (discrete fracture network combined with finite element method) was developed to simulate the multi-cluster complex fractures propagation within temporary plugging fracturing (TPF). Numerical results are matched with field injection and micro-seismic monitoring data. Based on geomechanical characteristics of Weiyuan deep shale gas reservoir in Sichuan Basin, SW China, a multi-cluster complex fractures propagation model is built for TPF. To study complex fractures propagation and the permeability-enhanced region evolution, intersecting and competition mechanisms between the fractures before and after TPF treatment are revealed. Simulation results show that: fracture from middle cluster is restricted by the fractures from side-clusters, and side-clusters plugging is benefit for multi fractures propagation in uniformity; optimized TPF timing should be delayed within a higher density or strike of natural fractures; Within a reservoir-featured natural fractures distribution, optimized TPF timing for most clustered method is 2/3 of total fluid injection time as the optimal plugging time under different clustering modes.

Three dimensional multi fracture induced stress model for highly deviated wells

ABSTRACT At present, the development of some oil and gas fields has gradually shifted from vertical to highly deviated wells. To improve the intensity of reservoir reconstruction, highly deviated wells mostly adopt staged fracturing for reservoir reconstruction. The fracture interference law of multiple fractures in highly deviated wells is not clear, and the fracture steering and stress interference problems in the fracturing process are obviously different from those in horizontal wells, resulting in a lack of a theoretical basis for fracture spacing design. Therefore, it is necessary to study the stresses induced by multiple fractures in highly deviated wells. To reproduce the spatial distribution of multiple fractures in highly deviated wells and analyze the changes in induced stress more accurately, a three-dimensional numerical model of the fracture-induced stress field in highly deviated wells was established based on the finite element method and elasticity theory, which can simulate a fracture-induced stress field under different angles of inclination. The results indicate that the key to mutual exclusion or proximity of the two fractures is whether the fractures overlap in the plane parallel to the fracture height direction. Based on the analysis of the control variables, the first fracturing fracture height is the key factor affecting the induced stress, and the fracturing fracture height has slight effect on the induced stress. As the angle of inclination increases, the probability of the two fractures overlapping in space increases and the critical distance from the induced tensile stress to the induced tensile stress decreases. The influence of the fracture half-length, net pressure, and stress difference on the induced stress is related to whether the spatial projection of the two fractures overlaps. The research results provide a theoretical basis for the optimal design of staged fracturing in highly deviated wells.

Experimental research on the heat transfer characteristics of fluid flowing through rock with intersecting fractures

The research on the seepage and heat transfer characteristics of fluid flowing through fractured rock mass is of great significance to the utilization of geothermal resources. The behaviors and characteristics of heat transfer when fluid flowing through a single fracture channel, two intersecting fractures channels, and three intersecting fractures channels were investigated and compared. Firstly, the calculation formula of convective heat transfer coefficient of water flowing through multi channels was deduced. Then the relationship between the seepage characteristic parameters and the confining pressure during the loading and unloading process was analyzed and discussed. Finally, the heat transfer mechanism of water flowing through rock sample with multi fractures was studied. This work supplements the law and theory of heat transfer when fluid flowing through fracture networks, which is of practical significance to the underground space utilization and geothermal resource exploitation.

Rate Transient Behavior of Wells Intercepting Non-Uniform Fractures in a Layered Tight Gas Reservoir

RTA (Rate Transient Analysis) is a valuable method for obtaining reservoir parameters and well performance, but current RTA models hardly consider the MLVF (Multi-Layer Vertical Fractured) well in a layered tight gas reservoir. To capture the production response caused by the fracture with non-uniform length and conductivity, a novel RTA model for an MLVF well in a layered tight reservoir was presented. In this paper, we present a novel tight gas reservoir RTA model, an extended MLVF well with non-uniform fracture length and conductivity to investigate the production decline feature by the combined RTA type curves. After that, the proposed RTA model is verified to ensure calculation accuracy. Sensitivity analysis is conducted on the crucial parameters, including the formation transmissibility, formation storability, fracture length, fracture conductivity, and fracture extension. Research results show that there are three rate decline stages caused by a multi fracture with non-uniform conductivity. The wellbore storage and formation skin can be ignored in the rate transient analysis work. The formation transmissibility affects the rate transient response more than the formation storability. The increase in fracture length, fracture conductivity, and the extension of a high conductivity fracture will improve the well’s production rate in a tight gas reservoir’s early production stage. Therefore, it is significant to incorporate how the effects of the MLVF well intercepting with non-uniform length fractures change conductivity. The RTA model proposed in this paper enables us to better evaluate well performance and capture the formation of complex fracture characteristics in a layered tight gas reservoir based on rate transient data.

