Hydrofracture Growth Research Articles

Overpressure‐driven hydrofracture growth in the northern South China Sea

Overpressure‐driven hydrofracture growth in the northern South China Sea

Stress heterogeneity in the Changning shale-gas field, southern Sichuan Basin: Implications for a hydraulic fracturing strategy

To evaluate the risk of induced seismicity due to hydraulic fracturing, a detailed stress analysis in a seismically active region, i.e., the Changning shale-gas field in the southern Sichuan Basin, was conducted utilizing sonic wave velocity logs and high-resolution electrical resistivity image logs of shale gas wells. Wells with relatively higher magnitudes of horizontal stresses are spatially located in the vicinity of thrust faults, in agreement with the stress concentration at the upper tip of the basement-involved thrust fault. The prevailing orientation of maximum horizontal principal stress (SHmax) is N110°–120°E on the southwestern Changning anticline, which aligns with the motion of the Sichuan Basin, as driven by the continuous eastward extrusion of the Tibet plateau, which initiated at approximately 50 Ma. However, the SHmax orientation is N30°–70°E along the northern Changning anticline, implying an inheritance of the N–S geodynamic compression during the Jurassic to Late Cretaceous. Drilling-induced tensile fractures primarily appear in compliant layers that are rich in clay or organic matter in wells located adjacent to thrust faults, suggesting that stiff layers can efficiently inhibit the hydrofracture growth height owing to more intense tectonic stress near thrust faults. A natural fault with a strike of approximately 30° to the SHmax orientation can be more easily activated by extreme pumping pressure in wells that are weakly affected by the stress concentration along the thrust fault. Nevertheless, a cyclic injection of fracturing fluid with a gradually increasing target pressure may effectively reduce the magnitude of induced seismicity.

In Situ Continuous Monitoring of Borehole Displacements Induced by Stimulated Hydrofracture Growth

AbstractWe provide direct observations of the three‐dimensional displacements of a fluid‐driven fracture during water injections in a borehole at ∼1.5 km depth in the crystalline rock at the Sanford Underground Research Facility (USA). Micro‐shearing of the borehole initiates on a foliation plane at 61% of the minimum principal stress σ3. As the fluid pressure increases further up to 112% of σ3, borehole axial and radial displacements increase with injection time highlighting the opening and sliding of a new hydrofracture growing ∼10 m away from the borehole, in accordance with the ambient normal stress regime and in alignment with the microseismicity. Our study reveals how important it is to consider monitoring zonal deformation in conjunction with pressure and flow to better manage the complex hydromechanical evolution of the growing fracture.

Numerical analysis of multiple hydro-fracture growth in layered media based on a non-differentiable energy minimization approach

In this study, the evolution of hydraulically driven fractures in naturally layered and fractured media is investigated. The non-differentiable energy minimization approach to cohesive fracture is employed with a Discontinuous Galerkin (DG) discretization in which every element edge in the mesh is a potential site of cracks. The coupled hydro-mechanical model accounts for the flow continuity equation governing the hydro-fractures inflow in which the opening-dependent permeability of the hydro-fractures is modelled based upon the well-known cubic law. The proposed framework provides great flexibility in modeling multiple fractures and is applied to study the interaction between hydro-fractures and material interfaces in naturally fractured layered domains. Results indicate robustness and versatility of the algorithm and exemplify several simulation aspects there were eluded in many of the works in previous literature.

An X-FEM investigation of hydro-fracture evolution in naturally-layered domains

In this paper, a computational model is developed for the simulation of hydro-fracture growth in naturally layered impervious media using the extended finite element method (X-FEM). The equilibrium equation of the bulk is solved in conjunction with the hydro-fracture inflow and continuity equations using the staggered Newton method. The hydro-fracture inflow is governed by the lubrication theory, where the permeability of the fracture is incorporated by taking advantage of the cubic law. The Eigen-function expansion method is utilised in order to develop enrichment functions suited for the asymptotic stress field in the vicinity of the singular points. An energy release rate-based criterion is used in order to study the competition between hydro-fracture penetration/deflection at the material interface. Finally, the robustness of the computational framework is explored by means of numerical simulation.

