Hydraulic Fracturing In Tight Reservoirs Research Articles

High-Order Models for Gas Transport in Multiscale Coal Rock.

The coal reservoirs exhibit great heterogeneity and strong anisotropy in multiscale pore/fracture structures. Developing highly accurate multiscale models for real-time prediction of microgaseous flow in complicated porous media with pronounced contrast in transport coefficients is crucial but not yet available, which is time-consuming, expensive, and even computationally impossible. In this study, a multiscale approximate solution of the gas flow and pressure field is derived in this paper for predicting coalbed methane (CBM) transport in macro-microscopic two-scale porous media of typical coal rocks in Guizhou Province, China, and the detailed finite element algorithm is established. The multiscale approximate solutions are constructed from the first- and second-order auxiliary cell functions and low- or high-order derivatives of the homogenized pressure field, which can accurately approximate the solution of the original problem to a certain extent. The numerical accuracy and validity of the multiscale approximate solutions under various working conditions are analyzed in detail by comparing and verifying the results with the direct numerical simulation results of fine-mesh high-precision finite elements. The results show that the homogenized approximate solution can only roughly capture the characteristics of the macroscopic pressure evolution, the first-order approximate solution can partially reproduce the macro-microscopic pressure oscillation phenomenon, and the second-order approximate solution can accurately predict the pressure profile and its evolution law with time in porous media with macro-micro configuration under various complex conditions. The second- and high-order two-scale approximate solutions constructed in this paper exhibit high numerical accuracy and excellent computational efficiency. We also develop multiscale approximate solutions for predicting microgaseous flow at a low reservoir pressure where the slippage effects are very pronounced. Potential applications of our high-order models to the coal reservoirs with a hierarchical matrix and fractures are discussed. The developed multiscale models exhibit excellent applicability to investigating the crossflow effects and coupling mechanisms between the matrix and cleats or fractures with multiple configurations in the CBM and shale gas reservoirs, which is crucial for the effective implementation of hydraulic fracturing technology. This study provides a fundamental understanding of gas transport in multiscale matrix-fracture networks, which helps to improve the optimization design of hydraulic fracturing in tight reservoirs.

Experimental Study on Connection Characteristics of Rough Fractures Induced by Multi-Stage Hydraulic Fracturing in Tight Reservoirs

The well spacing for the development of tight reservoirs by multi-stage fracturing is continuously narrowed. Consequently, interwell interference during fracturing is more and more serious, accompanied by a host of issues in fracturing design and oil and gas production. However, the mechanism of interwell interference during fracturing is not explicit. The corresponding laws of the connectivity of rough fractures during fracturing, which plays a critical role in interwell interference, are not fully understood. In this study, on the basis of characterizing the roughness of fractures, a laboratory evaluation method for fracture connectivity was established. The connectivity characteristics of rough fractures and factors affecting the fracture connectivity are studied. The time and scale effects of fracture connectivity were discussed and their application in interwell interference was analyzed. The results show that the connectivity performance of rough fractures can be characterized by the time for pressure decay. The upstream pressure gradually decreases over time, and the decline rate is related to the fracture aperture, the fracture surface roughness, the contact area of the closed fractures, and liquid properties. Specifically, the decrease in fracture aperture and the increase in fluid viscosity leads to a significant reduction in fracture connectivity. While larger fracture surface roughness and contact area can make fracture connectivity better. The connectivity of the fracture system is one of the significant mechanisms causing interwell interference during fracturing. The connectivity of rough fractures formed during fracturing has remarkable scale and time effects. This study can effectively guide the fracturing design and the evaluation of the impact of fracture connectivity on production.

Influence of natural fractures on propagation of hydraulic fractures in tight reservoirs during hydraulic fracturing

Natural fractures in tight reservoirs play an important role in affecting propagation of hydraulic fractures during hydraulic fracturing. In this study, simulated hydraulic fracturing is implemented on nine artificial samples. Within these artificial samples, the pre-existing fractures either showing different dip angles or exhibiting different distances from the simulated wellbore are created to investigate the influence of different types of natural fractures on fracture propagation in tight reservoirs during hydraulic fracturing. Based on the experimental results, propagation of hydraulic fractures varies with the presence of pre-existing fractures having different dip angles during the hydraulic fracturing. Specifically, hydraulic fractures easily go through the pre-existing fractures with high dip angles, while they are hindered by pre-existing fractures with small dip angles. In addition, propagation of hydraulic fractures changes with the distance of pre-existing fractures from simulated wellbore. With a small distance from simulated wellbore, hydraulic fractures are redirected by showing bifurcation from the pre-existing fractures. As the distance increases, hydraulic fractures are less affected by pre-existing fractures. On the contrary, effective stress is the main factor influencing propagation of hydraulic fractures. To validate the laboratory result, microseismic monitoring technology was used detect the influence of natural fractures on propagation of hydraulic fractures in tight reservoirs during a hydraulic fracturing process on Longmaxi Formation in the Fuling area of Sichuan Basin of China. This study is expected to inspire new understanding of the influence of natural fractures on propagation of hydraulic fractures during hydraulic fracturing in tight reservoirs.