Numerical Simulation of Fracture Propagation of Multi-Cluster Perforation and Fracturing in Horizontal Wells: A Case Study of Mahu Oilfield

Mahu oilfield is a typical tight gravel oil reservoir. At present, long-stage multi-cluster fracturing is widely used for reservoir stimulation, and multiple-clusters treatment is realized through cluster perforation. Field monitoring indicates that not all perforation clusters produce hydraulic fractures in the fracturing process, and each cluster of hydraulic fractures in the section will expand unevenly. The unbalanced expansion of multiple clusters of fractures in the section seriously affects the effect of reservoir reconstruction of horizontal wells. Aiming at the long-stage multi-cluster fracturing of horizontal wells, a multi-fracture propagation calculation model considering wellbore flow, performance friction, different fracture criteria at the tip, and the interaction stress among multiple fractures is established in this paper. In order to improve the calculation efficiency, an explicit Runge Kutta Legendre algorithm is proposed to solve the structural mesh, and the solution program is compiled, which provides a basis for the theoretical analysis and rapid solution of the mechanism of multiple fracture growth. Finally, taking Mahu oil field as an example, we calculate the multi fracture propagation and flow distribution under different geological conditions, perforation conditions, and construction parameters. The research results will help to improve the fracturing efficiency of long-stage multi-cluster fractures.

A new model for predicting fluid loss in fracture-porosity reservoir

Drilling fluid loss always occurs in fracture-porosity reservoirs and it causes severe problems. To reduce and prevent lost circulation, it is important to get to know the cause and the characteristic of drilling fluid loss. According to the approach in the reservoir simulation and well test analysis, a new model for drilling fluid loss in fracture-porosity reservoir is presented. Multi fractures in the formation and drilling fluid seepage between fracture and rock matrix have been considered in the model. The governing equations are derived based on the principle of conservation of mass. The model is solved numerically using Newton-Raphson iterative method. The obtained results indicate that drilling fluid leak-off has great influence on the total leakage volume. It is necessary to consider the impact of the drilling fluid leak-off. In addition, influence of formation properties, such as fracture stiffness, rock matrix porosity, rock matrix permeability, and operation factors, such as pressure difference between wellbore and formation, are also analysed in detail in the paper which could help better understand the factors that influence the drilling fluid loss during drilling operation.

The Minimum Safe Thickness and Catastrophe Process for Water Inrush of a Karst Tunnel Face with Multi Fractures

Water inrush of tunnel face is one of the most common geological disasters during tunnel construction in China. Aiming at the rock mass with multi fractures in water-resistant strata ahead of karst tunnel, the compressive-shear cracking property is analyzed by fracture mechanics theory and the change law of rock bridge shear strength with branch crack propagated length under karst water pressure and geo-stress is studied according to Mohr-Coulomb strength criterion. Moreover, the critical water pressure of water-resistant strata with multi fractures under tension-shear failure is deduced. The safe thickness of water-resistant strata with multi fractures ahead of karst tunnel is established based on two band theory and critical water pressure, and the influence of karst water pressure, initial crack length, crack spacing, array pitch of cracks, lateral pressure coefficient and the angle between the crack and the maximum principal stress on the minimum safe thickness of water-resistant strata are discussed. A 3 Dimension Distinct Element Code (3DEC) considering the fluid-solid coupling effect and structural characteristics of rock mass is adopted to study the catastrophe process and the influence of karst cavity scale on displacement and seepage field in water-resistant rock mass ahead of tunnel in the process of sequential excavation. The numerical simulation results show that: The transition from the single effect of unloading on the extrusion displacement of karst tunnel face to combined action of unloading and karst water pressure occurs with the tunnel face advance; The displacement at each measuring point in water-resistant strata continues to increase in the process of tunnel excavation; The extrusion displacement and water flow velocity in tunnel face suddenly increase when the water inrush pathway is about to form; With the increase of karst cavity size, the minimum thickness of water-resistant strata, the displacement of measuring point and pore pressure of crack increase. The study results provide a reference for early warning and prevention of water inrush in karst tunnel face.