A new mathematical model of asymmetric hydraulic fracture growth

ABSTRACTHydraulic fractures generated by fluid injection in rock formations are often mapped by seismic monitoring. In many cases, the microseismicity is asymmetric relative to the injection well, which has been interpreted by stress gradient along the direction of the hydraulic fracture. We present a mathematical model of asymmetric hydrofracture growth based on relations between the solid‐phase stress and the fracture hydraulics. For single fracture and single injection point, the model has three parameters, hydraulic conductivities of the fracture wings, and normalised stress gradient and predicts the positions of the fracture tips as functions of time. The model is applied to a set of microseismic event locations that occurred during and after an injection process. Two different methods are suggested that make it possible to delineate the fracture tips from the set of microseismic events. This makes it possible to determine the model parameters and to check the agreement between the model prediction and the measured data. The comparison of the measured and modelled growth of fracture wings supports both the assumption of the non‐zero stress gradient and the existence of the post‐injection unilateral growth.

Modeling fracture growth under multiple hydraulic fracturing using viscous fluid

Under computational investigation is the process of sequential growth of hydrofractures under conditions of plane strain. The working fluid is perfect and viscous. The authors analyze influence exerted on parameters and trajectories of growing fractures by spacing of the fractures, external compression field, fluid flow rate, fluid viscosity and leakage flow rate.

Semi-analytical solution for mode I penny-shaped crack in a soft inhomogeneous layer

An axisymmetric elastostatics problem for a penny-shaped crack placed in the middle of a inhomogeneous (FGM) elastic layer is considered. It is assumed that the elastic modulus of the layer varies through the thickness symmetrically with respect to the crack plane. Several specific distributions of the moduli variations have been analysed. We report a semi-analytical approximate solution for the determination of the stress intensity factor for the distributions considered. The obtained solution is accurate enough and can be applied in engineering applications for the analysis of crack propagation in FGM and hydrofracture growth in elastic reservoirs.

Hydrofracture growth in a compressed quasi-regular blocky rock mass

Effect of irregular structure of a compressed blocky rock mass on propagation of a hydrofracture is analyzed in the limit case when biaxial compression field is hydrostatic.

Seismicity-based estimation of the driving fluid pressure in the case of swarm activity in Western Bohemia

SUMMARY Two recent major swarms in Western Bohemia occurred in the years 2000 and 2008 within almost the same portion of a fault close to Novy Kostel. Previous analysis of the year 2000 earthquake swarm revealed that fluid intrusion seemed to initiate the activity whereas stress redistribution by the individual swarm earthquakes played a major role in the further swarm evolution. Here we analyse the new swarm, which occurred in the year 2008, with regard to its correlation to the previous swarm as well its spatiotemporal migration patterns. We find that (i) the main part of the year 2008 activity ruptured fault patches adjacent to the main activity of the swarm 2000, but that also (ii) a significant overlap exists where earthquakes occurred in patches in which stress had been already released by precursory events; (iii) the activity shows a clear migration which can be described by a 1-D (in up-dip direction) diffusion process; (iv) the migration pattern can be equally well explained by a hydrofracture growth, which additionally explains the faster migration in up-dip compared to the down-dip direction as well as the maximum up-dip extension of the activity. We use these observations to estimate theunderlyingfluidpressurechangeintwodifferentways:First,wecalculatethestresschanges induced by precursory events at the location of each swarm earthquake assuming that observed stress deficits had to be compensated by pore pressure increases; and secondly, we estimate the fluid overpressure by fitting a hydrofracture model to the asymmetric seismicity patterns. Both independent methods indicate that the fluid pressure increase was initially up to 30 MPa.

Growth of hydrofractures in an oil and gas stratum under impulse loading

It is analyzed how hydofractures grow from the circular hole boundary in the conditions of plane strain, as well as how amount of hydrofractures, their initial length, working fluid pressure and external compression parameters affect the fractured zone form, opening of the cracks and their volume. Depending on the basic parameters, development conditions are determined for two or more cracks.