Advances and challenges in hydraulic fracturing of tight reservoirs: A critical review

The hydraulic fracturing technology has been widely utilized to extract tight resources. Hydraulic fracturing involves rock failures, complex fracture generation, proppant transport and fracture closure. All these behaviors affect the productivity of fractured wells. In this work, the advances and challenges in hydraulic fracturing development of tight reservoirs are summarized from following aspects: the hydraulic fracture propagation, the proppant transport and distribution in hydraulic fractures, the calculation of hydraulic fracture conductivity, and productivity and/or pressure analysis model of multi-stages fractured horizontal wells. Current fracture propagation simulation methods generate only limited propagation paths and cannot truly reflect the complexity of the propagation. The current proppant migration and distribution research is mainly focused on indoor experimental studies of proppant migration in a single fracture or branched fracture, and simulation studies on proppant migration and distribution in a small-scale single slab fracture. Whereas fractures formed after hydraulic fracturing in tight reservoirs are generally complicated. There is a lack of models for calculating complex fracture conductivity that take into consideration the effect of proppant placement and proppant distribution in fractures, fracture surface roughness and dissolution, diffusion, deposition, elastic embedding, and creep caused by stress. The productivity models of fractured horizontal wells are mostly conducted based on the original reservoir fluid saturation and pressure distribution. Most of the studies are focused only on one aspect of the fracturing process. Predications of well performance after fracturing based on these studies are often inconsistent with actual field data. The paper also discusses the future research directions of fracturing in tight reservoirs and the results may be used to promote the development of tight reservoirs.

A new slick water system for hydraulic fracturing in tight reservoir to enhance imbibition oil recovery

In order to give full play to the role of imbibition of capillary force and enhance oil recovery of ultralow permeability sandstone reservoir after hydraulic fracturing, the mixed water fracture technology based on functional slick water is described and successfully applied to several wells in oilfield. The core of the technology is determination of influence factors of imbibition oil recovery, the development of new functional slick water system and optimization of volume fracturing parameters. The imbibition results show that it is significant effect of interfacial tension, wetting on imbibition oil recovery. The interfacial tension decreases by an order of magnitude, the imbibition oil recovery reduces by more than 10%. The imbibition oil recovery increases with the contact angle decreasing. The emulsifying ability has no obvious effect on imbibition oil recovery. The functional slick water system considering imbibition is developed based on the solution rheology and polymer chemistry. The system has introduced the active group and temperature resistant group into the polymer molecules. The molecular weight is controlled in 1.5 million. The viscosity is greater than 2mPa·s after shearing 2h under 170s-1 and 100℃. The interfacial tension could decrease to 10-2mN/m. The contact angle decreased from 58° to 22° and the core damage rate is less than 12%. The imbibition oil recovery could reach to 43%. The fracturing process includes slick water stage and linear gel stage. 10% 100 mesh ceramists and 8% temporary plugging agents are carried into the formation by functional slick water. 40-70 mesh ceramists are carried by linear gel. The liquid volume ratio is about 4:1 and the displacement is controlled at 10-12m3/min. The sand content and fracturing fluid volumes of single stage are 80m3 and 2500 m3 respectively. Compared with conventional fracturing, due to imbibition oil recovery, there is only 25% of the fracturing fluid flowback rate when the crude oil flew out. When the oil well is in normal production, about 50% of the fracturing fluid is not returned. It is useful to maintain the formation energy and slow down the production decline. The average cumulative production of vertical wells is greater than 2800t, and the effective period is more than 2 years. This technology overcoming the problem of high horizontal stress difference and lack of natural fracture has been successfully applied in Jidong Oilfield ultralow permeability reservoir. The successful application of this technology not only helps to promote the effective use of ultralow permeability reservoirs, but also helps to further clarify the role of imbibition recovery, energy storage and oil-water replacement mechanism.

Experimental study of the quantitative relationship between water uptake and fracture generation in tight reservoirs

Water uptake is an essential factor influencing the fate of fluids used during large-scale hydraulic fracturing in tight reservoirs. However, the relationship between water uptake and fracture generation in tight reservoirs is not well understood due to the special properties, such as extra-low porosity, permeability and high salinity. Water uptake, salinity evolution, and triaxial compression experiments were conducted on tight sandstone, shale, and volcanic rock to study the mechanisms of water uptake and its relationship with fracture generation. A new method for evaluating the generation of fractures based on the characteristics of water uptake was proposed. The results show that water uptake due to capillarity and osmosis leads to a decrease in rock compressive strength and Young's modulus but an increase in Poisson's ratio. The dilatancy of the sample changes with the water content for a similar lithology. With the increase of water uptake, the dilatancy of rock will occur earlier and more fractures will be produced. New fractures resulted from the high content of mixed layer of illite and smectite during water uptake lead to a significant increase in the salinity of the fracturing fluid. The difference in water uptake between matrix and fracture-matrix could increase with the larger contact areas and volume of fracture dilation. Owing to the larger contact areas between fractures and the fracturing fluid, the complex fractures lead to a larger final mass difference and more peak time of water uptake. The peak time and final mass difference of water uptake could reflect the generation and morphology of new formed fractures. This study is significant for hydraulic fracturing design and the evaluation of hydraulic fracture complexity in tight reservoirs.

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.

Numerical study of interaction between a hydraulic fracture and a weak plane using the bonded-particle model based on moment tensors

To better investigate hydraulic fracturing in tight reservoirs, interaction modes between a hydraulic fracture and a weak plane are studied with acoustic emission (AE) based on moment tensor under different differential stresses (Δσ) and approaching angles (β). Failure nature of hydraulic fracture is mainly tensile based on moment tensors except that the hydraulic fracture propagates into weak plane. At β = 30°, the hydraulic fracture is arrested by the weak plane. The failure nature of weak plane changes from ‘Shear’ (Δσ = 10, 7 and 5 MPa) to ‘Tensile’ (Δσ = 3 MPa). At β = 60°, the hydraulic fracture branches into weak plane as crossing, the nature of which is ‘Shear’ as differential stress decreases from 10 MPa to 5 MPa. At β = 90°, the hydraulic fracture crosses the weak plane perpendicularly.