Hydrodynamics Behaviour of Single and Multi Fracture with Different Orientations in Petroleum Reservoir

The studying of fluid flow throughout fracture in the reservoir is one of the most vital subjects attracted much attention from engineers and geologists. In the present paper, the Dual Porosity-Dual Permeability (DPDP) model has been applied to represent the fluid flow within the fractured reservoirs. This work aimed to demonstrate the utility of the fractures in the petroleum reservoir and how could be used the positive effect of these fractures on the productivity as well. The productivity of single-phase fluid flow within the single horizontal fracture, multi horizontal fractures, and inclined fracture with different orientations (20o, 30o, 45o, and 70o) have been implemented by using ANSYS-CFX program and compared with the productivity of conventional (without fractures) reservoirs. In addition to, visualize the velocity streamlines within fracture and matrix zones for the DPDP model. To verify this work the comparison has been made with published paper, which studies the fluid flow through fractures, and a good agreement has been obtained with each other. The study indicates that the presence of macro scale fractures in petroleum reservoirs contributes to increasing the total productivity of these reservoirs. Clearly, the productivity index of multi-horizontal fractures domain is more than twice of nonfractured domain. It is also clear that, when comparing the fractured and nonfractured reservoir, the improvement percentage of the productivity index reaches to (71.8) for a single horizontal fracture with 9 ft length. While this percentage would be about (116.88) if the fracture is inclined with 20o.

Micro failure analysis of adhesively bonded joints enhanced with natural cork particles: Impact of overlap length and particles volume fraction

In this work, the effects of natural particles on the micro failure mechanisms and static strength of an epoxy are studied using scanning electron microscope (SEM) technique. To this end, bulk specimens and adhesively bonded single lap joints (SLJs) containing different amount of micro cork particles were fabricated and tested. To investigate the toughening mechanisms, the fracture surfaces of the specimens were analyzed using SEM and a magnifying glass. It was shown that the deviation of the crack path and formation of micro-cracks generate multi fracture surfaces which is one of the main sources of energy absorption and resulted in higher fracture energy of the enhanced adhesives. Acting as a crack stopper and consequently stabilizing the crack propagation, prevents developing large and unstable cracks which is another source of energy consumption. However, the micro analysis showed that higher contents of cork lead to agglomeration of the particles which acts as a source of stress concentration, with a higher possibility of delamination between the matrix and the micro corks. Accordingly, there is an optimum amount of cork where the best adhesive fracture toughness is reached. The results showed that the optimum value of micro particles depends on the loading conditions.

The study on fracture network development in shale fracturing with considering capillary effect

Natural fractures widely exist in shale reservoirs with different scales, ranging from nano scale to micro scale, even to macro and engineering scale. These multi-scale natural fractures have a great influence on the fracture network propagation during a fracturing treatment. Meanwhile, the fracturing fluid flowing into these natural fractures will result in the co-existence of fracturing fluid and natural gas. As a result, the capillary force occurs at the gas-liquid interface in natural fractures. If the sizes of these natural fractures are different, the capillary force in the natural fractures is different as well. In particular, when the natural fracture is small, the capillary force will be very large and even affect the natural fracture initiation and propagation. As a consequence, the fracturing fluid flowing in natural fractures and the fracture network development will be significantly affected by these multiscale natural fractures. In this paper, a model about fracture network propagation was built which considered natural fractures sizes and fracturing fluid wettability. Besides, different methods were adopted to calculate the stress intensity of hydraulic fracture and natural fracture, respectively. In addition, the competitive growth theory of multi fractures is considered to describe the fracture network propagation. The studies found that with long length, small width natural fractures and strong wettability fracturing fluid, natural fractures were more likely to propagate, which resulted in a large fracture network. Besides, when the sizes of natural fractures were different, a more complex fracture network was tended to come out. An experimental study about fracture network propagation in shale was introduced from Suarez-Rivera’s work, in which a “fish bone” shape fracture propagation was observed. By comparing the numerical simulation with Suarez-Rivera’s work, it could be concluded that the simulation was in accordance with shale fracturing experiments results.