Control Model of Water Injection Into a Layered Formation

Summary Here we develop a new control model of water injection from a growing hydrofracture into a layered soft rock. We demonstrate that in transient flow, the optimal injection pressure depends not only on the instantaneous measurements, but also on the whole history of injection, growth of the hydrofracture, and the rock damage. Based on the new model, we design an optimal injection controller that manages the rate of water injection in accordance with hydrofracture growth and the formation properties. We conclude that maintaining the rate of water injection into a low-permeability rock above a reasonable minimum inevitably leads to hydrofracture growth, to establishment of steady-state flow between injectors and neighboring producers, or to a mixture of both. Analysis of field water injection rates and wellhead pressures leads us to believe that direct links between injectors and producers can be established at early stages of waterflood, especially if the injection policy is aggressive. Such links may develop in thin, highly permeable reservoir layers or may result from failure of the soft rock under stress exerted by injected water. These links may conduct a substantial part of injected water. Based on the field observations, we now consider a vertical hydrofracture in contact with a multilayer reservoir, where some layers have high permeability and quickly establish steady-state flow from an injector to neighboring producers. The main result of this paper is the development of an optimal injection controller for purely transient flow, and for mixed transient/steady-state flow in a layered formation. The objective of the controller is to maintain the prescribed injection rate in the presence of hydrofracture growth and injector/producer linkage. The history of injection pressure and cumulative injection, along with estimates of the hydrofracture size, are the controller inputs. By analyzing these inputs, the controller outputs an optimal injection pressure for each injector. When designing the controller, we keep in mind that it can be used either offline as a smart adviser, or online in a fully automated regime. Because our controller is process model-based, the dynamics of actual injection rate and pressure can be used to estimate effective area of the hydrofracture and the extent of the rock damage. The latter can be passed to the controller as one of the inputs. Finally, a comparison of the estimated fracture area with independent measurements leads to an estimate of the fraction of injected water that flows directly to the neighboring producers through links or thief-layers.

Water Injection into a Low-Permeability Rock - 1: Hydrofracture Growth

In this paper, we model water injection through a growing vertical hydrofracture penetrating a low-permeability reservoir. The results are useful in oilfield waterflood applications and in liquid waste disposal through reinjection. Using Duhamel's principle, we extend the Gordeyev and Entov (1997) self-similar 2D solution of pressure diffusion from a growing fracture to the case of variable injection pressure. The flow of water injected into a low-permeability rock is almost perpendicular to the fracture for a time sufficiently long to be of practical interest. We revisit Carter's model of 1D fluid injection (Howard and Fast, 1957) and extend it to the case of variable injection pressure. We express the cumulative injection through the injection pressure and effective fracture area. Maintaining fluid injection above a reasonable minimal value leads inevitably to fracture growth regardless of the injector design and the injection policy. The average rate of fracture growth can be predicted from early injection. A smart injection controller that can prevent rapid fracture growth is needed.

Lossy transmission line model of hydrofractured well dynamics

The real-time detection of hydrofracture growth is crucial to the successful operation of water, CO 2 or steam injection wells in low-permeability reservoirs and to the prevention of subsidence and well failure. In this paper, we describe propagation of very low frequency (1–10 to 100 Hz) Stoneley waves in a fluid-filled wellbore and their interactions with the fundamental wave mode in a vertical hydrofracture. We demonstrate that Stoneley-wave loses energy to the fracture and the energy transfer from the wellbore to the fracture opening is most efficient in soft rocks. We conclude that placing the wave source and receivers beneath the injection packer provides the most efficient means of hydrofracture monitoring. We then present the lossy transmission line model of wellbore and fracture for the detection and characterization of fracture state and volume. We show that this model captures the wellbore and fracture geometry, the physical properties of injected fluid and the wellbore-fracture system dynamics. The model is then compared with experimentally measured well responses. The simulated responses are in good agreement with published experimental data from several water injection wells with depths ranging from 1000 ft to 9000 ft. Hence, we conclude that the transmission line model of water injectors adequately captures wellbore and fracture dynamics. Using an extensive data set for the South Belridge Diatomite waterfloods, we demonstrate that even for very shallow wells the fracture size and state can be adequately recognized at wellhead. Finally, we simulate the effects of hydrofracture extension on the transient response to a pulse signal generated at wellhead. We show that hydrofracture extensions can indeed be detected by monitoring the wellhead pressure at sufficiently low frequencies.