Hydraulic fracturing mechanisms in coal: a review

Hydraulic fracturing mechanisms in coal differ in many aspects from those in inorganic rocks. Due to the lower Young's modulus and higher Poisson's ratio, the stress state in coal is higher than in adjacent formations. This leads to wider and shorter fractures than in inorganic rocks. Fractures are affected by the cleat system in coal, so that multiple fractures may follow in tortuous paths the coal cleats. Coal often has a fracture treatment pressure in excess of 22.6 kPa/m (1 psi/ft.). Several hydraulic fracturing mechanisms may account for that, namely fracture blocking by coal chips, fracture tip plugging by coal or the development of a complex multi fracture system. The orientation of the hydraulic fractures is predominantly horizontal at shallow depths ( 600 m. In between these depths, fracture orientation may be either horizontal or vertical. This paper gives a comprehensive overview of the proposed hydraulic fracturing mechanisms in coal. [Received: July 28, 2015; Accepted: January 14, 2016]

Hydraulic fracturing mechanisms in coal: a review

Hydraulic fracturing mechanisms in coal differ in many aspects from those in inorganic rocks. Due to the lower Young's modulus and higher Poisson's ratio, the stress state in coal is higher than in adjacent formations. This leads to wider and shorter fractures than in inorganic rocks. Fractures are affected by the cleat system in coal, so that multiple fractures may follow in tortuous paths the coal cleats. Coal often has a fracture treatment pressure in excess of 22.6 kPa/m (1 psi/ft.). Several hydraulic fracturing mechanisms may account for that, namely fracture blocking by coal chips, fracture tip plugging by coal or the development of a complex multi fracture system. The orientation of the hydraulic fractures is predominantly horizontal at shallow depths (< 200 m) and mostly vertical at depths > 600 m. In between these depths, fracture orientation may be either horizontal or vertical. This paper gives a comprehensive overview of the proposed hydraulic fracturing mechanisms in coal. [Received: July 28, 2015; Accepted: January 14, 2016]

Developing brittle transparent materials with 3D fractures and experimental study

The fracture propagation mechanism and fractured rock mass failure mechanism were important research in geotechnical engineering field. Many failures and instability in geotechnical engineering were related on fractures propagation, coalescence and interaction in rock mass under the external force. Most of the current research were limited to two-dimensional for the brittleness and transparency of three-dimensional fracture materials couldn't meet the requirements of the experiment. New materials with good transparent and brittleness were developed by authors. The making method of multi fracture specimens were established and made molds that could be reused. The tension-compression ratio of the material reached above 1/6 in normal temperature. Uniaxial and biaxial loading tests of single and double fracture specimens were carried out. Four new fractures were not found in the experiment of two-dimensional fractures such as the fin shaped crack, wrapping wing crack and petal crack and anti-wing crack. The relationship between stress and strain of the specimens were studied. The specimens with the load had experienced four stages of deformation and the process of the fracture propagation was clearly seen in each stage. The expansion characteristics of the fractured specimens were more obvious than the previous research.

OS4-30 Treatment of multi fragmentation fractures of the distal extremity of the radius with dorsal displacement (C type)

OS4-30 Treatment of multi fragmentation fractures of the distal extremity of the radius with dorsal displacement (C type)

A variational hydraulic fracturing model coupled to a reservoir simulator

A variational fracture model coupled to an external reservoir simulator through variable exchange is presented. While convergence is not optimal without Jacobian matrices with which fully coupling can provide, the presented coupling scheme is versatile enough that the reservoir simulator could be easily replaced with any other simulator. A variational approach to fracture is introduced first by comparison to the classic Griffith criteria, and is then expanded to include poro-elasticity and in-situ stresses that are required in hydraulic applications. The coupled code has been tested against existing analytical solutions of fluid-driven fracture propagation. Finally, illustrative examples are shown to demonstrate that the methodology's ability to simulate multi fracture interaction with the unified approach for turning and merging fractures.

Experimental Study of Fracture Propagation Mechanisms by Oriented Perforation Technology for SRV Fracturing

Due to the growing importance of shale oil and gas resources, more effective techniques to develop unconventional reservoirs are strongly required. In this paper, a new volumetric fracturing measure for unconventional resources based on oriented perforation technology has been introduced. Moreover, this innovative approach was validated through a series of laboratory experiments varying the parameters of physical models, such as perforation azimuth, perforation length and horizontal stress difference. The results showed that the artificial hydraulic fracture formed by using oriented perforation technology was not a satisfactory straight dual-wing fracture, instead, it was a bent dual-wing crack like “S” or “X”, which could significantly increase the stimulated reservoir volume. The perforation azimuth and horizontal stress difference were the primary factors that affected the formation of multi fractures. The reorientation path was extended with the increasing of the oriented perforation azimuth angle, but decreased with the increasing of horizontal stress difference. In addition, increasing the perforation length could greatly decrease the initiation pressure. Therefore, the oriented perforation technology could be as a powerful way to accelerate the exploitation of the unconventional reservoirs.

Production Simulate Analysis of Multiple-Fractured Horizontal Gas Wells

Productivity simulation of multiple-fractured horizontal wells is the core technique during optimizing fracture’s amount and parameters. According to the dissymmetric double wings of multiple fractured high productivity horizontal gas well which is equality discreted, applying the theory of point source, the formula of unsteady seepage of gas and the principle of superposition, this paper deduce the model of overall pressure drop in formation considering the mutual interference of multi fracture. Besides, considering the high velocity non-Darcy’s flow’s effect to the fracture productivity in high horizontal gas well, establishing a more sophisticated new model to stimulate the fracture productivity in horizontal gas well. Via stimulation calculating and comparative analysing the high velocity non-Darcy’s flow, the dissymmetry of the double wings fracture, the length of fracture and the conductivity’s effect on the multiple fracture gas well’s productivity. This offers a more accurate method to optimizing the fracture’s amount in landscape and the parameters of fracture in high productivity gas well and making sure the proper production’s differential pressure.

Rock specific hydraulic fracturing and matrix acidizing to enhance a geothermal system — Concepts and field results

Enhanced geothermal systems (EGS) are engineered reservoirs developed to extract economic amounts of heat from low permeability and/or porosity geothermal resources. To enhance the productivity of reservoirs, a site specific concept is necessary to actively make reservoir conditions profitable using specially adjusted stimulation treatments, such as multi fracture concepts and site specific well path design. The results of previously performed stimulation treatments in the geothermal research well GtGrSk4/05 at Groß Schönebeck, Germany are presented. The reservoir is located at a 4100–4300 m depth within the Lower Permian of the NE German Basin with a bottom-hole temperature of 150 °C. The reservoir rock is classified by two lithological units from bottom to top: volcanic rocks (andesitic rocks) and siliciclastics ranging from conglomerates to fine-grained sandstones (fluvial sediments). The stimulation treatments included multiple hydraulic stimulations and an acid treatment. In order to initiate a cross-flow from the sandstone layer, the hydraulic stimulations were performed in different depth sections (two in the sandstone section and one in the underlying volcanic section). In low permeability volcanic rocks, a cyclic hydraulic fracturing treatment was performed over 6 days in conjunction with adding quartz in low concentrations to maintain a sustainable fracture performance. Flow rates of up to 150 l/s were realized, and a total of 13,170 m 3 of water was injected. A hydraulic connection to the sandstone layer was successfully achieved in this way. However, monitoring of the water level in the offsetting well EGrSk3/90, which is 475 m apart at the final depth, showed a very rapid water level increase due to the stimulation treatment. This can be explained by a connected fault zone within the volcanic rocks. Two gel-proppant treatments were performed in the slightly higher permeability sandstones to obtain long-term access to the reservoir rocks. During each treatment, a total of 100 ton of high strength proppants was injected with 500 m 3 of cross-linked gel. The subsequent production test in conjunction with flowmeter logging showed an improvement of productivity by a factor of more than 4. Due to assumed residual drilling mud (constituents: calcite, dolomite, and aragonite) in the near-wellbore vicinity, an acid matrix stimulation was performed thereafter using a coil tubing unit. The following nitrogen lift test demonstrated another increase of productivity by 30–50% to an overall increase by a factor of 5.5–6.2.