Fracture Diagnostics Results for the First Multiwell Experiment's Paludal Zone Stimulation

Summary A dual seismic system designed to monitor the growth of hydrofractures was fielded during the first stimulation at the U.S. DOE-sponsored Multiwell Experiment (MWX) during Dec. 1983. The MWX consists of three 8,000-ft [2438-m] wells drilled in low-permeability gas reservoirs. The wellbores are spaced less than 200 ft [61 m] apart over most of their length. With this configuration, microseismic activity created by the fracture treatment in one well could be observed with downhole geophone packages in the two adjacent wells. Responses of the two triaxial borehole seismic tools (BST's) were monitored in real time. Upon detection of a seismic event of known location, orientations of the BST's were determined from the polarization of the compressional wave arrivals. From knowledge of the horizontal and vertical angles of incidence and an empirical estimate of seismic velocities, the locations of the fracture-induced seismic sources were estimated. Thus the location of the microseisms could be estimated and, in turn, the fracture height and extent inferred. This paper presents the results of fracture diagnostics by use of this borehole seismic system for the first MWX stimulation. This information is compared and combined with the results obtained from other diagnostic techniques to provide an integrated estimate of the fracture geometry resulting from the stimulation.

Studying hydrofractures by high frequency seismic monitoring

Two experiments were carried out in salt and granite to address several questions regarding the utility of seismological techniques for monitoring the propagation of a hydrofracture. More specifically, could techniques developed for earthquake analysis be used to characterize the detectable acoustic emission (AE) activity associated with the hydrofracture process, and could the AE activity be used to define the geometry of the hydrofracture, and characterize its manner of propagation? To answer these and other questions, 3-D arrays of piezoelectric sensors were placed around the immediate hydrofracture zones in a small scale laboratory experiment (300 x 300 x 450 mm triaxially confined salt block) and a shallow, 35 m, field experiment (5 x 5 x 5 m granite mine). Confirmation of the seismic techniques was provided by using dyed fluids for the hydrofracture experiments and later visually inspecting the hydrofracture zones by breaking the salt block apart and mining back the rock in the granite mine. Results of the experiments show that there is abundant detectable discrete seismic activity, and that high frequency seismicity can be very useful for determining the path and geometry of the hydrofracture. Tensile failure “events” can be directly related to the actual path of the hydrofracture, whereas the shear failures are not as closely related to the actual fracture path, but are nevertheless related to the overall dimensions of the hydrofracture. Source sizes of individual events as inferred from the seismic data varied from 5 to 7 mm in the salt block experiment (average grain size = 5 m) to 5 to 10 cm in the granite field experiment. Energy release of the events averaged 10 2 N-m (10) 9 dyne-cm) in the laboratory and 10 3 N-m (10 10 dyne-cm) in the field experiment. All events in the laboratory test were primarily tensile failure in nature as inferred from the waveform radiation patterns; however, in the field experiment, there were shear events as well as “harmonic tremor” type events in addition to the tensile failure. The occurrence rate, spatial, and temporal distribution of the events indicate that the hydrofracture growth pattern, at least for these two cases, is not symmetrical and does not follow the often assumed symmetric paths. The data indicate that the hydrofracture process is a complicated phenomenon and that the “fracture” is actually made up of a network of fractures which forms a zone around a main fracture. The overall direction of the hydrofracture and the surrounding zone is strongly influenced by rock inhomogeneities and anisotropy as well as the stress directions.